Inspections

Inspection

For the usage of the phrase "by inspection" in mathematics, see List of mathematical jargon#Proof techniques.
Maintenance check of electronic equipment on a U.S. Navy aircraft.

An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity. The results are usually compared to specified requirements and standards for determining whether the item or activity is in line with these targets, often with a Standard Inspection Procedure in place to ensure consistent checking. Inspections are usually non-destructive.

Inspections may be a visual inspection or involve sensing technologies such as ultrasonic testing, accomplished with a direct physical presence or remotely such as a remote visual inspection, and manually or automatically such as an automated optical inspection. Non-contact optical measurement and Photogrammetry have become common NDT methods for inspection of manufactured components and design optimisation.

A 2007 Scottish Government review of scrutiny of public services (the Crear Review, 2007[1][1]) defined inspection of public services as "... periodic, targeted scrutiny of specific services, to check whether they are meeting national and local performance standards, legislative and professional requirements, and the needs of service users."

A surprise inspection tends to have different results than an announced inspection. Leaders wanting to know how others in their organization perform can drop in without warning, to see directly what happens. If an inspection is made known in advance, it can give people a chance to cover up or to fix mistakes. This could lead to distorted and inaccurate findings. A surprise inspection, therefore, gives inspectors a better picture of the typical state of the inspected object or process than an announced inspection. It also enhances external confidence in the inspection process. See section 4.12 of the Crear report.[1]

Specific inspection

Manufacturing

Inspection and measurement of the thickness of the different layers of an electronic chip using THz and X-ray radiation. THz has the privilege of being non-ionizing (non-destructive) but the resolution of X-ray is higher.[2]

Quality related in-process inspection/verification is an essential part of quality control in manufacturing.

Inspection in manufacturing includes measuring, examining, testing, or gauging one or more characteristics of a product or process and comparing the results with specified requirements to determine whether is the requirements are met for each characteristic.[3][4] Common examples of inspection by measurement or gauging include using a caliper or micrometer to determine if a dimension of a manufactured part is within the dimensional tolerance specified in a drawing for that part, and is thus acceptable for use.

Design for Inspection (DFI) is a concept that should complement and work in collaboration with Design for Manufacturability (DFM) and Design for Assembly (DMA) to reduce product manufacturing cost and increase manufacturing practicality.

Photogrammetry is a modern way of visual inspection, delivering high accuracy and traceability for various industries. The portable 3D system is a versatile optical coordinate measuring machine (CMM) with a wide range of capabilities. Highly accurate point measurements can be taken with inspection carried out directly to CAD models, geometry or drawings.[5](DFI)

Fire Equipment

Most fire equipment needs to be inspected to make sure in the event of a fire, every effort has been taken to make sure it doesn't get out of control. Extinguishers are to be inspected every month by law and inspected by a servicing company at least once a year.Fire extinguishers can be heavy, so it's a good idea to practice picking up and holding an extinguisher to get an idea of the weight and feel.

Business

In international trade several destination countries require pre-shipment inspection. The importer instructs the shipper which inspection company should be used. The inspector makes pictures and a report to certify that the goods that are being shipped and produced are in accordance with the accompanying documents.

Commodity inspection is other term that is used between buyers and sellers. The scope of work for commodity inspection depends to the buyers. Some buyers hire the inspection agencies only for pre-shipment inspections i.e. visual quality, quantity, packing, marking and loading inspections and some others request for higher level inspections and ask inspection agencies to attend in the vendor shops and inspect commodities during manufacturing processes. Normally inspection is done based on an agreed inspection and test plan (ITP).[6]

Government

In government and politics, an inspection is the act of a monitoring authority administering an official review of various criteria (such as documents, facilities, records, and any other assets) that are deemed by the authority to be related to the inspection. Inspections are used for the purpose of determining if a body is complying with regulations. The inspector examines the criteria and talks with involved individuals. A report and evaluation follows such visits.

The Food Safety Inspection Service is charged with ensuring that all meat and egg products in the United States are safe to consume and accurately labeled. The Meat Inspection Act of 1906 authorized the Secretary of Agriculture to order meat inspections and condemn any found unfit for human consumption. The United Nations Monitoring, Verification and Inspection Commission is a regulatory body that inspects for weapons of mass destruction. The Scottish Commission for the Regulation of Care regulates and inspects care services in Scotland.

An Oregon Air National Guardsman makes an inspection of a radio-tower

Road vehicles

A vehicle inspection, e.g., an annual inspection, is a necessary inspection required on vehicles to conform with laws regarding safety, emissions, or both. It consists of an examination of a vehicle's components, usually done by a certified mechanic. Vehicles pass a pre-warranty inspection, if, and only if, a mechanic provide evidence for the proper working condition of the vehicle systems specified in the type of inspection.

Engineering, mechanics

A mechanical inspection is usually undertaken to ensure the safety or reliability of structures or machinery.[7]

In Europe bodies involved in engineering inspection may be assessed by accreditation bodies according to ISO 17020 "General criteria for the operation of various types of bodies performing inspection". This standard defines inspection as "examination of a product, process, service, or installation or their design and determination of its conformity with specific requirements or, on the basis of professional judgment, with general requirements".[8]

Non-destructive examination (NDE) or nondestructive testing (NDT) is a family of technologies used during inspection to analyze materials, components and products for either inherent defects (such as fractures or cracks), or service induced defects (damage from use). Some common methods are visual, industrial computed tomography scanning, microscopy, dye penetrant inspection, magnetic-particle inspection, X-ray or radiographic testing, ultrasonic testing, eddy-current testing, acoustic emission testing, and thermographic inspection. In addition, many non-destructive inspections can be performed by a precision scale, or when in motion, a checkweigher. Stereo microscopes are often used for examining small products like circuit boards for product defects.

Inspection and technical assistance during turnarounds helps to decrease costly downtime as well as ensures restart of operations quickly and safely. [9] [2]

Medical

A medical inspection is the thorough and unhurried visualization of a patient, this requires the use of the naked eye.

Military

An examination vessel is a craft used to inspect ships entering or leaving a port during wartime.

Railroad

The railroad's inspection locomotive were special types of steam locomotives designed to carry railroad officials on inspection tours of the railroad property.

Real estate

A property condition assessment is the examination for purposes of evaluating a commercial or business property's condition often as a part of a due diligence investigation for a company to know what it is buying. Building code officials do a building inspection to determine code compliance in new or altered buildings before issuing a certificate of occupancy. Residential inspections not for code compliance are called a home inspection. There are numerous types of more specific real estate and infrastructure inspections such as windstorm inspection, energy audit, and pipeline video inspection.

Software engineering

Software inspection, in software engineering, refers to peer review of any work product by trained individuals who look for defects using a well defined process.

See also

References

  1. ^ a b Crerar, Professor Lorne D, September 2007 The Crerar Review: The report of the independent review of regulation, audit, inspection and complaints handling of public services in Scotland.
  2. ^ Ahi, Kiarash (2015-05-13). "Terahertz characterization of electronic components and comparison of terahertz imaging with X-ray imaging techniques". SPIE Sensing Technology+ Applications: 94830K-94830K-15. doi:10.1117/12.2183128. 
  3. ^ "Quality Glossary". American Society for Quality (ASQ). Retrieved 2015-02-20. 
  4. ^ AS9100 Revision C, Clause 7.6 Control of Monitoring and Measurement Equipment
  5. ^ Ltd, Digital Parent Company. "3D Scanning Services | Physical Digital". www.physicaldigital.com. Retrieved 2016-05-09. 
  6. ^ "Commodity Inspection Services. Retrieved on 02-18-2013". Inspection-for-industry.com. Retrieved February 18, 2013. 
  7. ^ Antaki, George (2005). Fitness-for-Service Evaluations for Piping and Pressure Vessels: ASME Code Simplified. McGraw-Hill. ISBN 978-0071453998. 
  8. ^ BS EN ISO/IEC 17020: "Conformity assessment - Requirements for the operation of various types of bodies performing inspection", pp. 1 (2012)
  9. ^ Ben-Gal I., Herer Y. and Raz T. (2003). "Self-correcting inspection procedure under inspection errors" (PDF). IIE Transactions on Quality and Reliability, 34(6), pp. 529-540. 

Flux-Cored Arc Welding (FCAW)

Flux-cored arc welding

FCAW wire feeder

Flux-cored arc welding (FCAW or FCA) is a semi-automatic or automatic arc welding process. FCAW requires a continuously-fed consumable tubular electrode containing a flux and a constant-voltage or, less commonly, a constant-current welding power supply. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere, producing both gaseous protection and liquid slag protecting the weld. The process is widely used in construction because of its high welding speed and portability.

FCAW was first developed in the early 1950s as an alternative to shielded metal arc welding (SMAW). The advantage of FCAW over SMAW is that the use of the stick electrodes used in SMAW is unnecessary. This helped FCAW to overcome many of the restrictions associated with SMAW.

Types

One type of FCAW requires no shielding gas. This is made possible by the flux core in the tubular consumable electrode. However, this core contains more than just flux, it also contains various ingredients that when exposed to the high temperatures of welding generate a shielding gas for protecting the arc. This type of FCAW is attractive because it is portable and generally has good penetration into the base metal. Also, windy conditions need not be considered. Some disadvantages are that this process can produce excessive, noxious smoke (making it difficult to see the weld pool); As with all welding processes, the proper electrode must be chosen to obtain the required mechanical properties. Operator skill is a major factor as improper electrode manipulation or machine setup can cause porosity.

A drawing of FCAW at the weld point

Another type of FCAW uses a shielding gas that must be supplied by an external supply. This is known informally as "dual shield" welding. This type of FCAW was developed primarily for welding structural steels. In fact, since it uses both a flux-cored electrode and an external shielding gas, one might say that it is a combination of gas metal (GMAW) and flux-cored arc welding (FCAW). This particular style of FCAW is preferable for welding thicker and out-of-position metals. The slag created by the flux is also easy to remove. The main advantages of this process is that in a closed shop environment, it generally produces welds of better and more consistent mechanical properties, with fewer weld defects than either the SMAW or GMAW processes. In practice it also allows a higher production rate, since the operator does not need to stop periodically to fetch a new electrode, as is the case in SMAW. However, like GMAW, it cannot be used in a windy environment as the loss of the shielding gas from air flow will produce porosity in the weld.

Process variables

  • Wire feed speed (and current)
  • Arc voltage
  • Electrode extension
  • Travel speed and angle
  • Electrode angles
  • Electrode wire type
  • Shielding gas composition (if required)
  • Reverse polarity (Electrode Positive) is used for FCAW Gas-Shielded wire, Straight polarity (Electrode Negative) is used for self shielded FCAW

Advantages and applications

  • FCAW may be an "all-position" process with the right filler metals (the consumable electrode)
  • No shielding gas needed with some wires making it suitable for outdoor welding and/or windy conditions
  • A high-deposition rate process (speed at which the filler metal is applied) in the 1G/1F/2F
  • Some "high-speed" (e.g., automotive) applications
  • As compared to SMAW and GTAW, there is less skill required for operators.
  • Less precleaning of metal required
  • Metallurgical benefits from the flux such as the weld metal being protected initially from external factors until the slag is chipped away
  • Porosity chances very low

Used on the following alloys:

  • Mild and low alloy steels
  • Stainless steels
  • Some high nickel alloys
  • Some wearfacing/surfacing alloys

Disadvantages

Of course, all of the usual issues that occur in welding can occur in FCAW such as incomplete fusion between base metals, slag inclusion (non-metallic inclusions), and cracks in the welds. But there are a few concerns that come up with FCAW that are worth taking special note of:

  • Melted contact tip – when the contact tip actually contacts the base metal, fusing the two and melting the hole on the end
  • Irregular wire feed – typically a mechanical problem
  • Porosity – the gases (specifically those from the flux-core) don’t escape the welded area before the metal hardens, leaving holes in the welded metal
  • More costly filler material/wire as compared to GMAW
  • The equipment is less mobile and more costly as compared to SMAW or GTAW.
  • The amount of smoke generated can far exceed that of SMAW, GMAW, or GTAW.
  • Changing filler metals requires changing an entire spool. This can be slow and difficult as compared to changing filler metal for SMAW or GTAW.
  • Creates more fumes than SMAW.[1]

References

  1. ^ American Society of Safety Engineers, Are Welding Fumes an Occupational Health Risk Factor?

Gas Metal Arc Welding (GMAW)

Gas metal arc welding

Gas metal arc welding

Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a welding process in which an electric arc forms between a consumable wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to melt and join.

Along with the wire electrode, a shielding gas feeds through the welding gun, which shields the process from contaminants in the air. The process can be semi-automatic or automatic. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.

Originally developed for welding aluminium and other non-ferrous materials in the 1940s, GMAW was soon applied to steels because it provided faster welding time compared to other welding processes. The cost of inert gas limited its use in steels until several years later, when the use of semi-inert gases such as carbon dioxide became common. Further developments during the 1950s and 1960s gave the process more versatility and as a result, it became a highly used industrial process. Today, GMAW is the most common industrial welding process, preferred for its versatility, speed and the relative ease of adapting the process to robotic automation. Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding, it is rarely used outdoors or in other areas of air volatility. A related process, flux cored arc welding, often does not use a shielding gas, but instead employs an electrode wire that is hollow and filled with flux.


