Arc welding
Encyclopedia
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
that uses a welding power supply
Welding power supply
A welding power supply is a device that provides an electric current to perform welding. Welding usually requires high current and it can need above 12,000 amperes in spot welding. Low current can also be used; welding two razor blades together at 5 amps with gas tungsten arc welding is a good...
to create an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
between an electrode and the base material to melt the metals at the welding point. They can use either direct
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) or alternating
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) current, and consumable or non-consumable electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s. The welding region is usually protected by some type of shielding gas
Shielding gas
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 vapour...
, vapor, and/or slag.
Development
While examples of forge weldingForge welding
Forge welding is a solid-state welding process that joins two pieces of metal by heating them to a high temperature and then hammering them together. The process is one of the simplest methods of joining metals and has been used since ancient times. Forge welding is versatile, being able to join a...
go back to the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
and the Iron Age
Iron Age
The Iron Age is the archaeological period generally occurring after the Bronze Age, marked by the prevalent use of iron. The early period of the age is characterized by the widespread use of iron or steel. The adoption of such material coincided with other changes in society, including differing...
, arc welding did not come into practice until much later. In 1802, Vasily Petrov
Vasily Vladimirovich Petrov
Vasily Vladimirovich Petrov was a Russian experimental physicist, self-taught electrical technician, academician of Russian Academy of Sciences ....
discovered the continuous electric arc and subsequently proposed its possible practical applications, including welding. The French electrical inventor Auguste de Méritens
Auguste de Méritens
Baron Auguste de Méritens was a French electrical engineer of the 19th century.He was born in 1834.He is best known his work on magneto generators, particularly those used for arc lighting and lighthouses...
produced the first carbon arc torch, patented in 1881, which was successfully used for welding lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
in the manufacture of lead-acid batteries. In 1881-1882 a Russian inventor Nikolai Bernardos created the electric arc welding method for steel known as carbon arc welding
Carbon arc welding
Carbon arc welding is a process which produces coalescence of metals by heating them with an arc between a nonconsumable carbon electrode and the work-piece. It was the first arc-welding process ever developed but is not used for many applications today, having been replaced by twin-carbon-arc...
, using carbon electrodes. The advances in arc welding continued with the invention of metal electrodes in the late 19th century by a Russian, Nikolai Slavyanov (1888), and an American, C. L. Coffin
C. L. Coffin
Charles L. Coffin of Detroit was awarded for an arc welding process using a metal electrode. This was the first time that metal melted from the electrode carried across the arc to deposit filler metal in the joint to make a weld. About the same time, N.G. Slavianoff, a Russian, presented the same...
. Around 1900, A. P. Strohmenger released in Britain
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
a coated metal electrode which gave a more stable arc. In 1905 Russian scientist Vladimir Mitkevich proposed the usage of three-phase electric arc for welding. In 1919, alternating current welding was invented by C.J. Holslag but did not become popular for another decade.
Competing welding processes such as resistance welding
Resistance welding
Electric resistance welding refers to a group of welding processes such as spot and seam welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electical reistance of material vs the time and the force used to hold the materials together during welding...
and oxyfuel welding were developed during this time as well; but both, especially the latter, faced stiff competition from arc welding especially after metal coverings (known as flux
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
) for the electrode, to stabilize the arc and shield the base material from impurities, continued to be developed.
During World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
welding started to be used in shipbuilding
Shipbuilding
Shipbuilding is the construction of ships and floating vessels. It normally takes place in a specialized facility known as a shipyard. Shipbuilders, also called shipwrights, follow a specialized occupation that traces its roots to before recorded history.Shipbuilding and ship repairs, both...
in Great Britain in place of rivet
Rivet
A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...
ed steel plates. The Americans also became more accepting of the new technology when the process allowed them to repair their ships quickly after a German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
attack in the New York Harbor
New York Harbor
New York Harbor refers to the waterways of the estuary near the mouth of the Hudson River that empty into New York Bay. It is one of the largest natural harbors in the world. Although the U.S. Board of Geographic Names does not use the term, New York Harbor has important historical, governmental,...
at the beginning of the war. Arc welding was first applied to aircraft during the war as well, and some German airplane fuselages were constructed using this process. In 1919, the British shipbuilder Cammell Laird
Cammell Laird
Cammell Laird, one of the most famous names in British shipbuilding during the nineteenth and twentieth centuries, came about following the merger of Laird, Son & Co. of Birkenhead and Johnson Cammell & Co. of Sheffield at the turn of the twentieth century.- Founding of the business :The Company...
started construction of merchant ship, the Fullagar, with an entirely welded hull; she was launched in 1921.
