Welding

MACHINE PARTS

CHAPTER III

Welding is a process which provides a non-detachable joining of two like metal pieces¹ by heating them till melting condition or fusion without or with mechanical pressure. Fusion of two metal pieces may be brought about² by different types of welding, such as hammer welding, thermit welding, electric welding and gas welding.

Hammer welding is a process in which two metal pieces are joined and fused together by force from a hand or power hammer after having heated these pieces in a blacksmith's forge until they reached their plastic stage.

Thermit welding is a process consisting of a chemical reaction obtained by igniting finely divided aluminium and iron oxide. This type of welding is used in repairing rails, frames, etc.

Electric resistance welding is a process consisting of heating metals to be welded to their plastic temperature and then applying mechanical pressure for achieving a non-detachable joining of the metals.

Resistance welding processes form a group consisting of many types of welding. Selection of each type of welding depends on the kind and size of metals to be welded. Resistance welding embraces such processes as spot welding, butt welding, electric arc welding, etc.

Spot welding is a process in which two metals are held between electrodes passing a heavy current through the metals to be welded. The electrodes having been forced together by pressure join the surfaces of the metals in a spot, the size of which is about the same as that of the tip of the electrode. This kind of welding is suitable for welding parts of airplanes, refrigerators and automobiles.

In butt welding the parts to be welded are pressed together while heat is generated by passing a heavy current through the area of the joint. Butt welding may be applied for welding pipes, tubing, rods, etc.

Fig. 32. Electric Arc Welding:

1 - carbon electrode; 2 - additive;3 - plate; 4 - weld

Electric arc welding is a process in which surfaces to be joined are fused together by the heat of an electric arc. The electric arc was invented in 1802 by the famous Russian scientist V. V. Petrov, who demonstrated the possibility of utilizing its heat for fusing metals. Fig. 32 shows the process of electric arc welding. By bringing the work and the electrode together as conductors, an electric circuit is

established. When the conductors are separated, an electric arc is created in which the electrical energy is converted into heat, its temperature being as high as 7000°F³. An additive is placed into the flame of the electrical arc. An additive is a metal which is externally applied to the place of welding and melted to form a weld together with the material of the work. The electric arc melts both the edges of the parts to be melted and the additive used. The electrode having been removed from the place of welding, the molten metal cools, solidifies and forms a weld, joining the parts of the work.

In autogenous (gas) welding the source of heat is the gas flame obtained from a gas which is often acetylene. Acetylene mixed with oxygen in a torch when ignited gives a steady flame. The welding torch consists essentially of a gas mixing chamber and is designed in two types: the low-pressure injector type and the equal-pressure type. In the injector-type torch the acetylene is delivered at very low pressure and the oxygen enters the torch at high pressure and velocity expanding in the mixing chamber. Then the acetylene is drawn by suction created in the mixing chamber in a constant ratio to the oxygen. Autogenous welding is carried out in different ways and methods. According to the movement of the torch, welding may be divided into rightward welding and leftward welding. In rightward welding the torch moves ahead of the additive, and in leftward welding the additive moves in front of the torch. In practice rightward welding is mostly used. Gas welding may be used for cutting metals and repairing agricultural machines and implements.

¹. two like metal pieces - два однородных куска металла

². fusion of two metal pieces may be brought about - сплавление двух кусков металла может быть произведено (выполнено)

³. 7000°F=7000 degrees according to Fahrenheit ['fa:renhait] - 7000 градусов по Фаренгейту

Дата добавления: 2014-12-27