Development

The principles of gas metal arc welding began to be understood in the early 19th century, after Humphry Davy discovered the short pulsed electric arcs in 1800.[1] Vasily Petrov independently produced the continuous electric arc in 1802 (followed by Davy after 1808).[1] It was not until the 1880s that the technology became developed with the aim of industrial usage. At first, carbon electrodes were used in carbon arc welding. By 1890, metal electrodes had been invented by Nikolay Slavyanov and C. L. Coffin. In 1920, an early predecessor of GMAW was invented by P. O. Nobel of General Electric. It used a bare electrode wire and direct current, and used arc voltage to regulate the feed rate. It did not use a shielding gas to protect the weld, as developments in welding atmospheres did not take place until later that decade. In 1926 another forerunner of GMAW was released, but it was not suitable for practical use.[2]

In 1948, GMAW was developed by the Battelle Memorial Institute. It used a smaller diameter electrode and a constant voltage power source developed by . It offered a high deposition rate, but the high cost of inert gases limited its use to non-ferrous materials and prevented cost savings. In 1953, the use of carbon dioxide as a welding atmosphere was developed, and it quickly gained popularity in GMAW, since it made welding steel more economical. In 1958 and 1959, the short-arc variation of GMAW was released, which increased welding versatility and made the welding of thin materials possible while relying on smaller electrode wires and more advanced power supplies. It quickly became the most popular GMAW variation.

The spray-arc transfer variation was developed in the early 1960s, when experimenters added small amounts of oxygen to inert gases. More recently, pulsed current has been applied, giving rise to a new method called the pulsed spray-arc variation.[3]

GMAW is one of the most popular welding methods, especially in industrial environments.[4] It is used extensively by the sheet metal industry and, by extension, the automobile industry. There, the method is often used for arc spot welding, thereby replacing riveting or resistance spot welding. It is also popular for automated welding, in which robots handle the workpieces and the welding gun to speed up the manufacturing process.[5] GMAW can be difficult to perform well outdoors, since drafts can dissipate the shielding gas and allow contaminants into the weld;[6] flux cored arc welding is better suited for outdoor use such as in construction.[7][8] Likewise, GMAW's use of a shielding gas does not lend itself to underwater welding, which is more commonly performed via shielded metal arc welding, flux cored arc welding, or gas tungsten arc welding.[9]

Equipment

To perform gas metal arc welding, the basic necessary equipment is a welding gun, a wire feed unit, a welding power supply, a wire, and a shielding gas supply.[10]

Welding gun and wire feed unit

GMAW torch nozzle cutaway image. (1) Torch handle, (2) Molded phenolic dielectric (shown in white) and threaded metal nut insert (yellow), (3) Shielding gas diffuser, (4) Contact tip, (5) Nozzle output face
GMAW on stainless steel

The typical GMAW welding gun has a number of key parts—a control switch, a contact tip, a power cable, a gas nozzle, an electrode conduit and liner, and a gas hose. The control switch, or trigger, when pressed by the operator, initiates the wire feed, electric power, and the shielding gas flow, causing an electric arc to be struck. The contact tip, normally made of copper and sometimes chemically treated to reduce spatter, is connected to the welding power source through the power cable and transmits the electrical energy to the electrode while directing it to the weld area. It must be firmly secured and properly sized, since it must allow the electrode to pass while maintaining electrical contact. On the way to the contact tip, the wire is protected and guided by the electrode conduit and liner, which help prevent buckling and maintain an uninterrupted wire feed. The gas nozzle directs the shielding gas evenly into the welding zone. Inconsistent flow may not adequately protect the weld area. Larger nozzles provide greater shielding gas flow, which is useful for high current welding operations that develop a larger molten weld pool. A gas hose from the tanks of shielding gas supplies the gas to the nozzle. Sometimes, a water hose is also built into the welding gun, cooling the gun in high heat operations.[11]

The wire feed unit supplies the electrode to the work, driving it through the conduit and on to the contact tip. Most models provide the wire at a constant feed rate, but more advanced machines can vary the feed rate in response to the arc length and voltage. Some wire feeders can reach feed rates as high as 30.5 m/min (1200 in/min),[12] but feed rates for semiautomatic GMAW typically range from 2 to 10 m/min (75 – 400 in/min).[13]

Tool style

The most common electrode holder is a semiautomatic air-cooled holder. Compressed air circulates through it to maintain moderate temperatures. It is used with lower current levels for welding lap or butt joints. The second most common type of electrode holder is semiautomatic water-cooled, where the only difference is that water takes the place of air. It uses higher current levels for welding T or corner joints. The third typical holder type is a water cooled automatic electrode holder—which is typically used with automated equipment.[14]

Power supply

Most applications of gas metal arc welding use a constant voltage power supply. As a result, any change in arc length (which is directly related to voltage) results in a large change in heat input and current. A shorter arc length causes a much greater heat input, which makes the wire electrode melt more quickly and thereby restore the original arc length. This helps operators keep the arc length consistent even when manually welding with hand-held welding guns. To achieve a similar effect, sometimes a constant current power source is used in combination with an arc voltage-controlled wire feed unit. In this case, a change in arc length makes the wire feed rate adjust to maintain a relatively constant arc length. In rare circumstances, a constant current power source and a constant wire feed rate unit might be coupled, especially for the welding of metals with high thermal conductivities, such as aluminum. This grants the operator additional control over the heat input into the weld, but requires significant skill to perform successfully.[15]

Alternating current is rarely used with GMAW; instead, direct current is employed and the electrode is generally positively charged. Since the anode tends to have a greater heat concentration, this results in faster melting of the feed wire, which increases weld penetration and welding speed. The polarity can be reversed only when special emissive-coated electrode wires are used, but since these are not popular, a negatively charged electrode is rarely employed.[16]

Electrode

Electrode selection is based primarily on the composition of the metal being welded, the process variation being used, joint design and the material surface conditions. Electrode selection greatly influences the mechanical properties of the weld and is a key factor of weld quality. In general the finished weld metal should have mechanical properties similar to those of the base material with no defects such as discontinuities, entrained contaminants or porosity within the weld. To achieve these goals a wide variety of electrodes exist. All commercially available electrodes contain deoxidizing metals such as silicon, manganese, titanium and aluminum in small percentages to help prevent oxygen porosity. Some contain denitriding metals such as titanium and zirconium to avoid nitrogen porosity.[17] Depending on the process variation and base material being welded the diameters of the electrodes used in GMAW typically range from 0.7 to 2.4 mm (0.028 – 0.095 in) but can be as large as 4 mm (0.16 in). The smallest electrodes, generally up to 1.14 mm (0.045 in)[18] are associated with the short-circuiting metal transfer process, while the most common spray-transfer process mode electrodes are usually at least 0.9 mm (0.035 in).[19][20]

Shielding gas

GMAW Circuit diagram. (1) Welding torch, (2) Workpiece, (3) Power source, (4) Wire feed unit, (5) Electrode source, (6) Shielding gas supply.

Shielding gases are necessary for gas metal arc welding to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. This problem is common to all arc welding processes; for example, in the older Shielded-Metal Arc Welding process (SMAW), the electrode is coated with a solid flux which evolves a protective cloud of carbon dioxide when melted by the arc. In GMAW, however, the electrode wire does not have a flux coating, and a separate shielding gas is employed to protect the weld. This eliminates slag, the hard residue from the flux that builds up after welding and must be chipped off to reveal the completed weld.[21]

The choice of a shielding gas depends on several factors, most importantly the type of material being welded and the process variation being used. Pure inert gases such as argon and helium are only used for nonferrous welding; with steel they do not provide adequate weld penetration (argon) or cause an erratic arc and encourage spatter (with helium). Pure carbon dioxide, on the other hand, allows for deep penetration welds but encourages oxide formation, which adversely affect the mechanical properties of the weld. lts low cost makes it an attractive choice, but because of the reactivity of the arc plasma, spatter is unavoidable and welding thin materials is difficult. As a result, argon and carbon dioxide are frequently mixed in a 75%/25% to 90%/10% mixture. Generally, in short circuit GMAW, higher carbon dioxide content increases the weld heat and energy when all other weld parameters (volts, current, electrode type and diameter) are held the same. As the carbon dioxide content increases over 20%, spray transfer GMAW becomes increasingly problematic, especially with smaller electrode diameters.[22]

Argon is also commonly mixed with other gases, oxygen, helium, hydrogen and nitrogen. The addition of up to 5% oxygen (like the higher concentrations of carbon dioxide mentioned above) can be helpful in welding stainless steel, however, in most applications carbon dioxide is preferred.[23] Increased oxygen makes the shielding gas oxidize the electrode, which can lead to porosity in the deposit if the electrode does not contain sufficient deoxidizers. Excessive oxygen, especially when used in application for which it is not prescribed, can lead to brittleness in the heat affected zone. Argon-helium mixtures are extremely inert, and can be used on nonferrous materials. A helium concentration of 50–75% raises the required voltage and increases the heat in the arc, due to helium's higher ionization temperature. Hydrogen is sometimes added to argon in small concentrations (up to about 5%) for welding nickel and thick stainless steel workpieces. In higher concentrations (up to 25% hydrogen), it may be used for welding conductive materials such as copper. However, it should not be used on steel, aluminum or magnesium because it can cause porosity and hydrogen embrittlement.[21]

Shielding gas mixtures of three or more gases are also available. Mixtures of argon, carbon dioxide and oxygen are marketed for welding steels. Other mixtures add a small amount of helium to argon-oxygen combinations, these mixtures are claimed to allow higher arc voltages and welding speed. Helium also sometimes serves as the base gas, with small amounts of argon and carbon dioxide added. However, because it is less dense than air, helium is less effective at shielding the weld than argon—which is denser than air. It also can lead to arc stability and penetration issues, and increased spatter, due to its much more energetic arc plasma. Helium is also substantially more expensive than other shielding gases. Other specialized and often proprietary gas mixtures claim even greater benefits for specific applications.[21]

The desirable rate of shielding-gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode. Welding flat surfaces requires higher flow than welding grooved materials, since gas disperses more quickly. Faster welding speeds, in general, mean that more gas must be supplied to provide adequate coverage. Additionally, higher current requires greater flow, and generally, more helium is required to provide adequate coverage than if argon is used. Perhaps most importantly, the four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft³/h) is generally suitable, whereas for globular transfer, around 15 L/min (30 ft³/h) is preferred. The spray transfer variation normally requires more shielding-gas flow because of its higher heat input and thus larger weld pool. Typical gas-flow amounts are approximately 20–25 L/min (40–50 ft³/h).[13]

Flux-cored wire-fed

Flux-cored, self-shielding or gasless wire-fed welding had been developed for simplicity and portability.[24] This avoids the gas system of conventional GMAW and uses a cored wire containing a solid flux. This flux vaporises during welding and produces a plume of shielding gas. Although described as a 'flux', this compound has little activity and acts mostly as an inert shield. The wire is of slightly larger diameter than for a comparable gas-shielded weld, to allow room for the flux. The smallest available is 0.8 mm diameter, compared to 0.6 mm for solid wire. The shield vapor is slightly active, rather than inert, so the process is always MAGS but not MIG (inert gas shield). This limits the process to steel and not aluminium.

Vaporising the additional flux requires greater heat in the wire, so these gasless machines operate as DCEP, rather than the DCEN usually used for GMAW to give deeper penetration.[24] DCEP, or DC Electrode Positive, makes the welding wire into the positively-charged anode, which is the hotter side of the arc.[25] Provided that it is switchable from DCEN to DCEP, a gas-shielded wire-feed machine may also be used for flux-cored wire.

Flux-cored wire is considered to have some advantages for outdoor welding on-site, as the shielding gas plume is less likely to be blown away in a wind than shield gas from a conventional nozzle.[26][27] A slight drawback is that, like SMAW (stick) welding, there may be some flux deposited over the weld bead, requiring more of a cleaning process between passes.[26]

Flux-cored welding machines are most popular at the hobbyist level, as the machines are slightly simpler but mainly because they avoid the cost of providing shield gas, either through a rented cylinder or with the high cost of disposable cylinders.[26]

GMAW-based 3-D printing

GMAW has also been used as a low-cost method to 3-D print metal objects. [28][29][30] Various open source 3-D printers have been developed to use GMAW. [31] Such components fabricated from aluminum compete with more traditionally manufactured components on mechanical strength. [32] By forming a bad weld on the first layer, GMAW 3-D printed parts can be removed from the substrate with a hammer.[33][34]

Operation

GMAW weld area. (1) Direction of travel, (2) Contact tube, (3) Electrode, (4) Shielding gas, (5) Molten weld metal, (6) Solidified weld metal, (7) Workpiece.

