Microstructural Evolutions During Annealing of Plastically Deformed AISI 304 Austenitic Stainless Steel: Martensite Reversion, Grain Refinement, Recrystallization, and Grain Growth

Abstract

MEYSAM NAGHIZADEH and HAMED MIRZADEHMicrostructural evolutions during annealing of a plastically deformed AISI 304 stainless steel were investigated. Three distinct stages were identified for the reversion of strain-induced martensite to austenite, which were followed by the recrystallization of the retained austenite phase and overall grain growth. It was shown that the primary recrystallization of the retained austenite postpones the formation of an equiaxed microstructure, which coincides with the coarsening of the very fine reversed grains. The latter can effectively impair the usefulness of this thermomechanical treatment for grain refinement at both high and low annealing temperatures. The final grain growth stage, however, was found to be significant at high annealing temperatures, which makes it difficult to control the reversion annealing process for enhancement of mechanical properties. Conclusively, this work unravels the important microstructural evolution stages during reversion annealing and can shed light on the requirements and limitations of this efficient grain refining approach.

Effects of Grain Size on Mechanical Properties and Work‐Hardening Behavior of AISI 304 Austenitic Stainless Steel

“…It can be seen in Figure b that the microstructural evolutions have a profound effect on the hardness of the sheet (measured using a Wilson Tukon 1202 hardness tester by applying a load of 1 kgf with dwell time of 15 s on the carefully polished samples). The decrease in hardness up to 45 min was related to the transformation of martensite to austenite . The subsequent decrease up to 90 min can be attributed to the grain coarsening as shown in Figure a.…”

“…Based on the standard lineal intercept method (ASTM E112), the average grain size was 23 μm. Multipass cold rolling with 70% reduction in thicknesses was used to obtain 84 vol% martensite as shown in the earlier study by Naghizadeh and Mirzadeh . This was followed by heating the sample at the rate of 30 K s −1 to the annealing temperatures of 750 and 1000 °C, and the subsequent holding at these temperatures for various soaking times for the purpose of reversion of martensite to austenite and grain growth.…”

“…The choice of these temperatures was based on the earlier study, which showed that at 750 °C grain growth was insignificant and this temperature was appropriate to study the reversion of martensite whereas at 1000 °C the reversion process was fast with appreciable grain growth. For the annealing temperature of 750 °C and time of 45 min, a significant grain refinement ( D 1 = 0.7 μm, Figure a) was achieved with the formation of complete austenitic structure as shown in Figure b, which was completely characterized in the earlier study . Continuous annealing had led to grain coarsening with an almost saturated grain size of D 2 = 1.3 μm at ≈90 min .…”

“…As mentioned earlier, consideration of a wide range of grain sizes is required, which needs processing routes for grain refinement and grain growth. Both of these routes have been systematically studied for AISI 304 stainless steel based on the microstructural characterization by Naghizadeh and Mirzadeh . Therefore, in the current study, different grain sizes were considered to study the effect of grain size on the tensile behavior, work‐hardening rate, work‐hardening exponent, tensile toughness, and fracture surface appearance.…”

“…Both of these routes have been systematically studied for AISI 304 stainless steel based on the microstructural characterization by Naghizadeh and Mirzadeh. [56,57] Therefore, in the current study, different grain sizes were considered to study the effect of grain size on the tensile behavior, work-hardening rate, work-hardening exponent, tensile toughness, and fracture surface appearance. Besides the range of grain size, to the best of author's knowledge, there is no report on the three latter characteristics with respect to grain size.…”