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The many welding techniques that have been developed enable welding in many environments - even in space or under water.

16.11.2019 | 124

Welding is a jointing technique. 

Those who work professionally are called welders. Welding involves heating metal pieces and joining them through a fusion. In this way, a weld joint is formed. The method differs from soldering where the workpieces are joined by a non-similar material (solder), where only the additive material (solder) is melted during soldering.

The energy needed to melt the metals can come from a gas flame, an electric arc, a laser, an electron beam, friction, ultrasound or from high pressure, for example in an explosion. The many welding techniques that have been developed enable welding in many environments - even in space or under water. However, the usual thing is that the material to be welded must be free of dirt and oxides, and that welding electrodes should have a low moisture content, if there is to be a proper weld joint. In order to achieve a satisfactory weld joint, it is necessary to have a good so-called burn-through to avoid slag closures.

Due to rigid shrinkage in the welded material depending on the heat supply (tensile energy), residual stresses will remain in and around the welded material after completed welding. One way to reduce residual stresses is to heat treat the weld after finishing welding. Residual stresses affect toughness, fatigue strength and corrosion resistance (stress corrosion). One way to reduce residual stresses is to divide the weld joint into several smaller filling strands, and thus the diameter of the melt pool can be reduced, with the result that the shrinkage shrinkage becomes smaller.

History

Benardos and Olszewski's patented method of carbon welding.

The first technique that humans used to join metal is the scouring that was used at the Bronze and Iron Age. The steel is then heated to near the melting point to be processed with a hammer. In History of the ancient Greek historian Herodotus he described that Glaucus from Chios about 600 BC. independently invented welding (Greek: σιδήρου κόλλησις, literally translated "iron bonding") [1]. During the Middle Ages, the methods of dung developed and became more advanced. In 1540, the Italian Vannoccio Biringuccio published the book De la pirotechnia, which is considered to be the first printed European book on metallurgy in which methods of scourge are addressed.

In 1801, British chemist Humphry Davy discovered the electric arc. The Russian physicist Vasily Petrov also rediscovered the same thing in 1802 when he experimented with a large voltaic pile he built. In 1803 he published his discovery from the previous year in "News for Galvanic-Voltaic Experiment" (Russian: Известие о гальвани-вольтовских опытах). He described the experiment with his Volta's stack, and also how to get a stable electric arc as well as indications of future applications such as artificial light but above all melting and welding of metal. Petrov was forgotten after his death and his work fell into oblivion.

In 1830, the Englishman Michael Faraday discovered that one could convert mechanical energy to electrical using a simple single-pole generator that produced direct current. It was soon discovered that by connecting the poles to two pieces of metal and bringing them together, the transition resistance between them generated that the high heat fused the pieces where the current went. This is the basic principle of resistance welding.

American Elihu Thomson was granted a patent in 1885 for a method of joining metal according to the resistance welding method, which was later refined. It was not until 1881 before the Russian Nikolai Benardos and the pole Stanisław Olszewski invented carbon arc welding based on the principle Petrov described and which was the first practical method of arc welding. The method assumes that the workpiece was connected to one pole by a power source while the carbon rod was connected to the other. An electric arc is formed between the rod and the workpiece with such high thermal energy that metallic additives could be melted.

The development of arc welding continued to the development of metal arc welding by the Russian Nikolai Slavyanov in 1888 and the American Charles L. Coffin in 1890 where a metal wire served as both electrode and additive material. However, the weld was not protected from air, and therefore there were major problems with pores and poor strength.

Around 1900 Arthur Percy Strohmenger invented the coated electrode which had a clay and lime cover. About the same time, the Swedish Oscar Oscar Kjellberg experimented with the same thing and produced an iron electrode dipped in a mixture of silicate and carbonate, he founded the company Elektriska Svetsnings AB (ESAB). In 1906 he obtained a patent for the coated electrode, with which the welding quality increased considerably and now arc welding could be used for other than emergency repair. In 1912 Strohmenger began selling a heavily coated electrode, but a high price and a complex manufacturing method prevented the product from becoming popular. In the beginning, only direct current was used for welding and large batteries were used as power sources for welding experiments.