For most of its applications gas metal arc welding is a fairly simple welding process to learn requiring no more than a week or two to master basic welding technique. Even when welding is performed by well-trained operators weld quality can fluctuate since it depends on a number of external factors. All GMAW is dangerous, though perhaps less so than some other welding methods, such as shielded metal arc welding.[35]

Technique

The basic technique for GMAW is quite simple, since the electrode is fed automatically through the torch (head of tip). By contrast, in gas tungsten arc welding, the welder must handle a welding torch in one hand and a separate filler wire in the other, and in shielded metal arc welding, the operator must frequently chip off slag and change welding electrodes. GMAW requires only that the operator guide the welding gun with proper position and orientation along the area being welded. Keeping a consistent contact tip-to-work distance (the stick out distance) is important, because a long stickout distance can cause the electrode to overheat and also wastes shielding gas. Stickout distance varies for different GMAW weld processes and applications.[36][37][38][39] The orientation of the gun is also important—it should be held so as to bisect the angle between the workpieces; that is, at 45 degrees for a fillet weld and 90 degrees for welding a flat surface. The travel angle, or lead angle, is the angle of the torch with respect to the direction of travel, and it should generally remain approximately vertical. However, the desirable angle changes somewhat depending on the type of shielding gas used—with pure inert gases, the bottom of the torch is often slightly in front of the upper section, while the opposite is true when the welding atmosphere is carbon dioxide.[40]

Quality

Two of the most prevalent quality problems in GMAW are dross and porosity. If not controlled, they can lead to weaker, less ductile welds. Dross is an especially common problem in aluminium GMAW welds, normally coming from particles of aluminium oxide or aluminum nitride present in the electrode or base materials. Electrodes and workpieces must be brushed with a wire brush or chemically treated to remove oxides on the surface. Any oxygen in contact with the weld pool, whether from the atmosphere or the shielding gas, causes dross as well. As a result, sufficient flow of inert shielding gases is necessary, and welding in volatile air should be avoided.[41]

In GMAW the primary cause of porosity is gas entrapment in the weld pool, which occurs when the metal solidifies before the gas escapes. The gas can come from impurities in the shielding gas or on the workpiece, as well as from an excessively long or violent arc. Generally, the amount of gas entrapped is directly related to the cooling rate of the weld pool. Because of its higher thermal conductivity, aluminum welds are especially susceptible to greater cooling rates and thus additional porosity. To reduce it, the workpiece and electrode should be clean, the welding speed diminished and the current set high enough to provide sufficient heat input and stable metal transfer but low enough that the arc remains steady. Preheating can also help reduce the cooling rate in some cases by reducing the temperature gradient between the weld area and the base material.[42]

Safety

Gas metal arc welding can be dangerous if proper precautions are not taken. Since GMAW employs an electric arc, welders wear protective clothing, including heavy leather gloves and protective long sleeve jackets, to avoid exposure to extreme heat and flames. In addition, the brightness of the electric arc is a source of the condition known as arc eye, an inflammation of the cornea caused by ultraviolet light and, in prolonged exposure, possible burning of the retina in the eye. Conventional welding helmets contain dark face plates to prevent this exposure. Newer helmet designs feature a liquid crystal-type face plate that self-darken upon exposure to high amounts of UV light. Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.[43]

Welders are also often exposed to dangerous gases and particulate matter. GMAW produces smoke containing particles of various types of oxides, and the size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, carbon dioxide and ozone gases can prove dangerous if ventilation is inadequate. Furthermore, because the use of compressed gases in GMAW pose an explosion and fire risk, some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.[44]

Metal transfer modes

The three transfer modes in GMAW are globular, short-circuiting, and spray. There are a few recognized variations of these three transfer modes including modified short-circuiting and pulsed-spray.[45]

Globular

GMAW with globular metal transfer is considered the least desirable of the three major GMAW variations, because of its tendency to produce high heat, a poor weld surface, and spatter. The method was originally developed as a cost efficient way to weld steel using GMAW, because this variation uses carbon dioxide, a less expensive shielding gas than argon. Adding to its economic advantage was its high deposition rate, allowing welding speeds of up to 110 mm/s (250 in/min).[46] As the weld is made, a ball of molten metal from the electrode tends to build up on the end of the electrode, often in irregular shapes with a larger diameter than the electrode itself. When the droplet finally detaches either by gravity or short circuiting, it falls to the workpiece, leaving an uneven surface and often causing spatter.[47] As a result of the large molten droplet, the process is generally limited to flat and horizontal welding positions, requires thicker workpieces, and results in a larger weld pool.[48][49]

Short-circuiting

Further developments in welding steel with GMAW led to a variation known as short-circuit transfer (SCT) or short-arc GMAW, in which the current is lower than for the globular method. As a result of the lower current, the heat input for the short-arc variation is considerably reduced, making it possible to weld thinner materials while decreasing the amount of distortion and residual stress in the weld area. As in globular welding, molten droplets form on the tip of the electrode, but instead of dropping to the weld pool, they bridge the gap between the electrode and the weld pool as a result of the lower wire feed rate. This causes a short circuit and extinguishes the arc, but it is quickly reignited after the surface tension of the weld pool pulls the molten metal bead off the electrode tip. This process is repeated about 100 times per second, making the arc appear constant to the human eye. This type of metal transfer provides better weld quality and less spatter than the globular variation, and allows for welding in all positions, albeit with slower deposition of weld material. Setting the weld process parameters (volts, amps and wire feed rate) within a relatively narrow band is critical to maintaining a stable arc: generally between 100 and 200 amperes at 17 to 22 volts for most applications. Also, using short-arc transfer can result in lack of fusion and insufficient penetration when welding thicker materials, due to the lower arc energy and rapidly freezing weld pool.[50] Like the globular variation, it can only be used on ferrous metals.[20][51][52]

Cold Metal Transfer

For thin materials, Cold Metal Transfer (CMT) is used by reducing the current when a short circuit is registered, producing many drops per second. CMT can be used for aluminum.

Spray

Spray transfer GMAW was the first metal transfer method used in GMAW, and well-suited to welding aluminium and stainless steel while employing an inert shielding gas. In this GMAW process, the weld electrode metal is rapidly passed along the stable electric arc from the electrode to the workpiece, essentially eliminating spatter and resulting in a high-quality weld finish. As the current and voltage increases beyond the range of short circuit transfer the weld electrode metal transfer transitions from larger globules through small droplets to a vaporized stream at the highest energies.[53] Since this vaporized spray transfer variation of the GMAW weld process requires higher voltage and current than short circuit transfer, and as a result of the higher heat input and larger weld pool area (for a given weld electrode diameter), it is generally used only on workpieces of thicknesses above about 6.4 mm (0.25 in).[54]

Also, because of the large weld pool, it is often limited to flat and horizontal welding positions and sometimes also used for vertical-down welds. It is generally not practical for root pass welds.[55] When a smaller electrode is used in conjunction with lower heat input, its versatility increases. The maximum deposition rate for spray arc GMAW is relatively high—about 60 mm/s (150 in/min).[20][46][56]

Pulsed-spray

A variation of the spray transfer mode, pulse-spray is based on the principles of spray transfer but uses a pulsing current to melt the filler wire and allow one small molten droplet to fall with each pulse. The pulses allow the average current to be lower, decreasing the overall heat input and thereby decreasing the size of the weld pool and heat-affected zone while making it possible to weld thin workpieces. The pulse provides a stable arc and no spatter, since no short-circuiting takes place. This also makes the process suitable for nearly all metals, and thicker electrode wire can be used as well. The smaller weld pool gives the variation greater versatility, making it possible to weld in all positions. In comparison with short arc GMAW, this method has a somewhat slower maximum speed (85 mm/s or 200 in/min) and the process also requires that the shielding gas be primarily argon with a low carbon dioxide concentration. Additionally, it requires a special power source capable of providing current pulses with a frequency between 30 and 400 pulses per second. However, the method has gained popularity, since it requires lower heat input and can be used to weld thin workpieces, as well as nonferrous materials.[20][57][58][59]

References

  1. ^ a b Anders 2003, pp. 1060–9
  2. ^ Cary & Helzer 2005, p. 7
  3. ^ Cary & Helzer 2005, pp. 8–9
  4. ^ Jeffus 1997, p. 6
  5. ^ Kalpakjian & Schmid 2001, p. 783
  6. ^ Davies 2003, p. 174
  7. ^ Jeffus 1997, p. 264
  8. ^ Davies 2003, p. 118
  9. ^ Davies 2003, p. 253
  10. ^ Miller Electric Mfg Co 2012, p. 5
  11. ^ Nadzam 1997, pp. 5–6
  12. ^ Nadzam 1997, p. 6
  13. ^ a b Cary & Helzer 2005, pp. 123–5
  14. ^ Todd, Allen & Alting 1994, pp. 351–355.
  15. ^ Nadzam 1997, p. 1
  16. ^ Cary & Helzer 2005, pp. 118–9
  17. ^ Nadzam 1997, p. 15
  18. ^ Craig 1991, p. 22
  19. ^ Craig 1991, p. 105
  20. ^ a b c d Cary & Helzer 2005, p. 121
  21. ^ a b c Cary & Helzer 2005, pp. 357–9.
  22. ^ Craig 1991, p. 96
  23. ^ Craig 1991, pp. 40–1
  24. ^ a b Greg Holster. "Gasless wire welding is a breeze" (PDF). pp. 64–68. 
  25. ^ "Welding Metallurgy: Arc Physics and Weld Pool Behaviour" (PDF). Canteach. 
  26. ^ a b c "How to weld with flux cored wire". MIG Welding - The DIY Guide. 
  27. ^ "Gas Vs Gasless Mig Welding, what’s the difference". Welder's Warehouse. 4 October 2014. 
  28. ^ Loose screw? 3-D printer may soon forge you a new one http://www.nbcnews.com/technology/loose-screw-3-d-printer-may-soon-forge-you-new-2D11678840
  29. ^ You Can Now 3D Print with Metal at Home http://motherboard.vice.com/blog/you-can-now-3d-print-with-metal-at-home
  30. ^ Gerald C. Anzalone, Chenlong Zhang, Bas Wijnen, Paul G. Sanders and Joshua M. Pearce, “Low-Cost Open-Source 3-D Metal PrintingIEEE Access, 1, pp.803-810, (2013). doi: 10.1109/ACCESS.2013.2293018
  31. ^ Yuenyong Nilsiam, Amberlee Haselhuhn, Bas Wijnen, Paul Sanders, & Joshua M. Pearce. Integrated Voltage - Current Monitoring and Control of Gas Metal Arc Weld Magnetic Ball-Jointed Open Source 3-D Printer.Machines 3(4), 339-351 (2015). doi:10.3390/machines3040339
  32. ^ Amberlee S. Haselhuhn, Michael W. Buhr, Bas Wijnen, Paul G. Sanders, Joshua M. Pearce, Structure-Property Relationships of Common Aluminum Weld Alloys Utilized as Feedstock for GMAW-based 3-D Metal Printing. Materials Science and Engineering: A, 673, pp. 511–523 (2016). DOI: 10.1016/j.msea.2016.07.099
  33. ^ Amberlee S. Haselhuhn, Bas Wijnen, Gerald C. Anzalone, Paul G. Sanders, Joshua M. Pearce, In Situ Formation of Substrate Release Mechanisms for Gas Metal Arc Weld Metal 3-D Printing. Journal of Materials Processing Technology. 226, pp. 50–59 (2015).
  34. ^ Amberlee S. Haselhuhn, Eli J. Gooding, Alexandra G. Glover, Gerald C. Anzalone, Bas Wijnen, Paul G. Sanders, Joshua M. Pearce. Substrate Release Mechanisms for Gas Metal Arc 3-D Aluminum Metal Printing. 3D Printing and Additive Manufacturing. 1(4): 204-209 (2014). DOI: 10.1089/3dp.2014.0015
  35. ^ Cary & Helzer 2005, p. 126
  36. ^ Craig 1991, p. 29
  37. ^ Craig 1991, p. 52
  38. ^ Craig 1991, p. 109
  39. ^ Craig 1991, p. 141
  40. ^ Cary & Helzer 2005, p. 125
  41. ^ Lincoln Electric 1994, 9.3-5 – 9.3-6
  42. ^ Lincoln Electric 1994, 9.3-1 – 9.3-2
  43. ^ Cary & Helzer 2005, p. 42
  44. ^ Cary & Helzer 2005, pp. 52–62
  45. ^ American Welding Society 2004, p. 150
  46. ^ a b Cary & Helzer 2005, p. 117
  47. ^ Weman 2003, p. 50
  48. ^ Miller Electric Mfg Co 2012, p. 14
  49. ^ Nadzam 1997, p. 8
  50. ^ Craig 1991, p. 11
  51. ^ Cary & Helzer 2005, p. 98
  52. ^ Weman 2003, pp. 49–50
  53. ^ Craig 1991, p. 82
  54. ^ Craig 1991, p. 90
  55. ^ Craig 1991, p. 98
  56. ^ Cary & Helzer 2005, p. 96
  57. ^ Cary & Helzer 2005, p. 99
  58. ^ Cary & Helzer 2005, p. 118
  59. ^ American Welding Society 2004, p. 154

Bibliography

  • American Welding Society (2004). Welding Handbook, Welding Processes, Part 1. Miami: American Welding Society. ISBN 0-87171-729-8. 
  • Anders, A. (2003). "Tracking down the origin of arc plasma science-II. early continuous discharges". IEEE Transactions on Plasma Science. 31 (5): 1060–9. doi:10.1109/TPS.2003.815477. 
  • Cary, Howard B.; Helzer, Scott C. (2005). Modern Welding Technology. Upper Saddle River, New Jersey: Pearson Education. ISBN 0-13-113029-3. 
  • Craig, Ed (1991). Gas Metal Arc & Flux Cored Welding Parameters. Chicago: Weldtrain. ISBN 978-0-9753621-0-5. 
  • Davies, Arthur Cyril (2003). The Science and Practice of Welding. Cambridge University Press. ISBN 0-521-43566-8. 
  • Jeffus, Larry F. (1997). Welding: Principles and Applications. Cengage Learning. ISBN 978-08-2738-240-4. 
  • Kalpakjian, Serope; Schmid, Steven R. (2001). Manufacturing Engineering and Technology. Prentice Hall. ISBN 0-201-36131-0. 
  • Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 978-99949-25-82-7. 
  • Miller Electric Mfg Co (2012). Guidelines For Gas Metal Arc Welding (GMAW) (PDF). Appleton, WI: Miller Electric Mfg Co. 
  • Nadzam, Jeff, ed. (1997). Gas Metal Arc Welding Guidelines (PDF). Lincoln Electric. 
  • Todd, Robert H.; Allen, Dell K.; Alting, Leo (1994). Manufacturing processes reference guide. New York: Industrial Press. ISBN 978-0-8311-3049-7. 
  • Weman, Klas (2003). Welding processes handbook. New York: CRC Press LLC. ISBN 0-8493-1773-8. 

