Fine-grained Austenitic Stainless Steel Research Articles

Probing the impact of grain size distribution on the deformation behavior in fine-grained austenitic stainless steel: A critical analysis of unimodal structure versus bimodal structure

This study investigated the effect of grain size distribution on the deformation behavior in fine-grained (FG) austenitic stainless steels (γ-SSs). Commercial Cr–Mn–N γ-SS was selected as the coarse-grained (CG) specimen, which was cold rolled and then annealed at 1000 °C for 60 s and 700 °C for 3600 s to obtain FG specimens with similar grain sizes but different grain size distributions. The CG specimens annealed at 1000 °C for 60 s and 700 °C for 3600 s were referred to as unimodal-FG and bimodal-FG specimens, respectively. The tensile properties revealed that the yield strength was enhanced from 383 MPa to 685 MPa when the grain size was refined from CG (10.7 μm) to FG (∼1.8 μm). Moreover, the bimodal-FG specimen exhibited a higher total elongation of 48% than 36.5% for the unimodal-FG specimen. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction were used to identify the microstructural characteristics during tension. The results indicated that the main deformation mechanisms of γ-SSs specimens during tension were dislocation generation and accumulation at the initial stage, deformation twinning at the intermediate stage, and deformation-induced martensite (DIM) at the final stage. Compared with the unimodal-CG counterpart, grain refinement in the unimodal-FG specimens restricted the formation of DIM due to the increased austenite stability. A newly discovered phenomenon was that the bimodal-FG specimens promoted the formation of geometrically necessary dislocation (GND) and DIM compared to the unimodal-FG, which was due to the uneven deformation in grains located on hetero-boundaries (HBs). Moreover, the bimodal-FG specimen was preferentially formed DIM on HBs, especially in the smaller grains side rather than inside larger grains due to the higher stress concentration in smaller grains.

On the impacts of grain refinement and strain-induced deformation on three-body abrasive wear responses of 18Cr–8Ni austenitic stainless steel

The concept of phase reversion annealing involving cold deformation of metastable austenite to strain-induced martensite, followed by annealing was adopted to obtain fine grained 18Cr–8Ni austenitic stainless steel. The primary objective of the present study was to elucidate the wear performance of fine-grained austenitic stainless steel through three-body abrasive wear tests at room and high temperatures and compare with the coarse-grained counterpart. The quartzite stones (quartz content over 90 wt%) with diameter 5–15 mm and hardness of 1100HV were used as the abrasive in three-body abrasive wear tests. The study demonstrated that the microstructure consisting of near defect-free and equiaxed fine austenite grains with high yield strength and elongation exhibited superior wear resistance at high temperature (250 °C), which is attributed twinning induced plasticity deformation in fine austenite grains. The wear mechanism varied as a function of distance from the center of the steel sample and was characterized by microploughing and microcutting.

Tailoring mechanical behavior of a fine-grained metastable austenitic stainless steel by pre-straining

A fine-grained AISI 304 austenitic stainless steel (ASS) was fabricated by cryogenic-rolling and cycle annealing. The fine-grained ASS was pre-strained to various strains of 0–0.16, and the corresponding microstructures, kinetics of strain-induced α′-martensite (SIMα′) and work-hardening behavior were investigated based on quantitative characterization, interrupted-tensile tests and physical metallurgy. We find that the yield strength of the ASS can be enhanced by combining grain-refining with pre-straining due to the strengthening effect of grain-refinement, dislocations, shear bands and SIMα′. The formation rate of shear band increases with increasing pre-strain (εp) and the probability for an intersection of shear band to form a SIMα′ embryo is insensitive to the pre-strain, and as a result, the kinetics of SIMα′ is accelerated by pre-straining. The fine-grained ASS with εp ≈ 0–0.16 display a three-stage work-hardening behavior, and the corresponding Stage I, II and III are completed in advance mainly due to their faster kinetics of SIMα′ and severer dynamic recovery caused by pre-straining. The work-hardening rate (Θ) of pre-strained fine-grained ASS decreases faster than that of non-pre-strained counterpart at Stage I, accompanying with dislocations-dominated hardening. The Θ ascend with increasing strain to a peak value at Stage II and then decline continuously to necking at Stage III, following as SIMα′-governed hardening, which can be tailored by grain-refining and pre-straining. Our studies provide the direct experimental evidence for the pre-straining to trim the kinetics of SIMα′ in a fine-grained ASS. These observations help to deepen understanding of the effect of pre-strain on the mechanical behavior of other fine-grained strengthening ASS and transformation-induced-plasticity (TRIP) assisted alloy.

Crystal Plasticity Simulation Considering Microstructures of Austenitic Stainless Steel on Macroscopic Yield Function

In this study, yield surfaces of austenitic stainless steel produced by a cold-rolling process are measured using uniaxial and biaxial tensile tests. Using results obtained by electron backscatter diffraction, information on crystal orientation is introduced into a computational model for a multiscale crystal plasticity simulation. Finite element simulations for polycrystal of fine-grained austenitic stainless steel under biaxial tension are performed in order to predict yield surfaces of fine-grained austenitic stainless steel. Validity of predicted yield surfaces is evaluated by comparison between yield surfaces obtained by numerical simulations and experimental tensile tests.

