Deformation Of Austenitic Stainless Steel Research Articles

In-situ tensile deformation of austenitic stainless steels with various grain sizes during synchrotron and neutron diffraction

In-situ tensile deformation of austenitic stainless steels with various grain sizes during synchrotron and neutron diffraction

Study on the Low Plastic Behavior of Expansion Deformation of Austenitic Stainless Steel.

The plastic deformation of TWIP steel is greatly inhibited during the expansion process. The stress-strain curves obtained through expansion experiments and observations of fracture morphology confirmed the low plastic behavior of TWIP steel during expansion deformation. Through an analysis of the mechanical expansion model, it was found that the expansion process has a lower stress coefficient and a faster strain rate than stretching, which inhibits the plasticity of TWIP steel during expansion deformation. Using metallographic microscopy, transmission electron microscopy, and EBSD to observe the twin morphology during expansion deformation and tensile deformation, it was found that expansion deformation has a higher twin density, which is manifested in a denser twin arrangement and a large number of twin deliveries in the microscopic morphology. During the expansion deformation process, dislocation slips are hindered by twins, the free path of the slips is reduced, and dislocations accumulate significantly. The accumulation area is the initial point of crack expansion. The results show that the significant dislocation accumulation caused by the delivery of a large number of twins under expansion deformation is the main reason for the decrease in the plasticity of TWIP steel.

Surface roughness evolution and heterogeneous plastic deformation of austenitic stainless steel during micro deep drawing: Modeling and experiment

The present work involves the numerical simulation and experimental study on the surface roughness evolution and heterogeneous plastic deformation (strain localization and concomitant surface morphology evolution) in grain scale of micro deep drawn parts of austenitic stainless steel (ASS) 304 foils. A three-dimensional modeling framework was used to simulate the micro deep drawing (MDD) process. The simulation involved the development of a Voronoi-based polycrystalline geometry model (VPGM), a multi-stress-strain response model, and a crystal plasticity finite element methods (CPFEM) model. The results indicate that CPFEM model exhibits higher accuracy in the localized stress and strain, thickness distribution fluctuation and surface roughness evolution of the fabricated parts compared to multi-stress-strain response model. Therefore, the plastic deformation heterogeneity in MDD is in-depth discussed based on CPFEM model. The results show that the coarse-grained foils correspond to the intense strain localization, which is the main cause of surface roughening. Considering the average grain size is close to the thickness of ASS foils, mounding features on the rough surface may readily develop further into potentially fractured sites during deformation. On the other hand, crystal orientation influence plastic deformation by affecting the slip behavior of materials. The effect of crystal orientations including {011} 〈211〉 (Brass), {011} 〈100〉 (Goss), and {112} 〈110〉 components on deformation is discussed. The results indicate a strong correlation between the magnitude of the Schmid's factor and the ease of deformation. Additionally, variations in Schmid's factor across different directions result in differences in deformation accumulation, which affects the thickness distribution along the radial direction and the distribution of earrings and wrinkles along the circumferential direction of the fabricated micro parts.

Micromechanical Study of Heterogenous Deformation of Austenitic Stainless Steel Welded Joints at Different Temperatures

Micromechanical Study of Heterogenous Deformation of Austenitic Stainless Steel Welded Joints at Different Temperatures

A crystal plasticity model that accounts for grain size effects and slip system interactions on the deformation of austenitic stainless steels

To ensure a more representative simulation of meso‑scale and macro-scale deformation, it is important the underlying constitutive relations are advanced to more effectively incorporate the micro-mechanisms such as those operative near the grain boundaries. Critical features necessary for accurate predictions include the influence of grain size, morphology, and misorientation on the local deformation. In this study, a length scale dependence is incorporated into classical crystal plasticity simulations. The implementation is based on the influence of dislocation slip pile-up at grain boundaries based on the Hall-Petch theory in materials. This was achieved by applying a purpose-built algorithm to extract the slip distance in adjacent grains along each slip direction. This extracted data was used to adjust the critical resolved shear stress within the grain. The adjusted slip law was then implemented in constitutive models to predict the yield stress and hardening evolution of material for different grain sizes. The interaction of the slip systems between grains was also considered, which resulted in the adjustment in the extent of slip transfer permitted between grains based on misorientation. The proposed approach can be applied in both finite element and spectral methods for solving the continuum differential equations. The accuracy of the proposed model was investigated by considering the meso‑scale predictions using two-dimensional simulations, in addition to macro-scale predictions using three-dimensional models. Furthermore, the validity of the method was experimentally supported by tracking the development of intragranular residual elastic stresses, measured via high-resolution electron backscatter diffraction. The discussed results highlight a new potential of enhancing the way in which complex grain boundary interactions contribute to local deformation in crystal plasticity simulations.