Further reading

  • Blunt, Jane; Balchin, Nigel C. (2002). Health and Safety in Welding and Allied Processes. Cambridge, UK: Woodhead. ISBN 1-85573-538-5. 
  • Hicks, John (1999). Welded Joint Design. Industrial Press. ISBN 0-8311-3130-6. 
  • Minnick, William H. (2007). Gas Metal Arc Welding Handbook Textbook. Tinley Park: Goodheart–Willcox. ISBN 978-1-59070-866-8. 
  • Trends in Welding Research. Materials Park, Ohio: ASM International. 2003. ISBN 0-87170-780-2. 

External links

Gas tungsten arc welding (GTAW)

Gas tungsten arc welding

TIG welding of a bronze sculpture

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode is protected from oxidation or other atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.[1]

Development

After the discovery of the short pulsed electric arc in 1800 by Humphry Davy[2][3] and of the continuous electric arc in 1802 by Vasily Petrov,[3][4] arc welding developed slowly. C. L. Coffin had the idea of welding in an inert gas atmosphere in 1890, but even in the early 20th century, welding non-ferrous materials such as aluminum and magnesium remained difficult because these metals react rapidly with the air and result in porous, dross-filled welds.[5] Processes using flux-covered electrodes did not satisfactorily protect the weld area from contamination. To solve the problem, bottled inert gases were used in the beginning of the 1930s. A few years later, a direct current, gas-shielded welding process emerged in the aircraft industry for welding magnesium.[6]

Russell Meredith of Northrop Aircraft perfected the process in 1941.[7] Meredith named the process Heliarc because it used a tungsten electrode arc and helium as a shielding gas, but it is often referred to as tungsten inert gas welding (TIG). The American Welding Society's official term is gas tungsten arc welding (GTAW). Linde Air Products developed a wide range of air-cooled and water-cooled torches, gas lenses to improve shielding, and other accessories that increased the use of the process. Initially, the electrode overheated quickly and, despite tungsten's high melting temperature, particles of tungsten were transferred to the weld.[6] To address this problem, the polarity of the electrode was changed from positive to negative, but the change made it unsuitable for welding many non-ferrous materials. Finally, the development of alternating current units made it possible to stabilize the arc and produce high quality aluminum and magnesium welds.[6][8]

Developments continued during the following decades. Linde developed water-cooled torches that helped prevent overheating when welding with high currents.[9] During the 1950s, as the process continued to gain popularity, some users turned to carbon dioxide as an alternative to the more expensive welding atmospheres consisting of argon and helium, but this proved unacceptable for welding aluminum and magnesium because it reduced weld quality, so it is rarely used with GTAW today.[10] The use of any shielding gas containing an oxygen compound, such as carbon dioxide, quickly contaminates the tungsten electrode, making it unsuitable for the TIG process.[11] In 1953, a new process based on GTAW was developed, called plasma arc welding. It affords greater control and improves weld quality by using a nozzle to focus the electric arc, but is largely limited to automated systems, whereas GTAW remains primarily a manual, hand-held method.[10] Development within the GTAW process has continued as well, and today a number of variations exist. Among the most popular are the pulsed-current, manual programmed, hot-wire, dabber, and increased penetration GTAW methods.[12]

Operation

GTAW weld area

Manual gas tungsten arc welding is a relatively difficult welding method, due to the coordination required by the welder. Similar to torch welding, GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. Maintaining a short arc length, while preventing contact between the electrode and the workpiece, is also important.[13]

To strike the welding arc, a high frequency generator (similar to a Tesla coil) provides an electric spark. This spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the workpiece are separated, typically about 1.5–3 mm (0.06–0.12 in) apart.[14]

Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. While maintaining a constant separation between the electrode and the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed.[14]

Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is always kept inside the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with a low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld.[15][16]

Safety

Welders wear protective clothing, including light and thin leather gloves and protective long sleeve shirts with high collars, to avoid exposure to strong ultraviolet light. Due to the absence of smoke in GTAW, the electric arc light is not covered by fumes and particulate matter as in stick welding or shielded metal arc welding, and thus is a great deal brighter, subjecting operators to strong ultraviolet light. The welding arc has a different range and strength of UV light wavelengths from sunlight, but the welder is very close to the source and the light intensity is very strong. Potential arc light damage includes accidental flashes to the eye or arc eye and skin damage similar to strong sunburn. Operators wear opaque helmets with dark eye lenses and full head and neck coverage to prevent this exposure to UV light. Modern helmets often feature a liquid crystal-type face plate that self-darkens upon exposure to the bright light of the struck arc. Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.[17]

Welders are also often exposed to dangerous gases and particulate matter. While the process doesn't produce smoke, the brightness of the arc in GTAW can break down surrounding air to form ozone and nitric oxides. The ozone and nitric oxides react with lung tissue and moisture to create nitric acid and ozone burn. Ozone and nitric oxide levels are moderate, but exposure duration, repeated exposure, and the quality and quantity of fume extraction, and air change in the room must be monitored. Welders who do not work safely can contract emphysema and oedema of the lungs, which can lead to early death. Similarly, the heat from the arc can cause poisonous fumes to form from cleaning and degreasing materials. Cleaning operations using these agents should not be performed near the site of welding, and proper ventilation is necessary to protect the welder.[17]

Applications

While the aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing such as those used in the bicycle industry. In addition, GTAW is often used to make root or first-pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium.[18] Because the weld metal is not transferred directly across the electric arc like most open arc welding processes, a vast assortment of welding filler metal is available to the welding engineer. In fact, no other welding process permits the welding of so many alloys in so many product configurations. Filler metal alloys, such as elemental aluminum and chromium, can be lost through the electric arc from volatilization. This loss does not occur with the GTAW process. Because the resulting welds have the same chemical integrity as the original base metal or match the base metals more closely, GTAW welds are highly resistant to corrosion and cracking over long time periods, making GTAW the welding procedure of choice for critical operations like sealing spent nuclear fuel canisters before burial.[19]

Quality

GTAW fillet weld

Gas tungsten arc welding, because it affords greater control over the weld area than other welding processes, can produce high-quality welds when performed by skilled operators. Maximum weld quality is assured by maintaining cleanliness—all equipment and materials used must be free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality. To remove oil and grease, alcohol or similar commercial solvents may be used, while a stainless steel wire brush or chemical process can remove oxides from the surfaces of metals like aluminum. Rust on steels can be removed by first grit blasting the surface and then using a wire brush to remove any embedded grit. These steps are especially important when negative polarity direct current is used, because such a power supply provides no cleaning during the welding process, unlike positive polarity direct current or alternating current.[20] To maintain a clean weld pool during welding, the shielding gas flow should be sufficient and consistent so that the gas covers the weld and blocks impurities in the atmosphere. GTAW in windy or drafty environments increases the amount of shielding gas necessary to protect the weld, increasing the cost and making the process unpopular outdoors.[21]

The level of heat input also affects weld quality. Low heat input, caused by low welding current or high welding speed, can limit penetration and cause the weld bead to lift away from the surface being welded. If there is too much heat input, however, the weld bead grows in width while the likelihood of excessive penetration and spatter increase. Additionally, if the welding torch is too far from the workpiece the shielding gas becomes ineffective, causing porosity within the weld. This results in a weld with pinholes, which is weaker than a typical weld.[21]

If the amount of current used exceeds the capability of the electrode, tungsten inclusions in the weld may result. Known as tungsten spitting, this can be identified with radiography and can be prevented by changing the type of electrode or increasing the electrode diameter. In addition, if the electrode is not well protected by the gas shield or the operator accidentally allows it to contact the molten metal, it can become dirty or contaminated. This often causes the welding arc to become unstable, requiring that the electrode be ground with a diamond abrasive to remove the impurity.[21]

Equipment

GTAW torch with various electrodes, cups, collets and gas diffusers
GTAW torch, disassembled

The equipment required for the gas tungsten arc welding operation includes a welding torch utilizing a non-consumable tungsten electrode, a constant-current welding power supply, and a shielding gas source.

Welding torch

GTAW welding torches are designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. Air cooling systems are most often used for low-current operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A). The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.[22]

The internal metal parts of a torch are made of hard alloys of copper or brass so it can transmit current and heat effectively. The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. Collets are sized according to the diameter of the tungsten electrode they hold. The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder.[22]

The size of the welding torch nozzle depends on the amount of shielded area desired. The size of the gas nozzle depends upon the diameter of the electrode, the joint configuration, and the availability of access to the joint by the welder. The inside diameter of the nozzle is preferably at least three times the diameter of the electrode, but there are no hard rules. The welder judges the effectiveness of the shielding and increases the nozzle size to increase the area protected by the external gas shield as needed. The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a high purity glass, offers greater visibility. Devices can be inserted into the nozzle for special applications, such as gas lenses or valves to improve the control shielding gas flow to reduce turbulence and introduction of contaminated atmosphere into the shielded area. Hand switches to control welding current can be added to the manual GTAW torches.[22]

Power supply

Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.[23]

GTAW power supply

The preferred polarity of the GTAW system depends largely on the type of metal being welded. Direct current with a negatively charged electrode (DCEN) is often employed when welding steels, nickel, titanium, and other metals. It can also be used in automatic GTAW of aluminum or magnesium when helium is used as a shielding gas.[24] The negatively charged electrode generates heat by emitting electrons, which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material, and this can allow oxides to build on the surface of the weld.[24] Direct current with a positively charged electrode (DCEP) is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures.[24] To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance.[24]

Alternating current, commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material.[24] Surface oxides are still removed during the electrode-positive portion of the cycle and the base metal is heated more deeply during the electrode-negative portion of the cycle. Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source.[24] In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). To remedy the problem, a square wave power supply can be used, as can high-frequency voltage to encourage ignition.[24]

Electrode

ISO
Class
ISO
Color
AWS
Class
AWS
Color
Alloy[25]
WP Green EWP Green None
WC20 Gray EWCe-2 Orange ~2% CeO2
WL10 Black EWLa-1 Black ~1% La2O3
WL15 Gold EWLa-1.5 Gold ~1.5% La2O3
WL20 Sky-blue EWLa-2 Blue ~2% La2O3
WT10 Yellow EWTh-1 Yellow ~1% ThO2
WT20 Red EWTh-2 Red ~2% ThO2
WT30 Violet ~3% ThO2
WT40 Orange ~4% ThO2
WY20 Blue ~2% Y2O3
WZ3 Brown EWZr-1 Brown ~0.3% ZrO2
WZ8 White ~0.8% ZrO2

The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among pure metals, at 3,422 °C (6,192 °F). As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur. Electrodes can have either a clean finish or a ground finish—clean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction. The diameter of the electrode can vary between 0.5 and 6.4 millimetres (0.02 and 0.25 in), and their length can range from 75 to 610 millimetres (3.0 to 24.0 in).

A number of tungsten alloys have been standardized by the International Organization for Standardization and the American Welding Society in ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes, and are summarized in the adjacent table.

  • Pure tungsten electrodes (classified as WP or EWP) are general purpose and low cost electrodes. They have poor heat resistance and electron emission. They find limited use in AC welding of e.g. magnesium and aluminum.[26]
  • Cerium oxide (or ceria) as an alloying element improves arc stability and ease of starting while decreasing burn-off. Cerium addition is not as effective as thorium but works well,[27] and cerium is not radioactive.[28]
  • An alloy of lanthanum oxide (or lanthana) has a similar effect as cerium, and is also not radioactive.[28]
  • Thorium oxide (or thoria) alloy electrodes offer excellent arc performance and starting, making them popular general purpose electrodes. However, it is somewhat radioactive, making inhalation of thorium vapors and dust a health risk, and disposal an environmental risk.[28]
  • Electrodes containing zirconium oxide (or zirconia) increase the current capacity while improving arc stability and starting and increasing electrode life.[28]

Filler metals are also used in nearly all applications of GTAW, the major exception being the welding of thin materials. Filler metals are available with different diameters and are made of a variety of materials. In most cases, the filler metal in the form of a rod is added to the weld pool manually, but some applications call for an automatically fed filler metal, which often is stored on spools or coils.[29]

Shielding gas

GTAW system setup

As with other welding processes such as gas metal arc welding, shielding gases are necessary in GTAW to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc.[30]

The selection of a shielding gas depends on several factors, including the type of material being welded, joint design, and desired final weld appearance. Argon is the most commonly used shielding gas for GTAW, since it helps prevent defects due to a varying arc length. When used with alternating current, argon shielding results in high weld quality and good appearance. Another common shielding gas, helium, is most often used to increase the weld penetration in a joint, to increase the welding speed, and to weld metals with high heat conductivity, such as copper and aluminum. A significant disadvantage is the difficulty of striking an arc with helium gas, and the decreased weld quality associated with a varying arc length.[30]

Argon-helium mixtures are also frequently utilized in GTAW, since they can increase control of the heat input while maintaining the benefits of using argon. Normally, the mixtures are made with primarily helium (often about 75% or higher) and a balance of argon. These mixtures increase the speed and quality of the AC welding of aluminum, and also make it easier to strike an arc. Another shielding gas mixture, argon-hydrogen, is used in the mechanized welding of light gauge stainless steel, but because hydrogen can cause porosity, its uses are limited.[30] Similarly, nitrogen can sometimes be added to argon to help stabilize the austenite in austenitic stainless steels and increase penetration when welding copper. Due to porosity problems in ferritic steels and limited benefits, however, it is not a popular shielding gas additive.[31]

