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The influence of Laser shock peening without coating at a cryogenic temperature (CLPwC) on SS 304 alloy was investigated. Three different laser energies, such as 150 mJ, 300 mJ, and 450 mJ were chosen to investigate the effects of CLPwC in SS 304 steel. The impact of CLPwC on the residual stress, surface roughness, hardness, phase transformation and microstructural changes was studied. The XRD studies show the peak broadening and strain-induced martensite (γ→α′) of 36.3% after CLPwC at 9 GW cm−2. The work-hardening effect was observed up to 300–400 μm depth from the peened surface. The microhardness could effectively increase to 309.43 HV at 9 GW cm−2 when it compared to unpeened hardness (192.1 HV). Relaxation of compressive residual stress at cryogenic temperature was distinguished, and the maximum compressive stress of −138 MPa was obtained at 50 μm from the surface. Microstructural studies were carried out by different techniques such as EBSD and TEM. An increase in low angle grain boundary fraction (43%) and grain size reduction was observed using EBSD after CLPwC. The TEM investigations revealed deformation twins (width of 6–70 nm), shear bands, stacking faults, and martensitic phase. Strain induced martensitic phase is witnessed in XRD, EBSD, TEM studies.

The present research is part of a project that includes several grades of high‑manganese steels with different chemical compositions and various pre-treatment parameters. The main objective of this work was to determine favourable conditions for the occurrence of strain-induced phase transitions in unstable Fe-20Mn-3Al-3Si steel. After preliminary treatment the steel showed a two-phase microstructure consisting of austenite matrix and 29% of ferrite volume fraction. The deformation process of the examined steel was strongly influenced by the low stacking fault energy value of austenite matrix. Initially, the slip of dissociated dislocations led to the formation of numerous bands of stacking faults. Then, the phase transformations induced by plastic deformation took place, i.e., γ (fcc) → ε (hcp) and subsequently ε (hcp) → α' (bcc). The beginning and further progress of these phase transitions depended primarily on the crystallographic orientations of the austenite grains. The more favourable the orientation of the grains with respect to external stresses, the more effective was the activation of dislocation slip and the greater number of deformation bands was formed. These processes directly resulted in a locally induced phase transition γ → ε. Simultaneously, in grains showing the formation of ε-bands in at least two intersecting systems, the transformation ε → α' gradually occurred. • Unstable austenite microstructure in two-phase Fe-Mn-Si-Al steel subjected to cold rolling. • Appearance of deformation induced transformations γ (FCC) → ε (HCP) and γ (FCC) → ε (HCP) → α' (BCC). • Occurrence of phase transformations in grains with the most favourable orientations for slip activation. • Development of preferential Shoji-Nishiyama and Kurdjumov-Sachs relationships between the phases. [ABSTRACT FROM AUTHOR]

The present paper investigates the effect of cold working and electrochemical nitridation on corrosion resistance of 316L austenitic stainless steel (ASS). The X-ray diffraction results showed that the strain-induced martensite is formed on cold work. Also, the volumetric fraction of strain induced martensite and dislocation density increased with increase in degree of cold work. The cold working process adversely affects the corrosion resistance of 316L ASS due to the formation of the strain-induced martensite and increased dislocation density. On electrochemical nitridation, the electrochemical results show the formation of stable passive film, thus improving the corrosion resistance of 316L ASS attributed to the formation of nitrides on the surface. Moreover, the beneficial effect of electrochemical nitridation is more pronounced in cold worked samples, thus the adverse effects of cold working can be overcome by electrochemical nitridation. [ABSTRACT FROM AUTHOR]

This work presents an optimized thermo-mechanical processing route to produce ultrafine grained AISI 304 L stainless steel with enhanced strength. Here AISI 304 L was rolled to 93% thickness reduction at room temperature to form 70% strain induced martensite with the remaining phase being untransformed deformed austenite. Subsequently, this was subjected to a thermal treatment in the range of 650–750 °C with time ranging from 10 to 1440 min so as to revert the strain induced martensite to ultrafine grained austenite. The median grain size obtained after different reversion treatments varied from 0.42 to 1.2 μm. The major texture components observed after reversion at 650 and 700 °C were {110}〈112〉 Brass and {123}〈634〉 S, whereas those observed after reversion at 750 °C were {110}〈112〉 Brass, {123}〈634〉 S and {113}〈112〉 R. Tensile tests showed a higher strength (but lower ductility) along the transverse direction as compared to that along the rolling direction. The yield strengths obtained after reversions at different temperatures and times were correlated with grain size, dislocation density and texture. A combination of high tensile strength and good ductility was obtained in the rolling direction after reversion at 700 °C for 60 min (900 MPa and 45% ductility) and at 750 °C for 10 min (900 MPa and 55% ductility). • Ultrafine grained AISI 304 L was produced by reversion of strain induced martensite. • Reversion at 650 and 700 °C showed {110}<112 > Brass and {123}<634 > S texture components. • The transverse direction showed a high strength as compared to the rolling direction. • High strength is correlated to grain size, dislocation density and texture. • Strength of 900 MPa and 45–55% ductility was achieved along the rolling direction in tensile test. [ABSTRACT FROM AUTHOR]

