Abstract: The microstructure and mechanical properties of 9% chromium martensitic heat resistant steel tempered at different temperature were investigated with the OM，TEM， SEM and EPMA．The results showed that the dislocation network，precipitation phase and lath martensite fragmentation after tempering were the main factors for the variations of mechanical properties. It was found that after normalizing and tempering at 760℃， the test steel had an excellent integrated mechanical properties， where the tensile strength at room temperature and 550℃ reached 657MPa and 556MPa， respectively， and the impact absorbing energy at 0℃ reached 285J. Tempering at 700， 820 and 850℃， promoted a substantial reduction in toughness. Under a high temperature service， the MX phase was dispersed uniformly in the ferrite and martensite without coarsening， and metastable polygonal structure was formed during the recovery of martensite at high temperature. Such combination of substructure and dispersed distribution can effectively enhance the high-temperature creep resistance of this heat-resistant steel.

Key words: tempering temperature, martensite, precipitation, dislocation network, heat resistant steel