Metastable Austenitic Stainless Steel Research Articles

A novel method for the strain strengthening of metastable austenitic stainless steel dome by deep cryogenic forming

A novel method for the strain strengthening of metastable austenitic stainless steel dome by deep cryogenic forming

In-situ synchrotron X-ray diffraction study of the effects of grain orientation on the martensitic phase transformations during tensile loading at different strain rates in metastable austenitic stainless steel

In this work, in-situ high-energy X-ray diffraction was used to analyze the effects of strain rate and austenite (γ) grain orientation on the strain-induced martensitic transformation in metastable austenitic stainless steel 301LN. The diffraction measurements were carried out at strain rates ranging from 10–3 s–1 to 1 s–1 continuously without interrupting the experiment and thus creating nearly adiabatic conditions at the highest studied strain rate. The results indicate that <100>γ fiber-oriented grains preferentially transform at the strain rate of 10–3 s–1 when the true strain is above 0.10, whereas the <111>γ fiber-oriented grains transform only at later stages of plastic deformation. The phase transformation rate of the <111>γ and <100>γ fiber-oriented grains decreases with increase in strain rate. A theoretical model based on stacking fault width as a function of external stress and temperature (stacking fault energy) was used to predict lower-bound estimates for the critical tensile stress needed to start ε-martensite and α’-martensite phase transformations. The model can predict the experimentally observed phase transformation behavior of the <111>γ fiber orientations at all strain rates but is unable to predict the decrease of phase transformation rate of <100> fiber-oriented γ grains with increase in strain rate, which could be related to change in dislocation structure.

Residual Stress Engineering for Wire Drawing of Austenitic Stainless Steel X5CrNi18-10 by Variation in Die Geometries-Effect of Drawing Speed and Process Temperature.

As a result of conventional wire-forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent forming processes such as bending operations. High tensile residual stresses typically occur in the near-surface region of the wires and can limit further application and processability of the semi-finished products. This paper presents an approach for tailoring the residual stress distribution by modifying the forming process, especially with regard to the die geometry and the influence of the drawing velocity as well as the wire temperature. The aim is to mitigate the near-surface tensile residual stresses induced by the drawing process. Preliminary studies have shown that modifications in the forming zone of the dies have a significant impact on the plastic strain and deformation direction, and the approach can be applied to effectively reduce the process-induced near-surface residual stress distributions without affecting the diameter of the product geometry. In this first approach, the process variant using three different drawing die geometries was established for the metastable austenitic stainless steel X5CrNi18-10 (1.4301) using slow (20 mm/s) and fast (2000 mm/s) drawing velocities. The residual stress depth distributions were determined by means of incremental hole drilling. Complementary X-ray stress analysis was carried out to analyze the phase-specific residual stresses since strain-induced martensitic transformations occurred close to the surface as a consequence of the shear deformation and the frictional loading. This paper describes the setup of the drawing tools as well as the results of the experimental tests.

Influence of cryogenic turning strategies on the surface morphology and resulting wear behavior of metastable AISI 347 austenitic stainless steel

AbstractCryogenic turning of metastable austenitic stainless steels can improve wear resistance of the resulting surface due to the phase transformation of γ‐austenite into α’‐ and/or ϵ‐martensite in the near surface layer. By using a cryogenic two‐step turning process, the amount of deformation‐induced α’‐martensite in the subsurface regime can be further increased. To determine the influence of the implemented and optimized two‐step turning strategy on the tribological properties of countersurfaces for radial shaft seals, an evaluation of wear behavior of the shaft seal countersurface as well as microstructural analyses in subsurface regime is presented and compared to the cryogenic single step turning process. The results show that not only the integral phase transformation in the overall subsurface region, but also the local phase distribution plays an important role when it comes to the surface performance in tribological applications.