The present work investigates the microstructural changes in an AISI 304L austenitic stainless steel during the early stages of tensile deformation (where austenite does not transform to strain induced martensite). In situ tensile experiments were conducted to record grain orientation changes and slip activation in the steel. The effect of grain size, neighboring grains, and annealing twins on orientation changes during deformation was investigated. Results showed that at a given strain level, grains lying in relatively softer regions and possessing higher Schmid factor values accommodated the plastic deformation initially and showed orientation changes toward the stable orientation. The relatively larger grains changed their orientations only at higher strain levels. Grain orientation changes were also influenced by size and crystallographic orientation of neighboring grains. For grains containing annealing twins, the orientation changes of twin and its grain were in different directions during deformation at a given strain level. Further, grains containing multiple twins showed delayed deformation. The study of tensile deformation behavior in this respect opens up new routes to alter and hence enhance the mechanical properties of materials by engineering their microstructure. [ABSTRACT FROM AUTHOR]

The wear behavior of a non-standard cast cobalt chromium alloy (Co30Cr19Fe), during vibratory cavitation test, was investigated. Low stacking fault energies (SFE) cobalt alloys deform by complex plastic deformation mechanisms, which enhance the cavitation erosion (CE) resistance, since the onset of localized stress, leading to fatigue failure and material removal, is delayed. The purpose of this work is to characterize the main operating deformation mechanisms during cavitation erosion testing of the non-standard Co30Cr19Fe alloy. As cast, solution treated, and 15% and 30% cold rolled specimens were tested. The as cast microstructure consisted of ~2 mm fcc alpha grains and hcp epsilon-martensite. The 1473 K solubilization treatment led to primary recrystallization and formation of 250 µm new grains. The 15% cold worked microstructure consisted of heavily deformed fcc phase containing deformation twins and epsilon-martensite. Ultrasonic cavitation testing was carried out, during 40 h, according to ASTM G32-09. The solubilized specimens presented the worst behavior, whereas, the 30% cold worked specimens were the most CE resistant. The CE results are discussed based on the microstructural parameters: amount of alpha fcc and epsilon-hcp phases, grain sizes, relative amounts of twinning, slip lines and strain induced martensite formation. • Cavitation erosion resistance of Co30Cr19Fe alloy was determined. • Cold working was the most important mechanism improving wear resistance. • Wear mechanisms of Co30Cr19Fe alloy are dominated by plastic deformation. • Strain induced martensite also contributes to improve CE wear resistance. [ABSTRACT FROM AUTHOR]

Abstract A complete understanding of 'cyclic plasticity induced transformation' of austenitic stainless steels is documented in this article through different experiments, their critical analysis and comprehensive literature review. The formation and nucleation micro–mechanisms of deformation induced martensites (i.e., ϵ (hcp) and α / (bcc)) have been thoroughly examined by analytical transmission electron microscope after cyclic plastic deformation of AISI 304LN austenitic stainless steel at various total strain amplitudes (Δ ϵ t) under ambient environment. Magnetic measurements are performed for all fatigue failed specimens at different strain amplitudes. The measured characteristic of magnetization is sensitive to employed strain amplitude, corresponding deformed microstructural features and cyclic plastic response of the material. As strain amplitude increases, the extent of α / (bcc) martensite increases. It has also been found that deformation induced martensites can nucleate at a number of sites through multiplicity of mechanisms with different transformation sequences, i.e., γ (f c c) → ϵ (h c p) , γ (f c c) → ϵ (h c p) → α / (b c c) , γ (f c c) → deformation twin → α / (b c c) and γ (f c c) → α / (b c c). It is also investigated that these martensites can nucleate at a number of sites, such as: shear band intersections, isolated shear bands, grain boundary triple junctions, shear band–grain boundary intersections, dislocation piled–up etc., wherever total interaction energy favours to form. Deformation induced dislocation sub–structures are also characterized to show that cyclic plasticity towards higher strain amplitude loading results in the organization of more uniform dislocation cell structure. Dislocation sub–structure evolution (i.e., arrays, planar, veins, patches, ladders, wall, labyrinth, maze and cell) is also used to explain the shape of hardening/softening curves of the material for all strain amplitudes. The proposed study also quantitatively connects the strain amplitude dependency of the formation of deformation induced martensite and corresponding organizations, configurations and patterning of dislocation sub–structures. Two-dimensional striation spacing and crack density quantified from the fatigued fracture surfaces are also observed to predict the nature of variation in cyclic plastic response of the material as a function of strain amplitudes. Graphical Abstract [ABSTRACT FROM AUTHOR]

