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1. Flux-cored arc welding

Flux-cored arc welding (FCAW) is a semi-automatic or automatic arc welding process. FCAW requires a continuously fed consumable tubular electrode containing a flux and constant voltage or, less commonly, a constant electric 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. The process is widely used in construction because of its high welding speed and portability.

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

There are two types of FCAW. The first type 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 high temperatures of welding generate a shielding gas for protecting the arc. This type of FCAW is preferable because it is portable and has excellent penetration into the base metal. Also, the conditions of air flow do not need to be considered.

The second type of FCAW actually uses a shielding gas that must be supplied by an external device. This type of FCAW was developed primarily for welding 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. This particular style of FCAW is preferable for welding thicker metals. The slag created by the flux is also easier to remove. However, it cannot be used in a windy environment as the loss of the shielding gas from air flow will produce visible porosity on the surface of the weld.

2. GTAW weld area

Manual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. Unlike other welding processes, 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. However, some welds combining thin materials can be accomplished without filler metal; most notably edge, corner and butt joints.

To strike the welding arc, a high frequency generator provides a path for the welding current through the shielding gas, allowing the arc to be struck when the separation between the electrode and the workpiece is approximately 1.5-3 mm. Bringing the two into contact also serves to strike an arc, but this can cause contamination of the weld and electrode. 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 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 the vertical. Filler metal is added manually to the front end of the weld pool as it is needed.

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 never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with 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 pool. As the weld nears completion, the arc current is often gradually reduces to prevent the formation of a crater at the end of the weld.

3. Safety in GTAW

Like other welding processes, GTAW can be dangerous if proper precautions are not taken. The process produces intense ultraviolet radiation, which can cause a form of sunburn an, in a few cases, trigger the development of skin cancer. Flying sparks and droplets of molten metal can cause severe burns and start a fire, if flammable material is nearby.

It essential that the welder wear suitable protective clothing, including heavy leather gloves, a closed shirt collar to protect the neck and especially the throat, a protective long sleeve jacket and a suitable helmet to prevent arc eye. Due to the absence of smoke in GTAW, the electric arc can seem brighter than in shielded metal arc welding. Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby personnel from exposure.

Welders are also often exposed to dangerous gases and particulate matter. Shielding gases can displace oxygen and lead to asphyxiation, and while smoke is not produced, the brightness of the arc in GTAW can cause surrounding air to break down and form ozone. Similarly, the brightness and heat 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.

4.Oxy-fuel welding and cutting

Oxy-fuel welding of metal is commonly called oxyacetylene welding, since acetylene is the predominant choice for a fuel, or often simply oxy welding, or in America gas welding. In gas welding and cutting, the heat needed to melt the metal, comes from a fuel gas burning with oxygen in a torch.

Oxy-fuel cutting of metal is a similar process, using a different type of gas torch, called a cutting torch. Here the metal is heated until it glows orange ( about 980º C), and then a lever on the torch is pressed to pass a stream of oxygen through the workpiece, to burn the steel away where the cut is desired. The iron-oxide product of this combustion process falls to the floor as a dust. Once the process is started properly, there should be no globs of melted steel under the work-piece.

Sometimes a metal-cutting blowtorch is colloquially called a gas-axe, smoke wrench, hot wrench or hot-blue spanner. Many people mistakenly call a welding torch a blowtorch.

Torches that do not use pure oxygen with the fuel inside the torch, but burn it with atmosphere air, are not oxy-fuel torches and can be identified by their single gas tank. Oxy-fuel welding needs two tanks, fuel and oxygen. Most metals cannot be melted with such single tank torches, so they can only be used for soldering and brazing, not welding.

The apparatus used in gas welding consists basically of an oxygen source and a fuel gas source, usually cylinders, two pressure regulators and two flexible hoses – one for each cylinder, and a torch. The cylinders are usually carried in a special wheeled trolley.

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Combustion -горение

Glob – капля, шарик

Blowlamp – паяльная лампа

Spanner – гаечный ключ

Wrench – гаечный ключ

5. Electron beam welding

Electron beam welding (EBW) is a fusion welding process, in which a beam of high velocity electrons is applied to the materials being joined. The workpieces melts as the kinetic energy of electrons is transformed into heat upon impact, and the filler metal, if used, also melts to form part of the weld. Pressure is not applied, and a shielding gas is not used, though the welding is often done in conditions of vacuum to prevent dispersion of the electron beam.

