High temperature thermal stability of nanocrystalline 316L stainless steel processed by high-pressure torsion

Abstract

Differential scanning calorimetry (DSC) was used to study the thermal stability of the microstructure and the phase composition in nanocrystalline 316L stainless steel processed by high-pressure torsion (HPT) for ¼ and 10 turns. The DSC thermograms showed two characteristic peaks which were investigated by examining the dislocation densities, grain sizes and phase compositions after annealing at different temperatures. The first DSC peak was exothermic and was related to recovery of the dislocation structure without changing the phase composition and grain size. The activation energies for recovery after processing by ¼ and 10 turns were ~163 and ~106 kJ mol -1 , respectively, suggesting control by diffusion along grain boundaries and dislocations. The second DSC peak was endothermic and was caused by a reverse transformation of α'-martensite to γ-austenite. The hardness of annealed samples was determined primarily by the grain size and followed the Hall-Petch relationship.Nanocrystalline 316L steel processed by HPT exhibited good thermal stability with a grain size of ~200 nm after annealing at 1000 K and a very high hardness of ~4900 MPa.

Exceptionally high strength and good ductility in an ultrafine-grained 316L steel processed by severe plastic deformation and subsequent annealing

“…The thickness and the diameter of the HPT-processed disks were ~0.75 mm and ~10 mm, respectively. In former studies, the microstructure and the phase composition evolution during HPT processing and subsequent annealing were investigated in detail [2,5]. Differential scanning calorimetry (DSC) revealed that between ~590 and ~740 K there was only a recovery in the microstructure whereas between ~740 and ~1000 K recovery, grain growth and an almost complete reversion of α'-martensite to γ-austenite was also detected [5].…”

“…In former studies, the microstructure and the phase composition evolution during HPT processing and subsequent annealing were investigated in detail [2,5]. Differential scanning calorimetry (DSC) revealed that between ~590 and ~740 K there was only a recovery in the microstructure whereas between ~740 and ~1000 K recovery, grain growth and an almost complete reversion of α'-martensite to γ-austenite was also detected [5]. Therefore, in this study the tensile performance was investigated for the samples annealed to the two characteristic temperatures of the DSC thermograms, i.e., to ~740 and ~1000 K. Small specimens were cut from the regions between the half-radius and the periphery of the HPT disks and these samples were heated in the calorimeter to ~740 and ~1000 K at a heating rate of 20 K/min and then quenched to RT at ~300 K/min.…”

“…The grain size of the samples was determined by electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The experimental details and the results of these investigations were given in earlier reports [2,5]. For the tensile tests, two miniature specimens were cut from each HPT-processed disk by electro-discharge machining (EDM).…”

“…However, during SPD there is a simultaneous phase transformation from γaustenite to α'-martensite [2]. It was suggested that annealing after SPD may yield fine austenite grains [4,5] so that a combination of high strength and good ductility is feasible.…”

Recrystallization and grain growth of a nano/ultrafine structured austenitic stainless steel during annealing under high hydrostatic pressure

“…For this reason, many papers have been devoted to understanding these phenomena in microcrystalline [1,2], and recently in nanocrystalline, materials [3][4][5]. During the annealing of nanocrystalline materials produced by severe plastic deformation (SPD) techniques, one can observe typical processes of recovery, recrystallization and grain growth, but they follow a different course than in materials deformed by conventional techniques [6][7][8].…”