Welding

Richard Lofting

• Publisher: Crowood
• Year: 2013
• ISBN:
  9781847975133
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería mecánica

Welding is an essential technique for a wide range of jobs in the workshop. Whether you are new to welding or ready to try the more advanced techniques, this practical guide gives a thorough introduction to the method, and suggests ways of improving your skills to achieve professional and safe results.Explains the different types of welding and when they are best used.Advises on choosing equipment and its maintenance.Demonstrates the processes with clear, step-by-step photographs.Emphasizes safety and best practice.Aimed at everyone who needs to weld and has a workshop - modellers, theatre designers, automotive repair and restorers and farmers.A practical guide to the different types of welding and advise on equipment and its maintenance.Superbly illustrated with 280 step-by-step colour photographs.The first in a new series Crowood's Metalworking Guides.

1

A Brief History

The first evidence for welding dates from not long after the discovery that metals could be extracted from ore by heat. Examples of iron items being hammer welded in a hearth are known from before 1000BC. Under this process the two parts to be joined are heated to just below melting temperature and then quickly hammered together. The extra heat and pressure generated by the hammering enables the surfaces of the two components to fuse and become one.

Forge welding has been the mainstay of welding for centuries.

During the forge welding process, heat and pressure are used to join the parts as one.

One fine example of the ancient craft of forge welding is now at the Quwwat al-Islam Mosque in Delhi, to where it was moved at some point in its long history. Known as the Delhi Pillar, it is reputed to have been forge welded, by hand, from several billets of almost pure iron. It stands 23ft 8in (7m) above the ground with a further 3ft (1m) or so buried below, and weighs in the region of 6½ tonnes. It is 16in (400mm) in diameter at its base, tapering upwards to 11½in (300mm) with a fancy finial at the top, although it is believed that an ornate figure in the form of Garuda (Sunbird), the Vahana of the Hindu god Vishnu, originally stood on top of the column. According to the Sanskrit inscription at the base, the pillar was constructed during the fourth century AD in honour of the Gupta ruler Chandragupta Vikramaditya. In all the time that the pillar has stood at the mosque, there is little evidence of any rust appearing on the column. While this is a very arid region, it is now believed that this is the result of its phosphorus content, incorporated into the iron from charcoal used in the smelting process. This has caused an extremely thin oxide coating to form, preventing further rusting.

ELECTRIC WELDING

Forge welding and riveting remained the mainstay methods used by blacksmiths to join metal objects until the 1800s. Around this time it was found that carbon electrodes connected to an accumulator (battery) produced an arc. The DC current stored in the accumulators could then be utilized to make a brittle and porous weld. Since the introduction of mains electricity and AC current was still about a century away, recharging would have been carried out with a dynamo, possibly driven by a steam engine or waterwheel.

Coated electrodes were designed in the early twentieth century but did not become popular until 1927, when mass production techniques were used to drop the price.

In 1881 the French engineer Auguste de Méritens was awarded a patent for a method by which the plates of lead accumulators might be welded together with the carbon arc. In 1885 two of his Russian pupils, Nikolai Benardos and Stanislav Olszewski, obtained the first British patent in welding practices. Patent No. 12984 described the method of using the carbon arc with an electric power source to weld metals together. The apparatus used to achieve this was named the ‘Electrogefest’. Patents followed in Russia and the USA in 1886 and 1887, respectively.

Development was rapid. Another Russian, Nikolai Slavianov, developed arc welding with an iron electrode in 1888. Similar experiments in the United States led to Charles L. Coffin of Detroit obtaining patents in 1889 for flash-butt welding and in 1890 for spot welding equipment that he had been developing.

The first few years of the twentieth century saw further improvements to these techniques, including the use of hollow carbon rods, filled with metal particles, to act as filler in the carbon arc process. These never gained much popularity, however, although they were later used in specialized processes in the 1940s and ’50s. The great leap was made during the 1920s. The effective introduction of thick armour plating during the First World War, for example on warships and battle tanks, led to a demand for similar armaments. The riveting of such plating, however, was somewhat slow and a faster, more efficient, production method was urgently required. Alternating current (AC) was developed in the early 1920s and power stations were built to supply industry and homes with electricity, although steam power remained the main motive force for many more years.

It was understood that a means had to be found to protect the weld pool from oxygen and nitrogen in the atmosphere is required, since it was this lack of isolation of the weld pool from the atmosphere that made welds brittle and porous. In addition, the welding arc produced by the use of alternating current was shown to be very unstable.

