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The effects of deformation temperature on the transformation-induced plasticity (TRIP)-aided 304L, twinning-induced plasticity (TWIP)-assisted 316L, and highly alloyed stable 904L austenitic stainless steels were compared for the first time to tune the mechanical properties, strengthening mechanisms, and strength–ductility synergy. For this purpose, the scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), X-ray diffraction (XRD), tensile testing, work-hardening analysis, and thermodynamics calculations were used. The induced plasticity effects led to a high temperature-dependency of work-hardening behavior in the 304L and 316L stainless steels. As the deformation temperature increased, the metastable 304L stainless steel showed the sequence of TRIP, TWIP, and weakening of the induced plasticity mechanism; while the disappearance of the TWIP effect in the 316L stainless steel was also observed. However, the solid-solution strengthening in the 904L superaustenitic stainless steel maintained the tensile properties over a wide temperature range, surpassing the performance of 304L and 316L stainless steels. In this regard, the dependency of the total elongation on the deformation temperature was less pronounced for the 904L alloy due to the absence of additional plasticity mechanisms. These results revealed the importance of solid–solution strengthening and the associated high friction stress for superior mechanical behavior over a wide temperature range. [ABSTRACT FROM AUTHOR]

Controversy surrounds the potential of grain refinement in suppressing metal hydrogen embrittlement (HE). The literature presents two methods for introducing hydrogen into metals: hydrogen pre-charging (HPC) and hydrogen in-situ charging (HISC). This study compares the effect of grain refinement on HE under electrochemical HPC and HISC conditions, focusing on type 304 austenitic stainless steel with a grain size range of 0.5–35 μm. HE behaviors were evaluated through tensile testing and fracture morphology analyses, while hydrogen diffusion and concentration were examined using hydrogen uptake amount measurements and hydrogen permeation tests, complemented by finite element (FE) simulations. Results indicate that coupling with plastic deformation significantly accelerated hydrogen transport in the HISC condition, leading to higher apparent hydrogen diffusivity compared to the HPC condition, with grain boundaries (GBs) act as hydrogen traps. Under the HISC, grain refinement reduced HE susceptibility by decreasing the apparent diffusivity due to an increased trapping effect of GBs and suppressing intergranular fracture by reducing the hydrogen amount distributed to each GB (hydrogen occupancy of the GBs). However, grain refinement to below 1 μm increased HE under the HPC condition. Hydrogen permeation tests showed that under the HPC, grain refinement slightly decreased hydrogen diffusivity. In the ultrafine-grained specimen, as GBs trapped more hydrogen atoms, they not only provided hydrogen to stress-concentrated lattices and newly-formed dislocations by detrapping but also maintained a higher hydrogen occupancy of the GBs, thus increasing HE. It is concluded that whether grain refinement suppresses HE depends primarily on the resulting hydrogen occupancy of the GBs, which is influenced by the charging conditions rather than the quantity of traps. • A comparison of effect of grain size on HE in 304 steel is conducted under in-situ H charging and pre-charging. • Coupling with plastic deformation accelerates H transport in in-situ charging condition. • Grain refinement reduces HE in in-situ charging, but it slightly decreases and then increases HE in pre-charging. • The former is attributed to decreased H diffusivity and reduced H occupancy of grain boundaries (GBs). • The latter is due to that H-filled GBs act as H reservoirs, providing H to stress concentrations and dislocations. [ABSTRACT FROM AUTHOR]

Ultrafine/fine‐grained austenitic stainless steel (ASS) is prepared through severe cold rolling (CR) and subsequent annealing. The microstructure evolution and tensile deformation behavior is investigated, focusing on the influence of grain size distribution on the mechanical properties and strain‐hardening behavior of the ASS. In the results, it is revealed that obvious ultrafine/fine‐grained hierarchical microstructure forms after annealing at 650 °C for 80 and 110 min, respectively. And, the CR‐80 sample with more ultrafine grains (UFGs) (86%) exhibits good combination of strength and ductility. The yield strength (YS), ultimate tensile strength, and total elongation are 954 MPa, 1030 MPa, and 43.4%, respectively. The high YS mainly originates from the retained martensite and grain refinement strengthening, while the ductility enhancement can be attributed to the postponed transformation‐induced plasticity (TRIP) effect, which is a consequence of the significant number fraction of UFGs in the hierarchical microstructure. In this study, a new strategy is provided to solve the strength–ductility tradeoff in TRIP‐assisted metallic materials by tailoring the grain size distribution. [ABSTRACT FROM AUTHOR]