During the 1920s, major advances were made in welding technology, including the 1920 introduction of automatic welding in which electrode wire was continuously fed. Shielding gas became a subject receiving much attention as scientists attempted to protect welds from the effects of oxygen and nitrogen in the atmosphere. Porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
and brittleness were the primary problems and the solutions that developed included the use of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
, and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
as welding atmospheres. During the following decade, further advances allowed for the welding of reactive metals such as aluminum and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
. This, in conjunction with developments in automatic welding, alternating current, and fluxes fed a major expansion of arc welding during the 1930s and then during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
During the middle of the century, many new welding methods were invented. Submerged arc welding
Submerged arc welding
Submerged arc welding is a common arc welding process. Originally developed by the Linde - Union Carbide Company. It requires a continuously fed consumable solid or tubular electrode...
was invented in 1930 and continues to be popular today. In 1932 a Russian, Konstantin Khrenov
Konstantin Khrenov
Konstantin Konstantinovich Khrenov was a Soviet engineer and inventor who in 1932 introduced underwater welding and cutting of metals. For this method, extensively used by the Soviet Navy during World War II, Khrenov was awarded the State Stalin Prize in 1946....
successfully implemented the first underwater electric arc welding. Gas tungsten arc welding
Gas tungsten arc welding
Gas tungsten arc welding , also known as tungsten inert gas welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld...
, after decades of development, was finally perfected in 1941 and gas metal arc welding
Gas metal arc welding
Gas metal arc welding , sometimes referred to by its subtypes metal inert gas welding or metal active gas welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun...
followed in 1948, allowing for fast welding of non-ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
materials but requiring expensive shielding gases. Using a consumable electrode and a carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
atmosphere as a shielding gas, it quickly became the most popular metal arc welding process. In 1957, the flux-cored arc welding
Flux-cored arc welding
Flux-cored arc welding 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...
process debuted in which the self-shielded wire electrode could be used with automatic equipment, resulting in greatly increased welding speeds. In that same year, plasma arc welding
Plasma arc welding
Plasma arc welding is an arc welding process similar to gas tungsten arc welding . The electric arc is formed between an electrode and the workpiece...
was invented. Electroslag welding
Electroslag welding
Electroslag welding ' is a highly productive, single pass welding process for thick materials in a vertical or close to vertical position. is similar to electrogas welding, but the main difference is the arc starts in a different location...
was released in 1958 and was followed by its cousin, electrogas welding
Electrogas welding
Electrogas welding is a continuous vertical position arc welding process developed in 1961, in which an arc is struck between a consumable electrode and the workpiece. A shielding gas is sometimes used, but pressure is not applied...
, in 1961.
Power supplies
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.
The direction of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
will have a greater heat concentration and, as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, it will melt more quickly, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current (DC), as well as alternating current (AC). With direct current however, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
pattern instead of the normal sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
, eliminating low-voltage time after the zero crossings and minimizing the effects of the problem.
Home and Hobby Power supplies
Home and hobby arc welders for occasional light duty (under 1/4 inch plate) repair and construction are available from $100 and up as of 2011.In the $100 to $200 range, many choices are available in welding power supplies
Welding power supply
A welding power supply is a device that provides an electric current to perform welding. Welding usually requires high current and it can need above 12,000 amperes in spot welding. Low current can also be used; welding two razor blades together at 5 amps with gas tungsten arc welding is a good...
such as output current at a given duty cycle, 120 Volts "house current" or 220 VAC and differing input amperages. At these low prices, any positive factor typically weakens another important factor. One seller offers this specification: "Duty Cycle: 45% @ 60 amps, 25% @ 80 amps," for their 120 volts, 20 amp input, "90 Amp Flux Wire Welder."
