Initiation of LME crack in ferritic martensitic steel in liquid lead-bismuth

This work is aimed at fundamental understanding of the T91 steel cracking mechanisms in liquid PbBi eutectic. In particular, the crack initiation was studied by means of Constant Extension Rate Tensile (CERT) tests with flat tapered specimens in PbBi at 300 °C with oxygen content 3 × 10−7 - 6 × 10−6 wt. % and in air, up to rupture and maximum load. The tapered specimens were meant to create a uniform variation of stress along the gauge length and the threshold stress of the crack initiation was determined. The performance of the steel was affected by the environment. The threshold stress decreased and cracking character changed in PbBi when compared to the testing in air. Post-test examinations carrying out with SEM equipped with FIB found shallow cracks from broken oxide having ductile propagation and a grain-size-deep crack of inter-lath martensite path in the plastic strain region. The cracking stress-strain conditions and the role of oxide rupture causing wetting, as the cracking precursors, are discussed.