Nonlinear Rayleigh wave is allowed to propagate through cyclically damaged 304LN stainless steel for the characterization of microstructure evolved during plastic deformation. SS304LN is widely used in nuclear and petrochemical industries in piping components, heat exchangers etc. to withstand damages arising due to low cycle fatigue, thermal aging, corrosion etc. An initial monochromatic ultrasonic Rayleigh wave when interacts with degraded microstructure in material, distorts and generates higher order harmonics. Interrupted low cycle fatigue tests were carried out at different strain amplitudes and at each interruption, the distortion in ultrasound wave was measured by evaluating nonlinearity parameter which is the ratio of the amplitude of the second harmonic to the square of the amplitude of the fundamental frequencycomponent till failure. This study shows the effectiveness of β to characterize the microstructural evolution as well as the generation of strain induced α'-martensite in meta-stable 304LN stainless steel during cyclic deformation and fatigue damage accumulation. • Nonlinear Rayleigh surface wave has been used to evaluate the deformation behavior in SS304LN during low cycle fatigue. • Controlled level of fatigue deformation has been done at three strain amplitudes namely, ±0.4%, ±0.6% and ± 0.8% in this material. • Relative change in nonlinearity parameter (β΄) using propagating Rayleigh wave has been evaluated for the interrupted fatigue damaged samples at each strain amplitude. • Two peaks have been observed in the variation of β΄ with fatigue cycles; first peak corresponds to dislocation evolutions and the 2nd peak corresponds to generation of strain fields due to shear transformations from austenitic structure to martensitic structure. • β΄ is seen to be sensitive to microstructural evolution as well as the generation of strain induced α'-martensite in this meta stable SS304LN. [ABSTRACT FROM AUTHOR]

Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS. [ABSTRACT FROM AUTHOR]

The three-body abrasive wear behavior of three ferrous alloys with different microstructures but similar hardness has been investigated using a standard dry-sand rubber wheel test (ASTM G65-16). Although the hardness of the alloys was similar, the abrasion rates are radically different owing to differences in abrasion mechanisms in their microstructures. A carbide-free bainitic steel having fine bainitic laths exhibited better abrasion resistance owing to the strain- induced transformation of austenite into martensite. Nano-scaled martensitic laths that formed on the surface resisted plastic deformation during abrasion and thereby increased the abrasion resistance. The microstructure containing bainitic ferrite undergoes extensive plastic deformation associated with a large quantity of dislocations, which in turn accommodates the strain of abrasion. The steel with a blocky-martensitic microstructure had the least wear resistance. In that case, fragmentation and chipping comprised the prominent abrasion mechanism. The degeneration of a martensitic structure and its subsequent tempering radically reduces the hardness at the surface and makes it more susceptible to abrasion. Steel containing a mixture of bainite and martensite had intermediate abrasion resistance relative to the other two alloys. [ABSTRACT FROM AUTHOR]

The influence of strain state on the kinetics of martensitic phase transformation in stainless steel 304 is investigated. The methodology involves stretching various specimens with different widths over a hemispherical punch. The specimens are stretched to various depths by controlling the punch traveling in order to achieve different strain levels. The volume fraction of strain induced martensite is then measured using ferrite scope and plotted versus strain for different strain states. The results show that the martensitic transformation curve has the lowest value for plane strain condition among strain states used for determination of forming limit diagram. There are a close proximity between the two sets of the experimental data and those of the modified Olson‐Cohen and Santacreu's models. [ABSTRACT FROM AUTHOR]