Effect of Surface Roughness and Chemical Composition of Metastable Austenitic Stainless Steels on Electrochemical Polishing-Induced Martensitic Transformation

Herein, we analyzed the martensitic transformation kinetics during electrochemical polishing (EP) for stainless steel specimens with varying surface roughness and austenite stability. Martensite fraction measurement demonstrated that specimens with higher surface roughness and lower austenite stability exhibited relatively higher levels of martensitic transformation. To understand these phase transformation characteristics, the amount of charge build-up on the specimen surface during EP was calculated using COMSOL Multiphysics simulations for specimens with different surface roughness. The effect of charge build-up-induced stress was analyzed using previously published first-principles calculations. We found that specimens with higher surface roughness accumulated more charge build-up, resulting in greater stress and a martensitic transformation driving force. Furthermore, the critical energy required for the martensitic transformation was calculated using Thermo-Calc for specimens with different austenite stabilities. We demonstrated that the martensitic transformation kinetics during EP could be explained in terms of austenite stability, similar to the stress-induced martensitic transformation.Graphical

The Influence of Tempering Parameters on the Microstructure, Mechanical Property, and the Corresponding Strengthening Mechanism of Metastable 301 Austenitic Stainless Steel

Metastable austenitic stainless steels (MASS) are widely used in various industrial applications due to their exceptional mechanical properties. The microstructure of MASS can be regulated through severe plastic deformation, heat treatments, and surface treatments to achieve further improvement of mechanical properties. Herein, nanograin and quasiheterostructure are prepared by cold rolling and annealing process in Fe–17Cr–6Ni MASS. The mechanical response, deformation mechanism, and strengthening contribution of the two steels with representative microstructure are studied. The results indicate that nanograin austenitic steel (803 MPa, 22%) has superior yield strength and similar ductility compared with that of quasiheterostructure steel (600 MPa, 25%). In the nanograin steel, the deformation mechanism is mainly dislocation slip and deformation‐induced martensite transformation, while in quasiheterostructure steel, the deformation twinning is also included for the relative lower stacking fault energy in the coarse grain. The yield strength of nanograin structural steel is about 200 MPa higher than that of quasiheterostructure steel, which can be attributed to the differences in grain refinement strengthening, phase‐transformation strengthening, and precipitation strengthening of Cu particles. The findings presented in this study are informative for modulating MASS properties and ultimately improving the lifespan in a variety of industrial settings.

An experimental evaluation on rate sensitivity of fracture toughness in sheet specimens made of SUS304 metastable austenitic stainless steel

Utilizing transformation-induced plasticity (TRIP) steel with strain-induced martensitic transformation (SIMT) on automotive steel plates and developing products which have exceptional energy absorption capabilities can be used to reduce weight. For this purpose, it is important to clarify the fracture toughness of TRIP steels and its strain rate dependency to ensure the use of impact-absorbing members over a wide range of deformation rates. However, inconsistent rate dependent trends were observed in the previous studies, possibly as a result of the transition of the fracture mode. Therefore, the effect of fracture modes and strain rate on the fracture toughness values needs to be examined sufficiently. In the present study, the fracture toughness of the specimens made of SUS304 metastable austenitic stainless steel, which is a kind of TRIP steel, is evaluated by tensile tests at various strain rates. Direct current potential difference and digital image correlation methods are introduced to detect the time of fracture initiation during deformation, especially at high deformation rates by using the developed impact tensile tests based on the split Hopkinson pressure bar. The fracture surfaces of the fractured specimens are observed by using an electron scanning electron microscopy to determine not only the fracture modes and their rate dependent effects but also the reinforcement mechanism of SIMT in mixed modes. The results demonstrate that fracture toughness shows negative strain rate dependency, and mixed-mode fracture can be found near the crack in the case of the impact tensile test.

Effect of cold rolling deformation on the pitting corrosion behavior of high-strength metastable austenitic stainless steel 14Cr10Mn in simulated coastal atmospheric environments

The pitting corrosion behaviors of high-strength metastable austenitic stainless steel (MASS) 14Cr10Mn with different cold rolling reductions (0 %, 10 % and 20 %) were investigated in simulated coastal atmospheric environments by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), electrochemical measurements and cyclic corrosion test. The cold rolling deformation noticeably increases the dislocation density in the steel. As the cold rolling reduction changes from 0 % to 20 %, the pitting potential and the polarization resistance decrease to some extent in 1 wt% NaCl solution. The cold rolling deformation increases the defect density and the contents of (Fe,Cr)-oxyhydroxides/hydroxides in the passive film on the specimen surface. After the wet-dry cyclic corrosion test, the corrosion pits initiate at the deformation band regions. The quantity, diameter and depth of the pits gradually increase with enlarging the cold rolling reduction.

