Weld heat affected zone liquation cracking in type 347 stainless steel

The heat affected zone cracking-tendency of type 347 stainless steel was evaluated in five different samples of thick walled pipe or plate differing in Nb, C, and N content, Nb/(C + N) factor, P + S content, and base metal ferrite content. A combination of Gleeble hot ductility and spot varestraint tests, followed by optical and electron metallography, was employed to elucidate the role of Nb in such persistent cracking. Susceptibility to HAZ cracking as a result of liquation was always found to be associated with eutectics of Nb rich phases and austenite, but with a complex interplay between the Nb/(C + N) factor, the P + S impurity level, Nb carbide or carbonitride size and distribution, ferrite number, and ferrite form and distribution in the base metal. A key role for composition and also base metal thermomechanical processing is suggested that merits more intensive systematic study.