Materials

Gas tungsten arc welding is most commonly used to weld stainless steel and nonferrous materials, such as aluminum and magnesium, but it can be applied to nearly all metals, with a notable exception being zinc and its alloys. Its applications involving carbon steels are limited not because of process restrictions, but because of the existence of more economical steel welding techniques, such as gas metal arc welding and shielded metal arc welding. Furthermore, GTAW can be performed in a variety of other-than-flat positions, depending on the skill of the welder and the materials being welded.[32]

Aluminum and magnesium

A TIG weld showing an accentuated AC etched zone
Closeup view of an aluminum TIG weld AC etch zone

Aluminum and magnesium are most often welded using alternating current, but the use of direct current is also possible, depending on the properties desired. Before welding, the work area should be cleaned and may be preheated to 175 to 200 °C (347 to 392 °F) for aluminum or to a maximum of 150 °C (302 °F) for thick magnesium workpieces to improve penetration and increase travel speed.[33] AC current can provide a self-cleaning effect, removing the thin, refractory aluminum oxide (sapphire) layer that forms on aluminum metal within minutes of exposure to air. This oxide layer must be removed for welding to occur.[33] When alternating current is used, pure tungsten electrodes or zirconiated tungsten electrodes are preferred over thoriated electrodes, as the latter are more likely to "spit" electrode particles across the welding arc into the weld. Blunt electrode tips are preferred, and pure argon shielding gas should be employed for thin workpieces. Introducing helium allows for greater penetration in thicker workpieces, but can make arc starting difficult.[33]

Direct current of either polarity, positive or negative, can be used to weld aluminum and magnesium as well. Direct current with a negatively charged electrode (DCEN) allows for high penetration.[33] Argon is commonly used as a shielding gas for DCEN welding of aluminum. Shielding gases with high helium contents are often used for higher penetration in thicker materials. Thoriated electrodes are suitable for use in DCEN welding of aluminum. Direct current with a positively charged electrode (DCEP) is used primarily for shallow welds, especially those with a joint thickness of less than 1.6 mm (0.063 in). A thoriated tungsten electrode is commonly used, along with a pure argon shielding gas.[33]

Steels

For GTAW of carbon and stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface.[34] Preheating is generally not necessary for mild steels less than one inch thick, but low alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. Tool steels should also be preheated to prevent cracking in the heat-affected zone. Austenitic stainless steels do not require preheating, but martensitic and ferritic chromium stainless steels do. A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases.[34]

Dissimilar metals

Welding dissimilar metals often introduces new difficulties to GTAW welding, because most materials do not easily fuse to form a strong bond. However, welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention of corrosion and oxidation.[35] In some joints, a compatible filler metal is chosen to help form the bond, and this filler metal can be the same as one of the base materials (for example, using a stainless steel filler metal with stainless steel and carbon steel as base materials), or a different metal (such as the use of a nickel filler metal for joining steel and cast iron). Very different materials may be coated or "buttered" with a material compatible with a particular filler metal, and then welded. In addition, GTAW can be used in cladding or overlaying dissimilar materials.[35]

When welding dissimilar metals, the joint must have an accurate fit, with proper gap dimensions and bevel angles. Care should be taken to avoid melting excessive base material. Pulsed current is particularly useful for these applications, as it helps limit the heat input. The filler metal should be added quickly, and a large weld pool should be avoided to prevent dilution of the base materials.[35]

Process variations

Pulsed-current

In the pulsed-current mode, the welding current rapidly alternates between two levels. The higher current state is known as the pulse current, while the lower current level is called the background current. During the period of pulse current, the weld area is heated and fusion occurs. Upon dropping to the background current, the weld area is allowed to cool and solidify. Pulsed-current GTAW has a number of advantages, including lower heat input and consequently a reduction in distortion and warpage in thin workpieces. In addition, it allows for greater control of the weld pool, and can increase weld penetration, welding speed, and quality. A similar method, manual programmed GTAW, allows the operator to program a specific rate and magnitude of current variations, making it useful for specialized applications.[36]

Dabber

The dabber variation is used to precisely place weld metal on thin edges. The automatic process replicates the motions of manual welding by feeding a cold filler wire into the weld area and dabbing (or oscillating) it into the welding arc. It can be used in conjunction with pulsed current, and is used to weld a variety of alloys, including titanium, nickel, and tool steels. Common applications include rebuilding seals in jet engines and building up saw blades, milling cutters, drill bits, and mower blades.[37]

Notes

  1. ^ Weman 2003, pp. 31, 37–38
  2. ^ Hertha Ayrton. The Electric Arc, pp. 20 and 94. D. Van Nostrand Co., New York, 1902.
  3. ^ a b Anders, A. (2003). "Tracking down the origin of arc plasma science-II. early continuous discharges". IEEE Transactions on Plasma Science. 31 (5): 1060–9. Bibcode:2003ITPS...31.1060A. doi:10.1109/TPS.2003.815477. 
  4. ^ Great Soviet Encyclopedia, Article "Дуговой разряд" (eng. electric arc)
  5. ^ Cary & Helzer 2005, pp. 5–8
  6. ^ a b c Lincoln Electric 1994, pp. 1.1-7–1.1-8
  7. ^ Russell Meredith US Patent Number 2,274,631
  8. ^ Uttrachi, Gerald (2012). Advanced Automotive Welding. North Branch, Minnesota: CarTech. p. 32. ISBN 1934709964
  9. ^ Cary & Helzer 2005, p. 8
  10. ^ a b Lincoln Electric 1994, p. 1.1-8
  11. ^ Miller Electric 2013, pp. 14, 19
  12. ^ Cary & Helzer 2005, p. 75
  13. ^ Miller Electric 2013, pp. 5, 17
  14. ^ a b Lincoln Electric 1994, pp. 5.4-7–5.4-8
  15. ^ Jeffus 2002, p. 378
  16. ^ Lincoln Electric 1994, p. 9.4-7
  17. ^ a b Cary & Helzer 2005, pp. 42, 75
  18. ^ Cary & Helzer 2005, p. 77
  19. ^ Watkins & Mizia 2003, pp. 424–426
  20. ^ Minnick 1996, pp. 120–21
  21. ^ a b c Cary & Helzer 2005, pp. 74–75
  22. ^ a b c Cary & Helzer 2005, pp. 71–72
  23. ^ Cary & Helzer 2005, p. 71
  24. ^ a b c d e f g Minnick 1996, pp. 14–16
  25. ^ ISO 6848; AWS A5.12.
  26. ^ Jeffus 1997, p. 332
  27. ^ AWS D10.11M/D10.11 - An American National Standard - Guide for Root Pass Welding of Pipe Without Backing. American Welding Society. 2007. 
  28. ^ a b c d Arc-Zone.com 2009, p. 2
  29. ^ Cary & Helzer 2005, pp. 72–73
  30. ^ a b c Minnick 1996, pp. 71–73
  31. ^ Jeffus 2002, p. 361
  32. ^ Weman 2003, p. 31
  33. ^ a b c d e Minnick 1996, pp. 135–149
  34. ^ a b Minnick 1996, pp. 156–169
  35. ^ a b c Minnick 1996, pp. 197–206
  36. ^ Cary & Helzer 2005, pp. 75–76
  37. ^ Cary & Helzer 2005, pp. 76–77

References

  • American Welding Society (2004). Welding handbook, welding processes Part 1. Miami Florida: American Welding Society. ISBN 0-87171-729-8. 
  • Arc-Zone.com (2009). "Tungsten Selection" (PDF). Carlsbad, California: Arc-Zone.com. Retrieved 15 June 2015. 
  • Cary, Howard B.; Helzer, Scott C. (2005). Modern welding technology. Upper Saddle River, New Jersey: Pearson Education. ISBN 0-13-113029-3. 
  • Jeffus, Larry F. (1997). Welding: Principles and applications (Fourth ed.). Thomson Delmar. ISBN 978-0-8273-8240-4. 
  • Jeffus, Larry (2002). Welding: Principles and applications (Fifth ed.). Thomson Delmar. ISBN 1-4018-1046-2. 
  • Lincoln Electric (1994). The procedure handbook of arc welding. Cleveland: Lincoln Electric. ISBN 99949-25-82-2. 
  • Miller Electric Mfg Co (2013). Guidelines For Gas Tungsten Arc Welding (GTAW) (PDF). Appleton, Wisconsin: Miller Electric Mfg Co. 
  • Minnick, William H. (1996). Gas tungsten arc welding handbook. Tinley Park, Illinois: Goodheart–Willcox Company. ISBN 1-56637-206-2. 
  • Watkins, Arthur D.; Mizia, Ronald E (2003). Optimizing long-term stainless steel closure weld integrity in DOE standard spent nuclear canisters. Trends in Welding Research 2002: Proceedings of the 6th International Conference. ASM International. 
  • Weman, Klas (2003). Welding processes handbook. New York: CRC Press LLC. ISBN 0-8493-1773-8. 

External links

Shielded Metal Arc Welding (SMAW)

Shielded metal arc welding

Shielded metal arc welding

Shielded metal arc welding (SMAW), also known as manual metal arc welding (MMA or MMAW), flux shielded arc welding[1] or informally as stick welding, is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld.

An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. The workpiece and the electrode melts forming a pool of molten metal (weld pool) that cools to form a joint. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.

Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's first and most popular welding processes. It dominates other welding processes in the maintenance and repair industry, and though flux-cored arc welding is growing in popularity, SMAW continues to be used extensively in the construction of heavy steel structures and in industrial fabrication. The process is used primarily to weld iron and steels (including stainless steel) but aluminium, nickel and copper alloys can also be welded with this method.[2]

Development

After the discovery of the short pulsed electric arc in 1800 by Humphry Davy[3][4] and of the continuous electric arc in 1802 by Vasily Petrov,[4][5] there was little development in electrical welding until Auguste de Méritens developed a carbon arc torch that was patented in 1881.[1]

In 1885, Nikolay Benardos and Stanisław Olszewski developed carbon arc welding,[6] obtaining American patents from 1887 showing a rudimentary electrode holder. In 1888, the consumable metal electrode was invented by Nikolay Slavyanov. Later in 1890, C. L. Coffin received U.S. Patent 428,459 for his arc welding method that utilized a metal electrode. The process, like SMAW, deposited melted electrode metal into the weld as filler.[7]

Around 1900, A. P. Strohmenger and Oscar Kjellberg released the first coated electrodes. Strohmenger used clay and lime coating to stabilize the arc, while Kjellberg dipped iron wire into mixtures of carbonates and silicates to coat the electrode.[8] In 1912, Strohmenger released a heavily coated electrode, but high cost and complex production methods prevented these early electrodes from gaining popularity. In 1927, the development of an extrusion process reduced the cost of coating electrodes while allowing manufacturers to produce more complex coating mixtures designed for specific applications. In the 1950s, manufacturers introduced iron powder into the flux coating, making it possible to increase the welding speed.[9]

In 1938 K. K. Madsen described an automated variation of SMAW, now known as . It briefly gained popularity in the 1960s after receiving publicity for its use in Japanese shipyards though today its applications are limited. Another little used variation of the process, known as firecracker welding, was developed around the same time by in Austria.[10]

Operation

SMAW weld area

To strike the electric arc, the electrode is brought into contact with the workpiece by a very light touch with the electrode to the base metal then is pulled back slightly. This initiates the arc and thus the melting of the workpiece and the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the weld pool. Striking an arc, which varies widely based upon electrode and workpiece composition, can be the hardest skill for beginners. The orientation of the electrode to workpiece is where most stumble, if the electrode is held at a perpendicular angle to the workpiece the tip will likely stick to the metal which will fuse the electrode to the workpiece which will cause it to heat up very rapidly. The tip of the electrode needs to be at a lower angle to the workpiece, which allows the weld pool to flow out of the arc. As the electrode melts, the flux covering disintegrates, giving off shielding gases that protect the weld area from oxygen and other atmospheric gases. In addition, the flux provides molten slag which covers the filler metal as it travels from the electrode to the weld pool. Once part of the weld pool, the slag floats to the surface and protects the weld from contamination as it solidifies. Once hardened, it must be chipped away to reveal the finished weld. As welding progresses and the electrode melts, the welder must periodically stop welding to remove the remaining electrode stub and insert a new electrode into the electrode holder. This activity, combined with chipping away the slag, reduces the amount of time that the welder can spend laying the weld, making SMAW one of the least efficient welding processes. In general, the operator factor, or the percentage of operator's time spent laying weld, is approximately 25%.[11]

The actual welding technique utilized depends on the electrode, the composition of the workpiece, and the position of the joint being welded. The choice of electrode and welding position also determine the welding speed. Flat welds require the least operator skill, and can be done with electrodes that melt quickly but solidify slowly. This permits higher welding speeds.

Sloped, vertical or upside-down welding requires more operator skill, and often necessitates the use of an electrode that solidifies quickly to prevent the molten metal from flowing out of the weld pool. However, this generally means that the electrode melts less quickly, thus increasing the time required to lay the weld.[12]

Quality

The most common quality problems associated with SMAW include , porosity, poor fusion, shallow penetration, and cracking.

Weld spatter, while not affecting the integrity of the weld, damages its appearance and increases cleaning costs. It can be caused by excessively high current, a long arc, or arc blow, a condition associated with direct current characterized by the electric arc being deflected away from the weld pool by magnetic forces. Arc blow can also cause porosity in the weld, as can joint contamination, high welding speed, and a long welding arc, especially when low-hydrogen electrodes are used.

Porosity, often not visible without the use of advanced nondestructive testing methods, is a serious concern because it can potentially weaken the weld. Another defect affecting the strength of the weld is poor fusion, though it is often easily visible. It is caused by low current, contaminated joint surfaces, or the use of an improper electrode.

