Relation between microstructure, composition, and hot cracking in Ti-stabilized austenitic stainless steel weldments

A stabilized, fully austenitic alloy D9, a 15Cr-15Ni-2Mo stainless steel wi th a titanium addition corresponding to UNS 38660, is a candidate material for the fuel-clad and wrapper applications of the Prototype Fast Breeder Re actor (PFBR). The fully austenitic microstructure and the presence of titan ium in this alloy lead to high susceptibility to hot cracking during weldin g. The fusion-zone and the heat-affected zone (HAZ) cracking susceptibility of alloy D9 was studied at three titanium levels, 0.22, 0.32, and 0.42 pet . all other elements remaining constant. The longitudinal and transverse Va restraint (Transvarestraint) hot-cracking tests were used to evaluate fusio n-zone and HAZ cracking. The results showed that titanium increases crackin g in the fusion zone by 15 to 20 pet in the range of Ti levels studied. The microanalysis of fusion-zone hot cracks using electron probe microanalysis (EPMA) showed an enrichment of Ti, C, N, and S along cracks and in the int erdendritic regions. The corresponding phases were identified as TiC, TiC0. 3N0.7, and the carbosulfides Ti2CS and Ti4C2S2, which are believed to form eutectics with austenite to produce cracking. The amounts of these phases i ncreased with increasing Ti content. In the HAZ, a similar relation between titanium level and cracking was found. The comparison of the weldability o f the D9 with an FA mode type 321 revealed that Ti-bearing eutectics were r esponsible for a high degree of cracking irrespective of the solidification mode. The results show that in the D9, the ratio of Ti to C and N must be controlled to minimize cracking.