Study of the phase diagram and continuous cooling transformation of 12%Cr ultra-super-critical rotor steel

The pseudo-equilibrium phase diagram of a 12%Cr ultra-super-critical rotor steel was calculated by the use of Thermo-Calc software. The equilibrium microstructures at room temperature consist of ferrite (α) and carbides of M23C6 type, and the critical points A1 and A3 of the steel are 811 °C and 928 °C, respectively. The critical points Ac1, Ac3 and Ms determined by a dilatometry method are 806 °C, 930 °C and 395 °C, respectively, which are very similar to the calculated values. The continuous cooling transformation (CCT) curve of the steel was also determined, and the CCT products and microstructural morphologies were analyzed. The critical cooling rate without proeutectoid ferrite transformation is about 0.1 °C/s, and the quantitative expression between microhardness (Hv) of the CCT structure and cooling rate (v) for the steel is as follows:Hv=482−35exp⁡(−v/0.89)−30exp⁡(−v/3.80).Hv=482−35exp⁡(−v/0.89)−30exp⁡(−v/3.80).