Duty cycle is a welding equipment specification which defines the number of minutes, within a 10 minute period, during which a given arc welder can safely be used.
For example, an 80 amp. welder with a 60% duty cycle must be "rested" for at least 4 minutes after 6 minutes of continuous welding. Failure to observe duty cycle limitations could damage the welder. But professional welders can typically run at 100% duty cycle, so it's not a factor in professional or commercial welding.
Consumable electrode methods
Shielded metal arc welding
Shielded metal arc welding , also known as manual metal arc welding, flux shielded arc welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld...
(SMAW), which is also known as manual metal arc welding (MMAW) or stick welding. An electric current is used to strike an arc between the base material and a consumable electrode rod or 'stick'. The electrode rod is made of a material that is compatible with the base material being welded and is covered with a flux that gives off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination. The electrode core itself acts as filler material, making a separate filler unnecessary. The process is very versatile, requiring little operator training and inexpensive equipment. However, weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though specialty electrodes have made possible the welding of cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and other metals. The versatility of the method makes it popular in a number of applications including repair work and construction.
Gas metal arc welding
Gas metal arc welding
Gas metal arc welding , sometimes referred to by its subtypes metal inert gas welding or metal active gas welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun...
(GMAW), commonly called MIG (Metal Inert Gas), is a semi-automatic or automatic welding process with a continuously fed consumable wire acting as both electrode and filler metal, along with an inert or semi-inert shielding gas flowed around the wire to prevent the weld site from contamination. Constant voltage, direct current power source is most commonly used with GMAW, but constant current alternating current are used as well. With continuously fed filler electrodes, GMAW offers relatively high welding speeds, however the more complicated equipment reduces convenience and versatility in comparison to the SMAW process. Originally developed for welding aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
and other non-ferrous materials in the 1940s, GMAW was soon economically applied to steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
s. Today, GMAW is commonly used in industries such as the automobile industry for its quality, versatility and speed. Because of the need to maintain a stable shroud of shielding gas around the weld site, it can be problematic to use the GMAW process in areas of high air movement such as outdoors.
Flux-cored arc welding
Flux-cored arc welding
Flux-cored arc welding 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...
(FCAW) is a variation of the GMAW technique. FCAW wire is actually a fine metal tube filled with powdered flux materials. 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.
Submerged arc welding
Submerged arc welding
Submerged arc welding is a common arc welding process. Originally developed by the Linde - Union Carbide Company. It requires a continuously fed consumable solid or tubular electrode...
(SAW) is a high-productivity welding process in which the arc is struck beneath a covering layer of granular flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself and, combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes since the flux hides the arc and no smoke is produced. The process is commonly used in industry, especially for large products. As the arc is not visible, it typically automated. SAW is only possible in the 1F (flat fillet), 2F, (horizontal fillet), and 1G (flat groove) positions.
Non-consumable electrode methods
Gas tungsten arc weldingGas tungsten arc welding
Gas tungsten arc welding , also known as tungsten inert gas welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld...
(GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. It can be used on nearly all weldable metals, though it is most often applied to stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
and light metals. It is often used when quality welds are extremely important, such as in bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
, aircraft and naval applications. A related process, plasma arc welding
Plasma arc welding
Plasma arc welding is an arc welding process similar to gas tungsten arc welding . The electric arc is formed between an electrode and the workpiece...
, also uses a tungsten electrode but uses plasma gas
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process and is much faster. It can be applied to all of the same materials as GTAW except magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
; automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting
Plasma cutting
Plasma cutting is a process that is used to cut steel and other metals of different thicknesses using a plasma torch...
, an efficient steel cutting process.
Other arc welding processes include atomic hydrogen welding
Atomic hydrogen welding
Atomic hydrogen welding is an arc welding process that uses an arc between two metal tungsten electrodes in a shielding atmosphere of hydrogen. The process was invented by Irving Langmuir in the course of his studies of atomic hydrogen...
