Transformation In Stainless Steel Research Articles

Martensitic Transformations in Stainless Steels

Martensitic Transformations in Stainless Steels

Martensite formation during heating from cryogenic temperatures – A phase-field study

A 3D elastoplastic phase-field model is used to study the effect of isochronal heating on martensitic transformation in stainless steel quenched to cryogenic temperature. The results show that the slower the heating, the larger is the martensite volume fraction developed on heating. The simulated microstructures show that the transformation is driven by autocatalysis during the early stages and by coarsening of existing martensite units during later stages of the transformation. The internal stresses are mainly relaxed by autocatalysis during the initial stages of the transformation, whereas they are relaxed by plastic deformation during the later stages of the transformation. The temperature for attaining a certain martensite fraction increases with increasing heating rate, which is consistent with a thermally activated transformation. Kissinger-like analysis of the simulated transformation curves provides an activation energy of 11.9 kJ/mol.

Pulsed fusion grade deuterium plasma induced structural phase transformation in stainless steel using medium energy plasma focus device

Samples of ferrite -austenite stainless steel (SS) have been exposed to fusion grade plasma and radiations using a medium energy plasma focus device, ‘MEPF-12’. Three different polished samples of SS (10 mm diameter ×2 mm thick) were irradiated using single and multiple ( five and ten) plasma focus discharges. Each plasma focus discharge comprised energetic and high intensity deuterium ions, energetic deuterium plasma streams and 2.45 MeV neutrons produced via D(d, 3He)n fusion reactions. At sample position, typical values of deuterium ion fluence and neutron fluence were measured to be ∼1018ions/m2 and ∼1010 neutrons/m2, respectively, in each plasma focus discharge. Post irradiations, the samples have been characterized using different techniques for study of their surface modification characteristics, specifically, crystallinity, surface morphologies and surface roughness. The XRD analysis of the SS sample exposed to single or more plasma focus discharges revealed structural phase transformation from initial mixed ferrite [α(BCC)] -austenite [γ(FCC)] phase to austenite phase. Effects of deuterium ion concentration and transient rise in surface temperature on structural phase transformation have been discussed in detail. Surface morphologies studied using SEM has revealed erosion, melting and recrystallization of surface layer along with various other defects like pores, blisters, open bubbles and so on. Measurements using surface profilometer confirmed increase in surface roughness of the samples on irradiation. Detail discussion on the variation in the surface damage features with increasing number of plasma focus discharges used for irradiating samples and its comparison with virgin sample have been included in this report.

Phase-field modeling of austenite grain size effect on martensitic transformation in stainless steels

A 2D elastoplastic phase-field model is developed to study the effect of prior austenite grain size on martensitic microstructure evolution in stainless steel. The effects of strain hardening and strengthening by grain size reduction (Hall-Petch effect) have been included in the model. The results show that martensite units form in different packets oriented in different crystallographic directions in simulated coarse grains, whereas uni-directional martensitic growth is observed in simulated fine grains. The number of packets and martensite block width increase with increasing grain size. With a decreasing grain size the martensitic transformation start temperature decreases, indicating strengthening of austenite. Once the transformation is initiated, at a given time, simulated fine grains give rise to higher volume fraction of martensite compared to simulated coarse grains. The von Mises equivalent stress and plastic strain are large in simulated fine grains compared to those in simulated coarse grains. The simulation results are in good agreement with experimental results.

Influence of Strain State on the Kinetics of Martensitic Transformation Induced Plasticity (TRIP) in AISI 304 Stainless Steel

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.

ステンレス鋼の加工時に生成するナノサイズの結晶相

ステンレス鋼の加工時に生成するナノサイズの結晶相

Структура и свойства упрочненных нержавеющих сталей с эффектом памяти формы

A possibility of obtaining the shape memory effect has been investigated as a result of γ→e→γ transformations in stainless steels hardened by VC carbides. Age hardening conditions at 650 and 720 °C for stainless steels containing (in mass %) 0.20 – 0.35 C, 1 V, 3 – 4 Si, 9 and 13 – 14 Cr are presented. The values of reversible strain e determined after heating to 400 °C in specimens preliminarily deformed by 3.5 – 4 % vary from 0.15 to 2.7 % depending on the composition of the steels and the conditions of stabilizing and destabilizing aging.

