A Fe-Cr-C steel based on gradient scale precipitation hardening: Hardening and toughening mechanism of multistage heat treatment

In order to eliminate the common carbide networks in Fe-Cr-C tool steel, a low-cost Fe-Cr-C steel with high hardness and high toughness was designed. Correspondingly, a multistage heat treatment process is also designed. Multistage heat treatment includes homogenization, spheroidizing annealing, quenching and low temperature tempering. Compared to AISI D2 steel (58–62HRC, 24.3–27.9 J/cm2), the designed steel maintains high hardness (60.5HRC) and higher unnotched impact toughness (46.2 J/cm2). The hardening and toughening mechanisms of the designed Fe-Cr-C steel were studied by mechanical properties testing, multi-scale characterization and thermodynamic calculation. Experimental results show that the complete austenite phase region of Fe-Cr-C steel is the key factor to eliminate carbide networks by homogenization treatment. The gradient scale M7C3 produced by the spheroidizing annealing and quenching enhances the hardening effect through refining prior austenite grains size, promoting martensite nucleation, and limiting martensite growth. During the low temperature tempering, volume fraction reduction of retained austenite weakens the transformation induced plasticity (TRIP) effect and softens martensite, which is beneficial to improve the toughness. Finally, the decomposition of twinning martensite caused by nanoscale θ-Fe3C precipitation and the precipitation hardening of nanoscale θ-Fe3C play important role in maintaining hardness and improving toughness.