In a rail track structure, a turnout is used as a transition between straight and branching rails and therefore withstands complex dynamic rolling and impact loading. This paper reports a comprehensive failure investigation of a railway off-track turnout made from Hadfield austenitic cast steel. The worn turnout was examined using optical microscopy, scanning electron microscopy, micro-hardness, and quantitative X-ray diffraction analyses. A ball-on-disc sliding wear test was also applied to evaluate the impact of work-hardening on the wear property. The turnout exhibited surface embrittlement, severe cracking, delamination, and spalling characteristics. Severe plastic deformation of the worn turnout was observed with the formation of densely packed mechanical twins, which facilitated crack propagation. The rail top developed a gradient hardness (HV 0.1) profile from the rail top of 8.9 GPa, the cracked subsurface of 5.9 GPa to the bulk steel of 2 GPa. The rail top was also found to have accumulated an extremely high residual compressive stress of 500–650 MPa. Strain induced martensite transformation was repeatedly evidenced by X-ray diffraction peak of the ferritic (110) plane although the highly strained and nanocrystallised ferrite was only detected on the severely deformed rail top. Comparative sliding wear tests showed highly deteriorated wear resistance of the strain-hardened rail top as compared to the bulk steel. The failure investigation of a railway turnout was carried out by microscopy, X-ray diffraction, hardness testing, and wear testing: • Extensive brittle spalling failure, cracking and deformation found in a depth of 2 mm. • High-density twins observed in the deformed depth, along with XRD-detected tetragonal martensite in the rail top. • Extreme work-hardening to HV 6–9 GPa comparing to 2 GPa of the bulk steel. • Deformation induced residual compressive stress of 500–650 MPa. [ABSTRACT FROM AUTHOR]

The present study deals with the grain refinement of a high nitrogen duplex stainless steel through cold rolling up to 80% and subsequent reversion annealing in the range of 750 °C to 1100 °C at various duration times. The microstructural observations showed that the nano/ultrafine-grained (NG/UFG) austenite with the grain size of 400 nm was obtained by annealing at 900 °C for 30 s due to ά-martensite to austenite complete reverse transformation. The NG/UFG duplex steel indicated a superior mechanical properties consist of 1 GPa yield strength (YS) and 40% elongation to fracture. The transmission electron microscopy study of the tensile deformed NG/UFG material indicated that the nature of extraordinary plasticity in NG/UFG material is because of the dense planar defects, specifically, the twin/twin-like induced plasticity effect in austenite, accompanied with strain accommodation of coarse grain ferrite phase. [ABSTRACT FROM AUTHOR]

In this work, microstructure and sulfide stress corrosion cracking (SSCC) of 304 stainless steel (SS) pre-strained at cryogenic and room temperature were studied. 304 SS exhibits extremely high SSCC susceptibility, which increases as pre-strain rises for the acceleration of AD and HE. Cryogenic pre-strained specimens present higher SSCC resistance. Pre-strain leads to the formation of strain-induced ε and α' martensite and increase of crystal defects such as deformation bands, twins, stacking faults and dislocation, which accelerates SSCC. Cryogenic deformation produces more martensite, and the uniform occurrence of microstructure evolution only plays a limited role in SSCC. • The microstructure of 304 SS strain-strengthened at cryogenic/ambient temperature was studied. • The SSCC of strain-strengthened 304 SS was comprehensively evaluated. • Cryogenically pre-strained 304 SS is less susceptible to corrosion and SSCC benefiting from uniform microstructure evolution. [ABSTRACT FROM AUTHOR]

Direct γ→α′ martensitic transformation during cold rolling deformation was investigated for a high-Mn non-magnetic steel. Its influence on magnetic property was also analyzed. The magnetization under rolling reduction less than 50% almost presents a linear increase with the applied magnetic field. With deformation up to 73% and 93% thickness reductions, strain induced α′-martensite transformation starts to occur, causing the steel to be slightly magnetized. The α′-martensite prefers to nucleate directly at either microband–microband or microband-twin intersections without participation of intermediate ε-martensite. The volume fraction of α′-martensite is estimated as 0.070% and 0.17%, respectively, based on the magnetic hysteresis loops. Such a small fraction of ferromagnetic α′-martensite shows little influence on the magnetic induction intensity and low relative permeability. [ABSTRACT FROM AUTHOR]