Microstructure and mechanical behavior of an AISI 304 austenitic stainless steel prepared by cold- or cryogenic-rolling and annealing

An AISI 304 austenitic stainless steel (ASS) with various ultrafine- or fine-grained structures was fabricated by cold- or cryogenic-rolling and annealing. The microstructure and mechanical properties of the ultrafine- or fine-grained ASS were investigated based on statistical data and physical metallurgy. The results showed that much more volume fraction of α′-martensite can be obtained by cryogenic-rolling in comparison with cold-rolling under a similar rolling strain, and ε-martensite was a medium to transform into α′-martensite finally during cryogenic-rolling. The deformed ASS with larger volume fraction of α′-martensite was beneficial to obtaining finer structure with a narrow distribution of grain sizes after the similar annealing process. The cycle annealing was a feasible method to make reverse transformation completely and to inhibit the structural coarsening simultaneously for the cold- or cryogenic-rolling ASS. The yield strength was enhanced by cryogenic-rolling and cycle annealing to be approximately 2.7 times higher than that of solution-treatment state. The tensile strength was not changed evidently, and the uniform strain was apparently decreased with reducing grain size. There is no significant relevance between the mechanical stability of austenite and average grain size for the ultrafine- or fine-grained ASS; however, their mechanical stability was less than that of solution-treatment state.

Deformation and transformation behavior in micropiercing of SUS304

Processing technologies for fine-grained austenitic stainless steel SUS304, including the stability of boundary edge between burnished surface and fractured surface have been developed. However, the deformation mechanism related to the grain size is not clear. Here, we report the relationship between the grain deformation and transformation mechanism in micropiercing of SUS304. We investigated the effects on the grain phase and the misorientation angle in process-affected zones at circumferential direction by EBSD (electron backscatter diffraction) analysis. The result shows that the frequency of the austenitic phase and the strain-induced martensitic phase were affected by grain size strongly. This phenomenon was influenced by distribution of austenitic phase. The selection of an appropriate grain size may be required to enable micropiercing with high accuracy.

Work Hardening in a Fine Grained Austenitic Stainless Steel

Grain size refinement in austenitic stainless steels to approximately 1 μm can be readily achieved with benefits to material performance. Reducing the scale of the microstructure in these materials, however, leads to complex changes in the strengthening behaviour owing to the presence of several deformation mechanisms. Understanding the interaction between grain size and mechanical response in these materials is thus central to identifying optimal material properties. In this study, we have examined the work hardening response of a stable austenitic stainless steel with grain sizes between 2 μm and 27 μm. The strongly grain size dependent work hardening behaviour at room temperature is shown to arise from both the grain size dependence of the dislocation storage as well as the grain size dependence of twinning.

Deformation Mechanisms of a Micro-Sized Austenitic Stainless Steel with Fine Grains

ABSTRACTIn this study, deformation behavior of fine-grained austenitic stainless steel micro-cantilever beams was investigated using a newly developed testing machine for micro-sized specimens. The microbeams were deformed to different strain hardening stages of the material, and then the detailed deformation behavior on the specimen surface at the corresponding strain hardening stage was examined by scanning electron microscopy. Two deformation mechanisms corresponding to different strain hardening stages were found in the micro-sized austenitic stainless steel with fine grains. The dislocation slip mechanism characterized by the extensive dislocation slips and their interaction with grain boundaries resulted in the stage I strain hardening. With increasing deformation, the grain boundary sliding (GBS) mechanism at the stage II and subsequently intergranular cracking occurred. The differences in stress condition and work-hardening behavior on the top tension-side and rear compression-side surfaces of the micro-cantilever beam resulted in the different deformation behavior.

The influence of pipe diffusion on the creep of fine-grained materials

Pipe diffusion can be important in a number of creep mechanisms, including diffusional, grain boundary sliding and slip creep. When pipe diffusion is the rate-controlling process, the stress exponent increases by a factor of 2 over that obtained when lattice diffusion is the rate-controlling step. Experimental observations which show both a grain size dependence of creep rate ( ϵ ̇ ∝ d −2 ) and a relatively large stress exponent (of about 4) can be explained on the basis of a pipe-diffusion-controlled grain boundary sliding mechanism. This effect is predicted to be especially important for the creep of fine-grained materials at intermediate stresses and temperatures. The creep behavior of fine-grained austenitic stainless steel, as well as of Pb-5%Cd and a copper alloy, is shown to be in excellent agreement with the newly developed constitutive equations which incorporate pipe diffusion.

Low-Temperature Diffusion of Chromium in a Fine-Grained Austenitic Stainless Steel with Varying Dislocation Densities

AbstractChromium tracer diffusion in a fine-grain austenitic stainless steel with varying dislocation densities has been analysed using exact solutions to the grain-boundary and dislocation problem. Down to ∼700°C the results could be explained in terms of lattice and grain-boundary diffusion components. The concentration profiles of tracer below 700°C, however, also required a dislocation component. No systematic dependence of the penetration profile on dislocation density was detected. The dislocation component was dominant at short times and led to an apparent enhancement of the lattice diffusion coefficient. Enhanced lattice diffusion by dislocations according to the Hart model was not observed in the temperature range studied even at high dislocation densities. If the accepted value of the dislocation-pipe diameter is assumed then it can be postulated that the lack of enhancement is due to the ratio D p/D L not being sufficiently high (i.e. D p/D L < 103). This implies that the dislocation-pipe diffu...