In this study, ultrasonic nanocrystal surface modification (UNSM) was employed to process 316L stainless steel fabricated using selective laser melting (SLM). The surface integrity, microstructure evolution, and mechanical behavior of this alloy before and after UNSM treatment were systematically characterized. The results show that the surface integrity and wear resistance of the alloy has been significantly improved due to the synergistic effect of grain refinement, work hardening, and beneficial compressive residual stresses that resulted from UNSM processing. The effect of the gradient structure and residual stress introduced by UNSM processing on the strengthening mechanism and its effect on friction and wear behavior have been discussed. The improved surface finish, the grain refinement layer, and the presence of compressive residual stress of the UNSM SLM 316L stainless steel alloy successfully demonstrated UNSM to be a promising post-processing surface treatment technology to enhance the fatigue performance of additively manufactured metallic materials. [ABSTRACT FROM AUTHOR]

In this study, an austenitic CrMnNi–N steel is examined with regard to its solidification behavior of the weld metal. For this purpose, the cooling rate of the weld metal at different welding speeds is determined for tungsten inert gas welding without filler metal. The relationship between secondary dendrite arm spacing (λ2$\left(\lambda\right)_{2}$) and cooling rate (ε.$\overset{.}{\epsilon}$) can be described using λ2=18.56⋅ε.−0.269$\left(\lambda\right)_{2} = 18.56 \cdot \left(\overset{.}{\epsilon}\right)^{- 0.269}$. Moreover, the microstructure is characterized by a scanning electron microscope using energy‐dispersive X‐ray spectroscopy (EDS). Based on the EDS measurement, it is possible to describe the microsegregation of Cr, Ni, Mn, Mo, and N. The detected microsegregation behavior is compared with the results obtained from Thermo‐Calc calculations. The microsegregation varies depending on the present phase during solidification and correlate well with microstructure observations. By using the software Thermo‐Calc, the theoretical description of solidification is based on the classical Scheil model with fast diffusion of N. A primary ferritic solidification is calculated, which correlates with the experimental findings. [ABSTRACT FROM AUTHOR]

Low cost stainless steels (SS) with high performance, especially with good mechanical properties and oxidation resistance at high temperature, are continuous pursuit for manufacture industry. To achieve good combination of mechanical properties, oxidation resistance and cost, nanoprecipitate strengthened alumina forming austenitic (NSAFA) SS have been developed by removing expensive elements such as Nb and Ta, lowering Ni concentration in Fe–26Ni–15Cr-1.5Mo-(2.8–3.5)Al-(1–2.35)Ti (wt.%) alloys. Microstructure investigation demonstrates that NSAFA SS are strengthened by L12 structured (Ni, Fe)3(Al, Ti) γ′ nanoprecipitates with area fraction around 25%–40%. Oxidation experiments (750 °C/480 h) in air demonstrates that all the NSAFA steels have alumina forming ability. In addition, it is found that Al and Ti are all necessary for γ′ nanoprecipitates, but additional Al and Ti facilitate formation of the Ni2AlTi phase. To ensure high volume fraction of γ′ nanoprecipitates and reduce Ni2AlTi phase, 2.8–3.0 wt% Al and 1.8–2.35 wt% Ti are preferred. Benefit from the high volume fraction of γ′ nanoprecipitates, yield strength of NSAFA steels is much higher than that of Fe-base superalloy (e.g., A286) and can compare to some Ni-base superalloys (e.g., Hayners 282 & Inconel 740H).

