Deformation-induced Martensite Formation 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.

The significant role of bimodal lamellar heterostructure for Lüders deformation and TRIP effect in 18Cr–8Ni austenitic stainless steel

A series of 18Cr–8Ni ASSs with homogeneous or heterogeneous structures were prepared to reveal the unique deformation mechanism of hetero-deformation-induced (HDI) hardening in lamellar heterostructure. By adjusting the annealing temperature, the nano-grained (NG) and coarse-grained (CG) unimodal homogeneous structures were obtained by annealing at 600 °C and 1000 °C, respectively, while the nano-grained/ultrafine-grained (NG/UFG) and ultrafine-grained/fine-grained (UFG/FG) bimodal heterogeneous structures were achieved by annealing at 700 °C and 800 °C. Compared to the conventional bimodal homogeneous structure (UFG/FG), the hetero-zone boundaries (HBs) of bimodal lamellar heterostructure (NG/UFG) can not only provide extra HDI hardening effect, but also coordinate the inhomogeneous deformation for Lüders deformation. The HDI hardening and Lüders deformation greatly facilitate the transformation induced plasticity (TRIP) effect to achieve. This is due to the lower austenite stability in soft region (coarser grain), leading to the formation of deformation-induced martensite (DIM) under lower stress, on the other hand, the Lüders deformation achieved a dynamic balance between both sides of hetero-boundary, avoiding the stress concentration and promoting the TRIP effect. Consequently, the DIM content of the NG/UFG specimen is considerably higher than that of the UFG/FG specimen (78 vol% to 27 vol%).

Tailoring the austenite characteristics via dual nanoparticles to synergistically optimize the strength-ductility in cold rolled medium Mn steel

In this work, we proposed a novel Cu/θ dual nanoparticles strategy to tailor the austenite characteristics of a medium Mn steel via a tempering-annealing process to optimize the mechanical properties. We explored the effects of Cu-rich particles and cementite precipitated in the tempering process on the austenite reversion during the subsequent annealing process. Both experiments and numerical simulations verified that the austenite inherited from cementite had a finer size and a higher Mn enrichment compared with the austenite originating from the tempered martensite matrix. In addition, quantitative evaluations revealed that the pinning effect exerted by the Cu-rich particles could significantly hinder the α/γ interface migration and the recrystallized grain growth, thereby further refining the final microstructure. With contributions from the effects of dual nanoprecipitates on the austenite reversion, the heterogeneous austenite grains inherited from varying nucleation sites ensured the sustained and gradual deformation-induced martensite and twinning formation. Therefore, the Cu-added steels subjected to a tempering-annealing process achieved synergetic enhancement of the tensile strength from 1055 MPa to 1250 MPa and elongation from 33% to 45%. This strategy may provide new guidance for the development and alloy design of high-performance medium Mn steels.

Formation of deformation induced martensite in fatigued 316L steels manufactured by selective laser melting (SLM)

Destabilization of originally fully austenitic structure and formation of DIM (deformation induced martensite) were studied in SLMed 316L steels fabricated using different manufacturing systems and fatigued at room temperature in as-built state in two laboratories (TU Kaiserslautern and IPM Brno). Several microscopic techniques were adopted to reveal the distribution and morphology of DIM in the volume of fatigued steels. Results of microstructure investigations are confronted with the ferritescope measurements taken at different locations of fatigued specimens. Some factors which may impact the DIM formation in fatigued SLMed 316L steel are discussed.

Grain size dependent mechanical behavior and TRIP effect in a metastable austenitic stainless steel