Modeling cyclic deformation of austenitic stainless steels at elevated temperatures using a physically-based mesoscale crystal plasticity framework

Austenitic stainless steels demonstrate significant changes in key macroscale deformation responses at elevated temperatures; these changes are relatively well-understood at micro- and macroscales in the framework of dynamic strain aging (DSA). At the microscale, temperature-dependent behavior is attributed to enhanced solute mobility and subsequent pinning of dislocations by solute atmospheres. At the macroscale, phenomenological equations are used to approximate microscale material behavior. However, the connection between unit dislocation processes at the microscale and macroscale deformation behaviors is unclear. In the current study, a dislocation density-based crystal plasticity model is used to relate micro- and macroscale responses for cyclic loading of austenitic stainless steels at elevated temperatures. Specifically, the variation in deformation behavior at elevated temperatures is attributed to modifications in the heterogenous distribution of dislocations on the mesoscale. The proposed framework demonstrates good agreement with experiments for strain- and stress-controlled cyclic loading scenarios. The model is also shown to be able to address key phenomena associated with DSA, i.e., negative strain rate sensitivity and stress-strain curve serrations.

Flow behaviour and constitutive modeling for hot deformation of austenitic stainless steel

The flow behavior of 316H austenitic stainless steel is investigated using hot compression tests. The modified Johnson-Cook and Zerilli-Armstrong models are developed, and modified Arrhenius-type model is established using an approach by dividing low and high stress region for determining key material constant and an uncomplicated approach for compensating strain in which the activation energy is determined from peak stress and only other material constants are considered as strain-dependent constants. The performance of all developed constitutive models is comparatively analyzed. It is indicated that the significant sensitivity of flow stress to temperature and strain rate is exhibited, and at 900 and 950 °C, strain rate sensitivity is closely related to temperature and strain rate, which can be explained by low stacking fault energy for 316H austenitic stainless steel. The modified Arrhenius type model has a noticeably higher accuracy in predicting flow behaviour than other two developed models in spite of a good performance of all developed models according to visual examination and statistical analyses.

Tentative Analysis of Welding Deformation of Austenitic Stainless Steel 316 in TIG Welding

Tentative Analysis of Welding Deformation of Austenitic Stainless Steel 316 in TIG Welding

Inhomogeneity of plastic deformation in austenitic stainless steel after surface mechanical attrition treatment

Inhomogeneity of microstructure evolution in cold-rolled austenitic stainless steel after surface mechanical attrition treatment (SMAT) was investigated. A characteristic deformation pattern was obtained for all studied specimens. Selected areas were examined through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Estimation of the α′-martensite volume fraction below the treated surfaces showed, that some of the studied areas are characterised by significant different amount of this phase (from 10 to 22% - BB, from 19 to 31% - GIXD). The performed finite element (FE) numerical analysis showed, that the reason for this may be the presence of an air gap between the impacted material and fixation and also relatively short high stress time duration due to surface inclination during the surface treatment. Annealing at 550 and 650°C greatly increased the volume fraction of α′-martensite (up to 47% - BB) and formed Fe2O3 as well as Fe3O4, whereas annealing at 700°C resulted in both disappearance of α′-martensite and in reduction of oxides.

Grain boundary engineering: fatigue fracture

Grain boundary engineering has revealed significant enhancement of material properties by modifying the populations and connectivity of different types of grain boundaries within the polycrystals. The character and connectivity of grain boundaries in polycrystalline microstructures control the corrosion and mechanical behaviour of materials. A comprehensive review of the previous researches has been carried out to understand this philosophy. Present research thoroughly explores the effect of total strain amplitude on phase transformation, fatigue fracture features, grain size, annealing twinning, different grain connectivity and grain boundary network after strain controlled low cycle fatigue deformation of austenitic stainless steel under ambient temperature. Electron backscatter diffraction technique has been used extensively to investigate the grain boundary characteristics and morphologies. The nominal variation of strain amplitude through cyclic plastic deformation is quantitatively demonstrated completely in connection with the grain boundary microstructure and fractographic features to reveal the mechanism of fatigue fracture of polycrystalline austenite. The extent of boundary modifications has been found to be a function of the number of applied loading cycles and strain amplitudes. It is also investigated that cyclic plasticity induced martensitic transformation strongly influences grain boundary characteristics and modifications of the material’s microstructure/microtexture as a function of strain amplitudes. The experimental results presented here suggest a path to grain boundary engineering during fatigue fracture of austenite polycrystals.