Austenitic stainless steels, such as Sanicro 25 and Sanicro 28, exhibit excellent mechanical properties at elevated temperatures. This study investigates their time-dependent deformation behaviours through two distinct testing methods: slow strain rate tensile (SSRT) and creep testing at 650 °C. Microstructural analysis revealed that Sanicro 25 has a finer grain structure and superior high-temperature resistance compared to Sanicro 28. The deformation and fac­ture mechanics have been evaluated. A novel strain energy rate approach was developed to correlate SSRT and creep test data, providing insights into the materials’ high-temperature performance. This study offers a new perspective for evaluating and optimizing stainless steels for demanding applications.

This study aims to investigate the hydrogen embrittlement susceptibility of Cu-containing Fe–Cr–Ni austenitic stainless steels to examine the feasibility of Ni substitution in the steel used under a hydrogen atmosphere. The susceptibility to hydrogen embrittlement increased with replacement of Ni with Cu in N-free steels, while a complicated tendency appeared in N-added steels. Cu improved the austenite stability more effectively than Ni, but it promoted H diffusion and increased the adsorbed H content. Additionally, increase of Cu accelerated planar slip during tensile deformation, causing stress concentration and the accelerating H-induced cracks near the grain boundaries. In the metastable state, partial replacement of Ni with Cu improved hydrogen embrittlement resistance by suppressing the martensitic transformation. However, excessive replacement in stable steels resulted in the deterioration of hydrogen embrittlement resistance. • Increase of Cu/Ni ratio promoted the hydrogen diffusion. • Cu improved the austenite stability effectively than Ni. • Solute Cu promoted planar slip to induce stress localization. [ABSTRACT FROM AUTHOR]

Irradiation-induced defects and the precipitates in the wrapper material of the Indian Fast Breeder Test Reactor (FBTR), SS 316 are analyzed using the synchrotron source-based Angle Dispersive X-Ray Diffraction (ADXRD) technique with X-rays of energy 17.185 keV (wavelength ∼0.72146 Å). The differences and similarities in the high displacement damage samples as a function of dpa (displacement per atom) and dpa rate in the range of 2.9 × 10−7- 9 × 10−7 dpa/s are studied. Ferrite and M23C6 are commonly observed in the present set of high displacement damage 40–74 dpa SS 316 samples irradiated at temperatures in the range of 400–483 °C. Also, the dislocation density has increased as a function of the irradiation dose. The X-ray diffraction peak profile parameters quantified such as peak shift and asymmetry show that the irradiation-induced defects are sensitive to the dpa rate-irradiation temperature combinations. The increase in yield strength as a function of displacement damage is also correlated to the dislocation density.

This study involves data augmentation modeling using Generative Adversarial Networks (GAN) on the tensile test data of austenitic stainless steel, which encompasses chemical compositions, heat treatments, and mechanical properties. The synthetic data generated by GAN is subsequently used as the training dataset for six different algorithm models. The best-performing algorithm is selected based on the best evaluation metric values. The results of the Kolmogorov–Smirnov (KS) test indicate that the distribution of synthetic data does not significantly differ from the distribution of experimental data. Furthermore, the training results of predictive models employing synthetic data with the six machine learning algorithms demonstrate that the gradient boosting model exhibits superior performance in predicting the mechanical properties of austenitic stainless steel. [ABSTRACT FROM AUTHOR]

Rapidly quenched REP powders produced by melt atomization (spraying) are obviously characterized by the same effect of the quenching rate on structural features as in conventional solid-state quenching. However, the critical cooling rate determined from the TTT diagram for the phase transformation of the melt during atomization–crystallization is much higher than its value for the TTT diagram for conventional solid-state quenching. Important features of rapidly quenched powders, high dispersity of dendritic structures and formation of micro- and nanograin structures, determine the features of carbide and boride precipitation. Direct nuclear methods of activation autoradiography on carbon, track autoradiography on boron, metallography, SEM, EDX, etc., are used in the study. The structure features, including carbon and boron distribution, carbide and boride precipitation, the lattice parameter of an austenite solid solution for rapidly quenched REP powders, PM HIP compacts, and products such as nuclear fuel cladding tubes made of austenitic stainless steels and their counterparts produced by traditional technology, are revealed and analyzed taking into account the role of carbon and behaviour, precipitation of carbide and boride, and the influence of non-equilibrium states of the studied austenitic steels. [ABSTRACT FROM AUTHOR]

