Role of deformation induced martensite on mechanical properties of cryorolled 304 stainless steel

Abstract

The transformation of γ—austenite to complete α′—martensite (strain-induced martensite) through ε—martensite with no traces of deformation nanotwins was reported on rolling austenitic stainless steel (SS 304) with two different rolling strains (50% and 75%) at room and cryogenic temperatures. Transformation of complete α′—martensite was evaluated through the electron backscattered diffraction phase analysis and x-ray diffraction techniques. A proportional increment in hardness with an increase in volume fraction of α′—martensite was noted. The SS 304 rolled up to 75% reduction at cryogenic temperature (Cryo 75) that showed 99% α′—martensite in its x-ray diffraction pattern had exhibited a maximum hardness value of 491 HV. The effect of rolling strain and suppression of dynamic recovery resulted in reduced crystallite size, enhanced lattice strain and dislocation density values in cryorolled SS 304. The strength of cryorolled SS 304 improved due to increased dislocation density and finer lath shaped α′—martensite. Though the total elongation was lower in the cryorolled (CR) SS 304 compared to the as-received sample and there was no severe degradation of total elongation in comparison to room temperature rolled (RTR) SS304. The strength of RTR with 75% reduction was nearly equivalent to CR with 50% reduction. However, the Cryo 75 showed the maximum tensile strength of 2054 MPa among the rolled samples of SS 304. The enthalpy change was higher for the cryorolled sample with 75% reduction (290 J g−1–from DSC curve) due to the accumulation defect density, low stacking fault energy and rolling strain. The contribution of the strengthening mechanism to yield strength was evaluated and to substantiate, the tensile test was conducted on the rolled samples. The contribution of dislocation strength evaluated from the strengthening mechanisms was predominant for the CR 75 sample.