Strain-induced martensite (SIM) and its reversion in a cold-rolled AISI 201 austenitic stainless steel was studied by means of magnetic properties, light optical (LOM) and scanning electron (SEM) microscopy, electron backscatter diffraction (EBSD), texture measurements, and Vickers microhardness testing. According to Thermo-calc© predictions, the BCC phase (residual δ -ferrite and SIM) is expected to be stable until 600 °C. The current material was cold rolled up to 60% thickness reduction and submitted to both isothermal and stepwise annealing up to 800 °C. Magnetic measurements were taken during annealing ( in situ ) of the samples and also for their post mortem conditions. The Curie temperatures ( T c ) of residual δ -ferrite and SIM have similar values between 550 and 600 °C. Besides T c , the focused magnetic parameters were saturation magnetization ( M s ), remanent magnetization ( M R ), and coercive field ( H c ). SIM reversion was found to occur in the range of 600–700 °C in good agreement with Thermo-calc© predictions. The microstructures of the material, annealed at 600 and 700 °C for 1 h, were investigated via EBSD. Microtexture measurements for these samples revealed that the texture components were mainly those found for the 60% cold rolled material. This is an evidence that the SIM reversion occurred by an athermal mechanism. [ABSTRACT FROM AUTHOR]

Severe shot peening is a simple and effective method to synthesize nanocrystalline layer on metallic surfaces. A new reactor to synthesize thicker nanocrystalline layers on metallic surfaces by shot peening is presented. In this cylindrical reactor air enters inside the reactor tangentially, propels shots on the specimen to be shot peened and leaves the reactor after multiple rotations. Air vortex and very high turbulence formed inside the reactor dissipate heat generated during shot peening and thus suppress dynamic recovery which promotes formation of refined and thicker nanocrystalline structure on a metallic surface. Experiments were conducted on 5specimens of austenitic stainless steel with different shot velocities in the reactor. Characterization (X-ray diffraction profiles, cross-sectional optical micrographs, TEM and micro-hardness profiles) of shot peened surfaces showed that thicker nanocrystalline/nano-twinned layers were developed during all peening conditions used in the experiments. Microstrain in the lattice, dislocation density and volume fraction of strain induced martensite observed to increase with rise in shot velocity. Specimens with thicker nanocrystalline layers were prepared. These specimens contained high amount of strain induced martensite deep down the surface. Plasma nitriding of these pre-shot peened specimens were done at 425 °C and 500 °C for six hours. Presence of thicker nanocrystalline layer with high amount of martensite at the surface of austenitic stainless steel observed to have synergetic effect and significantly influenced kinetics of nitriding process leading to higher surface hardness and 6-fold thicker nitrided layers compare to untreated nitrided surface. Presence of nanocrystalline structure, however, also effects on precipitation of chromium nitrides during nitriding. [ABSTRACT FROM AUTHOR]

Hydrogen embrittlement (HE) together with the hydrogen transport behavior in hydrogen-charged type 304 stainless steel was investigated by combined tension and outgassing experiments. The hydrogen release rate and HE of hydrogen-charged 304 specimens increase with the hydrogen pressure for hydrogen-charging (or hydrogen content) and almost no HE is observed below the hydrogen content of 8.5 mass ppm. Baking at 433 K for 48 h can eliminate HE of the hydrogen-charged 304 specimen, while removing the surface layer will restore HE, which indicates that hydrogen in the surface layer plays the primary role in HE. Scanning electron microscopy (SEM) and scanning tunnel microscopy (STM) observations show that particles attributed to the strain-induced α′ martensite formation break away from the matrix and the small holes form during deformation on the specimen surface. With increasing strain, the connection among small holes along {111} slip planes of austenite will cause crack initiation on the surface, and then the hydrogen induced crack propagates from the surface to interior. [ABSTRACT FROM AUTHOR]

The coupled temperature-strain rate sensitivity (SRS) of Ti-29Nb-13Ta-4.6Zr β-type Ti-alloy has been studied. The martensitic transformation is identified as the main reason of negative rate sensitivity during deformation at room temperature and 100°C. In addition dynamic strain aging (DSA) is in charge of further decrease in SRS values at temperatures higher than 100°C. The SRS values start increasing beyond 200°C due to dissipating the DSA effects. This finally reaches the positive domain due to domination of dislocation slip. [ABSTRACT FROM AUTHOR]

Abstract: This rapid communication presents a novel approach for controlling the material property evolution for austenitic stainless steels during incremental sheet forming (ISF), offering a route to affect the strength and ductility of the parts produced. The method relies on the modelling and control of the formation of strain induced martensite. [Copyright &y& Elsevier]