The study and modelling of material degradation processes, such as the initiation and growth of creep cavities in high-temperature applications, require a correlative and comprehensive knowledge of the microstructure. However, individual microscopy is limited to a small region and specific microstructural information of the specimen. This work demonstrates a novel correlative microscopy approach for characterising creep cavitation and establishing correlations with local microstructural parameters in a statistical manner. This approach combines datasets from stitched higher-resolution backscattered electron (BSE) images, XeF2 Focused Ion Beam (FIB) images, and backscattered electron diffraction (EBSD) maps with advanced image correlation techniques. Deep-learning image segmentation techniques and statistical analysis are applied to find relations between creep cavitation and local microstructural environment. This approach is demonstrated in a cyclic creep-tested 316H stainless steel specimen with extensive creep cavities. The results show that in this material, strain localization, grain boundary misorientation, and substantial precipitation dominate the nucleation of cavities, whereas other microstructural properties such as grain size and Schmid factor play smaller roles. This study presents the use of the correlative microscopy approach to provide new insights into creep cavitation behaviour and its implications for establishing creep cavitation damage models.

For the titanium-stabilized austenitic stainless steels, the interaction of precipitation due to the addition of Ti and the recrystallization process is an important aspect of grain refinement, which needs more systematic investigations. Accordingly, in the present work, the effects of Ti microalloying and thermomechanical processing on the recrystallization behavior and mechanical properties of type 316 stainless steel were investigated. It was revealed that the strain-induced α′-martensitic transformation in these metastable alloys is sensitive to the presence of Ti and C in solid solution or their presence as TiC carbides in the aged condition, as well as the temperature of rolling and reduction in thickness. During annealing, the reversion of deformation-induced α′-martensite to austenite and primary recrystallization resulted in the formation of fine-grained equiaxed microstructures, where the precipitation of TiC and sigma-phase was found to be important in the retardation of recrystallization and grain growth (by Zener pinning mechanism) as well as refining the grain size. Annealing the cold rolled 316Ti stainless steel at 800 °C led to a fine grain size of ∼2.5 μm with the precipitation of sigma-phase; while annealing at 900 °C resulted in a coarser grain size of 4 μm but without the presence of the unfavorable sigma-phase. The yield stress of 476 MPa, ultimate tensile strength of 805 MPa, and total elongation of 68 % were obtained for the latter, revealing the merits of Ti addition and thermomechanical processing for the improvement of mechanical properties.

This study introduces a Multiple Linear Regression (MLR) model that functions as a virtual sensor for estimating the strain-hardening rate of austenitic stainless steels, represented by the Hardening Rate of Hot rolled and annealed Stainless steel sheet (HRHS) parameter. The model correlates tensile strength (Rm) with cold thickness reduction and chemical composition, evidencing a robust linear relationship with an R-coefficient above 0.9800 for most samples. Key variables influencing the HRHS value include Cr, Mo, Si, Ni, and Nb, with the MLR model achieving a correlation coefficient of 0.9983. The Leave-One-Out Cross-Validation confirms the model’s generalization for test examples, consistently yielding high R-values and low mean squared errors. Additionally, a simplified HRHS version is proposed for instances where complete chemical analyses are not feasible, offering a practical alternative with minimal error increase. The research demonstrates the potential of linear regression as a virtual sensor linking cold strain hardening to chemical composition, providing a cost-effective tool for assessing strain hardening behaviour across various austenitic grades. The HRHS parameter significantly aids in the understanding and optimization of steel behaviour during cold forming, offering valuable insights for the design of new steel grades and processing conditions.