Shallow penetration, another detriment to weld strength, can be addressed by decreasing welding speed, increasing the current or using a smaller electrode. Any of these weld-strength-related defects can make the weld prone to cracking, but other factors are involved as well. High carbon, alloy or sulfur content in the base material can lead to cracking, especially if low-hydrogen electrodes and preheating are not employed. Furthermore, the workpieces should not be excessively restrained, as this introduces residual stresses into the weld and can cause cracking as the weld cools and contracts.[13]

Safety

SMAW welding, like other welding methods, can be a dangerous and unhealthy practice if proper precautions are not taken. The process uses an open electric arc, which presents a risk of burns which are prevented by personal protective equipment in the form of heavy leather gloves and long sleeve jackets. Additionally, the brightness of the weld area can lead to a condition called arc eye, in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. Welding helmets with dark face plates are worn to prevent this exposure, and in recent years, new helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light. To protect bystanders, especially in industrial environments, translucent welding curtains often surround the welding area. These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets.[14]

In addition, the vaporizing metal and flux materials expose welders to dangerous gases and particulate matter. The smoke produced contains particles of various types of oxides. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, gases like carbon dioxide and ozone can form, which can prove dangerous if ventilation is inadequate. Some of the latest welding masks are fitted with an electric powered fan to help disperse harmful fumes.[15]

Application and materials

Shielded metal arc welding is one of the world's most popular welding processes, accounting for over half of all welding in some countries. Because of its versatility and simplicity, it is particularly dominant in the maintenance and repair industry, and is heavily used in the construction of steel structures and in industrial fabrication. In recent years its use has declined as flux-cored arc welding has expanded in the construction industry and gas metal arc welding has become more popular in industrial environments. However, because of the low equipment cost and wide applicability, the process will likely remain popular, especially among amateurs and small businesses where specialized welding processes are uneconomical and unnecessary.[16]

SMAW is often used to weld carbon steel, low and high alloy steel, stainless steel, cast iron, and ductile iron. While less popular for nonferrous materials, it can be used on nickel and copper and their alloys and, in rare cases, on aluminium. The thickness of the material being welded is bounded on the low end primarily by the skill of the welder, but rarely does it drop below 1.5 mm (0.06 in). No upper bound exists: with proper joint preparation and use of multiple passes, materials of virtually unlimited thicknesses can be joined. Furthermore, depending on the electrode used and the skill of the welder, SMAW can be used in any position.[17]

Equipment

SMAW system setup

Shielded metal arc welding equipment typically consists of a constant current welding power supply and an electrode, with an electrode holder, a 'ground' clamp, and welding cables (also known as welding leads) connecting the two.

Power supply

The power supply used in SMAW has constant current output, ensuring that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of SMAW are manual, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult. However, because the current is not maintained absolutely constant, skilled welders performing complicated welds can vary the arc length to cause minor fluctuations in the current.[18]

A high output welding power supply for Stick, GTAW, MIG, Flux-Cored, & Gouging

The preferred polarity of the SMAW system depends primarily upon the electrode being used and the desired properties of the weld. Direct current with a negatively charged electrode (DCEN) causes heat to build up on the electrode, increasing the electrode melting rate and decreasing the depth of the weld. Reversing the polarity so that the electrode is positively charged (DCEP) and the workpiece is negatively charged increases the weld penetration. With alternating current the polarity changes over 100 times per second, creating an even heat distribution and providing a balance between electrode melting rate and penetration.[19]

Typically, the equipment used for SMAW consists of a step-down transformer and for direct current models a rectifier, which converts alternating current into direct current. Because the power normally supplied to the welding machine is high-voltage alternating current, the welding transformer is used to reduce the voltage and increase the current. As a result, instead of 220 V at 50 A, for example, the power supplied by the transformer is around 17–45 V at currents up to 600 A. A number of different types of transformers can be used to produce this effect, including multiple coil and inverter machines, with each using a different method to manipulate the welding current. The multiple coil type adjusts the current by either varying the number of turns in the coil (in tap-type transformers) or by varying the distance between the primary and secondary coils (in movable coil or movable core transformers). Inverters, which are smaller and thus more portable, use electronic components to change the current characteristics.[20]

Electrical generators and alternators are frequently used as portable welding power supplies, but because of lower efficiency and greater costs, they are less frequently used in industry. Maintenance also tends to be more difficult, because of the complexities of using a combustion engine as a power source. However, in one sense they are simpler: the use of a separate rectifier is unnecessary because they can provide either AC or DC.[21] However, the engine driven units are most practical in field work where the welding often must be done out of doors and in locations where transformer type welders are not usable because there is no power source available to be transformed.

In some units the alternator is essentially the same as that used in portable generating sets used to supply mains power, modified to produce a higher current at a lower voltage but still at the 50 or 60 Hz grid frequency. In higher-quality units an alternator with more poles is used and supplies current at a higher frequency, such as 400 Hz. The smaller amount of time the high-frequency waveform spends near zero makes it much easier to strike and maintain a stable arc than with the cheaper grid-frequency sets or grid-frequency mains-powered units.

Electrode

Various accessories for SMAW

The choice of electrode for SMAW depends on a number of factors, including the weld material, welding position and the desired weld properties. The electrode is coated in a metal mixture called flux, which gives off gases as it decomposes to prevent weld contamination, introduces deoxidizers to purify the weld, causes weld-protecting slag to form, improves the arc stability, and provides alloying elements to improve the weld quality.[22] Electrodes can be divided into three groups—those designed to melt quickly are called "fast-fill" electrodes, those designed to solidify quickly are called "fast-freeze" electrodes, and intermediate electrodes go by the name "fill-freeze" or "fast-follow" electrodes. Fast-fill electrodes are designed to melt quickly so that the welding speed can be maximized, while fast-freeze electrodes supply filler metal that solidifies quickly, making welding in a variety of positions possible by preventing the weld pool from shifting significantly before solidifying.[23]

The composition of the electrode core is generally similar and sometimes identical to that of the base material. But even though a number of feasible options exist, a slight difference in alloy composition can strongly impact the properties of the resulting weld. This is especially true of alloy steels such as HSLA steels. Likewise, electrodes of compositions similar to those of the base materials are often used for welding nonferrous materials like aluminium and copper.[24] However, sometimes it is desirable to use electrodes with core materials significantly different from the base material. For example, stainless steel electrodes are sometimes used to weld two pieces of carbon steel, and are often utilized to weld stainless steel workpieces with carbon steel workpieces.[25]

Electrode coatings can consist of a number of different compounds, including rutile, calcium fluoride, cellulose, and iron powder. Rutile electrodes, coated with 25%–45% TiO2, are characterized by ease of use and good appearance of the resulting weld. However, they create welds with high hydrogen content, encouraging embrittlement and cracking. Electrodes containing calcium fluoride (CaF2), sometimes known as basic or low-hydrogen electrodes, are hygroscopic and must be stored in dry conditions. They produce strong welds, but with a coarse and convex-shaped joint surface. Electrodes coated with cellulose, especially when combined with rutile, provide deep weld penetration, but because of their high moisture content, special procedures must be used to prevent excessive risk of cracking. Finally, iron powder is a common coating additive that increases the rate at which the electrode fills the weld joint, up to twice as fast.[26]

To identify different electrodes, the American Welding Society established a system that assigns electrodes with a four- or five-digit number. Covered electrodes made of mild or low alloy steel carry the prefix E, followed by their number. The first two or three digits of the number specify the tensile strength of the weld metal, in thousand pounds per square inch (ksi). The penultimate digit generally identifies the welding positions permissible with the electrode, typically using the values 1 (normally fast-freeze electrodes, implying all position welding) and 2 (normally fast-fill electrodes, implying horizontal welding only). The welding current and type of electrode covering are specified by the last two digits together. When applicable, a suffix is used to denote the alloying element being contributed by the electrode.[27]

Common electrodes include the E6010, a fast-freeze, all-position electrode with a minimum tensile strength of 60 ksi (410 MPa) which is operated using DCEP. E6011 is similar except its flux coating allows it to be used with alternating current in addition to DCEP. E7024 is a fast-fill electrode, used primarily to make flat or horizontal welds using AC, DCEN, or DCEP. Examples of fill-freeze electrodes are the E6012, E6013, and E7014, all of which provide a compromise between fast welding speeds and all-position welding.[28]

Process variations

Though SMAW is almost exclusively a manual arc welding process, one notable process variation exists, known as gravity welding or gravity arc welding. It serves as an automated version of the traditional shielded metal arc welding process, employing an electrode holder attached to an inclined bar along the length of the weld. Once started, the process continues until the electrode is spent, allowing the operator to manage multiple gravity welding systems. The electrodes employed (often E6027 or E7024) are coated heavily in flux, and are typically 71 cm (28 in) in length and about 6.35 mm (0.25 in) thick. As in manual SMAW, a constant current welding power supply is used, with either negative polarity direct current or alternating current. Due to a rise in the use of semiautomatic welding processes such as flux-cored arc welding, the popularity of gravity welding has fallen as its economic advantage over such methods is often minimal. Other SMAW-related methods that are even less frequently used include firecracker welding, an automatic method for making butt and fillet welds, and massive electrode welding, a process for welding large components or structures that can deposit up to 27 kg (60 lb) of weld metal per hour.[10]

Notes

  1. ^ a b Houldcroft, P. T. (1973) [1967]. "Chapter 3: Flux-Shielded Arc Welding". Welding Processes. Cambridge University Press. p. 23. ISBN 0-521-05341-2. 
  2. ^ Cary & Helzer 2005, pp. 102–103
  3. ^ Hertha Ayrton. The Electric Arc, pp. 20 and 94. D. Van Nostrand Co., New York, 1902.
  4. ^ a b Anders, A. (2003). "Tracking down the origin of arc plasma science-II. early continuous discharges". IEEE Transactions on Plasma Science. 31 (5): 1060–9. doi:10.1109/TPS.2003.815477. 
  5. ^ Great Soviet Encyclopedia, Article "Дуговой разряд" (eng. electric arc)
  6. ^ US 363320, Benardos, Nikołaj & Stanisław Olszewski, "Process of and apparatus for working metals by the direct application of the electric current", issued 17 May 1887 
  7. ^ Cary & Helzer 2005, p. 5
  8. ^ Cary & Helzer 2005, p. 6
  9. ^ Lincoln Electric 1994, pp. 1.1-4–1.1-6, 1.1-8
  10. ^ a b Cary & Helzer 2005, pp. 115–116
  11. ^ Cary & Helzer 2005, pp. 102, 115
  12. ^ Lincoln Electric 1994, pp. 6.2-1
  13. ^ Lincoln Electric 1994, pp. 6.2-18–6.2-20, 3.2-1
  14. ^ Cary & Helzer 2005, pp. 42, 49–51
  15. ^ Cary & Helzer 2005, pp. 52–62
  16. ^ Lincoln Electric 1994, pp. 5.1-1–5.1-2
  17. ^ Cary & Helzer 2005, p. 103
  18. ^ Jeffus 1999, p. 47.
  19. ^ Jeffus 1999, pp. 46–47.
  20. ^ Jeffus 1999, pp. 49–53.
  21. ^ Jeffus 1999, pp. 49, 52–53.
  22. ^ Cary & Helzer 2005, p. 104
  23. ^ Lincoln Electric 1994, p. 6.2-1
  24. ^ Lincoln Electric 1994, pp. 6.2-13, 9.2-1, 10.1-3
  25. ^ Lincoln Electric 1994, pp. 7.2-5, 7.2-8
  26. ^ Weman 2003, pp. 65–66
  27. ^ Cary & Helzer 2005, p. 105
  28. ^ Lincoln Electric 1994, pp. 6.2-7–6.2-10

References

  • Cary, Howard B.; Helzer, Scott C. (2005), Modern Welding Technology, Upper Saddle River, New Jersey: Pearson Education, ISBN 0-13-113029-3 
  • Jeffus, Larry (1999), Welding: Principles and Applications (4th ed.), Albany, New York: Thomson Delmar, ISBN 0-8273-8240-5 
  • Lincoln Electric (1994), The Procedure Handbook of Arc Welding, Cleveland, Ohio: Lincoln Electric, ISBN 99949-25-82-2 
  • Miller Electric Mfg Co (2013). Guidelines For Shielded Metal Arc Welding (SMAW) (PDF). Appleton, Wisconsin: Miller Electric Mfg Co. 
  • Weman, Klas (2003), Welding processes handbook, New York: CRC Press, ISBN 0-8493-1773-8 

External links

Tensile testing

Tensile testing

Tensile testing on a coir composite. Specimen size is not to standard (Instron).

Tensile testing, is also known as tension testing,[1] is a fundamental materials science test in which a sample is subjected to a controlled tension until failure. The results from the test are commonly used to select a material for an application, for quality control, and to predict how a material will react under normal forces. Properties that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and reduction in area.[2] From these measurements the following properties can also be determined: Young's modulus, Poisson's ratio, yield strength, and strain-hardening characteristics.[3] Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic materials. For anisotropic materials, such as composite materials and textiles, biaxial tensile testing is required.

Tensile specimen

Tensile specimens made from an aluminum alloy. The left two specimens have a round cross-section and threaded shoulders. The right two are flat specimens designed to be used with serrated grips.

A tensile specimen is a standardized sample cross-section. It has two shoulders and a gage (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area.[2][4]

The shoulders of the test specimen can be manufactured in various ways to mate to various grips in the testing machine (see the image below). Each system has advantages and disadvantages; for example, shoulders designed for serrated grips are easy and cheap to manufacture, but the alignment of the specimen is dependent on the skill of the technician. On the other hand, a pinned grip assures good alignment. Threaded shoulders and grips also assure good alignment, but the technician must know to thread each shoulder into the grip at least one diameter's length, otherwise the threads can strip before the specimen fractures.[5]

In large castings and forgings it is common to add extra material, which is designed to be removed from the casting so that test specimens can be made from it. These specimens may not be exact representation of the whole workpiece because the grain structure may be different throughout. In smaller workpieces or when critical parts of the casting must be tested, a workpiece may be sacrificed to make the test specimens.[6] For workpieces that are machined from bar stock, the test specimen can be made from the same piece as the bar stock.

