Improvement of strength–ductility balance by copper addition in 9%Ni steels

Effect of Cu addition on mechanical properties of a cryogenic 9%Ni steel was studied by means of tensile testing, and then strength–ductility balance was discussed in the Cu bearing 9%Ni steel in connection with microstructural characteristics. Cu particles precipitated finely within martensite matrix on tempering. The Cu addition also increased the amount of reversed austenite formed along lath boundaries, because the Cu was condensed into the reversed austenite and changed the stability of austenite. On the other hand, yield strength and tensile strength of 9%Ni steel were increased by Cu addition owing to precipitation strengthening by the Cu particles without losing ductility. The improvement of uniform elongation by Cu addition was explained in terms of deformation-induced martensitic transformation of reversed austenite (TRIP assisted effect). It was concluded that the Cu addition increases the work-hardening rate through the TRIP assisted effect and suppresses the occurrence of necking deformation.