Ultrafine-grained Austenitic Stainless Steels Research Articles

Quantitative Characterization of Grain Boundaries in Ultrafine-Grained Austenitic Stainless Steel by Cluster Analysis

Quantitative Characterization of Grain Boundaries in Ultrafine-Grained Austenitic Stainless Steel by Cluster Analysis

Additive manufacturing of ultrafine-grained austenitic stainless steel matrix composite via vanadium carbide reinforcement addition and selective laser melting: Formation mechanism and strengthening effect

Selective Laser Melting (SLM) was applied to Additive Manufacturing (AM) of carbide ceramic particulate reinforced 316 L stainless steel matrix composites. The dense and ultrafine-grained Austenite matrix with uniformly distributed and nano-sized VCx particulate reinforcements were obtained according to the tailored SLM process parameters and 3 wt% submicron-sized V8C7 ceramic particle addition into 97 wt% 316 L powder via ball-milling treatment. The influences of SLM process parameters on the SLM printed composites in terms of density, surface roughness, phase constitution, microstructure characteristics and mechanical tensile performance were investigated in detail, comparing with the SLM printed 316 L in absence of vanadium carbides. Based on the electron back-scattered diffraction analyses, the number of grains with grain size less than 2 µm accounted for more than 90% in the printed composites. The formation mechanisms of the printed composites with their unique microstructure characteristics were elaborated. The room-temperature ultimate tensile strength of the printed 316 L matrix composites achieved higher than 1400 MPa, which was more than twice as high as that of printed 316 L stainless steel. It contributed substantially to the strengthening effect on the printed composites of the ultrafast laser-induced and highly nonequilibrium melting-solidification nature of SLM process, the dispersed carbide ceramic particulate reinforcements, the nanoscaled nucleation sites of VCx for the equiaxed Austenite grains, and the inhibition against Austenite matrix growth by particulate dispersedly distributed along Austenite grain boundaries.

Characterization of coarse and ultrafine-grained austenitic stainless steel subjected to dynamic impact load: XRD, SEM, TEM and EBSD analyses

A comparative study of the dynamic impact responses of coarse (37 µm) and ultrafine-grained (0.24 µm) AISI 321 austenitic stainless steel was conducted using a split Hopkinson pressure bar system. An increase in firing pressure of the system's projectile results in the corresponding increase in impact momentum, strain rate, total strain and maximum flow stress of the specimens. Although ultrafine-grained (UFG) specimens show higher compressive strength, coarse-grained (CG) specimens possess higher strain hardenability than the UFG specimens under the same impact loading conditions. UFG specimens exhibit lower critical strain and strain rate at which adiabatic shear bands (ASBs) are observed; hence, UFG specimens are more susceptible to ASB formation than CG specimens. Using the XRD, SEM, TEM and EBSD characterization techniques, the underlying deformation and strengthening mechanisms were studied. Slip and twinning are the active deformation mechanisms in CG specimens and they are highly suppressed in UFG specimens due to spatial restriction effect. At higher strain rates, CG specimens are only susceptible to the formation of multiple transformed shear band (MTSB), while UFG specimens are susceptible to both MTSB and TSB bifurcation. Although the mechanism behind the formation of MTSB is not completely clear, bifurcation of TSB was geometrically necessary and the presence of precipitate at the bifurcation point could play major role. EBSD analysis of inside, interface and outside the TSB revealed the development of equiaxed ultra-fine grain structure (∼0.17–0.21 µm in CG and ∼0.14–0.15 µm in UFG specimens) inside TSB by rotational dynamic recrystallization mechanism.

