热作模具钢DM的高温稳定性和热疲劳性能

研究了热作模具钢DM的高温稳定性和热疲劳性能。结果表明，DM钢在620℃热稳保温过程中马氏体板条内的薄片状M₃C型碳化物逐渐向条块状M₇C₃型碳化物转变，在板条的边界生成M₇C₃、M₂₃C₆型碳化物。DM钢的短循环周次热疲劳性能受控于位错重排和湮灭，长循环周次热疲劳性能受控于碳化物的粗化程度。DM钢中M₃C、M₇C₃、M₆C型碳化物的生成自由能分别为27765.5 J/mol、3841.5 J/mol、-7138.1 J/mol，表明在热稳保温与热疲劳试验过程中碳化物的演变机理一致，发生了M3C→M7C3→M6C类型演变。

High Temperature Stability and Thermal Fatigue Behavior of DM Hot Working Die Steel

The microstructure and performance of a novel DM steel for hot forging dies were systematically investigated by means of scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD) and thermal fatigue tester. Results show that the laminar M₃C carbides gradually transform into blocky carbides M₇C₃ inside the martensitic slabs, while carbides M₇C₃ and M₂₃C₆ are found at boundaries of slabs. Based on the Uddeholm self-restricting thermal fatigue test results, the short cyclic thermal fatigue performance was controlled by dislocation rearrangement and annihilation. Whereas, the long cyclic one was affected by the temper resistance of the DM steel and strongly depended on the carbide morphology and their resistance to over-ageing. In addition, the free energies of formation for carbides M₃C, M₇C₃ and M₆C in the DM steel are 236.4, 212.0, and 228.9 kJ/mol, respectively. The mechanism of carbides evolution during the thermal stability test is consistent with thermal fatigue test, the transformation of the carbides follows the sequence as M₃C→M₇C₃→M₆C.