[Display omitted] • Fatigue behavior of the thick electron beam welded titanium alloy joint was studied from high cycle to very high cycle domain. • Site specific deformation microstructures were analyzed using FIB and TEM. • Cracking grain boundary and slip-driven fatigue crack formation mechanisms were unraveled. • Critical parameters of porosity defects in assisting very high cycle fatigue crack initiation were clarified. High cycle and very high cycle fatigue (VHCF) crack initiation mechanism of TC17 alloy joint with heterogeneous microstructures in FZ has been studied at the stress ratio of 0.1. Intrinsic deficiencies of slip deformation in β grains with few martensites and cracking coarse grain boundary between prior β grains with martensite colonies account for the high cycle fatigue (HCF) of joints. The average diameter of intergranular welding pores regarding VHCF crack nucleation is measured to be ∼36 μm. The driving force governing the crack nucleation from intrinsic deficiencies in HCF to porosity defects in VHCF is ∼1.66 MPa·m1/2. [ABSTRACT FROM AUTHOR]

Deformation metastability of chromium-nickel steels of austenitic and austenitic-ferritic classes is studied directly during specimen extension in a magnetic measurement facility. Structural changes and their correlation with steel magnetic properties are analyzed. [ABSTRACT FROM AUTHOR]

Austenitic stainless steels represent a significant portion of the alloys used in the aeronautical, chemical, shipbuilding, food processing and biomechanical industries. They combine good mechanical properties with high corrosion resistance. When subjected to cold deformation, these steels exhibit a metastable phase called: strain induced martensite (ferromagnetic), whose formation increases mechanical strength and formability, allowing for a wide range of applications. Heated from room temperature, the strain induced martensite transforms to austenite (non-magnetic). It is easy to find information in literature about the strain induced martensite for 18Cr/8Ni austenitic steels, but there is no data for high nickel alloys like A286 (26Ni, 15Cr), Incoloy 800 (30-40 Ni, 21Cr) and Inconel (50Ni, 19Cr). Therefore, this study aimed to verify the formation of strain induced martensite after cold working in Fe-18Cr base alloys with the addition of up to 60 %Ni. The reversion of this phase to austenite after annealing up to 600 °C was also studied. Optical microscopy, magnetic characterization tests, and x-ray diffraction were used to analyze the transformations. [ABSTRACT FROM AUTHOR]

Abstract: A near-Ni-free, Mn–N bearing duplex stainless steel (D-SS) that shows transformation induced plasticity was developed. The present D-SS exhibited an excellent strength–ductility combination over 1000MPa tensile strength and 50% elongation. An analysis of the element partitioning during annealing revealed that the stacking fault energy of austenite was low enough for a strain induced martensite (SIM) transformation to occur. The strain hardening rate began to increase at ∼10% strain with the same manner of SIM fraction. The TEM and EBSD analyses showed that not only the ɛ martensite band intersections but the austenite grain boundaries acted as the SIM nucleation sites. The SIM transformation was saturated because of the austenite grain refinement and the corresponding austenite stabilization. The austenite grain refinement was caused by the mutual impingement of growing SIM and as a result by the engulfment of remaining austenite by SIM. The deformation behavior of the present D-SS was characterized by analyzing the kernel average misorientation (KAM) of the constituent phases with strain. The KAM distribution of austenite, ferrite and SIM exhibited different characteristics. The average KAM of austenite and ferrite increased as the strain increased, but its increasing rate of austenite was higher than that of ferrite. These KAM characteristics were discussed along with the dislocation glide modes of austenite and ferrite. By contrast, the average KAM of SIM was insensitive to strain and higher than that of the other two phases. [Copyright &y& Elsevier]

Abstract: Near Ni-free duplex stainless steels (D-SS) containing the different N contents of 0.1–0.35wt.% were tensile-tested. Microstructure of D-SSs consisted of a contiguous band structure of coarse ferrite and fine austenite. The austenite fraction increased almost linearly as the N content increased. All D-SSs exhibited transformation induced plasticity (TRIP), resulting in an enhancement of both strength and ductility, especially over 1GPa and 60% for the 0.35 N D-SS. Strain induced martensite (SIM) that causes TRIP nucleated at not only in-grain ɛ martensite band intersections but austenite grain boundaries (AGBs) and annealing twin boundaries (TBs). Growth of SIM at AGBs and TBs was found to be faster than that of in-grain SIM. The apparent SIM fraction increased as the N content increased (correspondingly, the austenite fraction), leading to a better strain hardening. The overall strain hardening was dominated by the amount of accumulated SIM and the strain hardening of the remaining untransformed austenite, not the SIM transformation rate. Analysis using the SIM fraction normalized to the initial austenite fraction revealed that the SIM transformation was delayed as the N content increased due to the austenite stabilization increase. Also, it was found that regardless of the austenite fraction, there is a critical SIM fraction showing the transition of the flow mechanism of remaining untransformed austenite. [Copyright &y& Elsevier]