Characterization of carbon ion implantation induced graded microstructure and phase transformation in stainless steel

Austenitic stainless steel 316L is ion implanted by carbon with implantation fluences of 1.2×1017ions-cm−2, 2.4×1017ions-cm−2, and 4.8×1017ions-cm−2. The ion implantation induced graded microstructure and phase transformation in stainless steel is investigated by X-ray diffraction, X-ray photoelectron spectroscopy and high resolution transmission electron microscopy. The corrosion resistance is evaluated by potentiodynamic test. It is found that the initial phase is austenite with a small amount of ferrite. After low fluence carbon ion implantation, an amorphous layer and ferrite phase enriched region underneath are formed. Nanophase particles precipitate from the amorphous layer due to energy minimization and irradiation at larger ion implantation fluence. The morphology of the precipitated nanophase particles changes from circular to dumbbell-like with increasing implantation fluence. The corrosion resistance of stainless steel is enhanced by the formation of amorphous layer and graphitic solid state carbon after carbon ion implantation.

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.

Multi-length scale modeling of martensitic transformations in stainless steels

In the present work a multi-length scale model is developed to study both the athermal and stress-assisted martensitic transformations in a single crystal of 301 type stainless steel. The microstructure evolution is simulated using elastoplastic phase-field simulations in three dimensions. The input data for the simulations is acquired from a combination of computational techniques and experimental works. The driving force for the transformation is calculated by using the CALPHAD technique and the elastic constants of the body-centered cubic phase are calculated by using ab initio method. The other input data is acquired from experimental works. The simulated microstructures resemble a lath-type martensitic microstructure, which is in good agreement with the experimental results obtained for a stainless steel of similar composition. The martensite habit plane predicted by the model is in accordance with experimental results. The Magee effect, i.e. formation of favorable martensite variants depending on the loading conditions, is observed in the simulations. The results also indicate that anisotropic loading conditions give rise to a significant anisotropy in the martensitic microstructure.

Analysis of deformation induced martensitic transformation in stainless steels

Many studies monitoring the formation of martensite during the tensile deformation of austenite report data which are, in principle, affected by both the applied stress and the resulting plastic strain. It is not clear in these circumstances whether the transformation is stress induced (i.e. the stress provides a mechanical driving force) or whether the generation of defects during deformation helps nucleate martensite in a scenario better described as strain induced transformation. The authors demonstrate in the present work that a large amount of published data relating the fraction of martensite to plastic strain can in fact be described in terms of the pure thermodynamic effect of applied stress.

Identification of the Direction-Dependency of the Martensitic Transformation in Stainless Steel Using In Situ Magnetic Permeability Measurements

The evolution of the martensite content is monitored throughout uniaxial tensile experiments on anisotropic temper-rolled stainless steel 301LN. Several martensite content measurement techniques are discussed. It is found that micrography, basic X-ray diffraction and EBSD provide good qualitative results, but the absolute errors in the estimated absolute martensite content can be greater than 10%. Magnetic saturation induction measurements provide the spatial average of the martensite content over a large volume, which eliminates inaccuracies associated with metallographic surface preparation. Inverse magnetostriction of the ferromagnetic martensitic phase has a strong effect on the results from magnetic permeability measurements. It is critically important to remove all elastic strains before measuring the magnetic permeability. Neutron diffraction is used to quantify the residual lattice strains in the martensite after removing all macroscopic elastic strains. The results demonstrate that the linear relationship between the magnetic permeability and the martensite content holds true despite the presence of small residual strains. In situ measurements of the martensite content evolution during tensile tests along the rolling, the cross-rolling and the 45° direction of the anisotropic sheet material reveal that the transformation kinetics are independent of the loading direction in stainless steel 301LN under uniaxial tension.

Development of SQUID-Based System for Nondestructive Evaluation

This article describes the development of a superconducting quantum interference device (SQUID)-based system for nondestructive evaluation. The setup incorporates an in-house developed thin-film-based Nb SQUID with readout flux locked loop electronics and consists of a liquid helium cryostat with adjustable stand-off distance, a precision XY- thetas scanner for studying both flat and cylindrical samples, and a data acquisition system. The system has been used for the detection of artificially engineered subsurface defects in aluminum plates and to track magnetic-to-nonmagnetic phase transformation in stainless steel [grade 316L(N)] weldment specimens subjected to low cycle fatigue deformation.

