Pre Heat for 1.25 Cr - 0.5 Mo

[roniprabowo]

Dear all,
I wonder why we require pre heat arround 150 C when we weld P4 (for example A-387). Is there any metallurgy reason for it ?

 

[Metalguy]

Yes. The reason for it is to slow the cooling rate of the HAZ, especially as it passes down thru ~650 deg C. If the rate is slow enough pearlite will form instead of crack-prone martensite/bainite. The Cr and Mo have a strong effect on delaying the formation of pearlite.

 

[TomEun]

Excellent answer.

Additionally,
min. 250 deg.F (121 deg.C) may be applied when MSTS > 60 ksi or thickness > 1/2" and
min. 50 deg.F (10 deg.F) may be applied when MSTS </ 60 ksi and thickness </ 1/2".
----- in ASME Section VIII, Div.2, App. D-120

Thomas Eun

 

[sayee1]

For the same reason, welds have generally a post heat maintenance, for these materials, at the same temperature as the preheat for typically half an hour followed by wrapping in asbestos to further slow the cooling rates. Also that is why all the welds in CrMo materials are typically PWHTed to further convert the unsuitable microstructure to suitable form (in particular in the HAZ) and degas the welds to further mitigate cracking problems.

Thanks and regards
Sayee Prasad R
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If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[stanweld]

In addition to the above comments, slowing the cooling rate provides additional time for hydrogen, dissolved in the solidifying weld and HAZ, to dissociate into the atmosphere at temperatures between approximately 400 F and 600 F.

 

[rido]

Additionaly I can say that there are standards that strictly regulates usage of prheating for CrMo alloyed steel welding. It depends on: thickness of welding ant conditions of welding - temperature.