Effect of long-term ageing at 475 °C on microstructure and properties of a Cr-Ni-Ti-Mo stainless martensitic steel

Abstract

This study deals with the effect of long-term ageing at 475 °C on microstructure and properties of a Custom 465 alloy. The theoretical part is about the microstructure and mechanical properties of the precipitation hardened stainless steels, the formation of reverted austenite and its effect on the mechanical properties of steels, and the process of 475 °C embrittlement in Fe-Cr steels. Experimental studies using TEM & XRD reveal that quality heat treatment results in the precipitation of η-Ni3Ti particles and the formation of a small fraction of reverted austenite in tempered lath martensite. Subsequent aging treatment of the Custom 465 alloy at 475 °C for 1000 hrs. and 2000 hrs. results in additional precipitation of η-Ni3Ti particles and α´-Cr rich phase due to spinodal decomposition of the α-solid solution. Furthermore, the fraction of reverted austenite in tempered martensite increases with increasing annealing duration. Prolonged ageing results in the decreasing of strength values. However, elongation and impact energy remain almost unaffected. Increasing the fraction of reverted austenite during aging helps to overcome the hardening effect of the α´-Cr rich phase.