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.

Quenching and partitioning (Q&P) treatment has been proven effective in manufacturing advanced high strength steels with high content of retained austenite, showing the improved balance of high strength and sufficient ductility. This method has been very well elaborated for carbon steel processing over the last two decades. Though it can also be potentially applied for processing other steel families, this has been scarcely studied. This article focuses on the effect of chemistry and heat treatment parameters on the microstructure and properties of Q&P treated martensitic stainless steels. Three different martensitic stainless steels with different contents of alloying elements are subjected to Q&P processing with varying quenching temperature or partitioning temperature and partitioning time. The tensile behavior of the Q&P treated steels is studied. The effect of chemistry and Q&P treatment parameters on the microstructure and tensile properties is analyzed. The effect of plastic deformation on the microstructure of the Q&P treated steels is also investigated. It is demonstrated that the Q&P treated martensitic stainless steels can show a good combination of enhanced strength and sufficient tensile ductility. Their uniform elongation increases with the increasing volume fraction of retained austenite due to the transformation induced plasticity (TRIP) effect. The ability of the martensitic matrix to accumulate plastic deformation also plays an important role. The Q&P process - microstructure - property relationship is discussed.

Recent studies have demonstrated the viability of quenching and partitioning (Q&P) treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, to date, the fatigue behaviour of these materials has not been explored. This study examines the effect of their complex hierarchic microstructure on high cycle fatigue performance. Three steels with different alloying element contents underwent Q&P processing, resulting in multiphase microstructures rich in retained austenite. High cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high cycle fatigue performance in Q&P treated martensitic stainless steels, surpassing traditional counterparts. Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries and MnS inclusions have minimal influence on fatigue crack formation and growth. Microplastic deformation at the fatigue crack tip enhances local KAM values and triggers localized transformation of retained austenite grains. It is hypothesized that the developed Q&P treated martensitic stainless steels exhibit improved resistance to low cycle fatigue.