By the forging process the parts to be united are heated to a temperature considerably less than that of fusion.

It is sufficient to put into contact the two metals brought to white heat and hammer them together. The hammering may be substituted for pressing or rolling, but these means are inferior, as it cannot produce a proper weld.

The weld by forging has some disadvantages. The temperature which is required easily causes the iron to oxidise; the presence of oxide of iron offers great resistance to the welding and renders difficulty for the metals to join with one another; during the operation of the welding, which ought to be done in the open air, the oxidation cannot be prevented. The welder, in order to minimise the oxidation, employs some sand or borax, but in both cases a silicate is formed, which is very fusible and must be removed by the hammering.

The inconvenience of the oxidation makes the forging unsuitable for articles of small and thin dimensions, such as tools, wires, etc., because the oxidation being too rapid, the metal burns but does not weld.

The necessity of hammering makes it also difficult to apply forging to various objects by reason of their form or the difficulty of their removal; for instance, tubes, cisterns, or objects of a voluminous form.

It is in such cases, where the forge is insufficient, or the application difficult, that the welding by fusion is resorted to.