Modelling kinetics of strain-induced martensite transformation during plastic deformation of austenitic stainless steel

A model to simulate the kinetics of formation of strain-induced martensite (α′) in austenitic stainless steel (SS) 304 is derived. The model predicts sigmoidal behaviour of the γ→α′ transformation. The model was successfully applied to various levels of α′ obtained after cold-rolling of type 304 SS and to many previously reported experimental data set with the volume fraction of α′ as high as 1.0. This model was compared with two other sigmoidal models and it was shown that the fitting parameters for the proposed model can be correlated to physical parameters associated with γ→α′ transformation. The fitting parameters of the model consist of the saturated level of α′ for a given condition, the strain value associated with the maximum rate of γ→α′ transformation, and a parameter that characterises the rate of γ→α′ transformation.

Intervention of martensite variants on the spatial aspect of microvoids

The spatial aspect of microvoids’ distribution at different strained austenite grains has been investigated through strain rate variation during tensile deformation of metastable austenitic stainless steel at ambient temperature. Ductile fracture micromechanisms of metastable austenite have been investigated through direct measurements of void density, its fraction and their distribution at various levels of stresses/strains for all strain rates. The nature of the distribution of microvoids’ fraction closely corresponds to the local stress-state and strain-state variation and hence is strongly attributed to the crystallographic variant selection of martensite and their spatial nature of distribution. The direct intervention of martensite variants has been looked into for influencing void nucleation, growth and coalescence under tensile deformation of austenitic stainless steel.

Elastic interaction between twins during tensile deformation of austenitic stainless steel

In austenite, the twin boundary normal is a common elastically stiff direction shared by the two twins, which may induce special interactions. By means of three-dimensional X-ray diffraction this elastic interaction has been analysed and compared to grains separated by conventional grain boundaries. However, the components of the Type II stress normal to the twin boundary plane exhibit the same large variations as for the grain boundaries. Elastic grain interactions are therefore complex and must involve the entire set of neighbouring grains. The elastic-regime stress along the tensile direction qualitatively depends on the grain orientation, but grain-to-grain variations are large.

On the role of dislocation characters influencing ratcheting deformation of austenitic stainless steel

Ratcheting (i.e., asymmetric stress-controlled fatigue) behavior of a non-conventional stainless steel X12CrMnNiN17-7-5 has been investigated with various combinations of stress amplitudes, mean stresses and number of cycles. XRD profile analysis following modified Williamson-Hall method has been carried out to estimate the dislocation characters and their respective densities in the ratcheted specimens. The precise role of these substructural features on the extent of strain produced during the course of ratcheting has been investigated. The increase in strain owing to ratcheting has been explained by the presence of type of dislocations and their respective densities in the constituent phases of the microstructure. The accumulation of cyclic plastic strain during ratcheting deformation is predominantly progressed by the presence of screw dislocations. The significantly increased fraction of edge dislocations in martensite phase resulted higher work hardening in the ratcheted specimens. Accordingly, the extent of strain produced is relatively less during subsequent ratcheting deformation.

Modeling and simulation of temperature-dependent cyclic plastic deformation of austenitic stainless steels at the VHCF limit

The exploration of fatigue mechanisms in the VHCF regime is gaining importance since many components have to withstand a very high number of loading cycles due to high frequency or long product life. In this regime, the period of fatigue crack initiation and thus the localization of plastic deformation play an important role. The material that was investigated in this study is the metastable austenitic stainless steel AISI 304 in the initially purely austenitic condition. The experimental investigations during quasi-isothermal fatigue tests revealed that a moderate increase of temperature from room temperature up to 150°C led to a reduced VHCF strength. At both temperatures the 304 grade still undergoes a pronounced localization of plastic deformation in shear bands accompanied by a deformation-induced martensitic phase transformation from the γ-austenite to the α’-martensite during VHCF loading. In the present study, the experimental study is extended by modeling and simulation of the relevant temperature-dependent VHCF deformation mechanisms in order to provide a more profound understanding of the observed cyclic deformation. For this purpose, two-dimensional (2-D) morphologies of microstructures are modeled in the mesoscopic scale by the use of the boundary element method (BEM), and cyclic plastic deformation is considered by certain mechanisms defined in a simulation model. It describes the localization of plastic deformation in shear bands taking into account the formation, plastic sliding deformation and cyclic slip irreversibility of each shear band. The deformation-induced martensitic phase transformation is represented by introducing martensitic nuclei into the modeled microstructure depending on the plastic deformation in shear bands. The influence of temperature is incorporated into the simulation model by the use of empirical data and a kinetic model, each related to the tensile test. The simulated cyclic deformation and phase transformation is compared to experimental observations and allows for assessing the individual influence of deformation mechanisms on the temperature-dependent fatigue behavior. Finally, a temperature-independent ‘limit curve’ for the accumulated irreversible plastic sliding deformation regarding failure in the VHCF regime is proposed.

