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. Recent research, albeit limited, has shown the viability of Q&P treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, their application-related properties have never been investigated.

This thesis 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 Q&P parameters. It is demonstrated that the Q&P-treated martensitic stainless steels can show a favorable combination of enhanced strength and sufficient tensile ductility. Their uniform elongation increases with the increasing volume fraction of retained austenite, attributed to the transformation induced plasticity (TRIP) effect. The alloy-process-microstructure-property relationship is discussed. High-cycle fatigue performance and formability of the Q&P-treated martensitic stainless steels is explored. The results reveal satisfactory high-cycle fatigue performance, surpassing that of their traditional counterparts.

Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries, MnS inclusions and nanocarbides have minimal influence on fatigue crack formation and growth.
To assess the mechanical behavior of the Q&P-treated martensitic stainless steels during cold forming, a computational approach is developed. The Johnson‐Cook (J‐C) constitutive model and Forming Limit Diagram (FLD) are derived and validated for each alloy using experimental data from tensile and Nakajima tests. From the simulated FLD, a theoretical Forming Limit Stress Diagram (FLSD) is calculated. The latter is used as a fracture criterion for the cold‐forming process. Cold stamping simulations of two automotive parts, the B‐pillar and tunnel, are conducted using the J‐C constitutive model and the FLSD. The study demonstrates successful cold forming of the tunnel using all three studied steels. In contrast, extensive cracking is expected during the cold forming of the B‐pillar for all materials.

The life cycle cost analysis reveals that initial materials acquisition costs dominate total material costs, followed by final heat treatments, casting, and hot rolling. Q&P-treated grades are more cost-effective than traditional austenitic stainless steel, as the latter is comparatively expensive due to its high nickel content. The Q&P process demonstrates low energy intensity and cost, indicating minimal impact on overall manufacturing energy and material costs upon implementation. While the Q&P process has no direct effect on carbon emissions, its adoption may indirectly reduce manufacturing-related emissions by utilizing lower nickel content materials compared to standard austenitic stainless steels.