What is Welding?
Welding is the joining of metal pieces by the application of heat, or pressure, or both. The metal pieces coalesce into a single metal along a common edge (joint).
Welding is simpler if the workpieces are of the same metal. Additional, or filler metal, must be added for some types of joint. Welded joints are usually as strong as the metal itself, and make an airtight seal. Thus, welding is very important in the manufacture of metal products, construction, and maintenance and repair.
There are certain basic requirements which must be fulfilled in all welding processes. Energy in some form, usually heat, must be supplied to the joint so that the parts can be united.
It is also a feature of every welding process that the contaminating surface film must be dissolved or dispersed. This may be done by the chemical action of a flux, by the impingement of an electric arc, or by mechanical means such as rubbing. Once removed, surface films must be prevented from forming during the welding process. In almost all processes therefore, there must be some means of excluding the atmosphere whilst the weld is being made. If a flux is used for cleaning the fusion faces of the joint this also performs the function of shielding. Alternatively, an inert gas cover may be provided or the weld may be carried out in a vacuum.
In certain processes the atmosphere may be excluded by the fact that the parts to be joined are in intimate contact. There are two main classes of process, pressure welding and fusion welding, and there are several variations of each.
Low-carbon steels are the metals easiest to weld, but practically all metals can now be welded.
Types of Joints
Welding is used chiefly for: (1) butt joints, to join parallel metal edges, (2) lap joints, where two edges overlap each other, (3) tee joints, in which the two parts form a "T", (4) corner joints, where two parts form a corner, and (5) edge joints, where two pieces are joined at their edges.
There are many types of weld used in the joints described above. The most common are groove, fillet, spot, and seam. These welds are useful for making welded seams for joining flat plates, closing cylinders, joining bars, rods, or pipes, and welding one piece to another face-to-face.
Different Welding Processes
The most important welding processes used are arc welding, resistance welding, gas welding, and brazing. Other modern welding processes include friction welding, electron-beam welding, ultrasonic welding, diffusion welding, and laser-beam welding. The earliest welding process, and the only one used for thousands of years, was forge welding, in which the parts to be welded were heated in a forge and then hammered together on an anvil.
In manual shielded metal-arc welding (the most common process) heat alone is used to make the weld. Heat is generated by an electric arc formed between an electrode and the metal being welded. The work and the electrode are connected to a high-current electric source, either direct or alternating current. The arc is formed by touching the electrode to the work and then withdrawing it to form a gap.
Arc-welded joints generally require filler metal, in the form of rods covered by heavy coatings. The coatings are extruded over the electrode wire in its manufacture. The coatings contain a variety of substances and may serve several purposes. They may contain a fluxing material to cover the weld or a substance that will vaporize to provide a gaseous protective blanket. Powdered filler metal, as well as various elements to improve the quality of a weld, may also be in the coating. Since the coating generally melts more slowly than the core of the electrode, it protects and forms the arc and prevents splattering.
One wire from the transformer (in the A.C. system) or the electric generator (in the D.C. system) is connected to the work, and the other to a clamp which holds the filler rod or electrode, as it is termed. The heat of the arc is sufficient to melt the rod and the edges of the work. By choosing the correct size of electrode, adjusting the arc current (at the power source) and controlling the rate of travel of the arc along the work, a perfect joint can be made.
Gas Metal Arc Welding
An arc is created between a base metal electrode and the work. This electrode is usually a coil of wire. The heat from the arc melts the base metal close to the arc, and also melts the electrode. This electrode is "fed" into the arc at the same rate at which it is melted, sustaining the arc and providing the correct amount of filler metal. Throughout the process, a jet of shielding gas (usually argon, helium, or carbon dioxide) is directed over the weld zone to shield it from the atmosphere.
Gas Tungsten Arc Welding
Nonconsumable electrodes usually made of tungsten are protected by argon or another inert gas that flows over the welding area. Filler metal in rods is applied to welds made with these electrodes. Another welding process is the submerged-arc process, in which the tip of the electrode, the weld, and the entire arc are buried in a granular flux that melts to form a slag over the weld. The slag formed by various fluxes is easy to remove.
Here, the source of heat is the resistance offered by the metal to an electric current. Two electrodes are used, one on each side of the joint to be made. The electrodes are pressed onto the work during welding by pneumatic or hydraulic devices that exert high pressures. For the welding of a continuous seam, the electrodes are in the form of rollers through which the work is passed.
When the thicknesses of metal are equal or when differences are compensated, the greatest heat caused by electrical resistance is generated at the interface between the two pieces. The weld is thus entirely protected by solid metal and no fluxing is required. The electrodes in resistance welding are generally made of copper alloy and conduct heat away from the surface of the metal to prevent melting there.
This includes any welding process whose source of heat is a gas flame. Various flames of different temperature are used. The most common mixture is one of oxygen and acetylene, mixed and directed by an oxyacetylene torch.
The heat required is obtained from an oxy-acetylene or oxy-hydrogen flame burning at a special blow-pipe. The two gases are supplied usually from high pressure cylinders and mixed in the blow-pipe. In the normal fusion weld, the edges to be joined are melted by the blow-pipe while molten metal from a special filler rod held in the flame fills the space between the pieces. The composition of the filler rod is similar to that of the pieces, but usually of higher quality.
Gas-welding devices produce much lower temperatures than electric devices, and a weld therefore takes longer to form. The weld is protected with a powdered flux. The equipment for gas welding is less expensive and more versatile than that used in other methods and is therefore popular for repair and maintenance work.
Brazing encompasses a group of processes in which a very close-fitting joint is prepared and a special flux is placed in or at this joint prior to heating. The flux cleans and protects the metal. The work-piece and filler metal are heated simultaneously to a temperature just high enough to melt the filler metal but not the base metal. The liquid filler metal displaces the flux and flows through the joint by capillary attraction, then fuses to the surfaces of the base metal. The heat is usually supplied by a gas flame, or the entire assembly can be heated in an oven.
In this process the two pieces, usually sheet metal, are pressed very firmly together, and a sufficient electric current is passed between them to generate locally a very high temperature. The term spot welding is used where the electrode area is fairly small; it is often used in place of riveting.
A fine jet of oxygen passed across the edge of a piece of steel which is at a bright red heat, oxidizes the metal adjacent to it, so that by steady movement of the jet it is possible to cut the steel into pieces of any desired shape. A special cutting blowpipe is used for this work. The heating jets are arranged around the central oxygen jet, which is turned on by depressing a lever at the top of the blowpipe. Specially designed machines, which guide the blowpipe over the desired course at a steady rate, are capable of cutting shapes to a surprising degree of accuracy.
Occupational Health & Safety
Some welding processes can involve temperatures as high as 10,000° F (5500° C), electric currents as large as 2,000 amperes, or devices capable of exerting heavy pressure.
Though some types of welding do not generate heat. In ultra
sonic welding, a very-high-frequency oscillating device is applied to the joint. Vibration and pressure cause the structure of the metals' surfaces to loosen; the surfaces then coalesce to form a weld.
Welding tools range in size from hand models with pistol grips to large automatic machines. Safety measures are needed with all welding tools.
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