Texture of Ultrafine-Grained Austenitic Stainless Steels Produced by Martensite Treatment

Texture of Ultrafine-Grained Austenitic Stainless Steels Produced by Martensite Treatment

Austenite stability in reversion-treated structures of a 301LN steel under tensile loading

Ultrafine-grained austenitic stainless steels can be produced by the martensitic reversion process, but the factors affecting the stability of refined austenite in subsequent deformation are still unclear. To clarify this, fully and partially austenitic reversed structures with the average grain size between 24 and 0.6μm were created in a 60% cold-rolled 301LN type (18Cr-7Ni-0.16N) austenitic stainless steel by varying the annealing conditions. The amount of strain-induced α′-martensite (SIM) during tensile loading was determined by magnetic measurements and the microstructure evolution and texture examined by electron backscatter diffraction and X-ray diffraction methods. The extensive experimental data evidenced firmly that in completely austenitic structures the austenite stability increases with decreasing grain size down to about 1μm, obtained at 900°C, but the stability decreases drastically in the ultrafine-grained and partially reversed structures, with the average grain size of 0.6–0.7μm obtained at 800–700°C. However, these structures are nonuniform also containing larger micron-size grains transformed from slightly deformed SIM. The low stability of austenite is not a result from the ultrafine grain size, neither due to retained phases nor texture, but the main reason is concluded to be the precipitation of CrN during the reversion at low temperatures of 800–700°C. Due to this precipitation, micron-size grains in the ultrafine and partially reversed structures show most unstable behavior under tensile deformation.

Hydrogen Embrittlement of Ultrafine-grained Austenitic Stainless Steels Processed by High-pressure Torsion at Moderate Temperature

Hydrogen Embrittlement of Ultrafine-grained Austenitic Stainless Steels Processed by High-pressure Torsion at Moderate Temperature

Ultrafine-Grained Austenitic Stainless Steels X4CrNi18-12 and X8CrMnNi19-6-3 Produced by Accumulative Roll Bonding

Austenitic stainless steels X4CrNi18-12 and X8CrMnNi19-6-3 were processed by accumulative roll bonding (ARB). Both materials show an extremely high yield strength of 1.25 GPa accompanied by a satisfactory elongation to failure of up to 14% and a positive strain rate sensitivity after two ARB cycles. The strain-hardening rate of the austenitic steels reveals a stabilization of the stress-strain behavior during tensile testing. Especially for X8CrMnNi19-6-3, which has an elevated manganese content of 6.7 wt.%, necking is prevented up to comparatively high plastic strains. Microstructural investigations showed that the microstructure is separated into ultrafine-grained channel like areas and relatively larger grains where pronounced nano-twinning and martensite formation is observed.

Plastic Deformation Modes in Mono- and Bimodal-Type Ultrafine-Grained Austenitic Stainless Steel

An attempt was made to track plastic tensile deformation modes operating in bulk ultrafine-grained austenitic stainless steel with mono- (maximum at ~0.6 μm) and bimodal-type (minimum at ~0.5 μm and maximum at ~1.65 μm) grain size distributions produced by cyclic thermal processing. Post tensile deformation electron backscatter diffraction studies were conducted to analyse the impact of grain size distribution on plastic deformation characteristics. The study revealed extensive strain localisation in the monomodal-type grain size distribution, leading to poor strain-hardening behaviour. On the other hand, the bimodal-type distribution disclosed a conventional dislocation-mediated deformation mechanism operating in the coarse grains, while it was restricted to initial small strains in ultrafine austenite grains. The subsequent deformation process in these ultrafine austenite grains was dictated by nucleation and autocatalytic growth of strain-induced α′-martensite. The observed martensitic transformation of ultrafine austenite grains in preference to coarse grains was attributed to activation of local ‘grain to grain’ interactions.

Grain boundary segregation induced strengthening of an ultrafine-grained austenitic stainless steel

The influence of grain boundary segregation on the strength of a nanostructured austenitic stainless steel was investigated. The steel was nanostructured by severe plastic deformation at two different temperatures to form ultrafine-grained states different by microstructure parameters. It is shown that despite the difference in grain size both nanostructured steels demonstrated the same level of strength. For the first time it is directly observed that severe plastic deformation at elevated temperature leads to formation of Mo–Cr–Si rich grain boundary segregations in the steel. Considering different contributions to the material strengthening, we demonstrate that grain boundary segregations can lead to significant enhancement of the yield stress.