Heat input, a crucial factor in the optimization of high-temperature thermocouple laser welding, has a significant impact on the appearance and mechanical properties of dissimilar welded joints involving stainless-steel- and nickel-based alloys. This study focuses on laser overlay welding of austenitic stainless steels and nickel-based alloys. The findings indicate that an increase in heat input has a more pronounced effect on the penetration depth and dilution rate. Under high heat input, the weld has cracks, spatter, and other defects. Additionally, considerable amounts of chromium (Cr) and nickel (Ni) elements are observed outside the grain near the crack, and their presence increases with higher heat input levels. Phase analysis reveals the presence of numerous Cr2Fe14C and Fe3Ni2 phases within the weld. The heat input increases to the range of 30–35 J/mm, and the weld changes from shear fracture to tensile fracture. In the center of the molten pool, the Vickers hardness is greater than that of the base metal, while in the fusion zone, the Vickers hardness is lower than that of the base metal. The overall hardness is in a downward trend with the increase of heat input, and the minimum hardness is only 159 HV0.3 at 40 J/mm. The heat input falls within the range of 28–30 J/mm, and the temperature shock resistance is at its peak. [ABSTRACT FROM AUTHOR]

The structure of a welded joint of stainless steel 316L is studied by the methods of scanning electron microscopy including microprobe x-ray spectrum analysis and diffraction of backscattered electrons (EBSD). It is shown that a double-phase structure forms in the region of the weld including high-temperature δ-ferrite and austenite. The austenite and the ferrite undergo recrystallization in direction ⟨ 100 ⟩ parallel to the heat removal. The presence of high-temperature δ-ferrite in the steel results in nonuniform distribution of the alloying elements (Cr, Ni, Mo) and in appearance of phase boundaries, which can reduce the corrosion resistance of the welded joint. [ABSTRACT FROM AUTHOR]

The plastic deformation and martensite evolution of austenitic CrMnNi–N stainless steel with 8.6 wt% Mn and 4 wt% Ni is investigated in a temperature range of −40 to 100 °C. Martensite evolution is determined by in situ magnetic measurements during tensile test. The triggering stress and strain for martensite formation decrease with decreasing temperature. Ex situ volumetric magnetic measurements are used to determine the strain‐induced α′‐martensite volume fractions. The characterization of the microstructure is carried out with scanning electron microscope. Using electron backscatter diffraction, strain‐induced α′‐martensite is detected within deformation bands in the austenite. The strain hardening curve at −40 °C shows a typical progression for metastable austenitic stainless steels with pronounced strain‐induced α′‐martensite formation and can be divided into four hardening stages. At this temperature, the studied steel achieves the highest strain hardening rate. The amount of martensite increases with decreasing test temperature and reaches a maximum volume fraction of 76 vol% at −40 °C. The highest ductility of 83% is achieved at 40 °C, accompanied by a tensile strength of 732 MPa. The in situ magnetic measurement confirms that the inflection point in the strain hardening curve coincides with the maximum martensite formation rate. [ABSTRACT FROM AUTHOR]

Herein, experimental and modeling tools are employed to understand the joint area for a diffusion‐bonded 316L stainless steel. Detailed microstructure characterizations by means of optical microscopy, electron backscatter diffraction, and energy‐dispersive X‐ray spectroscopy are coupled to in situ micromechanical testing and micro‐ and nano‐indentation to fully reveal the properties at the joint area. Crystal plasticity finite‐element modeling is performed utilizing the exact microstructure to understand the effect of individual slip systems on transgranular strain fields. It is revealed that the diffusion line is only marginally harder. The final failure of the sample occurred away from the joint area, and both the base metal and bond line, shows evidence of considerable twinning and plastic deformation by (dislocation) slip initiating at grain boundaries. Additional slip systems and slip bands form to propagate across all matrices, blocking further dislocations after the ultimate tensile strength point. The presence of flaws within the weldment is found to be negligible. [ABSTRACT FROM AUTHOR]

