Deformation-induced γ → αʹ-martensitic transformation in austenitic stainless steel obtained by electron beam additive manufacture

Abstract

The relationship between strain hardening and kinetics of deformation γ → αʹ phase transformation in chromium-nickel steel Fe–19Cr–9Ni–0.7Ti–0.06C wt. % obtained by electron beam additive manufacture was studied under uniaxial static tension at room temperature and at liquid nitrogen temperature. Additively-produced steel had a two-phase (γ + δ) structure with an increased content of δ-ferrite (≈14 %). Post-production heat treatment at 1100 °С (for 1 h) allowed to reduce its volume content down to 6 %, that is, a predominantly austenitic structure in steel was close to those for analogues obtained by traditional metallurgical methods. Plastic deformation of additively-produced steel was accompanied by the formation of deformation αʹ-martensite, the volume fraction of which increased with an increase in the strain and with a decrease in the test temperature. Using the method of magnetophase analysis, it was shown that at room temperature, kinetics of the deformation γ → αʹ transformation was sluggish and it, as well as the stage and magnitude of the strain hardening, weakly depended on the content of δ-ferrite in the structure of steel obtained by the additive method. At the same time, increased content of the δ-phase under these deformation conditions contributed to an increase in the yield strength and reduced elongation to failure of the additively obtained samples. At low-temperature deformation, when the rapid kinetics of deformation γ → αʹ transformation was observed, the formation rate of αʹ-martensite under plastic deformation was slower and strain hardening was weaker in steel with a larger volume fraction of δ-ferrite than those in the samples with low content of δ-phase.