Page 141 - CITS - Welder - Trade Theory

WELDER - CITS

The preheating reduces the rate of cooling after welding. This is necessary to prevent the weld metal from
cracking in restrained/rigid joints. Also some of the non-ferrous metals like copper, brass, aluminium, etc. expand
more due to heating and ferrous metals like cast iron, medium and high carbon steels require preheating as they
are too brittle. These materials are necessarily to be preheated to avoid cracking or distortion. In some cases, it
is also necessary to preheat during welding between each layer of deposition.
The minimum preheating temperature for satisfactory welds of different grades of steel, cast iron, non-ferrous
metals will depend upon the: (Fig 1)

Fig 1

a Type of metal
b Composition and properties of the parent metal

c Thickness of the plate
d Type of joint
e Degree of restraint of the joint
f Rate of heat input.

Do not allow the temperature to drop below the minimum preheating temperature between each weld run.
The preheating temperature can be checked by temperature indicating crayons. (Fig 2)
If the job and area to be preheated are large, then it is done in a preheating furnace (Fig 3).
If it is small localised preheating is applied to the joint area only. This is called local preheating. (Fig 4)
Post heating: Post heating means that the part is heated immediately after welding. The reasons for post
heating are to prevent hard and brittle spots from forming in the weldment. It also relieves the residual stresses
caused by the welding heat and due to welding of a rigid joint.

The important aspects to be considered while post– heating are:
a the rate of heating

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CITS : C G & M - Welder - Lesson 33 - 43