Its very kind of you ,GRoberts, for ur response.
The wall thickness of the pressure vessel(coke drum in an oil refinery )is 28mm,1Cr-1/2Mo(B2 type metal) ,and the thickness of the overlay is 3mm,ENiCrFe-3.It is said when u r using the electrode of ENiCrFe-3,u have to keep the interpass temperature no higher than 100Ž,otherwise, it is easy to make a hot cracking.is it?if so,is it necessary to preheat before overlaying on 1Cr-1/2Mo(28mm)?
I have not heard about causing hot cracking with an interpass temp over 100(C?), but that does not mean it does not happen. Your application, however, seems to be ideal for requiring preheat. Thick plate, high residual stresses, and an application where a soft HAZ is essential. A good preheat would help all these circumstances. Is this a new fabrication or a repair? If it is a repair, the base metal could already have a high hydrogen content, and a pre &/or post weld hydrogen bake would also help.
Very wise words indeed! I've heard of a case where boiler waterwall tubes were overlayed with nickel-base material and, unknown to the contractor, there was some hydrogen damage to the tubes. After the unit started back up they experienced so many tube failures that all of the waterwall panels had to be replaced.