Mechanisms of the Reverse Martensite-to-Austenite Transformation in a Metastable Austenitic Stainless Steel

Abstract

The martensite-to-austenite reversion mechanisms under continuous heating and annealing of metastable austenitic stainless steel subjected to cold swaging were studied. The reversion-temperature-time diagram was constructed using high-resolution dilatometry. The diagram revealed a sequence of martensitic and diffusional reversion and recrystallization. Martensitic and diffusional reversion might be separated by using the heating rate of >10 °C/s. The reversion started via the martensitic mechanism, and the diffusional mechanism developed during subsequent heating. However, both mechanisms enhance simultaneously during continuous heating at slow heating rates (<10 °C/s). At higher temperatures, recrystallization occurred. Post-mortem microstructure analysis has allowed classifying the reverse annealing modes into low- (500–650 °C), medium- (~700 °C), and high-temperature (~800 °C) regimes. During low-temperature annealing, the development of the phase reversion, recovery, recrystallization, and carbide precipitation was characterized by both a high amount of new austenite grains and restriction of their growth that resulted in the formation of an ultrafine grain structure with an average grain size of 100–200 nm. Medium-temperature annealing was associated with the formation of almost a fully recrystallized austenitic structure, but the lamellar regions were still detected. Austenitic grain growth and dissolution of carbide particles were enhanced during high-temperature annealing.

Excellent strength-toughness synergy in metastable austenitic stainless steel due to gradient structure formation

“…1e). Annealing at 500 • C (~30 • C below the reverse martensite-to-austenite phase transformation temperature [6]) caused the development of recovery and primary recrystallization (Fig. 1c).…”

“…2a) due to a gradual decrease in strain from the edge to the center during swaging [4]. Annealing at 500 • C was associated with a minor decrease in the amount of the α-phase (~4%) caused by the reverse transformation [6].…”

“…Post-deformation phase reverse annealing might also lead to the heterogeneous structure formation, thereby resulting in an attractive strength-ductility combination [5]. Meanwhile, annealing of cold-deformed MASS at temperatures which is slightly below the lower bound (A S ) of the interval of the reverse martensite-to-austenite phase transformation, might be a promising approach because such annealing can cause additional structure refinement and hardening [6]. Furthermore, partial recovery might also improve strength [7].…”

Formation of a Gradient Structure in Metastable Austenitic Steel During Deformation-Heat Treatment

“…Однако, если относительно крупные заготовки подвергнуть деформации методом радиальной ковки с высокими степенями, то в них также можно получить структурный градиент [4,5]. Последующая термическая обработка при этом дает широкие возможности управления структурой исходно холоднодеформированной заготовки за счет реализации обратного мартенситного превращения, полигонизации, рекристаллизации, а также выделения избыточных фаз [6]. Таким образом, данная работа посвящена исследованию влияния деформационно-термической обработки на структуру и механические свойства метастабильной аустенитной нержавеющей стали.…”

Microstructure Evolution and Mechanical Properties of Austenite Stainless Steel with Gradient Twinned Structure by Surface Mechanical Attrition Treatment

“…[ 1 , 2 ]. However, the low strength of the austenitic SSs is a dominant drawback in the fields of commercial reactor, vessel internals, and coolant piping [ 3 , 4 , 5 ]. Conventionally, austenitic SSs cannot be hardened by heat treatment, but by cold working, which results in a significant increase of dislocation density, and strain-induced martensite transformation in some types of austenitic SSs [ 6 , 7 ].…”