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.

The article presents the influence of structure formation on the properties of 321 metastable austenitic stainless steel in the process of cold radial forging (CRF). The steel under study after austenitization was subjected to CRF at room temperature with degrees of true strain (e) 0.26, 0.56, 1.00, 1.71 and 2.14. It has been shown that structure formation of the studied steel during CRF consists of three stages: formation of the lamellar structure of austenite, formation of the trapezoidal structure, and formation of the equiaxial grain structure. The kinetics of the strain-induced α′-martensitic transformation is related to the stages of structure evolution. Hardness, ultimate tensile strength and yield strength uniformly increase in all stages of structure formation with a significant decrease of elongation to fracture during the first stage of structure formation while the value of elongation to fracture remains constant in the subsequent stages of deformation. Impact strength of fatigue cracked specimens (KCT) decreases sharply at the first stage of structure formation and smoothly increases at the second and third stages. However, the impact strength of V-notch specimens (KCV) continuously decreases when deformation degree increases in the overall investigated deformation range.

The paper aimed to study the evolution of the microstructure and texture gradient of a 321-type metastable austenitic stainless steel during cold rotary swaging. Cold rotary swaging was carried out with a reduction of up to 90% at ambient temperature. Pronounced gradients of the α’-martensite volume fraction, the axial texture of austenite (⟨111⟩ and ⟨001⟩) and α’-martensite (⟨101⟩), and non-uniform microhardness distribution along the rod diameter were obtained after a reduction of 80–90%. According to the finite element analysis, moderate tensile stresses were attained in the center, whereas high compressive stresses operated at the edge. Due to water cooling of the rod surface and heating of the rod center during processing, a temperature gradient was also derived. Features of strain-induced martensitic transformation, microstructure and texture evolution, and non-uniform hardening during cold rotary swaging were discussed.

The microstructure, texture, and mechanical properties of Inconel 718 fabricated via hybrid wire-arc additive manufacturing (WAAM) with inter-pass forging, and the subsequent modified post-deposition heat treatment (PDHT), were investigated. The modified PDHT included homogenization at 1185 °C and double ageing at 720 °C, with furnace-cooling to 620 °C; this process was first used for Inconel 718 obtained via WAAM and inter-pass forging. In the as-printed material, two characteristic zones were distinguished, as follows: (i) columnar grains with a preferable orientation and (ii) fine grains with a random crystallographic orientation. The development of static recrystallization induced via inter-pass forging and further heating during the deposition of the next (upper) layer provoked the formation of the fine-grained zone. In the as-printed material, particles of (Nb,Ti)C and TiN, and precipitates of a Nb-rich Laves phase that caused premature cracking and failure during mechanical testing, were detected. In the PDHT material, two zones were found, as follows: (i) a zone with coarse uniaxial grains and (ii) a zone with a gradient grain size distribution. PDHT resulted in the precipitation of γ″ nanoparticles in the γ-Ni matrix and the dissolution of the brittle Laves phase. Therefore, significant hardening and strengthening, as well as increases in ductility and impact toughness, occurred.