The change in the trends of mechanical behavior and martensite formation of austenitic steels versus grain size has not yet been explained for the coarse-grained regime. Moreover, the effect of grain size needs systematic investigation by considering a wide range of grain sizes from the ultrafine-grained (UFG) regime to the coarse-grained one. In response, the effect of a wide range of austenite average grain sizes from 0.5 to 192 µm on the mechanical behavior and transformation-induced plasticity (TRIP) effect in an AISI 304L stainless steel was systematically investigated in the present work. Analysis of the tensile properties revealed that there is a transition grain size range of 34 to 90 µm, where a meaningful change in the trends of the ultimate tensile strength (UTS), total elongation, tensile toughness, work-hardening capacity, and yield ratio was observed. For instance, the total elongation increased with increasing average grain size up to the transition range but decreased at coarser grain sizes. However, the yield stress followed the Hall-Petch relationship of YS(MPa)=134.2+632.6/D(μm) for the whole grain size range, indicating that the presence of the transition grain size range is related to the work-hardening behavior, especially the TRIP effect. The apparent stacking fault energy decreased with increasing the grain size and reached a plateau after the transition grain size range. The same trends were found for the critical strain for the onset of the TRIP effect. Moreover, increasing the grain size up to this transition range promoted the formation of deformation-induced martensite, which was followed by its suppression at coarser grain sizes. The same trends were found for the maximum work-hardening rate related to the TRIP effect. The evolution of mechanical properties and TRIP effect with the grain size up to the transition range was explained based on the dependency of apparent stacking fault energy on the grain size. However, with the constancy of apparent stacking fault energy at coarser grain sizes, the decline in the formation of shear band intersections was found to play a significant role. The latter effect was further supported by Olson-Cohen analysis and investigation of the grain size dependency of the rate of shear-band formation (α) and the probability of generation of the martensite embryo at shear-band intersections (β).

Influence of Cold Working on Hydrogen Diffusion Behaviour in an Austenitic Steel with Transformation-Induced Plasticity

The main objective of this work was to obtain information about the hydrogen diffusion behaviour in a cold-worked austenitic stainless steel (X3CrMnNiMoN17-8-4) in which deformation-induced martensite formation occurs during mechanical deformation. Three different states of pre-deformation (31 %, 39 % and 49 %) that showed induced phase transformation from austenite to α’martensite as well as the solution-annealed material were part of this study. All samples were charged with hydrogen in a 0.1 M NaOH solution. This charging took place electrolytic with 10 mA cm-2 at three different temperatures (50 °C; 65 °C and 80 °C) in the double cell according to Devanathan and Stachurski. Due to the very slow diffusion of hydrogen through austenite, the samples were not charged until the equilibrium state was reached. To find out the necessary diffusion parameters, the data were fitted with numerical optimisation. Using this method, the effective diffusion coefficients of charging could be determined for all material states. The study also contains microscopic analyses to visualize the effect of cold working on the microstructure of the material. The appearance of α’-martensite significantly contributes to the susceptibility to hydrogen uptake leading to increasing diffusion coefficients in relation to higher pre-deformation.

Effective grain size refinement of an Fe-24Ni-0.3C metastable austenitic steel by a modified two-step cold rolling and annealing process utilizing the deformation-induced martensitic transformation and its reverse transformation

Metastable austenitic steels having ultrafine grained (UFG) microstructures can be fabricated by conventional cold rolling and annealing processes by utilizing the deformation-induced martensitic transformation during cold rolling and its reverse transformation to austenite upon annealing. However, such processes are not applicable when the austenite has high mechanical stability against deformation-induced martensitic transformation, since there is no sufficient amount of martensite formed during cold rolling. In the present study, a two-step cold rolling and annealing process was applied to an Fe-24Ni-0.3C metastable austenitic steel having high mechanical stability. Prior to the cold rolling, a repetitive subzero treatment and reverse annealing treatment were applied. Such a treatment dramatically decreased the mechanical stability of the austenite and greatly accelerated the formation of deformation-induced martensite during the following cold rolling processes. As a result, the grain refinement was significantly promoted, and a fully recrystallized specimen with a mean austenite grain size of 0.5 μm was successfully fabricated, which exhibited both high strength and high ductility.

Setting of deformation-induced martensite content in cryogenic external longitudinal turning

In metastable austenitic steels like AISI 304, martensitic surface layers can be created by cryogenic external longitudinal turning which results in a hardening of the surface. It is possible to identify the correlation between process parameters and the formation of deformation-induced martensite with machine learning methods. Based on this, evolutionary algorithms are used to determine the appropriate process parameters in order to achieve different defined martensite contents. In order to be able to even control the martensite content within the turning process, an eddy current sensor is integrated into the machine tool. In-situ measurements can be conducted and are presented here.