As the electrons strike the workpiece, their energy is converted into heat, instantly vaporizing the metal under temperatures near 25 000º C. The heat penetrates deeply, making it possible to weld much thicker workpieces than it is possible with most other welding processes. However, because the electron beam is tightly focused, the total heat input is actually much lower than that of any arc welding processes. As a result, the effect of welding on the surrounding material is minimal, and the heat-affected zone is small. Distortion is slight, and the work-piece cools rapidly, and while normally an advantage, this can lead to cracking in high-carbon steel. Almost all metals can be welded by the process, but the most commonly welded are stainless steels, super-alloys and reactive and refractory metals. The process is also widely used to perform welds of a variety of dissimilar metals combinations. However, attempting to weld plain carbon steel in a vacuum causes the metal to emit gases as it melts, so deoxidizers must be used to prevent weld porosity. The amount of heat input, and thus the penetration, depends on several variables, most notably the number and speed of electrons impacting the workpiece, the diameter of the electron beam, the travel speed. Greater beam current causes an increase in heat output and penetration, while higher travel speed decreases the amount of heat input and reduces penetration. The diameter of the beam can be varied by moving the focal point with respect to the workpiece - focusing the beam below the surface of the workpiece increases the penetration, while placing the focal point above the surface increases the width of the weld.

6. Laser beam welding

Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications, such as in the automotive industry.

Like electron beam welding, laser beam welding has high density (about 1 MW\cm 2) resulting in small heat affected zones and high heating and cooling rates. The spot size of the laser can vary between0.2 mm and 13mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece.

LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of the power supplied but also depends on the type and thickness of the workpieces.

A derivative of LBW, laser-hybrid welding, combines the laser of LBW with the arc welding method such as gas metal arc welding. This combination allows for greater poisoning flexibility, since GMAW supplies molten metal to fill the point, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Wels quality tends to be higher as well, since the potential for undercutting is reduced.

The two types of lasers commonly used in metalworking are solid-state lasers and gas lasers, especially carbon dioxide lasers. The first uses only one of several solid media, including synthetic ruby and chromium in aluminum dioxide, neodimium in glass and the most common type, crystal composed of yttrium, aluminum and nitrogen, and carbon dioxide as a medium. Regardless of the type, however, when the medium is exited, it emits photons and forms the laser beam.

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HSLA steel – high strength low alloy steel

7. Resistance welding

Resistance welding refers to a group of welding processes that produce coalescence of surfaces where heat to form the weld is generated by the resistance of the welding current through the workpieces. Some factors influencing heat or welding temperature are the proportions of the workpieces, the electrode material, electrode geometry, electrode pressing force, weld current and weld time etc. Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current (100-100 000 A) is passed through the metal. In general resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high.

Spot welding is a popular resistance welding method used to join two to four overlapping metal sheets which are up to 3 mm thick each. In some applications with only two overlapping metal sheets, the sheet thickness may be up to 6 mm. Two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated in the air gap at the contact points. At the contact points between electrodes and workpiece the heat dissipates throughout the copper electrodes quickly, since the copper is an excellent conductor. However at the air gap between metal sheets the heat has nowhere to go, as the metal is a comparatively poor conductor. Therefore the heat remains in the one location, which melts the metal at the spot. As the heat dissipates throughout the workpiece over a second or so, it cools the spot weld, causing the metal to solidify.

The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in the shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the shear strength of each weld is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often significantly lower than with other welding methods. This limits the usefulness of the process. It is used extensively in the automotive industry – ordinary cars can have several thousand spot welds. A specialized process, called shot welding, can be used to spot weld stainless steels.

• shear – кромка, срез

8. Shielding gases

Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, most notably gas metal arc welding and gas tungsten arc welding. Their purpose is to protect the weld area from atmospheric gases, such as oxygen, nitrogen, carbon dioxide and water vapor. Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make the welding process more difficult to use. Other arc welding processes use other methods of protecting the weld from the atmosphere as well – shielded metal arc welding, for example, uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.

Shielding gases fall into two categories – inert or semi-inert. Only two of the noble gases, helium and argon, are cost-effective enough to be used in welding. These inert gases are used in gas tungsten arc welding, and also in gas metal arc welding for the welding of non-ferrous materials. Semi-inert shielding gases, or active shield gases, include carbon dioxide, nitrogen and hydrogen. Most of them in large quantities, would damage the weld, but when used in small, controlled quantities, can improve weld characteristics.