A multi-scale self-consistent model describing the lattice deformation in austenitic stainless steels

Differently oriented grains within a polycrystalline material exhibit different micro-mechanical lattice responses for a given macroscopic stress due to the local elastic and plastic anisotropy. A physical understanding of the lattice deformation mechanism during plastic flow is still lacking. In this study, a three-dimensional multi-scale self-consistent model is developed to examine the micro-mechanical deformation behaviour of F.C.C. polycrystalline austenitic stainless steels under uniaxial tensile loading at ambient temperature. The model is formulated in a crystal based plasticity framework and takes into account the detailed kinematics of dislocation slip and its influence on the evolution of the dislocation distribution on different crystallographic planes of individual grains within a polycrystalline material. The effect of athermal solute strengthening is also incorporated. Predictions of the microscopic lattice response developed within individual grains are compared with various available experimental results obtained using neutron diffraction (ND). There are significant differences in the way in which the lattice strain evolves with macroscopic strain obtained by different groups. This is explained in terms of the variation in initial residual stress state in the samples tested by these different groups.

Aspects of ion irradiations to study localized deformation in austenitic stainless steels

The effect of irradiation depth on localized deformation in strained SUS304 and SUS316 was assessed by irradiation to various doses using different energies of Fe++ and protons. The average height and spacing of slip steps formed on the surfaces after 2% plastic strain in 300°C argon were characterized using AFM and SEM. The step height and spacing were nearly unchanged for irradiation depth greater than 1/3 of the average grain size. 2.8MeV Fe++ irradiation, with an irradiation depth of <1μm, resulted in slightly larger step spacing compared to the unirradiated samples but it was much smaller compared to that in proton-irradiated samples with much greater irradiation depths. Step height and spacing also appeared to be affected by the grain size as they were nearly double in magnitude for 30μm compared to 14μm grain sizes in SUS304.

Effects of martensite development on lattice strain evolution during the in situ deformation of austenitic stainless steels at cryogenic temperatures

The effects of martensite development on lattice strain evolution during the in situ compression of austenitic stainless steels, Type 304L, at ambient and cryogenic temperatures were investigated. Engin-X neutron diffractometer was used to study the change in the lattice strain during the uniaxial compression to 10% strain in situ. The report found no evidence of martensite during the compression test at ambient temperature. Similarly, no martensite was observed at cryogenic temperatures up to the applied load of 200 MPa in the elastic region. Martensite was only observed during the plastic deformation at cryogenic temperatures up to the applied strain of 10%. The presence of martensite in the austenitic stainless steels microstructure reinforces the austenite matrix. This was evident from the decrease in the lattice strain as the deformation temperature decreases.

An Internal State Variable Constitutive Model for Deformation of Austenitic Stainless Steels

Austenitic stainless steels—particularly the 304 and 316 families of alloys—exhibit similar trends in the dependence of yield stress on temperature. Analysis of temperature and strain-rate dependent yield stress literature data in alloys with varying nitrogen content and grain size has enabled the definition of two internal state variables characterizing defect populations. The analysis is based on an internal state variable constitutive law termed the mechanical threshold stress model. One of the state variables varies solely with nitrogen content and is characterized with a larger activation volume. The other state variable is characterized by a much smaller activation volume and may represent interaction of dislocations with solute and interstitial atoms. Analysis of the entire stress–strain curve requires addition of a third internal state variable characterizing the evolving stored dislocation density. Predictions of the model are compared to measurements in 304, 304L, 316, and 316L stainless steels deformed over a wide range of temperatures (up to one-half the melting temperature) and strain rates. Model predictions and experimental measurements deviate at temperatures above ∼600 K where dynamic strain aging has been observed. Application of the model is demonstrated in irradiated 316LN where the defect population induced by irradiation damage is analyzed. This defect population has similarities with the stored dislocation density. The proposed model offers a framework for modeling deformation in stable austenitic stainless steels (i.e., those not prone to a martensitic phase transformation).

고 Mn 오스테나이트계 스테인리스강의 기계적성질에 미치는 가공유기 마르텐사이트 변태의 영향

The effect of deformation induced martensite transformation on the mechanical properties in austenitic stainless steel with high Mn was studied. `-martensite was formed by deformation in austenitic stainless steel with high Mn. Deformation induced `-martensite was formed with surface relief by cold rolling. With the increase of deformation degree, volume fraction of deformation induced martensite was increased rapidly in early stage of deformation and then, increased slowly. With the increase of deformation degree, hardness and tensile strength were rapidly increased with linear relations, while elongation was rapidly decreased and then slowly decreased. Hardness, tensile strengths and elongation were influenced strongly by deformation induced martensite.