Various shoulder styles for tensile specimens. Keys A through C are for round specimens, whereas keys D and E are for flat specimens. Key:

A. A Threaded shoulder for use with a threaded grip
B. A round shoulder for use with serrated grips
C. A butt end shoulder for use with a split collar
D. A flat shoulder for used with serrated grips

E. A flat shoulder with a through hole for a pinned grip
Test specimen nomenclature

The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.[6]

A standard specimen is prepared in a round or a square section along the gauge length, depending on the standard used. Both ends of the specimens should have sufficient length and a surface condition such that they are firmly gripped during testing. The initial gauge length Lo is standardized (in several countries) and varies with the diameter (Do) or the cross-sectional area (Ao) of the specimen as listed

Type specimen United States(ASTM) Britain Germany
Sheet ( Lo / √Ao) 4.5 5.65 11.3
Rod ( Lo / Do) 4.0 5.00 10.0

The following tables gives examples of test specimen dimensions and tolerances per standard ASTM E8.

Flat test specimen[7]
All values in inches Plate type (1.5 in. wide) Sheet type (0.5 in. wide) Sub-size specimen (0.25 in. wide)
Gauge length 8.00±0.01 2.00±0.005 1.000±0.003
Width 1.5 +0.125–0.25 0.500±0.010 0.250±0.005
Thickness 0.188 ≤ T 0.005 ≤ T ≤ 0.75 0.005 ≤ T ≤ 0.25
Fillet radius (min.) 1 0.25 0.25
Overall length (min.) 18 8 4
Length of reduced section (min.) 9 2.25 1.25
Length of grip section (min.) 3 2 1.25
Width of grip section (approx.) 2 0.75 38
Round test specimen[7]
All values in inches Standard specimen at nominal diameter: Small specimen at nominal diameter:
0.500 0.350 0.25 0.160 0.113
Gauge length 2.00±0.005 1.400±0.005 1.000±0.005 0.640±0.005 0.450±0.005
Diameter tolerance ±0.010 ±0.007 ±0.005 ±0.003 ±0.002
Fillet radius (min.) 38 0.25 516 532 332
Length of reduced section (min.) 2.5 1.75 1.25 0.75 58

Equipment

A universal testing machine (Hegewald & Peschke)

The most common testing machine used in tensile testing is the universal testing machine. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic powered and electromagnetically powered machines.[4]

The machine must have the proper capabilities for the test specimen being tested. There are four main parameters: force capacity, speed, precision and accuracy. Force capacity refers to the fact that the machine must be able to generate enough force to fracture the specimen. The machine must be able to apply the force quickly or slowly enough to properly mimic the actual application. Finally, the machine must be able to accurately and precisely measure the gauge length and forces applied; for instance, a large machine that is designed to measure long elongations may not work with a brittle material that experiences short elongations prior to fracturing.[5]

Alignment of the test specimen in the testing machine is critical, because if the specimen is misaligned, either at an angle or offset to one side, the machine will exert a bending force on the specimen. This is especially bad for brittle materials, because it will dramatically skew the results. This situation can be minimized by using spherical seats or U-joints between the grips and the test machine.[5] If the initial portion of the stress–strain curve is curved and not linear, it indicates the specimen is misaligned in the testing machine.[8]

The strain measurements are most commonly measured with an extensometer, but strain gauges are also frequently used on small test specimen or when Poisson's ratio is being measured.[5] Newer test machines have digital time, force, and elongation measurement systems consisting of electronic sensors connected to a data collection device (often a computer) and software to manipulate and output the data. However, analog machines continue to meet and exceed ASTM, NIST, and ASM metal tensile testing accuracy requirements, continuing to be used today.[citation needed]

Process

The test process involves placing the test specimen in the testing machine and slowly extending it until it fractures. During this process, the elongation of the gauge section is recorded against the applied force. The data is manipulated so that it is not specific to the geometry of the test sample. The elongation measurement is used to calculate the engineering strain, ε, using the following equation:[4]

where ΔL is the change in gauge length, L0 is the initial gauge length, and L is the final length. The force measurement is used to calculate the engineering stress, σ, using the following equation:[4]

where F is the tensile force and A is the nominal cross-section of the specimen. The machine does these calculations as the force increases, so that the data points can be graphed into a stress–strain curve.[4]

Standards

Metals

  • ASTM E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" (2013)
  • ISO 6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)
  • ISO 6892-2: "Metallic materials. Tensile testing. Method of test at elevated temperature" (2011)
  • JIS Z2241 Method of tensile test for metallic materials

Flexible materials

  • ASTM D638 Standard Test Method for Tensile Properties of Plastics
  • ASTM D828 Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus
  • ASTM D882 Standard test method for tensile properties of thin plastic sheeting
  • ISO 37 rubber, vulcanized or thermoplastic—determination of tensile stress–strain properties

References

  1. ^ Czichos, Horst (2006). Springer Handbook of Materials Measurement Methods. Berlin: Springer. pp. 303–304. ISBN 978-3-540-20785-6. 
  2. ^ a b Davis, Joseph R. (2004). Tensile testing (2nd ed.). ASM International. ISBN 978-0-87170-806-9. 
  3. ^ Davis 2004, p. 33.
  4. ^ a b c d e Davis 2004, p. 2.
  5. ^ a b c d Davis 2004, p. 9.
  6. ^ a b Davis 2004, p. 8.
  7. ^ a b Davis 2004, p. 52.
  8. ^ Davis 2004, p. 11.

External links

ATEX Directive

ATEX directive

The ATEX directive consists of two EU directives describing what equipment and work environment is allowed in an environment with an explosive atmosphere. ATEX derives its name from the French title of the 94/9/EC directive: Appareils destinés à être utilisés en ATmosphères EXplosibles.

Directives

The CE mark which should be attached to EU certified equipment
Mark for ATEX certified electrical equipment for explosive atmospheres.

As of July 2003, organizations in EU must follow the directives to protect employees from explosion risk in areas with an explosive atmosphere.

There are two ATEX directives (one for the manufacturer and one for the user of the equipment):

  • the ATEX 95 equipment directive 94/9/EC, Equipment and protective systems intended for use in potentially explosive atmospheres;
  • the ATEX 137 workplace directive 99/92/EC, Minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres.

The ATEX 94/9/EU that is dedicated to the manufacturer has changed. Still applicable up to 19 April 2016 the ATEX 94/9/EC will be removed and replaced by a new directive.

This new ATEX directive was published on Saturday 29 March 2014, under the new reference : Directive 2014/34/EU of the European Parliament and of the Council of 26 February 2014 on the harmonisation of the laws of the Member States relating to equipment and protective systems intended for use in potentially explosive atmospheres (recast) Text with EEA relevance - Official Journal of the European Union L 96 from 29/03/2014.

This new ATEX directive 2014/34/EU will be mandatory for manufacturer on 20 April 2016 as is stated in article 44 of the directive.

Promised for a long time, this new ATEX directive has been published together with 8 other directives in the frame of NEW LEGISLATIVE FRAMEWORK (NLF) ALIGNMENT PACKAGE (Implementation of the Goods Package). It was the subject of a "COMMUNICATION FROM THE COMMISSION TO THE EUROPEAN PARLIAMENT AND THE COUNCIL" for the Alignment of ten technical harmonisation directives to Decision No 768/2008/EC of the European Parliament and of the Council of 9 July 2008 on a common framework for the marketing of products, in Brussels, 21.11.2011 under reference COM(2011) 763 final.

This texts aims to align at its origin 10 directives who were :

  • Civil Explosives Directive: Directive 93/15/EEC on the harmonisation of the provisions relating to the placing on the market and supervision of explosives for civil use;
  • Directive on equipment for use in explosive atmospheres (ATEX): Directive 94/9/EC on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres;
  • Lifts Directive: Directive 95/16/EC on the approximation of the laws of the Member States relating to lifts;
  • Pressure Equipment Directive: Directive 97/23/EC on the approximation of the laws of the Member States concerning pressure equipment;
  • Measuring Instruments Directive: Directive 2004/22/EC on measuring instruments;
  • Electromagnetic Compatibility Directive (EMC): Directive 2004/108/EC on the approximation of the laws of the Member States relating to electromagnetic compatibility;
  • Low Voltage Directive (LVD): Directive 2006/95/EC on the harmonisation of the laws of Member States relating to electrical equipment designed for use within certain voltage limits;
  • Pyrotechnic Articles Directive: Directive 2007/23/EC on the placing on the market of pyrotechnic articles;
  • Non-automatic Weighing Instruments Directive: Directive 2009/23/EC on non-automatic weighing instruments;
  • Simple Pressure Vessels Directive: Directive 2009/105/EC relating to simple pressure vessels

The text of the new ATEX 2014/34/EU in available on the following web site :

Regarding ATEX 99/92/EC directive, the requirement is that Employers must classify areas where hazardous explosive atmospheres may occur into zones. The classification given to a particular zone, and its size and location, depends on the likelihood of an explosive atmosphere occurring and its persistence if it does.

Areas classified into zones (0, 1, 2 for gas-vapor-mist and 20, 21, 22 for dust) must be protected from effective sources of ignition.[1] Equipment and protective systems intended to be used in zoned areas must meet the requirements of the directive. Zone 0 and 20 require Category 1 marked equipment, zone 1 and 21 require Category 2 marked equipment and zone 2 and 22 require Category 3 marked equipment. Zone 0 and 20 are the zones with the highest risk of an explosive atmosphere being present.[1]

Equipment in use before July 2003 is allowed to be used indefinitely provided a risk assessment shows it is safe to do so.

The aim of directive 94/9/EC is to allow the free trade of ‘ATEX’ equipment and protective systems within the EU by removing the need for separate testing and documentation for each member state.

The regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or mechanical, including protective systems. There are two categories of equipment 'I' for mining and 'II' for surface industries. Manufacturers who apply its provisions and affix the CE marking and the Ex marking are able to sell their equipment anywhere within the European union without any further requirements with respect to the risks covered being applied. The directive covers a large range of equipment, potentially including equipment used on fixed offshore platforms, in petrochemical plants, mines, flour mills and other areas where a potentially explosive atmosphere may be present.

In very broad terms, there are three preconditions for the directive to apply: the equipment a) must have its own effective source of ignition; b) be intended for use in a potentially explosive atmosphere (air mixtures); and c) be under normal atmospheric conditions.

The directive also covers components essential for the safe use and safety devices directly contributing to the safe use of the equipment in scope. These latter devices may be outside the potentially explosive environment.

Manufacturers/suppliers (or importers, if the manufacturers are outside the EU) must ensure that their products meet essential health and safety requirements and undergo appropriate conformity procedures. This usually involves testing and certification by a ‘third-party’ certification body (known as a Notified Body e.g. Intertek, Sira, Baseefa, Lloyd's, TUV ICQC) but manufacturers/suppliers can ‘self-certify’ Category 3 equipment (technical dossier including drawings, hazard analysis and users manual in the local language) and Category 2 non-electrical equipment but for Category 2 the technical dossier must be lodged with a notified body. Once certified, the equipment is marked by the ‘CE’ (meaning it complies with ATEX and all other relevant directives) and ‘Ex’ symbol to identify it as approved under the ATEX directive. The technical dossier must be kept for a period of 10 years.

Certification ensures that the equipment or protective system is fit for its intended purpose and that adequate information is supplied with it to ensure that it can be used safely. There are four ATEX classification to ensure that a specific piece of equipment or protective system is appropriate and can be safely used in a particular application: 1. Industrial or Mining Application; 2. Equipment Category; 3. Atmosphere; and 4. Temperature.

The ATEX as an EU directive finds its US equivalent under the HAZLOC standard. This standard given by the Occupational Safety and Health Administration defines and classifies hazardous locations such as explosive atmospheres.

Technical definitions

In DSEAR, an explosive atmosphere is defined as a mixture of dangerous substances with air, under atmospheric conditions, in the form of gases, vapours, mist or dust in which, after ignition has occurred, combustion spreads to the entire unburned mixture.

Atmospheric conditions are commonly referred to as ambient temperatures and pressures. That is to say temperatures of –20 °C to 40 °C and pressures of 0.8 to 1.1 bar.[2]

Zone classification

Zone 0 - A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapor or mist is present continuously or for long periods or frequently

Zone 1 - A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapor or mist is likely to occur in normal operation occasionally.

Zone 2 - A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapor or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Hazard - Dusts

Zone 20 - A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.

