The Effect of heat treatment on the microstructure and hardness in ۴۴۰C+۰.۲%Nb martensitic stainless steel

The ۴۴۰C martensitic stainless steel is considered one of the high-performance steels due to its combination of hardness and toughness due to its high carbon and chromium content. The high amounts of carbon alongside the chromium and molybdenum open an aperture to optimize the microstructure of this material with respect to its application. The ۴۴۰C stainless steel, modified by adding niobium, was used in the investment casting process and then heat-treated to produce a martensitic matrix. Optical microscopy (OM) and the phase diagram generated by JMatPro were used to analyze the microstructure and phases. Additionally, hardness tests were conducted to be evaluated as an indicator of mechanical properties comparison. In this research, NbC carbides act as an effective refining phase that resists the coarsening of the microstructure during solidification, austenitization, and tempering, thereby maintaining higher hardness and mechanical stability than Nb-free ۴۴۰C steel. The results show that martensite plays a major role in the hardness increment of the ۱۰۶۵°C austenitized sample, while the combination of carbides and martensite contributes to increased hardness in the ۱۰۲۰°C solution-treated sample.