Modeling of strain-induced γ → α′ phase transformation for 201Cu metastable austenitic stainless steel and its verification in rolling processes

Modeling of strain-induced γ → α′ phase transformation for 201Cu metastable austenitic stainless steel and its verification in rolling processes

Fatigue life evaluation of metastable austenitic stainless steel AISI 347 based on nondestructive testing methods for different environmental conditions

Due to aging nuclear power plants there is an increasing need for methods to evaluate the integrity of components and structures of nuclear engineering. During power plant operation, nuclear materials are exposed to the effects of temperature, corrosion and cyclic loading. These lead to microstructural changes resulting in material degradation, which can be detected by suitable nondestructive testing (NDT) methods. Fatigue testing setups for ambient temperature, distilled water and 300 °C were designed to represent relevant environmental conditions and instrumented with in-situ measurements of temperature, electric, micromagnetic and electrochemical NDT parameters. Constant amplitude and strain increase tests with specimens of metastable austenitic stainless steel AISI 347 (X6CrNiNb18-10, 1.4550) typically used for pipe components were conducted in total strain control. Data obtained were employed in short-time evaluation procedure StrainLife to generate fatigue life data. To establish an understanding between microstructure evolution and NDT data, scanning electron microscopic methods such as electron backscatter diffraction and energy dispersive X-ray spectroscopy were used. Electrochemical data yields fatigue life results as good as conventionally used parameters like stress and strain. These findings could enable usage of presented parameters for mobile or even online testing systems, since accessibility depending on application case can be much improved.

Evolution of Gradient Structure Under Heat Treatment of Metastable Austenitic Stainless Steel Subjected to Cold Radial Forging

Evolution of Gradient Structure Under Heat Treatment of Metastable Austenitic Stainless Steel Subjected to Cold Radial Forging

Experimental and numerical investigation of martensite phase transformation in austenitic stainless steel subjected to ultrasonic nanocrystal surface modification at room and cryogenic temperatures

This study aims to investigate the evolution of martensite volume fraction (ξ) following ultrasonic nanocrystal surface modification (UNSM) on metastable austenitic stainless steel 304L specimens at room and cryogenic temperatures using both numerical and experimental methods. A user subroutine (VUMAT) was utilized to implement a constitutive model that describes the phase transformation occurring in the target during the UNSM process. An alternative method was employed to simulate UNSM. The numerical predictions of residual stress (σres) and ξ exhibited good agreement with those measured experimentally. Finite element models were employed to undertake a comprehensive study of the impact of UNSM parameters on σres and ξ. The findings demonstrated that UNSM at cryogenic temperature produced significantly higher levels of compressive σres in comparison with UNSM at room temperature, while the depth of the layer with compressive σres and the extent of plastic deformation decreased. Based on the experimental and numerical findings, a significant value of ξ was observed to form within the subsurface region subsequent to UNSM at both temperature conditions. The value of ξ on the surface of UNSMed specimens at room and cryogenic temperatures was quantified using X-ray diffraction, resulting in respective values of 82 % and 88 %. Moreover, a significant increase in near-surface layer hardness for the UNSMed specimen at cryogenic temperature was observed, with a value of approximately 600 Vickers. This value is notably higher than the hardness of 480 Vickers obtained for the UNSMed specimens at room temperature.

Modelling of hydrogen distribution at the fatigue crack tip of austenitic stainless steels in internal and external hydrogen tests

A hydrogen diffusion model was developed, which accounts realistically for the kinetics of hydrogen adsorption, absorption and their reverse processes at the gas-solid interface. Using the model, the hydrogen distribution at the fatigue crack tip of austenitic stainless steels (304, 316 L) in internal and external hydrogen tests were analyzed. Results indicate that for stable austenitic stainless steel, the hydrogen concentration at the fatigue crack tip is much higher in internal hydrogen test than that in external hydrogen test, which is attributed to the extremely low adsorption, desorption and diffusion rates of γ phase. For metastable austenitic stainless steel, hydrogen rapidly enters into strain-induced α′ martensite around the fatigue crack tip, forming an enrichment peak at the α′/γ interface in external hydrogen test. While significant hydrogen outgassing occurs in α′ phase in internal hydrogen test, leading to a much lower crack tip concentration compared with external hydrogen test.

