Preheating involves applying heat to a base metal prior to welding to bring and maintain the temperature at a suitable level for welding. It can be done using gas burners, oxygen flames, electric blankets, or induction heating. Preheating is necessary to remove moisture from the weld area, lower the thermal gradient and cooling rate, remove excess hydrogen to prevent cracking, and avoid cold cracking. Whether preheating is essential depends on factors like moisture content, existing residual stresses, hydrogen content, and carbon content as determined by a formula for carbon equivalents and cracking index.

1. Preheating (MANUFACTURING) By: ranjeetnitj@gmail.com
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What is Preheating?
Ans- The heat applied to the base metal or Substrate to attain & maintain the temperature
(Surrounded Temp.) Suitable for beginning the welding Process. When there is Subsequent
welding going on then it is also the process to attain Interpass temperature.(Temperature required
for begin welding process to New Material).
How it is done ?
Ans- Preheating May be Performed by the use of Gas burners, Oxygen flames ,Electric Blankets,
Induction Heating or by heating in the furnace & to obtain a uniform temp. Throughout the
material Thickness, it is desirable to apply the heating Sources to one side of the material surfaces
& measure the material temperature on the opposite side.
Reasons for Preheating-
1. To Remove the moisture from the weld area.
2. To lower the thermal gradient or to lower the cooling Rate.
3. To remove the extra Hydrogen in materials or to avoid Hydrogen cracking or cold cracking
How to know Preheating is essential?
Preheating is essential when:-
1.There is an extra moisture present in work piece.
2.There is a lot of Residual stress already present in work piece.
3.There is More Hydrogen content in Work piece.
4.On the basis of Carbon content:- when there is no above criteria appear in the work piece then
we should concentrate on the present carbon content in the work piece, which directly can be
initialized by the formula:-
Catbon Equivalents(CE)= C + A(C){ Si/24+ Mn/6 +Cu/15 +Ni/20+
Cr+Mo+Nb+V/ 5 + 5B}
& A(C)= 0.75+0.25 tanh {20(C-0.12)}
C= % carbon content in work piece
h= thickness of work piece
All other symbol have standard element meaning.
A(C) increases with increase in carbon content. It approaches
A(C) = 0.5 ; when carbon content decreases below 0.08%.
& A(C)= 1 ; when the carbon content is greater than 0.18 %.
Now based upon these carbon content the cracking index(Probability of Cold cracking) will-
Cracking Index(CI)= C.E + K
K= constant depend upon hydrogen content & stress present in work piece.