Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals when cool result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between a metal stick " electrode " and the base material to melt the metals at the point of contact. Arc welders can use either direct DC or alternating AC current, and consumable or non-consumable electrodes. The welding area is usually protected by some type of shielding gas , vapor, or slag. Arc welding processes may be manual, semi-automatic, or fully automated.
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Gas tungsten arc weldingVIDEO ON THE TOPIC: Stick Welding Stainless Steel with 308L-16 Electrodes
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.
When helium is used, this is known as heliarc welding. 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.
After the discovery of the short pulsed electric arc in by Humphry Davy   and of the continuous electric arc in by Vasily Petrov ,   arc welding developed slowly. Coffin had the idea of welding in an inert gas atmosphere in , 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, resulting in porous, dross -filled welds.
To solve the problem, bottled inert gases were used in the beginning of the s. A few years later, a direct current , gas-shielded welding process emerged in the aircraft industry for welding magnesium.
Russell Meredith of Northrop Aircraft perfected the process in 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. Finally, the development of alternating current units made it possible to stabilize the arc and produce high quality aluminum and magnesium welds.
Developments continued during the following decades. Linde developed water-cooled torches that helped prevent overheating when welding with high currents. 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. Among the most popular are the pulsed-current, manual programmed, hot-wire, dabber, and increased penetration GTAW methods. 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. 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.
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.
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.
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. 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.
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 that 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. 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.
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.
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. 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 increases. 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. 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.
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.
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.
The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply. 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.
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. 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. 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. 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. 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. 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.
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. 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.
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Welding Different Metals
For years fabricators have questioned the need for properly storing opened containers of electrodes in a controlled environment. The cost of holding ovens, as well as the discipline required to use them, has had fabricators second-guessing the benefits of such a practice. Regardless of the type of filler metal you use, the need to following the practice has never been greater. Covered electrodes and cored wire are the most sensitive to moisture. Many of the ingredients used to extrude electrodes or fabricate wires are sensitive to moisture pickup. Exposed electrodes can absorb moisture as the temperature and humidity rise. An electrode exposed at 70 degrees F and 70 percent humidity will absorb moisture more slowly than one at 90 degrees F and 90 percent humidity.
Metrode Products Ltd. Keywords : Austenitic stainless steel , cryogenic, LNG, lateral expansion , toughness, welding consumables. The storage and distribution of various gases including liquefied natural gas LNG requires materials that have good mechanical properties, particularly toughness, at low temperatures. Gases are generally stored as liquids at low pressure and this requires that the materials used for storage tanks and pipework are capable of withstanding the low temperatures encountered with liquefied gases. Some examples of the liquefaction temperatures of various gases are shown in Table 1. The most important criterion for service at cryogenic temperatures is normally toughness, and it is important that the weld metals used are capable of achieving good toughness. The materials and welding consumables suitable for use at various nominal temperatures are shown in Table 2. As explained already, many different alloys are selected for LNG applications. The use of fully austenitic weld metals eg.
Stainless Steel Workshop: Storing and reconditioning filler metals
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Clarification over tungsten electrodes for TIG/GTAW welding
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Я бы хотела попросить об одной любезности. Не знаю только, к кому обращаться - к тебе или к Арчи. я даже не знаю, возможно ли выполнить мою просьбу.
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Поэтому не свидетельствует ли их присутствие здесь о существовании третьего поселения. - поинтересовалась Николь.