preheat requirement for alloy steel

[g.venkatesan06]

Dear experts

why preheat is mandatory for alloy steel and not mandatory for
stainless steel. Give some metallurgical reason and code reference.

Is joining alloy with stainless steel what is the preheat requirement.

Regards
venkatguru

[RAJA SEKAR]

Dear Sir,

Alloy steel has phase transformation while heating or cooling, so it require heat tratement. But for SS(P8 group only) it has no phase transformation, so it does not require any heat treatment.

If welding alloy steel with SS(P8) preheat is done on alloy steel side only as per the requirement.

Regards
Rajasekar

[John Henning]

Purposes of preheat for alloy steel are:
(1) allow time for hydrogen to evolve from the weld
(2) promote a softer microstructure
(3) reduce residual stress
(4) help to control distortion

For stainless steels, hydrogen assisted cracking is typically not a
issue (there is no step change in solubility as occurs with alloy
steels). A greater concern is that a slower cooling rate increases the
probability of sensitization thereby increasing the risk of SCC and
other corrosion.

John A. Henning
Welding & Materials

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[Geoff Rogers]

Alloy steels can form martensite (hard microstructure susceptible to cracking) when it cools to fast.  Preheat slows the cooling rate and helps to prevent the formation of martensite during the phase transformation that occur.

SS can become sensitized (formation of chromium carbides along grain boundaries) if it cools off too slow.  Typically sensitization is prevented by using an "L" type SS such as 304L or 316L.  Duplex SS steels are not susceptible to sensitization.

The preheat requirement should consider they type of SS used and the chemistry of the alloy steel.  The higher the CE value for the allow steel, the more likely martensite will form.

Regards,

Geoff

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[Octane Mac]

venkatguru,

Reasons behind performing pre-heat have been adequately explained by the fellow members. 

More to that, pre-heat requirements are governed by the construction codes based upon the material thickness, P-number, Group number and their minimum tensile strength. We just need to follow that.

For welding of low alloy to stainless steel, ferritic side (low alloy) has to be pre-heated at the temperature recommended by the construction code. Stainless steels do not require any pre-heating.

Octane