Deformation induced martensitic transformation in stainless steels

Abstract Deformation induced martensitic transformation was investigated in metastable austenitic stainless steel. This steel can present a microstructure of austenite (γ), α′ martensite and non magnetic e martensite. Uni-axial tensile test was used for loading at different temperatures below room temperature (from −120 to 20 °C). During the deformation the transformation takes place at certain places in an anisotropic way and texture also develops. Quantitative phase analysis was done by X-ray diffraction (XRD) and magnetic methods while the texture was described by X-ray diffraction using a special inverse pole figure. The quantitative phase analysis has shown that the formation of α′ and e martensite from austenite is the function of deformation rate, and deformation temperature. The transformation of the textured austenite takes place in an anisotropic way and a well defined crystallographic relationship between the parent and α′ martensite phase has been measured.

Deforming Texture Effect on Phase Transformation in Stainless Steels

Deforming Texture Effect on Phase Transformation in Stainless Steels

Modeling of Plastic Strain-Induced Martensitic Transformation for Cryogenic Applications

A simplified model of martensitic transformation in stainless steels at cryogenic temperatures is proposed. The constitutive modeling of plastic flow under cryogenic conditions is based on the assumption of small strains (⩽0.2). The hardening law for the biphase material (α′ martensite platelets embedded in the γ austenite matrix) has been obtained from the Mori-Tanaka homogenization. A mixed hardening with combined isotropic and kinematic contributions is proposed. The constitutive model, containing a reasonable number of parameters, has been numerically implemented and checked with respect to experimental data. Finally, the model is applied to compute the martensite evolution in thin-walled corrugated shells designed for cryogenic temperatures (mechanical compensation system of the Large Hadron Collider at CERN).

Study of martensitic transformation in stainless steel by CEMS and RBS channeling

The effect of Xe ion irradiation in a single crystal of 17/13 stainless steel has been studied, using RBS channeling techniques and conversion electron Mössbauer spectroscopy (CEMS). 300 keV Xe ions were used to induce martensitic transformation in the austenitic steel. A dynamic behavior of the transformation was observed as functions of the fluence and depth dependence. The martensite appears abruptly at a critical fluence, in contrast with polycrystalline 17/7 stainless steel.

Mössbauer study of martensitic transformation in Xe ion-irradiated type 304 austenitic stainless steel

Foil specimens of type 304 stainless steel have been irradiated with Xe+ ions in the range of 100–400 keV and 1×1020−1×1021 ions/m2 to elucidate the dynamics of the ion-induced martensitic phase transformation in stainless steel. It has been clearly shown by depth selective conversion electron Mossbauer spectroscopy (DCEMS) that the ion-induced martensitic phase in type 304 stainless steel has grown from the surface to a depth dependent both on the ion energy and the fluence of the Xe+ ions. Especially, we observed by means of DCEMS that the extension of the martensitic phase into the interior of stainless steel has been induced with increasing ion energy. It is concluded from these results that the depth distribution of the ion-induced martensitic phase is stress-induced by the formation of the highly pressurized Xe+ inclusion in type 304 stainless steel.

MeV-ion-beam induced localized enhancement of magnetization in stainless steel foils

MeV ions were implanted into 301 and 304 stainless steel foils to investigate the localized modification of magnetic properties associated with the phase transformation. The intensity of the magnetization was found to increase by MeV Au + implantation because of the localized martensitic transformation. The increase in magnetization for 301 stainless steel was greater than that for 304 stainless steel because 301 stainless steel more easily transformed austenite into martensite. The beam-induced localized phase transformation in stainless steel can be applied to permanent magnetic recording.

Irradiation effect of rare-gas ions in type 304 austenitic stainless steel

Ar, Kr and Xe rare-gas ions were irradiated in foil specimens of type 304 stainless steel. An irradiation effect in the surface region of the foil specimens has been investigated by means of glancing angle X-ray diffraction (GXRD), Rutherford backscattering (RBS) and particle-induced X-ray emission (PIXE) in order to observe the retained rare-gas atoms and elucidate their correlation with the ion-induced martensitic phase transformation of γ phase → α phase. It has been seen that the amounts of Ar, Kr and Xe retained in stainless steel as estimated from PIXE spectra were affected by the sputtering under ion irradiation. It has been shown by GXRD measurements that the retained rare-gas atoms agglomerated and formed a highly-pressurized solid phase inclusion. It is clear from GXRD measurements that the amount of the phase transformation at saturation increased linearly with increasing ion range of the implanted rare-gas atoms irrespective of the ion species. We speculate from this linearity that the depth distribution of the ion-induced martensitic phase may overlap with that of the rare-gas inclusion in stainless steel, and the formation of the rare-gas inclusion has stress-induced the phase transformation of γ → α. However, before saturation of the ion-induced phase transformation, the degree of the phase transformation in stainless steel as estimated from the amount of retained rare-gas atoms clearly differed for the different ion species.