Abstract: The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material’s susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material’s ductility compared to other alloying elements. [Copyright &y& Elsevier]

Abstract: The effects of nitrogen addition on the strain induced martensitic transformation (SIMT) behavior of duplex stainless steel (D-STS) with the low nickel content were examined in a wide range of strain by means of cold rolling. Nitrogen of 0.1, 0.2 and 0.3wt.% was added into Fe–20Cr–5Mn–0.2Ni D-STS (in wt.%) and cold rolling was conducted up to the effective strain of ∼1.45 after annealing at 1000°C for 30min. In the as-annealed state, the austenite fraction increased with increasing the N content. Regardless of the N content, the ferrite grain size was coarser than that of austenite. The stacking fault energy of austenite of the present D-STSs inferred by the element partitioning analysis was low enough so that SIM transformation is available. Accordingly, during cold rolling, SIMT occurred in austenite with a sequential manner of austenite→ ɛ martensite→ α′ martensite with increasing strain. By contrast, the deformed microstructure of ferrite was dominated by dislocation cells. The SIM fraction, which was normalized with reference to the initial austenite fraction in order to eliminate its effect, increased with increasing the N content. Along with the present results, the factors influencing the SIMT kinetics in the present D-STSs with the different N content were discussed. [Copyright &y& Elsevier]

Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness. [ABSTRACT FROM AUTHOR]

The cyclic plastic deformation of the weldment of superaustenitic stainless steel with 6% molybdenum (UNS S31254) demonstrates that the strain rate does not affect the strength, but affects the ductility significantly. The response of tensile peak stress versus number of cycles for the weldment shows the cyclic hardening followed by the cyclic softening. A secondary hardening that appears at 1·5% strain amplitude is related to the strain induced martensite formation. The dislocations piled-up at the δ-ferrite grain boundary due to the essential rigid property of ferrite have a higher content of chromium and molybdenum. With a high fatigue strain rate, the austenite matrix of the weld metal exhibits persistent slip bands after fatigue at 0·6% strain amplitude and evolves to dislocation cell structures at 1·5% strain amplitude. The martensite band or the martensite sheath is formed at 1·5% strain, depending on the magnitude of the strain rate. [ABSTRACT FROM AUTHOR]

This paper presents a study of strain induced martensite formed in a Ti modified SS 316 bellow by electron microscopy methods. This bellow is part of a bellows sealed valve used in a sodium testing facility. It is known that Ti is added to austenitic stainless steel to overcome the problem of intergranular corrosion due to sensitization. However, the addition of Ti, a strong ferrite stabilizer can influence the stability of γ phase especially under strained conditions. Detailed electron microscopy studies showed the presence of strain induced martensite in an austenite matrix. The reasons for its formation and stability at the operating temperature are discussed, since sensitization kinetics is reported to be accelerated by the formation of martensite. [ABSTRACT FROM AUTHOR]

Forming Limit Curves (FLCs) were determined experimentally through limiting dome height (LDH) tests in AA1050, AISI 316L and AISI 304L. FLCs were also simulated through FE (finite element) analysis. Simulations involved both constant and varying (with strain and strain path) material properties — namely, strain hardening exponent ( n) and normal anisotropy ( $$\bar r$$ ).Varying n values were estimated from limited experimental data (from tensile tests) and the implicit assumption that n scales with in-grain misorientation developments and formation of strain induced martensite. $$\bar r$$ , on the other hand, could be estimated from crystallographic texture only for AA1050. Simulations with varying $$\bar r$$ in AA1050 had shown a clear, though numerically marginal, improvement. On the other hand, varying n could remarkably improve the FLC predictability in 316L and 304L, especially in the biaxial region . [ABSTRACT FROM AUTHOR]

The strain induced martensite transformation in austenitic stainless steels is of considerable interest, because it results in materials with attractive combinations of strength and ductility. The present work examines the mechanical response for a variety of strain and temperature paths, and relates these to microstructural observations. New evidence of the detailed transformation sequence is presented, along with direct evidence of codeformation of the austenite and martensite. Using different deformation temperature sequences enables the transformation to be changed from one that is heterogeneous to one that propagates axially along the sample. The strain hardening that occurs due to combined plasticity and martensitic transformation results in high kinematic hardening that is revealed by microstructural observations here, and which are linked directly to the mechanical response of these materials described in Part II of the present work. [ABSTRACT FROM AUTHOR]

