##Welding
Welding processes may be divided into two main groups: pressure welding and non-pressure welding.
Any welding process which requires pressure is generally referred to as a forge welding process and these processes do not usually require a filler metal or flux The parts to be welded however should be clean and free from grease, etc.
The oldest form of forge welding is blacksmiths forge welding.The process consists of heating the metal components to be welded in a blacksmiths fire until the parts to be united are plastic then the parts of the components are removed from the heat source and hammered together to form a union.
Resistance welding is another forge welding process, current and pressure are supplied to the parts being welded but no filler metal or flux is required. The heat which is generated in order to form the weld depends upon (1) the square of the current supplied, (2) the metal to be welded and the contact resistance, (3) the time of application of current and pressure. Examples of resistance welding are studs welded to decks or to boiler tubes in water tube boilers (Table 1.6)
Welding processes which do not require any pressure are often referred to as fusion welding processes. Fusion welding processes require a filler metal and often a flux is used. The most popular and most convenient form of fusion welding is the electric a welding process, sometimes called the metal arc welding process.