Engineering mechanical properties by controlling the microstructure of an Fe–Ni–Mn martensitic steel through pre-cold rolling and subsequent heat treatment

Abstract

Experiments were conducted to examine the effects of prior cold rolling and subsequent heat treatment on the microstructure, phase evolution and mechanical properties of an Fe–10Ni–7Mn martensitic steel. The results show that 70% cold rolling leads to deformation-induced austenite formation in the microstructure and reduces the size of the martensite blocks. Prior cold rolling increases the reversed austenite volume fraction under post-deformation intercritical annealing (PDIA) at 600 °C compared with the solution annealed condition. Electron back scattering diffraction and dilatometric studies showed that under PDIA there is a reverse transformation of martensite to austenite through a sequential combination of martensitic and diffusional mechanisms. There is also a precipitation of θ-NiMn particles in the PDIA specimen after subsequent ageing leading to an increase in the ultimate tensile strength in tensile testing. Cyclic tensile testing revealed pseudoelastic behavior in the cold-rolled specimen but this disappeared after PDIA and appeared again after subsequent ageing. • Cold rolling produces deformation-induced austenite formation and also refines the martensite blocks. • The prior austenite and the smaller martensite blocks are the primary potential sites for austenite nucleation during PDIA. • Ultimate tensile strength improves from ~695 MPa for the solution annealed sample to ~1215 MPa for the PDIA processed specimen followed by ageing. • Observed pseudoelasticity in the cold rolled specimen disappears after PDIA. • Pseudoelasticity appears again in the PDIA processed specimen followed by ageing.