History of Welding
Originally the term welding was applied to the joining of pieces of metal by hammering or pressing them together when hot. It is now also, and more particularly, applied to processes in which the edges to be joined are completely melted, and to bronze welding.
The ability to be drawn is a characteristic of metals, stemming from their tensile strength and ductility (ability to change shape without breaking).
Because the earliest metals were all fairly soft, unless especially treated such as sword edges, they could be cut and pierced readily. This had to be done with large items while they were hot. but small objects could be shaped cold.
The most delicate work of all was the beautiful technique of inlaying patterns of one metal into recesses in another metal of contrasting color. Sometimes the inlaid pieces were secured by hammering the edges of the recesses.
But in other cases soldering or brazing was employed, the latter using a brass (an alloy of copper and zinc), which was melted and run into the joints.
By 1800 the technique of welding metals using a hot gas flame was being developed and by the end of the century it had reached a high degree of perfection and had been supplemented by welding with an electric arc. Welding, in which some of the metal is melted , exerts a profound effect on the metal's properties and much patient research was needed to find the best methods while simultaneously avoiding distortion caused by the severe heating.
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.
For butt welds in material over 1/8 of an inch in thickness the edges must be specially prepared with a "V" or "U" section.
Bronze welding is a form of gas welding used for joining cast iron. It is not a fusion weld, the temperature being always below the melting point of cast iron. A strong joint is obtained by virtue of the fact that molten bronze from the filler rod enters the spaces between the crystals of the iron; thus, when solidification has taken place, both edges are firmly locked to the bronze core of the weld.
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.
This process consists in striking and maintaining an electric arc between the work and the filler rod. 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 fill er 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.
Small amounts of grease and other impurities present on the work would weaken the joint if steps were not taken to remove it. This is done by introducing into the flame a flux, which combines with the impurities to form a slag. This floats on the molten metal, and solidifies there when the joint cools. The flux is introduced, in the arc process, by having the electrode coated with it, and, in the gas process, by periodically dipping the hot filler rod into a flux powder.
Joints in material of large section are often made by performing a series of small runs rather than a single run. This is done, sometimes because there is no alternative, but often because it results in a stronger weld, since each successive run normalizes the previous ones.
The thermite, or Goldschmidt process, depends upon the production, by the almost instantaneous action of finely divided aluminum upon ironoxide, of a mass of molten iron at an exceedingly high temperature, approximately 5000Â° F. Arrangements are made by which this mass of iron, the moment it is formed, enters a narrow space between the two surfaces to be united, previously heated to a red heat by a blowpipe.
Electric Resistance and Spot Welding
In resistance welding, which is mainly used for joining overlapping sheets, two electrodes are used to apply pressure and current is passed between them. Resistance heating causes local melting at the interface between the sheets and a weld results.
The two main variants of this process are spot welding and seam welding. In spot welding the electrodes are stationary relative to the workpiece and cause a local fused spot, whereas in seam welding the workpiece is drawn between rolling electrodes to give a continuous weld. On certain metals a weld may be formed without the application of heat if the surfaces are relatively clean and deformed sufficiently by the applied pressure. This process, which is known as cold pressure welding, is suitable only for ductile metals of relatively low melting point, notably aluminum and copper. In a process known as friction welding one part is rotated under load against a stationary workpiece.
When the frictional heat generated at the interface has caused the joint area to become sufficiently plastic, rotation is stopped and the end force increased to forge the joint. Friction welding is used for high-speed mechanized joining of rods, tubes, and any cylindrical parts and is most effective for joining dissimilar metals. One of the more recent developments is explosive welding. By setting two plates at a slight angle and detonating an explosive charge adjacent to one of them, a weld can be effected between them. The process is finding increasing use for cladding materials previously considered difficult to weld.