Microstructural evolution in adiabatic shear band in the ultrafine-grained austenitic stainless steel processed by multi-axial compression

Adiabatic shear bands play the most important role in the deformation and failure of high strength AISI 201 austenitic stainless steel. The microstructure and microtexture of the adiabatic shear band in ultrafine-grained stainless steel are investigated by means of optical microscopy, TEM and EBSD. The shear bands can be generated at about 54μs after the true flow stress reaches the peak value of about 1135MPa. The width of the shear bands is about 9μm, and the grains sizes 50–200nm in the boundary of the shear band are elongated along the shear direction; the grain subdivision on approaching the shear band can be observed, and the core of the shear band consists of recrystallized equiaxed grains (sizes 30–80nm) with new microtextures and the ultrafine grains with deformed microtextures and with high dislocation density. The grain boundaries in the adiabatic shear band are geometrical necessary boundaries with high-angles. The calculated temperature in the shear band is estimated to reach 0.58 Tm (968K). It takes 7μs for the shear band to cool down from 968K to the room temperature when the shear localization ceases, and the cooling strain rate is calculated as high as 9.6×107K/s. Kinetic calculations indicate that during the deformation process, the recrystallized nanosized grains can be formed in the shear band by way of the subgrain boundaries rotation when the subgrains׳ sizes are lower than 80nm or the temperature in the shear band is higher than 0.5 Tm (834K), and they do not undergo significant growth by grain boundary migration at the cooling stage. These results indicate that the microstructure development within shear band is controlled by both dynamic recovery and rotational dynamic recrystallization.

Development of Ultrafine Grained Austenitic Stainless Steels by Large Strain Deformation and Annealing

The development of ultrafine grained structures in 316L and 304-type austenitic stainless steels subjected to large strain cold working and subsequent annealing and their effect on mechanical properties were studied. The cold rolling was accompanied by a mechanical twinning and a partial martensitic transformation and resulted in the development of elongated austenite/ferrite grains with the transverse size of about 50 nm at a strain of 4. The grain refinement by large strain cold working resulted in an increase of tensile strength above 2000 MPa in the both steels. Annealing at temperatures above 500°C resulted in ferrite-austenite reversion. However, the transverse grain/subgrain size remained on the level of about 100-150 nm after annealing at temperatures up to 700°C.

The influence of grain size on the strain-induced martensite formation in tensile straining of an austenitic 15Cr–9Mn–Ni–Cu stainless steel

In order to improve understanding on the behavior of ultrafine-grained austenitic stainless steels during deformation, the influence of the austenite grain size and microstructure on the strain-induced martensite transformation was investigated in an austenitic 15Cr–9Mn–Ni–Cu (Type 204Cu) stainless steel. By different reversion treatments of the 60% cold-rolled sheet, varying grain sizes from ultrafine (0.5μm), micron-scale (1.5μm), fine (4μm) to coarse (18μm) were obtained. Some microstructures also contained a mixture of ultrafine or micron-scale and coarse initially cold-worked austenite grains. Samples were tested in tensile loading and deformation structures were analyzed after 2%, 10% and 20% engineering strains by means of martensite content measurements, scanning electron microscope together with a electron backscatter diffraction device and transmission electron microscope. The results showed that the martensite nucleation sites and the rate of transformation vary. In ultrafine grains strain-induced α′-martensite nucleates at grain boundaries and twins, whereas in coarser grains as well as in coarse-grained retained austenite, α′-martensite formation occurs at shear bands, sometimes via ε-martensite. The transformation rate of strain-induced α′-martensite decreases with decreasing grain size to 1.5μm. However, the rate is fastest in the microstructure containing a mixture of ultrafine and retained cold-worked austenite grains. There the ultrafine grains transform quite readily to martensite similarly as the coarse retained austenite grains, where the previous cold-worked microstructure is still partly remaining.

