Huibin Wu

Annealing of strain-induced martensite to obtain micro/nanometre grains in austenitic stainless

Micro/nanometre grain sizes appear to improve the biocompatibility of austenitic stainless steel. In order to realise the reverse transformation (from strain-induced martensite) austenite structure control with micro/nanometre size, the influence of annealing parameters on the microstructure evolution and mechanical properties of 316L-Nb austenitic stainless steel were investigated. Furthermore, the role of Nb in the annealing process was also studied. The results showed that the closely 100% reversion transformation austenite structures were obtained in the samples after annealing at 850°C, where the grains with the grain diameter ≤500 nm accounted for 25% and the grains with the grain diameter >0.5 µm accounted for 75%. The micro/nanometre grain steel not only exhibited a high strength level but also exhibited a desirable elongation. Moreover, the Nb demonstrates a remarkable effect on grain-refining and a significant role in improving the stability of the microstructure.

Effects of annealing temperature on nano/ultrafine-grained structure in austenite stainless steel

ABSTRACT A nano/ultrafine-grained (NG/UFG) structure was obtained by heavy cold deformation (80%) and annealing in the range between 700 and 950°C for 60 s to explore the effects of temperature on the development of NG/UFG structures in austenitic stainless steel. Results showed that martensite was reversibly transformed to austenite, with the accumulation of twins, dislocations and subgrain boundaries. At 700°C, the microstructure exhibited low elongation and consisted of 65% austenite. Above 750°C, the amount of reversed austenite was nearly 100%. The tensile strength of the sample decreased slightly, whereas the elongation increased further, showing co-dependent strengthening and toughening. At 850°C, micrometre-sized grains were embedded in the nanocrystalline/ultrafine grains. In this case, both the microstructure and mechanical properties were optimal.

Effect of tempering temperatures on microstructures and properties of 0.28C–0.22Ti wear-resistant steel

ABSTRACT The microstructures and properties of a 0.28C–0.22Ti low-alloy wear-resistant steel at different temperatures from 200 to 600°C was experimentally studied. It is shown that the wear resistance of the steel is not monotone changing with its hardness and strength. With the increase of the tempering temperature, the tensile strength and the hardness of the steels were gradually declined; however, the wear resistance was first decreased and then increased. The TiC particles can be divided into two classes: the small TiC particles (about 0.3–0.4 µm in diameter) and the coarse TiC particles (1–5 µm in diameter). The small TiC particles can improve the yield strength of the steels, and the coarse TiC particles can improve the wear resistance of the tested steels.

An improved constitutive model for high-temperature flow behaviour of the Fe–Mn–Al duplex steel

ABSTRACT The high-temperature flow curves of the Fe–Mn–Al duplex steel showed an uncommon yield-like behaviour and an abnormal dynamic recrystallisation behaviour that occurred at low temperatures rather than high temperatures. The interaction of strain partitioning and unsynchronised softening behaviour in δ-ferrite and austenite caused this peculiar flow behaviour. By discussing the stress exponent and apparent activation energy, respectively, at low and high temperatures, a modified hyperbolic sine function was developed to predict the characteristic stresses. By simplifying the material constant θ and compensating the microstructural evolution in the exponential saturation work-hardening law, an improved constitutive model was developed to predict the transient stress. The comparison between the experimental and calculated values confirmed a high prediction accuracy of this improved model.

Effect of martensitic transformation on nano/ultrafine-grained structure in 304 austenitic stainless steel

Abstract304 austenitic stainless steel was cold rolled in the range of 20%–80% reductions and then annealed at 700–900 °C for 60 s to obtain nano/ultrafine-grained (NG/UFG) structure. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction were used to characterize the resulting microstructures. The results showed that with the increase of cold reduction, the content of martensite was increased. The steel performed work hardening during cold-working owing to the occurrence of strain induced martensite which nucleated in single shear bands. Further rolling broke up the lath-type martensite into dislocation-cell type martensite because of the formation of slip bands. Samples annealed at 800–960 °C for 60 s were of NG/UFG structure with different percentage of nanocrystalline (60–100 nm) and ultrafine (100–500 nm) grains, submicron size (500–1000 nm) grains and micron size (>1000 nm) grains. The value of the Gibbs free energy exhibited that the reversion mechanism of the reversion process was shear controlled by the annealing temperature. For a certain annealing time during the reversion process, austenite nucleated first on dislocation-cell type martensite and the grains grew up subsequently and eventually to be micrometer/submicrometer grains, while the nucleation of austenite on lath-type martensite occurred later resulting in nanocrystalline/ultrafine grains. The existence of the NG/UFG structure led to a higher strength and toughness during tensile test.

Determination of Intergranular Corrosion Susceptibility of Z3CN20.09M Cast Duplex Stainless Steel by a Modified Double Loop Electrochemical Potentiokinetic Reactivation Method

In this study, a modified double loop electrochemical potentiokinetic reactivation (DL-EPR) test was applied to determine the degree of intergranular corrosion (IGC) susceptibility of a Z3CN20.09M cast duplex stainless steel (CDSS) which has been used widely in primary coolant pipes of nuclear power plants. An optimized DL-EPR measurement was successfully used to characterize the interactions among precipitates, chromium depletion and IGC of steel Z3CN20.09M with high sensitivity. The experimental results showed that the degree of sensitization increased gradually from 1h to 50h due to the formation of chromium-depleted phase γ2 around the M23C6 and σ phases precipitated at the ferrite/austenite boundaries or in ferrite during aging.