The Effect of Long-Term Ageing at 475 °C on Microstructure and Properties of a Precipitation Hardening MartensiticStainless Steel

Abstract

The effect of long-term ageing (1000, 2000, and 3000 h) at 475 °C on mechanical properties, microstructure, and substructure of CUSTOM 465® maraging stainless steel was studied. The additional precipitation of nanometric particles of η-Ni3Ti phase in partly recovered lath martensite and decomposition of the BCC solid solution accompanied by the formation of nanometric Cr-rich α’particles were identified. The fraction of reverted austenite in the final microstructure gradually increased with time of ageing at 475 °C. Ageing resulted in a gradual slight decline (up to 10%) in yield strength, ultimate tensile strength, and hardness. On the other hand, for all ageing, dwells ductility and impact energy values remained almost unchanged. The reason for this phenomenon lies in the gradual increase in the fraction of reverted austenite during long-term ageing at 475 °C and at the same time in the sluggish kinetics of microstructural changes in lath martensite. No susceptibility to 475 °C embrittlement was proved.