In Part 1 of the present study attention was given to the influence of the deformation temperature on the structural aspects of the strain induced martensite transformation in austenitic stainless steels. In this part the mechanical response is examined, both during and after the martensite transformation, with particular attention to the development of internal stress. The influence of internal stresses on the bulk mechanical response concerns both the nature of the elastoplastic transition and the extent of kinematic hardening. The approach taken to assess the development of internal stresses is to use strain reversal (Bauschinger) tests and tensile experiments performed on a neutron diffractometer. In this way the evolution of stresses in partially transformed samples can be measured in the absence of any further martensite transformation. [ABSTRACT FROM AUTHOR]

Presented here is a technique of producing an ultra-high strength steel starting with an annealed low strength AISI 304L stainless steel. Annealed AISI 304L stainless steel was severely rolled at room temperature to a thickness reduction of 93%. Microstructure after rolling of SS304L showed subgrain formation with high dislocation density. The resulting strength of this steel after rolling was 1.7 GPa. Subsequent aging of this at 400 °C for 120 h further increased the strength to 2.2 GPa. The ductility of the as-rolled and subsequently aged conditions was similar and in the range of 5 to 7%. Electron back scatter diffraction analysis and vibrating-sample magnetometer showed that in the as-rolled condition the fraction of strain induced martensite was ≈ 72% with the remaining being untransformed austenite in the deformed state. The median spacing between boundaries (low as well as high angle) varied between 700 nm for the as-rolled condition to 900 nm after rolling and subsequent aging at 400 °C for 120 h. Atom probe tomography along with small angle neutron scattering showed that the process of aging resulted in a spinodal phase separation of Cr rich regions with 7 nm wavelength and precipitation of the G-phase having 1.75 nm radius. It was estimated that G-phase precipitation, rather than spinodally decomposed Cr-rich regions, is the dominant hardening phase. [Display omitted] [ABSTRACT FROM AUTHOR]

Type 304 stainless steel is a metastable austenitic stainless steel that undergoes strain induced transformation from austenite to α ′ martensite during deformation at room temperature. Role of prior deformation, by rolling at room temperature and at 200 °C (a temperature at which the strain induced martensitic transformation does not occur), on the subsequent room temperature tensile deformation behaviour of the austenite phase is investigated. The changes in dislocation density of the parent phase and the volume fraction of the martensite formed due to deformation were estimated by X-ray diffraction line profile analysis of data obtained from Beam Line - 12 of the Indus 2 synchrotron. The contrast caused by dislocations in the austenite phase has been addressed using the modified Williamson-Hall method. The character of dislocations changes from screw to edge above a critical equivalent strain of ~0.14. The influence of martensite formation on the increase in dislocation density in the austenite phase is confirmed from XRDLPA. An alternative approach to determine dislocation density is proposed which is independent of elastic constants of the material and is found to correlate well with the values determined by the Williamson-Smallman approach. The results obtained from XRDLPA are corroborated with EBSD analysis. It is also seen that the composite strength of the steel, which has undergone different levels of deformation, is related to the changes in dislocation density of austenite and volume fraction of martensite based on Taylor's equation enabling correlation of structure-property. • Deformation induced change in dislocation density (ρ) and character are studied by XRDLPA. • Strengthening due to α′ martensite and ρ in austenite are related through rule of mixtures. • SIM contributes to strengthening by introducing extra dislocations into γ phase. • Results from EBSD analysis corroborate the findings of XRDLPA. • Alternative approach to determine ρ from different forms of mWH equation is proposed. [ABSTRACT FROM AUTHOR]

Abstract: FCGR behaviour of 304LN stainless steel was studied employing (i) constant R and (ii) constant K max cyclic loading envelopes. The closure free crack growth rates obtained by the constant K max method were always lower than that of the constant R procedure. Martensitic transformation at the crack tip was observed in both the cases. Qualitatively, the constant K max method produced more martensite than the constant R procedure. The martensitic transformation is thought to decrease the applied stress amplitude due to volumetric expansion and strain associated with phase transformation. The dissimilarity in the martensitic transformation and the resultant reduction in the applied stress cycle produced by the two methods were thought to be the reasons for the observed difference in the crack growth rates. [Copyright &y& Elsevier]