Fatigue crack formation and growth in a quenched and partitioned (Q&P) martensitic stainless steel

Abstract

Quenching and partitioning (Q&P) treatment of martensitic stainless steels offers an improved balance of high strength and ductility through the formation of multiphase microstructures containing retained austenite. However, the fatigue behavior of these materials and the underlying crack-microstructure interactions have not been investigated. This study focuses on fatigue crack initiation and propagation mechanisms in Q&P-treated martensitic stainless steel containing a high fraction of retained austenite. High-cycle fatigue tests combined with scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) characterization reveal that fatigue cracks preferentially initiate and propagate along martensite packet and block boundaries rather than prior austenite grain boundaries. This boundary-dominated crack path results from plastic strain incompatibility between adjacent martensite variants with different Schmid factors. Crack branching also occurs along these crystallographic interfaces. Progressive mechanically-induced transformation of retained austenite is observed in the subsurface region adjacent to propagating cracks, with the austenite volume fraction decreasing substantially as the crack extends. Blocky retained austenite transforms preferentially compared to fine interlath austenite due to lower mechanical stability. The plastic zone expands with crack extension, accompanied by increased kernel average misorientation (KAM) values, reflecting cumulative plastic deformation and dislocation accumulation. While micron-sized TiN particles fracture when encountered by cracks and can induce secondary cracking, nanocarbides do not noticeably influence crack behavior due to their very small size relative to the crack tip deformation field. These findings provide fundamental insights into fatigue damage mechanisms in Q&P martensitic stainless steels and highlight the critical role of martensite substructure boundaries in controlling crack development.