Zone 21 - A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

Zone 22 - A place in which an explosive atmosphere in the form or a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Effective ignition source

Effective ignition source is a term defined in the European ATEX directive as an event which, in combination with sufficient oxygen and fuel in gas, mist, vapor or dust form, can cause an explosion. Methane, hydrogen or coal dust are examples of possible fuels.[3]

Effective ignition sources are:

  • Lightning strikes.
  • Open flames. This varies from a lit cigarette to welding activity.
  • Mechanically generated impact sparks. For example, a hammer blow on a rusty steel surface compared to a hammer blow on a flint stone. The speed and impact angle (between surface and hammer) are important; a 90 degree blow on a surface is relatively harmless.
  • Mechanically generated friction sparks. The combination of materials and speed determine the effectiveness of the ignition source. For example, 4.5 m/s steel-steel friction with a force greater than 2 kN is an effective ignition source. The combination of aluminium and rust is also notoriously dangerous. More than one red hot spark is often necessary in order to have an effective ignition source.
  • Electric sparks. For example, a bad electrical connection or a faulty pressure transmitter. The electric energy content of the spark determines the effectiveness of the ignition source.
  • High surface temperature. This can be the result of milling, grinding, rubbing, mechanical friction in a stuffing box or bearing, or a hot liquid pumped into a vessel. For example, the tip of a lathe cutting tool can easily be 600 Celsius (1100 °F); a high pressure steam pipe may be above the autoignition temperature of some fuel/air mixtures.
  • Electrostatic discharge. Static electricity can be generated by air sliding over a wing, or a non-conductive liquid flowing through a filter screen.
  • Radiation.
  • Adiabatic compression. Air is pumped into a vessel and the vessel surface heats up.

See also

References

  1. ^ a b "Explosive Atmospheres - classification of hazardous areas (zoning) and selection of equipment" (PDF). HSE UK. 
  2. ^ "ATEX and explosive atmospheres". HSE UK. 
  3. ^ Michelis, J.: "Explosionsschutz im Bergbau unter Tage", Verlag Glückauf Essen, 1998, ISBN 3-7739-0900-4

External links

Pressure Equipment Directive (PED)

Pressure Equipment Directive

The Pressure Equipment Directive (PED) 2014/68/EU (formerly97/23/EC) [1] of the EU sets out the standards for the design and fabrication of pressure equipment ("pressure equipment" means steam boilers, pressure vessels, piping, safety valves and other components and assemblies subject to pressure loading) generally over one litre in volume and having a maximum pressure more than 0.5 bar gauge. It also sets the administrative procedures requirements for the "conformity assessment" of pressure equipment, for the free placing on the European market without local legislative barriers. It has been mandatory throughout the EU since 30 May 2002, with 2014 revision fully effective as of 19 July 2016.[2] This is enacted in the UK as the Pressure Equipment Regulations (PER). The set out standards and regulations regarding pressure vessels and boilers safety is also very close to the US standards defined by the American Society of Mechanical Engineers (ASME). This enables most international inspection agencies to provide both verification and certification services to assess compliance to the different pressure equipment directives.[3]

Contents

  1. Scope and Definitions (including exemptions of its scope)
  2. Market surveillance
  3. Technical requirements: classification of pressure equipment according to type and content.
  4. Free movement
  5. Presumption of conformity
  6. Committee on technical standards and regulations
  7. Committee on Pressure Equipment
  8. Safeguard clause
  9. Classification of pressure equipment
  10. Conformity assessment
  11. European approval for materials
  12. Notified bodies
  13. Recognized third-party organizations
  14. User inspectorates
  15. CE marking
  16. Unduly affixed CE marking
  17. International cooperation
  18. Decisions entailing refusal or restriction
  19. Repeal
  20. Transposition and transitional provisions
  21. Addressees of the Directive: the EU member states for implementation in national laws and/or regulations.
  • Annex I: Essential safety requirements
    1. General
    2. Design
    3. Manufacturing
    4. Materials
    5. Fired or otherwise heated pressure equipment with a risk of overheating (article 3.1)
    6. Piping
    7. Specific quantitative requirements for certain pressure equipment
  • Appendix II: Conformity assessment tables. Actually diagrams of pressure vs. volume (or diameter for pipes), for classification of equipment in four classes.
  • Appendix III: Conformity assessment procedures
  • Appendix IV: Minimum criteria to be met when designating the notified bodies (article 12) and the recognized third party organizations (article 13)
  • Appendix V: Criteria to be met when authorizing user inspectorates (article 14)
  • Appendix VI: CE marking
  • Appendix VII: Declaration of conformity

Transition to Directive 2014/68/EU

Directive 97/23/EC was fully superseded by directive 2014/68/EU from July 20, 2016 onwards. Article 13 of the new directive (classification of pressure equipment) became effective June 1, 2015, replacing article 9 of directive 97/23/EC.[4]

See also

References

External links


Charpy impact test

Charpy impact test

The Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's notch toughness and acts as a tool to study temperature-dependent ductile-brittle transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. A disadvantage is that some results are only comparative.[1]

The test was developed around 1900 by S.B. Russell (1898, American) and Georges Charpy (1901, French).[2] The test became known as the Charpy test in the early 1900s due to the technical contributions and standardization efforts by Charpy. The test was pivotal in understanding the fracture problems of ships during World War II.[3][4]

Today it is utilized in many industries for testing materials, for example the construction of pressure vessels and bridges to determine how storms will affect the materials used.[3][5][6]

History

In 1896 S. B. Russell introduced the idea of residual fracture energy and devised a pendulum fracture test. Russell's initial tests measured un-notched samples. In 1897 Frémont introduced a test trying to measure the same phenomenon using a spring-loaded machine. In 1901 Georges Charpy proposed a standardized method improving Russell's by introducing a redesigned pendulum, notched sample and generally giving precise specifications.[7]

Definition

A vintage impact test machine. Yellow cage on the left is meant to prevent accidents during pendulum swing, pendulum is seen at rest at the bottom

The apparatus consists of a pendulum of known mass and length that is dropped from a known height to impact a notched specimen of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after the fracture (energy absorbed by the fracture event).

The notch in the sample affects the results of the impact test,[8] thus it is necessary for the notch to be of regular dimensions and geometry. The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made.[9]

The "Standard methods for Notched Bar Impact Testing of Metallic Materials" can be found in ASTM E23,[10] ISO 148-1[11] or EN 10045-1,[12] where all the aspects of the test and equipment used are described in detail.

Quantitative results

The quantitative result of the impact tests the energy needed to fracture a material and can be used to measure the toughness of the material. There is a connection to the yield strength but it cannot be expressed by a standard formula. Also, the strain rate may be studied and analyzed for its effect on fracture.

The ductile-brittle transition temperature (DBTT) may be derived from the temperature where the energy needed to fracture the material drastically changes. However, in practice there is no sharp transition and it is difficult to obtain a precise transition temperature (it is really a transition region). An exact DBTT may be empirically derived in many ways: a specific absorbed energy, change in aspect of fracture (such as 50% of the area is cleavage), etc.[1]

Qualitative results

The qualitative results of the impact test can be used to determine the ductility of a material.[13] If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture.[1]

Sample sizes

According to ASTM A370,[14] the standard specimen size for Charpy impact testing is 10 mm × 10mm × 55mm. Subsize specimen sizes are: 10 mm × 7.5 mm × 55mm, 10 mm × 6.7 mm × 55 mm, 10 mm × 5 mm × 55 mm, 10 mm × 3.3 mm × 55 mm, 10 mm × 2.5 mm × 55 mm. Details of specimens as per ASTM A370 (Standard Test Method and Definitions for Mechanical Testing of Steel Products).

According to EN 10045-1,[12] standard specimen sizes are 10 mm × 10 mm × 55 mm. Subsize specimens are: 10 mm × 7.5 mm × 55 mm and 10 mm × 5 mm × 55 mm.

According to ISO 148,[11] standard specimen sizes are 10 mm × 10 mm × 55 mm. Subsize specimens are: 10 mm × 7.5 mm × 55 mm, 10 mm × 5 mm × 55 mm and 10 mm × 2.5 mm × 55mm.

Impact test results on low- and high-strength materials

The impact energy of low-strength metals that do not show change of fracture mode with temperature is usually high and insensitive to temperature. For these reasons, impact tests are not widely used for assessing the fracture-resistance of low-strength materials whose fracture modes remain unchanged with temperature. Impact tests typically show a ductile-brittle transition for low-strength materials that do exhibit change in fracture mode with temperature such as body-centered cubic (BCC) transition metals.

Generally high-strength materials have low impact energies which attest to the fact that fractures easily initiate and propagate in high-strength materials. The impact energies of high-strength materials other than steels or BCC transition metals are usually insensitive to temperature. High-strength BCC steels display a wider variation of impact energy than high-strength metal that do not have a BCC structure because steels undergo microscopic ductile-brittle transition. Regardless, the maximum impact energy of high-strength steels is still low due to their brittleness. [15]

See also

Notes

  1. ^ a b c Meyers Marc A; Chawla Krishan Kumar (1998). Mechanical Behaviors of Materials. Prentice Hall. ISBN 978-0-13-262817-4. 
  2. ^ Siewert
  3. ^ a b The Design and Methods of Construction of Welded Steel Merchant Vessels: Final Report of a (U.S. Navy) Board of Investigation (July 1947). "Welding Journal". 26 (7,). Welding Journal: 569. 
  4. ^ Williams, M. L. & Ellinger, G. A (1948). Investigation of Fractured Steel Plates Removed from Welded Ships. National Bureau of Standards Rep. 
  5. ^ James A Jacobs & Thomas F Kilduff (2005). Engineering Materials Technology (5th ed.). Pearson Prentice Hall. pp. 153–155. ISBN 978-0-13-048185-6. 
  6. ^ Siewert, T. A.; Manahan, M. P.; McCowan, C. N.; Holt, J. M.; Marsh, F. J. & Ruth, E. A (1999). Pendulum Impact Testing: A Century of Progress, ASTM STP 1380. ASTM. 
  7. ^ Cedric W. Richards (1968). Engineering materials science. Wadsworth Publishing Company, Inc. 
  8. ^ Kurishita H, Kayano H, Narui M, Yamazaki M, Kano Y, Shibahara I (1993). "Effects of V-notch dimensions on Charpy impact test results for differently sized miniature specimens of ferritic steel". Materials Transactions - JIM. Japan Institute of Metals. 34 (11): 1042–52. ISSN 0916-1821. 
  9. ^ Mills NJ (February 1976). "The mechanism of brittle fracture in notched impact tests on polycarbonate". Journal of Materials Science. 11 (2): 363–75. Bibcode:1976JMatS..11..363M. doi:10.1007/BF00551448. 
  10. ^ ASTM E23 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
  11. ^ a b ISO 148-1 Metallic materials - Charpy pendulum impact test - Part 1: Test method
  12. ^ a b EN 10045-1 Charpy impact test on metallic materials. Test method (V- and U-notches)
  13. ^ Mathurt KK, Needleman A, Tvergaard V (May 1994). "3D analysis of failure modes in the Charpy impact test". Modeling and Simulation in Materials Science Engineering. 2 (3A): 617–35. Bibcode:1994MSMSE...2..617M. doi:10.1088/0965-0393/2/3A/014. 
  14. ^ ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products
  15. ^ Courtney, Thomas H. (2000). Mechanical Behavior of Materials. Waveland Press, Inc. ISBN 978-1-57766-425-3. 

External links

Brinnel Test

Brinell scale

Force diagram

The Brinell scale /brəˈnɛl/ characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness. However it also had the useful feature that the hardness value divided by two gave the approximate UTS in ksi for steels. This feature contributed to its early adoption over competing hardness tests.

The typical test uses a 10 millimetres (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

where:

BHN = Brinell Hardness Number (kgf/mm2)
P = applied load in kilogram-force (kgf)
D = diameter of indenter (mm)
d = diameter of indentation (mm)

Brinell hardness is sometimes quoted in megapascals, the Brinell hardness number is multiplied by the acceleration due to gravity, 9.80665 m/s2, to convert it to megapascals. The BHN can be converted into the ultimate tensile strength (UTS), although the relationship is dependent on the material, and therefore determined empirically. The relationship is based on Meyer's index (n) from Meyer's law. If Meyer's index is less than 2.2 then the ratio of UTS to BHN is 0.36. If Meyer's index is greater than 2.2, then the ratio increases.[1]

BHN is designated by the most commonly used test standards (ASTM E10-14[2] and ISO 6506–1:2005[3]) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters.

HBW is calculated in both standards using the SI units as

where:

F = applied load (Newtons)
D = diameter of indenter (mm)
d = diameter of indentation (mm)

Common values

When quoting a Brinell hardness number (BHN or more commonly HB), the conditions of the test used to obtain the number must be specified. The standard format for specifying tests can be seen in the example "HBW 10/3000". "HBW" means that a tungsten carbide (from the chemical symbol for tungsten or from the Swedish/German name for tungsten, "Wolfram") ball indenter was used, as opposed to "HBS", which means a hardened steel ball. The "10" is the ball diameter in millimeters. The "3000" is the force in kilograms force.

The hardness may also be shown as XXX HB YYD2. The XXX is the force to apply (in kgf) on a material of type YY (5 for aluminum alloys, 10 for copper alloys, 30 for steels). Thus a typical steel hardness could be written: 250 HB 30D2. It could be a maximum or a minimum.

Brinell hardness numbers
Material Hardness
Softwood (e.g., pine) 1.6 HBS 10/100
Hardwood 2.6–7.0 HBS 1.6 10/100
Lead 5.0 HB (pure lead; alloyed lead typically can range from 5.0 HB to values in excess of 22.0 HB)
Pure Aluminium 15 HB
Copper 35 HB
Hardened AW-6060 Aluminium 75 HB
Mild steel 120 HB
18–8 (304) stainless steel annealed 200 HB[4]
Glass 1550 HB
Hardened tool steel 600–900 HB (HBW 10/3000)
Rhenium diboride 4600 HB
Note: Standard test conditions unless otherwise stated

Standards

See also

(Multi use Hardness Test)

References

Notes

  1. ^ Tabor, p. 17.
  2. ^ ASTM E10 – 14 Standard Test Method for Brinell Hardness of Metallic Materials
  3. ^ ISO 6506–1:2005 Metallic materials – Brinell hardness test – Part 1: Test method
  4. ^ 304: the place to start, retrieved 2009-03-31 .

Bibliography

External links

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