Dynamic Strain Aging Induced by Synergistic Effects of Deformation-Induced Martensite and Deformation Twins in Fe–Cr–Ni Metastable Austenitic Stainless Steel

Dynamic Strain Aging Induced by Synergistic Effects of Deformation-Induced Martensite and Deformation Twins in Fe–Cr–Ni Metastable Austenitic Stainless Steel

Tension-compression asymmetry of metastable austenitic stainless steel studied by in-situ high-energy X-ray diffraction

This work studies the tension-compression asymmetry (TCA) of metastable austenitic stainless steel (MASS) in uniaxial loading depending on temperature. In-situ high-energy X-ray diffraction was used to simultaneously probe phase fractions, transformation kinetics, crystallographic texture, lattice strains, strain and stress partitioning between austenite and martensites during quasi-static tensile and compressive deformation at 24, 60 and 100 °C. Complementary relaxed-constraint crystal plasticity simulations and calculations of the mechanical driving force related to the formation of α’ and ε martensites were performed. At 24 °C, martensitic transformations (MTs) prevail, while at 100 °C dislocation slip is the dominant deformation mechanism for both load senses. Macroscopic stress-strain response and transformation behaviour exhibit TCA, with compression promoting the conversion of ε into α’. Transformation kinetics were analyzed in relation to shear banding and the geometric alignment of ε lamellas depending on load sense and temperature. A strong TCA was found for crystallographic texture, bearing signatures of grain rotation due to plastic slip and of MT in case of austenite (γ). For both load senses, the relative strengths of austenite and martensite texture fibres were related to the driving force anisotropy for α’ formation calculated based on the phenomenological theory of martensite crystallography. Texture evolution of α’ is largely controlled by the MT itself, not by grain rotation. Analysis of differently orientated austenite grain families revealed a pronounced TCA of the lattice strains, linked to the γ → ε MT. This was found to be a direct consequence of driving force and volume change related to ε formation. Furthermore, stress is shared differently between austenite and martensites in tension vs. in compression. γ hardens more and hence carries a larger portion of the total stress in compression than in tension. The origin for this TCA could be found in the elasto-plastic accommodation of the volume change related to α’ formation. These findings can aid the development of new material laws for MASSs that are sensitive to load-sense and temperature for advanced forming simulations.

Role of Ni content on microstructural and mechanical responses of Nb-stabilized metastable austenitic stainless steel weld metals

Three Nb-stabilized metastable austenitic stainless steel (15Cr–9Ni–Nb steel) weld metals with Ni contents of 8.4 wt%, 9.2 wt%, and 10.2 wt% were designed and prepared by gas tungsten arc welding. The effects of Ni content on the solidification behavior, microstructure, and the mechanism of deformation-induced martensitic transformation in the tensile process were investigated via scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, and high-resolution transmission electron microscopy, and their effects on mechanical properties were also discussed. As the Ni content increased, the solidification mode of weld metals changed from ferrite-austenite mode to austenite-ferrite mode, which made the segregation of Nb and Si at the interdendritic region more obvious and increased the total volume and the average size of primary NbC significantly. The large amount of coarse primary NbC in the AF weld metal damaged the impact toughness and reduction of area by inducing intergranular cracks and intergranular fractures. Moreover, both the three weld metals had poor mechanical stability and the deformation-induced martensitic transformation (DIMT) (γ→α′) occurred during the tensile process, which contributed to increased tensile strength. As the Ni content increased, the mechanical stability of austenite increased, and the strengthing effect of DIMT decreased, while the uniform elongation of weld metals increased.

Can Martensitic Phase Transformation Measured by Magnetic Methods be an Indicator of Fatigue Damage in Austenitic Steel at Elevated Temperature and Thermo-Mechanical Loading?