Characterization of deformation-induced martensite by cryogenic turning using eddy current testing

The formation of deformation-induced martensite upon cryogenic turning results in a substantial hardening of the surface. In this study, eddy current testing was used to gain a deeper insight in the microstructural evolution. Electromagnetic simulations were used to separate the various contributions to the measurement signal. The overall martensite content is strongly correlated with the measurement signal. However, the actual martensite structure also affects the signals. Thus, eddy current testing not only has the potential to control the evolution of the martensite content during the cutting process but also can provide information about the finer details of the martensitic microstructure.

Novel Fe-0.3Cr-0.4Mo-1.5Mn–3Ni-0.6C tool steel with superior properties under quasi-static and dynamic loading

In the present work, the correlation between microstructure and mechanical behaviour under quasi-static and dynamic loading of a novel designed high strength Fe-0.3Cr-0.4Mo-1.5Mn–3Ni-0.6C tool steel is presented. The alloy was manufactured under elevated cooling rates promoting a fine microstructure composed of martensite and retained austenite already in the as-cast state as detected via electron backscattered diffraction and X-ray diffraction. Due to this phase combination, a high compressive strength of about 3700 MPa combined with a total compressive strain of 24% can be observed as well as a pronounced work hardening. This behaviour was mainly traced back to deformation-induced martensite formation (transformation induced plasticity (TRIP) effect). Tests under dynamic loading conditions at a strain rate of 102 s−1, which are also of high importance for later tool applications, display no significant change regarding the compressive strength and total deformation. Furthermore, an increased compressive yield strength and a pronounced work hardening could be detected under dynamic conditions, which was also attributed to the TRIP effect as for the quasi-static tests. Thereby, a decreasing rate of martensite formation with increasing deformation could be observed for both states. Furthermore, adiabatic shear bands (ASB) were detected in compression samples under dynamic loading via scanning electron microscopy and the failure behaviour was discussed with a focus on adiabatic shearing (also including transmission electron microscopy investigations). Concluding, the newly developed cast Fe-0.3Cr-0.4Mo-1.5Mn–3Ni-0.6C steel shows no significant strain rate sensitivity, which is an outstanding property of cast tool steels.

The effect of thin surface layer of nitrogen-expanded austenite on bulk γ-α’ phase transformation in low-temperature deformation of 316L stainless steel

Deformation-induced γ-α’ phase transformation was studied in austenitic stainless 316L steel in low-temperature deformation regime (tension at 77 K). Before deformation, the surface of the steel specimens was saturated with nitrogen by ion-plasma nitriding at 673 K for 12 h. Thin 4-μm surface layer of nitrogen-expanded austenite (S-phase) does not influence mechanical properties but affects phase transformation in the bulk 1.5 mm-thick specimens. Surface nitriding effectively suppresses the inhomogeneity in deformation-induced martensite nucleation and formation of the α’-phase prior the macroscopic neck formation.

Effect of solution annealing on low cycle fatigue of 304L stainless steel

The present study investigates low cycle fatigue behaviour of the metastable austenitic 304L steel in two variants: i) as-received and ii) after additional solution annealing at 1050 °C/30 min. The applied heat treatment resulted in a decrease of δ-ferrite volume fraction and slight grain coarsening. After the solution annealing, significantly prolonged fatigue life within the whole range of studied total strain amplitudes was observed. Such improvement is attributed to the larger grain size possessing better ductility and enhanced formation of deformation-induced martensite. The phase transformation led to a significant secondary cyclic hardening stage at high strain amplitude loading. The low cycle fatigue behaviour and principal fatigue life stages were characterized by the analysis of cyclic response. A vital supplementary data were obtained by the analysis of hysteresis loop shape carried out, for the first time on a phase-transforming material, by the generalized statistical theory. The analysis was able to follow the evolution of cyclic stress sources during the fatigue life, especially the increasing contribution of deformation-induced martensite. The identified features of cyclic plasticity were supported by electron channeling contrast imaging observations. Furthermore, electron backscatter diffraction revealed a distinct preferential localization of α'-martensite into macrobands parallel to rolling direction. Energy dispersive spectroscopy mapping identified pronounced chemical segregation of Cr and Ni which correlated well with the observed macrobands. These compositional fluctuations led to local austenite metastability which is enhanced further by the applied solution annealing.

