The effect of nitrogen on the formation of phase reversion-induced nanograined/ultrafine-grained structure and mechanical behavior of a Cr–Ni–N steel

In the study described here, we have used the concept of phase reversion involving combination of cold working and annealing to examine the effect of nitrogen on the reversion transformation of strain-induced martensite to austenite and tensile behavior of a Cr–Ni–N steel. The behavior is compared with that of a Cr–Ni steel with extremely low levels of interstitial elements C and N. Consistent with the high metastability of the investigated Cr–Ni–N steel, only 40% cold rolling reduction was adequate to transform 100% austenite to martensite, while Cr–Ni steel required ∼73% reduction for the complete transformation. The cold deformed austenite was characterized by a combination of lath and dislocation cell-type martensite. On annealing in the temperature range of 800–900 °C for 1–100 s, the phase reversion of martensite to austenite occurred by shear mechanism involving the following steps: (a) transformation of strain-induced martensite to lath-type austenite grains with high dislocation density, (b) refinement of reverted austenite grains due to recovery, i.e., dislocation cell formation to produce defect-free austenite subgrains, and (c) finally recrystallization of austenite grains following coalescence of sub-grains to form NG/UFG structure with grain size in the range of ∼200–600 nm. Some nucleation of secondary precipitates also occurred. Interestingly, the phase reversion mechanism was similar to that of the Cr–Ni steel. In spite of the similarity in the phase reversion mechanism and the grain structure between the two steels, the Cr–Ni–N steel exhibited extensive strain hardening ability during tensile straining, consistent with its higher metastability index. Both steels showed an inflexion in the strain hardening rate plots. This occurred at ∼2.5% strain in the Cr–Ni–N steel and at ∼10% in the Cr–Ni steel, again consistent with their metastability indexes.