Austenitic steels used for components in high-pressure hydrogen storage systems in the automotive sector have to meet high requirements in terms of material properties and cost efficiency. The commonly used 1.4435/AISI 316L type steels fulfil the technological requirements but are comparatively expensive and resource-intensive. Lower alloyed steel grades are less costly, though prone to α-martensite formation and therefore sensitive to hydrogen embrittlement. Segregation-related fluctuations of the local element concentrations exert a strong impact on the austenite stability, thus controlling the segregation behavior can improve the austenite stability of lean alloyed steel grades, making them suitable for hydrogen applications. In this work, a novel approach for the optimization of alloy compositions with the aim of improving the homogeneity of the austenite stability is developed. The approach is based on combining automated Scheil–Gulliver solidification simulations with a multi-objective optimization algorithm. The solidification simulations provide information about the influence of the segregation profiles on the local austenite stability, which are then used to optimize the alloy composition automatically. The approach is exemplarily used for an optimization within the compositional range of 1.4307/AISI 304L. It is shown that a significant increase in the homogeneity of the austenite stability can be achieved solely by adjusting the global element concentrations, which has been validated experimentally. [ABSTRACT FROM AUTHOR]

The sequence of structural changes produced in two deformed microstructures of austenitic stainless steel elaborated by a multipass unidirectional cold rolling (CR) and a two‐step one, to a 75% thickness reduction, is followed by dilatometric experiments. The two materials show different dilatometric behaviors. X‐ray diffraction and microhardness measurements are performed to underlie the observed dilatometric behaviors. The material subjected to multipass unidirectional CR shows an unusual dilatometric behavior. The first heating stage leads to the occurrence of the recovery reaction in competition with the ε‐martensite reversion. When the temperature increases between 550 and 780 °C, the reversion of deformation‐induced α‐martensite takes place and leads to a complicated dilatometric anomaly. Further increase in temperature leads to the occurrence of the recrystallization transformation. However, the material subjected to two‐step CR shows a quite usual dilatometric behavior which is explained by the occurrence of several reactions in the following order: 1) T < 300 °C, the recovery reaction, 2) 300 < T < 680 °C, the ferro‐ to paramagnetic transformation of α‐martensite, the reversion of ε‐martensite, and the athermal reversion of α‐martensite, 3) 680 < T < 760 °C, thermal reversion of DIM, and then 4) at T > 760 °C, the recrystallization. [ABSTRACT FROM AUTHOR]

The influence of nitrogen on the mechanical properties of two high Ni containing advanced austenitic stainless steels with low stacking fault energies is investigated. The results show that increase of nitrogen content greatly increases both strength and elongation of the steel at the same time. At the cryogenic temperature, the steels show a twin induced plasticity behavior. Ab initio calculations indicate that the increase of nitrogen slightly increases the stacking fault energy and consequently the critical shear stress for twin initiation in the steel. However, addition of nitrogen significantly increases the flow stress. This leads to a smaller critical strain for twin initiation and promotes deformation twinning in the high nitrogen steel. This is confirmed by the microstructure investigation. Deformation in steels is a competitive process between slip and twinning. Dislocation slip is dominant at low strain range, but formation of stacking fault and twinning become important in the later stages of deformation. At cryogenic temperature, it is mainly deformation twinning. The influence of nitrogen addition on magnetic property and its effect on deformation twinning are also discussed. The present study increases the understanding for the development of high‐performance and low‐cost advanced austenitic stainless steels. [ABSTRACT FROM AUTHOR]

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The metastable pitting behavior of austenitic stainless steel (304 SS) and duplex stainless steel (2205 DSS) under U‐bending deformation is comparatively investigated. This study highlights the effects of differences in inclusions and matrix between 304 SS and 2205 DSS with regard to the metastable pitting behavior under tensile and compressive deformation. The results show that the pit initiation frequency of 304 SS increases in the order: tensile < as‐received < compressive; in contrast, the initiation frequency of 2205 DSS increases in the order: as‐received < tensile < compressive. In addition, tensile and compressive deformation change the preferential initiation sites of metastable pitting in 2205 DSS. It is proposed that the types of transient signals can be comprehensively and objectively distinguished by the two parameters (tg/trep) and [(Ip/Qg):(Ip/Qrep)]. The correlation relationship between transient signals and pitting morphologies is clarified by cluster analysis. Both tensile and compressive deformations promote the growth of a few metastable pits, thus increasing the transition probability from metastability to stability. Overall, both tensile and compressive deformations are detrimental to the pitting corrosion resistance of the experimental steels. [ABSTRACT FROM AUTHOR]