Correlation between mechanical stability and hardness of austenite in martensite/austenite mixtures

The hardness of austenite in Fe−1.5%Mn−(0.44–1.46)%C steels with microstructures consisting of austenite and martensite was estimated by an extrapolation procedure. The procedure involved heat treatments to obtain different fractions of athermal martensite. For each carbon concentration, the variation of hardness with martensite fraction was non-linear. Linear and inverse rules of mixtures could not account for the observed hardness variations, which was justified by the metastability of austenite. The hardness variations could be described by exponential fits, which were subsequently extrapolated to zero percent martensite to obtain the hardness of austenite. The variation of extrapolated hardness with carbon concentration in the range of 0.44–1.46% indicated maximum hardness at intermediate carbon concentrations. This behavior was interpreted in the context of the influence of carbon on the mechanical stability of austenite and on the hardness of the product martensite. On this basis, the lowest extrapolated austenite hardness was obtained in the steel with the highest carbon concentration due to the suppression of the deformation-induced martensite formation.

Effect of Mo addition on aging behavior of TRIP-aided duplex stainless steel

The effects of Mo addition and an aging heat treatment on microstructural evolution and transformation-induced plasticity (TRIP) in cold-rolled lean duplex stainless steel (DSS) samples were investigated. 21Cr lean DSS with 0–2% Mo was cold-rolled into 1-mm-thick plates and solution-treated at 1050 °C for 5 min. With increasing Mo content, a smaller thermal α′ phase formed after solutionization. During the subsequent aging treatment, redistribution of alloying elements, such as chromium (Cr), nickel (Ni) and Nitrogen (N), occurred in the α and γ phases, which resulted in local segregation and precipitation of Cr2N in the α/γ phase boundary. In turn, this led to a decrease in the stability of the γ phase, allowing deformation-induced martensite transformation (DIMT) to occur more easily. Due to the instability of the γ phase, the fraction of the thermal α′ phase increased with aging. Steels with a higher Mo content exhibited a more stable γ phase, which suppressed the formation of deformation-induced martensite (DIM). The effect of DIM (caused by TRIP) on the strength and elongation of the steels according to the fraction of the thermal α′ phase was evaluated.

Critical assessment of the strain-rate dependent work hardening behaviour of AISI 304 stainless steel

Abstract In the present work, the effect of strain rate on the micro-mechanism of plastic deformation and ensuing work hardening behaviour of AISI 304 stainless steel has been critically investigated. The defining role of the early stage of deformation ( γ → α ′ in 3% strained specimen at HSR was higher contributing to lower transition strain from stage 1 to 2 of work hardening compared to SSR, the extent of work hardening was higher in the latter case during the stage 2 due to a combined effect of extensive twinning and deformation induced martensite formation, which was limited in HSR due to occurrence of cross-slip. These differences in the work hardening mechanisms were manifested as an improvement in UTS/YS ratio from 2.66 to 2.99 in HSR and SSR specimens, respectively. Furthermore, changes in the mechanism of plastic deformation was also reflected as ~4 fold increase in dislocation density for SSR specimen compared to HSR specimen at any given strain level and nearly 27% increase in hardness at the point of fracture. A significant improvement in UTS/YS ratio and ductility for SSR specimen left its signature on the fracture surface in form of three-fold increase in the number density of finer dimples compared to HSR specimen.

Strain rate and mechanical stability in determining deformation behavior of beta Ti alloys

The effect of strain rate on flow behaviors of three beta Ti alloys with different beta phase stability was studied. The alloys were subjected to compression deformation at four strain rates ranging from 7 × 10−5 to 7 × 10−2 s−1 at room temperature. Microstructure evolution was characterized using laser scanning microscopy and electron backscattered diffraction methods. The effect of strain rate on the yield strength was found to vary between the alloys increasing with increased stability. The microstructural observations showed that the alloys exhibited different deformation mechanisms: deformation induced martensite formation, twinning and slip band formation. The strain hardening exponent (n) depended on the deformation mechanism, i.e., the stability. In the low and intermediate stability alloys showing martensite formation and combination of the deformation mechanisms respectively, n was about 0.07 independent of strain rate. In the alloy with the highest stability it was only 0.01 when the deformation mechanism was dislocation slip at low strain rates but increased significantly to ≈0.13 at high strain rates when also twinning occurred. The strain rate sensitivity factor (m) was about 0.014 and 0.017 for the low and intermediate stability alloy respectively independent of strain, but very low ≈ 0.002 for the highest stability alloy though increasing to ≈ 0.011 with increasing strain. Based on the experiments, the stability-strain rate-mechanism diagram was introduced to predict the effect of phase stability and strain rate on the deformation mechanism of beta Ti alloys.

