Microstructure Evolution and Modelling During Tempering Heat Treatment at 670 °C in 9–12 wt.% Cr Ferritic/Martensitic Heat-Resistant Steel

This study aimed to investigate the growth behavior of key precipitates in 9–12 wt.% Cr ferritic/martensitic heat-resistant steel as a function of tempering duration. Tempering was carried out at 670 °C for durations between 1 h and 300 h. The precipitates identified were M23C6, Laves phase, Nb-rich MX, and V-rich MX, each exhibiting distinct growth behavior with increasing tempering time. Transmission electron microscopy (TEM) was used to quantitatively analyze their growth behavior. The results revealed that M23C6, Laves phase, and V-rich MX underwent progressive coarsening with time, whereas Nb-rich MX showed negligible growth even after prolonged tempering. Based on the quantified data, a precipitate-growth model was implemented in MatCalc, and the key parameters were derived and validated against experimental data. The modeling results showed excellent agreement with experimental observations, confirming that the proposed model can reliably predict the microstructural evolution of ferritic/martensitic heat-resistant steel under long-term high-temperature exposure relevant to USC applications.