In 1905, German AEG produced DC generators that worked for welding, these weighed closer to a ton and took up a lot of space. The same year, the Russian scientist Vladimir Mitkevich proposed to use three-phase system for arc welding. In 1919 C. J. Holslag invented alternating current welding, the transformers were smaller and cheaper than the DC converters, and required less energy.

Gas welding 1918

Termite welding based on an alumina thermal reduction was invented in 1894 by the German Hans Goldschmidt: The method was widely used when joining rail rails. In 1903, gas welding was invented in France by Edmond Fouché and Charles Picard. It was discovered that a mixture of acetylene and oxygen provided a sufficiently high heat to melt various metals, the fouché burner made it possible to use the method practically. Gas welding became popular because it was relatively inexpensive and portable. However, the risks of transporting and handling the explosive acetylene tubes made many skeptical at first. It was known that acetone could dissolve large amounts of acetylene in liquid form. However, as the liquid decreased when used in the tubes, the amount of free acetylene increased and the tubes again became explosive. It was only when Swedish inventor Gustaf Dalén invented the AGA pulp that the gas became safe to handle. The AGA pulp was able to absorb larger amounts of acetylene and now the tubes could also be filled with higher pressure than before.

During the 1920s, much happened within the welding development, experiments were started with automatic welding in 1920 where a metal wire electrode was continuously fed to increase the speed and where noble gases were allowed to flow around the melt. The method was called Metal Inert Gas (MIG) welding and was broken in the 40s and 50s. Protective gases became an interesting subject for researchers and welding gases, such as hydrogen, helium and, above all, argon. However, the cost of the noble gases was expensive and carbon dioxide was used as protective gas in most cases mixed with noble gas. Carbon dioxide is partially chemically active and the method was renamed to Metal Active Gas (MAG) welding.

World War II set new demands for welding in light metals such as aluminum and magnesium for the aircraft industry. One needed a method that could break up the oxide layer that the metals have on the surface, this through. In the American aerospace industry, a method was developed that used a graphite rod similar to Benardo's previous invention, using helium as a shielding gas, and later argon. The graphite rod was replaced with a tungsten electrode that can withstand a very high temperature before it melts. The method is called TIG welding (Tungsten Inert Gas) Initially, the light metals were welded with direct current and the electrode connected to the positive pole so the current went from the light metal to the electrode and thus the oxide breaks up. Later it was started to use alternating current for this and finally also direct current with the electrode connected to the negative coil to weld other metal alloys such as e.g. steel.

While the development of these new and popular methods was developed in the middle of the century, other methods have been refined and developed, production began to be automated and specialized for various purposes. In 1930, bolt welding was invented, which became popular in ships and building construction. In the same year, powder arc welding was also invented which gave a high weld production. In 1932, the Russian Konstantin Khrenov invented a method of arc welding under the water surface. During the 1950s, metal arc welding with added metal powder was developed in the electrode coating to increase production. In 1957, automatic flux wire feed was developed with pipe wire similar to the MAG method. In the same year, plasma welding was also invented, which is largely similar to TIG. Electroslag welding was invented in 1958 and was followed by Electro gas welding in 1961.

The highly power-concentrated welding method electron beam welding was also invented in 1958, and not long afterwards laser welding was also developed in the 1960s. Magnetic pulse welding has been used since 1967. Friction stir welding is a relatively new method invented in 1991 by Wayne Thomas at The Welding Institute (TWI) in the United Kingdom.

Quality requirements

Some welding must meet the quality requirement according to SS-EN ISO 3834 (formerly EN 729) and EN 1090-1 and 2, which can be an addition to the quality systems ISO 9001 and ISO 9002, but can also be used independently. A welder is certified according to ISO 9606-1 for steel and SS-EN ISO 9606 -2, -3, -4 or -5 for aluminum, copper, nickel or titanium and zirconium. The certificate is also called welding test certificate.

Some product areas or applications have special requirements with regard to welding. There may be requirements in product standards and from e.g. authorities or classification societies. (Of course, in all product areas, demands can be made from a customer that goes beyond standard requirements for the product.)

This applies, for example:

Pressure-bearing devices (Pressure vessels)

Nuclear power

building constructions

bridges

Reinforcing Bars / -steel

Lifting devices (Cranes and lifts)

District heating pipes

Iron

Wikipedia


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