Influence of Microstructures on Crack Propagation in Stainless Steel Weld Metal

Abstract

In certain welded fabrications, austenitic stainless steel E309 Nb is overlayed on ferritic steel SA 515 Grade 70 as a buffer layer for subsequent overlay with E347 grade filler material. This was subjected to Stress Relief Heat Treatment (SRHT) cycle with holding at 600°C for 12 hours. E309 Nb weld metal showed rapid crack initiation and propagation in 180° bend tests. To study the influence of microstructure on crack initiation and propagation, bend deformation stage was used in SEM. The metallographically polished and etched specimens were fatigue precracked and bent in the deformation stage. The crack propagation was preferential along delta ferrite and austenite boundaries, even though the path was longer. Whereas in 347 weld, random crack propagation with appreciable crack tip blunting was seen. However, these two materials without SRHT showed random crack propagation with reference to microstructure and crack tip blunting was similar. X ray diffraction studies revealed appreciable transformation of delta ferrite to secondary phases in the case of 309 Nb. Detailed thin foil studies in Transmission Electron Microscope revealed selective needle-like precipitation of secondary phases along the interface of delta and gamma phases of 309 Nb. This weakened the interfacial strength. This was the reason for easy crack propagation in 309 Nb after SRHT. Whereas such precipitates were not observed in 347 weld after SRHT and hence it was resisting crack propagation by crack tip blunting.