Characteristics of Material Degradation for Thermal Aged X20CrMoV12 1 Steel

X20CrMoV12 1 steels were exposed at three different high temperatures and various durations. Aging parameter, LMP _s, was used to describe long-term thermal history. Creep rupture, hardness and tensile tests for aged X20CrMoV12 1 steels were carried out. Yield strength was slightly decreased and hardness was rapidly decreased afterLMP _s reached 20.6. Creep strength of aged X20CrMoV12 1 steel decreased rapidly after the LMP_s reached 20.6. It was shown that the creep strength decreased due to the coarsening of acicular type M₂₃C₆ at subgrain boundaries and the agglomeration of M₂₃C₆ at prior austenite grain boundaries. After LMP_s reached 20.6, the assumed linear regression line of stress and Larson-Miller parameter,LMP_f, also moved toward a lower LMP_f region as aging parameter increased. The dependency of creep rupture life on the microstructural degradation could be represented by C_LMPs, which is LMP_f at stress of 1 kg/mm². It was found that the creep rupture life of X20CrMoV12 1 steel could be predicted by considering the material degradation during and prior to creep test.