, carbon arc welding, electroslag welding
Electroslag welding
Electroslag welding ' is a highly productive, single pass welding process for thick materials in a vertical or close to vertical position. is similar to electrogas welding, but the main difference is the arc starts in a different location...
, electrogas welding
Electrogas welding
Electrogas welding is a continuous vertical position arc welding process developed in 1961, in which an arc is struck between a consumable electrode and the workpiece. A shielding gas is sometimes used, but pressure is not applied...
, and stud arc welding.
Corrosion issues
Some materials, notably high-strength steels, aluminium, and titanium alloys, are susceptible to hydrogen embrittlementHydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
. If the electrodes used for welding contain traces of moisture, the water decomposes in the heat of the arc and the liberated hydrogen enters the lattice of the material, causing its brittleness. Stick electrodes for such materials, with special low-hydrogen coating, are delivered in sealed moisture-proof packaging. New electrodes can be used straight from the can, but when moisture absorption may be suspected, they have to be dried by baking (usually at 800 to 1000 °F (425 to 550 °C)) in a drying oven. Flux used has to be kept dry as well.
Some austenitic stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
s and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-based alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s are prone to intergranular corrosion
Intergranular corrosion
Intergranular corrosion , also known as intergranular attack , is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides...
. When subjected to temperatures around 700 °C (1,292 °F) for too long time, chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
reacts with carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in the material, forming chromium carbide
Chromium carbide
Chromium carbide is an extremely hard refractory ceramic material. It is usually processed by sintering. It has the appearance of a gray powder with orthorhombic crystal structure. The orthorhombic Cr3C2 occurs extremely rarely as mineral tongbaite....
and depleting the crystal edges of chromium, impairing their corrosion resistance in a process called sensitization. Such sensitized steel undergoes corrosion in the areas near the welds where the temperature-time was favorable for forming the carbide. This kind of corrosion is often termed weld decay.
Knifeline attack (KLA) is another kind of corrosion affecting welds, impacting steels stabilized by niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
. Niobium and niobium carbide
Niobium carbide
Niobium carbide is an extremely hard refractory ceramic material, commercially used in tool bits for cutting tools. It is usually processed by sintering and is a frequent additive in cemented carbides. It has the appearance of a brown-gray metallic powder with purple lustre...
dissolves in steel at very high temperatures. At some cooling regimes, niobium carbide does not precipitate, and the steel then behaves like unstabilized steel, forming chromium carbide instead. This affects only a thin zone several millimeters wide in the very vicinity of the weld, making it difficult to spot and increasing the corrosion speed. Structures made of such steels have to be heated in a whole to about 1950 °F (1,065.6 °C), when the chromium carbide dissolves and niobium carbide forms. The cooling rate after this treatment is not important.
Filler metal (electrode material) improperly chosen for the environmental conditions can make them corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
-sensitive as well. There are also issues of galvanic corrosion if the electrode composition is sufficiently dissimilar to the materials welded, or the materials are dissimilar themselves. Even between different grades of nickel-based stainless steels, corrosion of welded joints
Welding joints
Welding joints are formed by welding two or more workpieces, made of metals or plastics, according to a particular geometry. The most common types are butt and lap joints; there are various lesser used welding joints including flange and corner joints....
can be severe, despite that they rarely undergo galvanic corrosion when mechanically joined.
Safety issues
Welding can be a dangerous and unhealthy practice without the proper precautions; however, with the use of new technology and proper protection the risks of injury or death associated with welding can be greatly reduced.Heat and sparks
Because many common welding procedures involve an open electric arc or flame, the risk of burns from heat and sparksSpark (fire)
A spark is an incandescent particle. Such sparks may be produced by pyrotechnics, by metalworking or as a by-product of fires, especially when burning wood.-Pyrotechnics:...
is significant. To prevent them, welder
Welder
A welder is a tradesman who specializes in welding materials together. The materials to be joined can be metals or varieties of plastic or polymer...
s wear protective clothing in the form of heavy leather
Leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...
glove
Glove
A glove is a garment covering the hand. Gloves have separate sheaths or openings for each finger and the thumb; if there is an opening but no covering sheath for each finger they are called "fingerless gloves". Fingerless gloves with one large opening rather than individual openings for each...
s and protective long sleeve jackets to avoid exposure to extreme heat, flames, and sparks.
