Why Pre-heat Required in low alloy steel (e.g. 5cr, 0.5 Mo)

By Yagnesh Purohit Date 07-08-2009 04:25

During one technical interview question was asked to me that why low alloy steel (e.g. Grade P5; 5Cr, 0.5 Mo) get cracked during welding if pre-heat not done and maintained.

I answered because of temperature gradient in parent metal is stiff OR in other words heat distribution in parent metal is not uniform and rapid heating and subsequent cooling effect because of welding results in cracking in HAZ.

Please send probable reasons for this type of cracking.

By 3.2 Inspector (***)

Date 07-08-2009 11:55

Yes this kind of steel can get martensitic if cooling rates are high (8/5 cooling rate)
I would not agree that it cracks during welding, but rather after welding. Cracking during welding is generally caused by a poor width to depth ratio.

Cracking can happen up to 24-48 hours after welding, which is why a bakeout or go straight in to PWHT is a good idea.

Filler material storage and handeling can also be related to cracking.

NDT must be performed after the PWHT.

Grade P5 is not rocket science but it must be treated with some respect, also because it is usually "nasty" operating temperatures, which is the reason for using P5.

Good luck with your new job?

3.2