Deformation and Damage Mechanisms in Ultrafine-Grained Austenitic Stainless Steel During Cyclic Straining

The ultrafine-grained (UFG) structure of an austenitic stainless steel (Type 301LN), processed by controlled phase-reversion annealing, was fatigued to study the deformation and damage mechanisms during cyclic straining. Fatigue cracking along the grain boundaries and the formation of extended persistent slip band-like shear bands (SBs) were observed to be the fatigue-induced microstructural features in the ultrafine-grained structure. Characterization of SBs was performed by electron backscattered diffraction and atomic force microscopy to study the fine features.

Tensile deformation characteristics of bulk ultrafine-grained austenitic stainless steel produced by thermal cycling

Deformation microstructures of bulk ultrafine-grained austenitic AISI 304L stainless steels were analyzed by electron backscatter diffraction. Samples with grain sizes below 500 nm showed transition from grain-scale deformation to the collective phenomenon of shear banding. This was assisted by strain-induced grain rotation and coalescence. This phenomenon was suppressed in samples with a bimodal grain size distribution (maxima at ∼650 and ∼1400 nm) due to deformation-induced martensite formation, yielding high tensile strength and ductility (1348 MPa ultimate tensile strength at 0.36 max. true strain).

Grain-boundary diffusion and precipitate trapping of hydrogen in ultrafine-grained austenitic stainless steels processed by high-pressure torsion

Abstract This study was conducted to clarify the effects of grain boundaries and precipitates on room-temperature hydrogen transport in two types of austenitic stainless steels with ultrafine-grained structures produced by high-pressure torsion (HPT) and subsequent annealing. The grains in the Fe–25Ni–15Cr (in mass%) alloy containing Ti and the Fe–25Cr–20Ni alloy were refined by the HPT-processing to ∼150 and ∼85 nm, respectively. The high-temperature annealing after the HPT processing led to the precipitation of η-Ni3Ti for the former and σ-FeCr for the latter. In the HPT-processed specimens, hydrogen diffusivity was enhanced through short-circuit diffusion because of the increased population of grain boundaries in comparison with the increased opportunity of hydrogen trapping on dislocations. As for the post-HPT-annealed specimens having the precipitates, the hydrogen diffusion was hindered by the hydrogen trapping on η-Ni3Ti precipitates, but was not affected by σ-FeCr precipitation. This depends on the affinity between hydrogen and constituting elements.

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Luders deformation. Measurements showed that Luders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Luders bands and large uniform plastic deformation. During tensile deformation, the Luders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.

Martensite → austenite phase transformation kinetics in an ultrafine-grained metastable austenitic stainless steel

A generalized phase transformation kinetics model is used to understand the martensite to austenite transformation in a cold-rolled and annealed metastable AISI 301LN ultrafine-grained austenitic stainless steel. The model shows that the presence of interstitial nitrogen and heavy cold-rolling is important to promote fast transformation kinetics, through rapid nitrogen-diffusion and austenite nucleation at austenite/martensite phase boundaries.

Thermal stability of ultrafine-grained austenitic stainless steels

Ultrafine-grained 316 and 304 austenitic stainless steel samples have been produced by high pressure torsion. Their microstructure, after deformation and annealing at a temperature in the 350–900 °C range, has been characterized using several techniques (transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy). The average grain size in the ultrafine-grained 316 is about 40 nm while it is larger in the ultrafine-grained 304 due to a smaller deformation. Results show the formation of α′-martensite during deformation in both steels while ɛ-martensite is formed only in the 304 steel. Annealing at 350 °C induces the decrease of α′-martensite content in the 316 steel. The trend is different in the 304 steel, in which the α′-martensite content increases. Recrystallization of grains is observed from 700 °C. Mössbauer spectroscopy shows a reduction of the level of solute atoms in α′-martensite during annealing.

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.

Development of Ultrafine Grained Austenitic Stainless Steels by High Strain Powder Metallurgy Process

Development of Ultrafine Grained Austenitic Stainless Steels by High Strain Powder Metallurgy Process