Hydrogen-assisted cracking in 2205 duplex stainless steel: Initiation, propagation and interaction with deformation-induced martensite

The present work evaluates hydrogen-assisted cracking in 2205 duplex stainless steel by performing electron backscatter diffraction (EBSD) on in-situ hydrogen charged tensile tested specimens. The hydrogen embrittlement sensitivity increases with the electrochemical hydrogen charging time. The hydrogen presence results in deformation-induced martensite formation in the austenite during in-situ tensile straining, which starts immediately after yielding. Hydrogen-assisted cracking starts at a later stage in the tensile test and the cracks mainly initiate in austenite. The deformation-induced martensite is not vulnerable for crack initiation, neither is its interface with the austenite matrix. The presence of hydrogen-assisted cracks depends on the strain level reached in the specimen and is therefore most pronounced in the necked region. Crack propagation occurs through austenite, ferrite and their interface and is mainly governed by strain fields. Finally, propagation also occurs by coalescence of several smaller cracks.

Simultaneous enhancement of stress- and strain-controlled fatigue properties in 316L stainless steel with gradient nanostructure

A gradient nanostructured (GNS) surface layer with full austenitic phase was synthesized on AISI 316L stainless steel by surface mechanical rolling treatment at ∼280 °C. The mean grain size is ∼45 nm at the top surface and increases gradually with depth. Deformation-induced martensite (DIM) transformation was suppressed and the microstructural refinement was dominated by dislocation activities and twinning during the formation of the GNS layer. Axial tension-compression fatigue tests showed that fatigue strength and life are simultaneously enhanced in the GNS samples relative to the corresponding coarse-grained counterparts in both stress- and strain-controlled tests. This is very different from fatigue behavior of conventional nanostructured materials, i.e. an enhanced stress-controlled fatigue strength with a decreased strain-controlled fatigue life. Besides contributions from the enhanced mechanical properties and the suppressed surface defects formation, analyses of fatigue mechanisms demonstrated that the promoted formation of DIM during cyclic strain plays a crucial role in enhancing fatigue properties of the GNS samples in strain-controlled tests.

Laminated TRIP/TWIP Steel Composites Produced by Roll Bonding

In order to investigate the roll bonding of high-alloy transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) steel, roll-bonded sheets of the TRIP and TWIP steel were manufactured starting from hot rolling, followed by brushing and cold rolling. Both, the microstructure and mechanical properties of the roll-bonded sheets were characterized by metallographic investigations, and tensile and T-peel tests. Preliminary results, such as an occurrence of an adhesive bonding between two TWIP steel sheets and between TRIP and TWIP steel sheet after a thickness reduction of approximately 50% were obtained. Moreover, the formation of deformation-induced martensite leads to outstanding mechanical properties of the roll-bonded composite sheet. An ultra-fine grained microstructure was observed in the bonding zone after only one roll-bonding process. The obtained promising results demonstrate the possibility of the development of an accumulative roll-bonding process for TRIP/TWIP steel composites.

Effect of Pipe Diameter on Electrochemical Behavior of Stainless Steel Type 304 Pipes in Tap Water

We investigated the effects of pipe diameter on the corrosion resistance of stainless steel type 304 pipes using electrochemical measurements. Compared to plate steel, pipes have harder physical properties and tend to be harder and showed greater permeability with decreasing inner diameter. We found that the maximum corrosion current density in the secondary active state, which is the starting point of secondary passivation, appeared in the polarization curve measurement in tap water. Similar to the Vickers hardness and the maximum current density in the secondary active state, the permeability tended to increase as the diameter decreased. This is thought to increase the amount of deformation-induced martensitic and increase corrosion susceptibility. The peak of the secondary active current density was clearly seen as the potential sweep speed was increased. In addition, potential sweep speed dependence was observed in the corrosion susceptibility evaluation of deformation-induced martensite. In comparison with acid treatment, the formation of deformation-induced martensite was considered to occur in the extreme surface layer. The maximum corrosion current density in the secondary active state is expected to be a new susceptibility evaluation method for evaluating the deformation-induced martensitic transformation.