The applications of shielding gases are limited primarily by the cost of the gas, cost of the equipment and by the location of the welding. Some shielding gases, like argon, are expensive which limits their use. The equipment used for the delivery of the gas is also an added cost, and as a result, processes like shielded metal arc welding which require less expensive equipment, might be preferred in certain situations. Finally, because atmospheric movements can cause the dispersion of the shielding gas around the weld, welding processes that require shielding gases are only done indoors, where the environment is stable and atmospheric gases can be effectively prevented from entering the weld area.

Unit I5. WELDING DEFECTS

Common welding defects include lack of fusion, lack of penetration or excess penetration, porosity, inclusions, cracking, undercut, lamellar tearing. Any of these defects are potentially disastrous as they can give rise to high stress intensities which may result in sudden unexpected failure below the design load.

To achieve a good quality joint it is essential that the fusion zone extends to the full thickness of the sheets being joined. Thin sheet material can be joined with a single pass and a clean square edge will be a satisfactory basis for a joint. How-ever, thicker material will normally need edges cut at a V- angle and may need several passes to fill the V with weld metal. Where both sides are accessible one or more passes may be made along the reverse side to ensure the joint extends to the full thickness of the metal. Lack of fusion results from too little heat input and \ or too rapid traverse of the welding torch (gas or electric). Excess penetration or burning through arises from too high a heat input and \ or too slow traverse of the welding torch. It is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal.

Porosity occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal, or from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and the workpiece is carefully cleaned and degreased prior to welding.

Inclusions occur when several runs are made along a V-joint when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run.

Cracking can occur due to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with a carbon content greater than 0.2% are being welded, self-cooling may be rapid enough to cause some brittle martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating may be needed, and after welding a slow controlled post-cooling in stages will be required. This can greatly increase the cost of welded joints, but for high strength steels, such as those used in petrochemical plants piping, there may well be no alternative.

Solidifying cracking is also called centerline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulfur and phosphorus, are more likely to occur in higher carbon steels. Solidification cracks are normally distinguishable from other types of cracks by the following features: 1. they occur only in the weld metal – although the parent metal is almost always the source of the low melting point contaminants associated with the cracking; 2.They normally appear in straight lines along the centerline of the weld bead, but may occasionally appear as transverse cracking; 3.Solidification cracks in the final crater may have a branching appearance; 4. as the cracks are open they are visible to the naked eye. A schematic diagram of a centerline crack is shown below:

On breaking open the weld the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries. There may be evidence of segregation associated with the solidification boundary. The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses as the weld pool solidifies. Factors which increase the risk include insufficient weld bead size or inappropriate form, welding under excessive restraint, material properties, such as a high impurity content or a relatively large shrinkage on solidification.

Joint design can have an influence on the level of residual stresses. Large gaps between components will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking. In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese can help to reduce the risk. To minimize the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. However when welding a highly restrained joint using high strength steels, a combined level below 0.03 might be needed.

VOCABULARY

Lack of fusion – непровар (шва)

Lack of penetration – недостаточная глубина провара

Undercut - подрез

Lamellar tearing – расслаивание, образование продольных трещин

Single pass \ run – однократный \единичный проход

Edge cutting - срез кромки

Heat input – эффективная тепловая мощность, погонная энергия

Transverse – пересечение, (поперечное) движение

Torch weld – шов, полученный при газовой сварке

Rapid traverse – быстрый ход, форсированная продольная подача

Burn through – проплавление, прожог

Core rod – сердцевина электрода

Flux cored rod – электрод \ стержень с флюсовой сердцевиной

Flux coating – электрод \ стержень с минеральным \ флюсовым покрытием

Shrinkage - усадка

Stiff frame – жесткая рама

Residual stress – остаточное напряжение

Segregation – расслоение, сегрегация

Hot crack – горячая трещина, горячий крекинг

Parent metal – основной металл

Weld bead – наплавленный валик (металла)

Transverse cracking – поперечное растрескивание

Open weld – шов с зазором между кромками

Solidification cracking – образование усадочных трещин

To withstand – выдерживать, противостоять

Contraction – напряжение сжатия

Ratio – коэффициент, соотношение

Impurity – примесь, постороннее включение