THE COATED ELECTRODE

At the turn of the twentieth century Swedish engineers were developing coatings to cover filler rods: A. P. Strohmenger used clay and lime, while Oscar Kjellberg used carbonates and silicates. The coated electrode was found to do the job. The heat of the arc vaporizes the coating of various clays and silicates into gases that shield the weld pool from the detrimental effects of the atmosphere until the liquid metal beneath has cooled and solidified, leaving a deposit from the remains of the rod coating and any impurities, known as slag, on top of the weld. Coated electrodes were originally produced by dipping lengths of filler wire into a liquid mixture of the coating and setting them aside to dry. The use of these rods did not become widespread until around 1927, when an extrusion process was designed to speed up production, so reducing the price and extending the range of tasks covered.

GAS WELDING

The English scientist Edmund Davy discovered acetylene, a hot burning gas, in 1836. This was followed in 1900 by the development by two Frenchmen, Edmond Fouché and Charles Picard, of an oxyacetylene torch able to create a flame of 3500°C that is ideal for welding. The acetylene obtained for this early development was produced by dripping water on to calcium carbide to release acetylene, just as early carriage lamps used the acetylene gas liberated by this method to produce a bright white light.

In 1904 Percy Avery and Carl Fisher founded the Concentrated Acetylene Company in Indianapolis to develop ways of storing acetylene as a gas. Acetylene is extremely unstable and, unlike other gases, cannot be directly compressed into an empty cylinder since it quickly becomes unstable at a pressure in excess of around 20 psi and explodes. The cylinder is instead filled with a porous medium, such as balsa wood or asbestos fibre. This is then saturated with acetone, a common solvent. The acetylene gas is very slowly introduced into the acetone and is readily absorbed, like ink on blotting paper, as the pressure is increased. When the pressure in the cylinder is released during burning of the gas, the acetone gives up the acetylene, leaving just the acetone-soaked medium behind in the cylinder, ready for its next charge of gas. Even with the porous medium and acetone in the cylinder, the acetylene gas must be introduced very slowly if it is not to become unstable.

HELIARC PROCESS

From the early days of the Second World War lighter non-ferrous materials, such as aluminium and magnesium, were increasingly used in the new fighter and bomber aircraft and techniques were soon developed to weld these newfangled materials.

Acetylene was not available in a cylinder until 1913, when Avery and Fisher developed a cylinder with a porous mass inside, soaked in acetone.

Today the engineering world is fairly familiar with the term TIG welding and it is generally supposed that it was developed after the modern MIG welder. Its origins, however, go back to work during the 1920s on using inert gases to shield the weld pool from the atmosphere. It was known as the Heliarc process, after patents taken out in 1941 using helium as the inert shielding gas and a tungsten electrode, as in the modern TIG welder (although nowadays the shielding gas is argon). Modern equipment, of course, is now more compact than the original machines and a lot more versatile.

MIG WELDER

The now ubiquitous MIG welder was developed in 1948. The fixed tungsten electrode in the handset of the Heliarc process was replaced by a continuously fed wire electrode from a roll within the machine. In its initial form it was used for non-ferrous metals, but in the early 1950s it became popular for use with ferrous metals after it was discovered that carbon dioxide – much cheaper and more easily obtainable – made a very suitable shielding gas on steel, although technically it is an active gas and is known as MAG welding (Metal Active Gas). The other main advance was the development of much thinner electrode wires, which made the process more versatile across the range from thin sheet work to heavier sections.

EXOTIC TECHNIQUES

During the 1950s technological advances in the world of welding were being made almost on a daily basis as new ways were developed to produce welds fit for harsher and more extreme environments.

Electron Beam Welding

It was revealed during this decade that electron beam welding was being used in France’s growing nuclear power industry, for which good, reliable welds are a necessity, not just desirable. Plasma arc welding, first experimented with in the 1920s, was also developed for industrial use in the 1950s. In this process a stream of gas is heated in a tungsten arc, creating plasma that is half as hot again as a tungsten arc alone can produce. There are many specialist uses and high-grade steels, in particular, benefit from these techniques. The process can also be used for cutting. The plasma cutter itself has now developed into a portable machine that can be used to cut not only steel, but also stainless steel, aluminium, brass and copper. The plasma heats a spot on the surface of the item to b...