The phenomenon of phase transformation from paramagnetic γ austenite to ferromagnetic α' martensite in metastable austenitic stainless steels under fatigue loading is well investigated, especially at ambient temperature. Generally, it can be said that at ambient temperature, the amount of martensite is proportional to the accumulated plastic strain and, therefore, can be an early indicator of fatigue damage. The problem arises at elevated temperatures when the thermal energy can severely or even completely inhibit the phase transformation. In this case, the amount of α'-martensite can no longer be a good indicator of fatigue damage. The question is, where is the limit the deformation-induced martensite can still be used as a fatigue damage indicator? To answer this question, several fatigue tests have been performed on AISI 347 (X6CrNiNb18-10), a metastable austenitic stainless steel often used in the piping of European nuclear powerplants. Tests included isothermal fatigue tests as well as thermo-mechanical fatigue (TMF) loading in the range of 25 to 320 °C. Deformation-induced martensite was measured in situ by means of a uniaxial magnetic balance, from which the rate of phase transformation was obtained. Fatigue specimens were then cut to investigate the distribution of martensite in the gauge length by means of magnetic force imaging. The results showed that isothermal fatigue loading results in a steady decrease in phase transformation rate with temperature. Initial martensite nucleation was observed to be different at elevated temperatures and occurred only at the surface, where sufficient shear deformation was present. TMF tests showed higher than isothermal rates of phase transformation. Peak temperatures above 300 °C were nevertheless enough to severely reduce the formation of α' martensite, even when cooled back to ambient temperature in the other half of the TMF cycle.

Overview of Surface Modification Strategies for Improving the Properties of Metastable Austenitic Stainless Steels

Metastable austenitic stainless steels (MASS) are widely used in various industrial applications due to their exceptional compromise between mechanical properties and corrosion resistance. However, the mechanical properties of these materials can be further enhanced by surface treatments. This paper reviews various surface treatment methodologies used to improve the mechanical properties of MASS, with particular attention to laser treatments. The effects of these surface treatments on the microstructure and chemical composition in the thermal affected zone of the MASS are discussed, and their impact on the material’s mechanical properties, such as hardness, tensile strength, and fatigue life, are investigated in detail. Additionally, the paper highlights the limitations of these surface treatments and points out some areas where further research is needed. The findings presented can be used to guide the selection of appropriate surface treatment techniques for specific applications, ultimately improving the performance and lifespan of MASS in various industrial settings.

Micro-Mechanical Characterisation of Hydrogen Embrittlement and Fatigue Crack Growth Behaviours in Metastable Austenitic Stainless Steels with Microstructure Refinement

This article reviews the microstructural evolution in ultrafine-grained and nanotwinned austenitic stainless steels that have been subjected to hydrogen embrittlement (HE) and fatigue cracking. It provides guidelines for the development of high-strength austenitic steels without sacrificing HE and fatigue performance. The author focuses on the hydrogen-induced ductility loss and short fatigue crack growth associated with deformation-induced martensitic transformation, using micro-tension and micro-fatigue testing technologies. In type 304 metastable austenitic stainless steel, the microstructure produced by high-pressure torsion depends strongly on the processing temperature. Nanocrystalline austenite with enhanced strength and moderate ductility can be obtained at a processing temperature of ~423–573 K, whereas dual-phase microstructures comprising austenite and martensite are formed by processing at room temperature. Introducing ultrafine grains and nanotwin bundles mitigates the hydrogen-induced ductility loss in metastable austenitic steel by controlling the dynamic martensitic transformation. The microstructure refinement also contributes to enhanced resistance to short fatigue crack growth by changing the route of the damage accumulation process via phase transformation and detwinning.

Development of two-dimensional temperature field solution method based on the stress–strain response of 316LN stainless steel at cryogenic temperatures

To better understand the tensile fracture behavior and temperature variation during the tension testing of the 316LN at cryogenic temperatures, a two-dimensional temperature field solution method is developed based on two-dimensional strain field measured by the digital image correlation technology combined with the Taylor Quinney coefficient model and the intrinsic model of strain-induced martensitic phase transformation. The method allows for visualization of the strain field and temperature variation of the 316LN austenitic stainless steel at cryogenic temperature. By numerical simulation, the localized temperature elevation of the 316LN is determined to be 13.3 K during the tensile test at 77 K, which is in good agreement with the experimental result of 10.2 K. This model reconstructed the two-dimensional temperature field variation throughout the entire tensile test, which is beneficial for facilitating the analysis of the mutual relation between the mechanical properties and microstructural evolution of the 316LN and other metastable austenitic stainless steels in cryogenic temperatures.