Eye damage
The brightness of the weld area leads to a condition called arc eye in which ultraviolet light causes inflammation of the corneaCornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
and can burn the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
s of the eyes. Welding goggles
Welding goggles
Welding goggles provide a degree of eye protection while some forms of welding and cutting are being done. They are intended to protect the eyes not only from the heat and optical radiation produced by the welding, such as the intense ultraviolet light produced by an electric arc, but from sparks...
and helmets
Welding helmet
Welding helmets are headgear used when performing certain types of welding to protect the eyes, face and neck from flash burn, ultraviolet light, sparks, infrared light, and heat. Most commonly used with arc welding processes such as shielded metal arc welding, gas tungsten arc welding, and gas...
with dark face plates are worn to prevent this exposure and, in recent years, new helmet models have been produced featuring a face plate that self-darkens upon exposure to high amounts of UV light. To protect bystanders, transparent welding curtains often surround the welding area. These curtains, made of a polyvinyl chloride
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...
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.
Those dark face plates must be much darker than those in sunglasses or oxy-fuel goggles. Sunglasses and oxy-fuel goggles are not adequate for arc welding protection.
In 1970, a Swedish doctor, Åke Sandén, developed a new type of welding goggles that used a multilayer interference filter to block most of the light from the arc. He had observed that most welders could not see well enough with the mask on to strike the arc, so they would flip the mask up to strike then flip it down again once the arc was going; this exposed their naked eyes to the intense light for a while. By coincidence, the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of an electric arc has a notch in it which coincides with the yellow sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
line. Thus, a welding shop could be lit by sodium vapor lamp
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure...
s or daylight, and the welder could see well to strike the arc. The Swedish government required these masks to be used for arc welding, but they were not used in the United States. They may have disappeared.
Inhaled matter
Welders are also often exposed to dangerous gases and particulate matter. Processes like flux-cored arc welding and shielded metal arc welding produce smokeSmoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
containing particles of various types of oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, many processes produce various gases (most commonly carbon dioxide and ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, but others as well) that can prove dangerous if ventilation is inadequate. Furthermore, the use of compressed gases and flames in many welding processes 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.
Interference with pacemakers
Certain welding machines which use a high frequency AC current component have been found to affect pacemaker operation when within 2 meters of the power unit and 1 meter of the weld site.Sources
- Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology. Upper Saddle River, New JerseyNew JerseyNew Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
: Pearson Education. ISBN 0-1311-3029-3 - Lincoln ElectricLincoln ElectricLincoln Electric is a company in Euclid, Ohio, United States that manufactures arc welding equipment and consumables, plasma and oxy-fuel cutting equipment and robotic welding systems. They are a worldwide leader in production of welding products and have 39 manufacturing locations, including...
(1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 99949-25-82-2. - Weman, Klas (2003). Welding processes handbook. New York: CRC Press LLC. ISBN 0-8493-1773-8.
Further reading
- ASM International (society) (2003). Trends in Welding Research. Materials Park, OhioOhioOhio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
: ASM International. ISBN 0-87170-780-2 - Blunt, Jane and Nigel C. Balchin (2002). Health and Safety in Welding and Allied Processes. CambridgeCambridgeThe city of Cambridge is a university town and the administrative centre of the county of Cambridgeshire, England. It lies in East Anglia about north of London. Cambridge is at the heart of the high-technology centre known as Silicon Fen – a play on Silicon Valley and the fens surrounding the...
: Woodhead. ISBN 1-85573-538-5. - Hicks, John (1999). Welded Joint Design. New YorkNew York CityNew York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
: Industrial Press. ISBN 0-8311-3130-6.
External links
- Arc Flash Awareness video (25:39) from U.S. National Institute for Occupational Safety and Health
- The American Welding Society
- Canadian Welding Association
- The Welding Institute