Effect of Cu addition on formation of reversed austenite and hardness in 9% Ni steels

Copper (Cu) was added to a cryogenic 9% Ni steel as an alloying element for improving strength and controlling microstructure, and then the softening behavior and microstructural development during tempering were investigated in the 9% Ni steels with various amounts of Cu. Hardness of the specimens decreased with an increase in tempering time owing to recovery of martensite. However, the addition of Cu more than 1% significantly retarded the softening. In particular, the steels containing 2% Cu or more exhibited clear age hardening phenomena after tempering at 873K for 1.8 ks. The retardation of softening by Cu is found to be due to precipitation of fine Cu particles within martensite matrix. On the other hand, Cu addition promoted the formation of reversed austenite at tempering temperature, thus the volume fraction of the reversed austenite became much larger in a 3% Cu bearing steel than in a Cu free 9% Ni steel under a same tempering condition. Chemical analysis using XEDS revealed that not only Ni but also Cu concentrate into the reversed austenite and make the austenite more stable in the Cu bearing steel than in the Cu free steel. As a result, the hardness of Cu bearing 9% Ni steels was found to be estimated by the law of mixture of each hardness in martensite and reversed austenite.