δ‐Ferrite plays a controversial role in austenitic stainless steel and controlling its morphology and distribution is essential for the optimization of the properties of stainless steel. Herein, the solidification microstructure of S321 stainless steel with the addition of Nb and Y is studied by electron probe microanalysis, nanoindentation, and transmission electron microscopy. The results show that the addition of Nb promotes the precipitation of (Nb, Ti)(C, N) and coarsens the grain boundary carbides in S321 stainless steel, resulting in a decrease in the solute carbon content near the grain boundaries and a delay in the δ → γ phase transformation. The addition of Y inhibits the segregation of Ni along the original austenite grain boundaries and promotes the δ → γ phase transformation. Furthermore, the morphology of δ‐ferrite changes from lathy δ to skeletal δ and the grain boundary carbide network is broken. The higher density dislocation due to the phase transformation leads to an increase in the hardness of S321 steel containing Y. [ABSTRACT FROM AUTHOR]

Direct-fired supercritical CO2 (sCO2) power cycles are being commercialised to revolutionise fossil energy as a low-emission power source. To lower the cost of this technology, less expensive steels are needed in the lower temperature segments of the cycle. However, there are concerns about internal carburisation of steels in sCO2.1 A consistent observation is that thin, Cr-rich oxides appear to reduce C ingress compared to thick Fe-rich oxides formed on 9–12% Cr ferritic-martensitic steels. Advanced austenitic stainless steels (SS) like alloy 709 (20Cr-25Ni) are able to continue to form Cr-rich oxides at 650°C, while a conventional type 316 H SS formed a Fe-rich scale. The C diffusion profiles in SS specimens were quantified at 550°C–650°C using glow discharge optical emission spectroscopy and electron probe microanalysis. Analytical transmission electron microscopy was used to compare the thin protective Cr-rich oxide formed on alloy 709 in sCO2 at 650°C to that formed in ambient air. [ABSTRACT FROM AUTHOR]

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 ~423-573K, 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. [ABSTRACT FROM AUTHOR]

Creep rupture extrapolation is crucial for high-temperature materials served in power plants. Many analytical models can be used for creep rupture analysis, and fundamental models are also available. Machine learning is also an alternative. However, unphysical prediction curves occur readily in common machine learning algorithms, where one must manipulate the best results or ignore the less satisfactory ones. Using just high regression coefficients and low errors is not enough to obtain high accuracy of the methods. Nevertheless, five soft constrained machine learning algorithms (SCMLAs), where soft constraints, stability analysis by culling long-time or low-stress data, extrapolation from short to long times, and errors of solutions and algorithms are considered, are used for creep rupture prediction in this work. The models can generate reasonable results for fitting all data, extrapolating from short to long times, and stability analysis for Sanicro 25 after a number of tests. The errors of solutions for all the analyses are in a quite reasonable range, including extrapolation and stability analysis. The average relative standard deviation of the five SCMLAs is less than 2.5% at three times the maximum experimental creep rupture time. Creep rupture strength of the austenitic stainless steel Sanicro 25 can be predicted quantitatively by taking the average predicted stresses of the five SCMLAs. The method can also be used for other high-temperature alloys with similar creep degradation mechanisms.

Austenitic stainless steels that exhibit good corrosion resistance have recently found increasing applications in industry and transportation. This article addresses the influence of cold rolling and deformation on the pitting corrosion resistance of AISI 301LN and 316L stainless steels. The results indicate that the content of martensite increases as the cold rolling reduction also increases. The current work combined different techniques such as optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) analyses. Corrosion tests were carried out, in accordance with the ASTM standards. The results confirm that the 316L steel performed better than the 301LN, regarding pitting corrosion, even when deformed. This is due to the high molybdenum (Mo) content, which guarantees greater corrosion resistance. The conducted corrosion tests showed that the increase of cold deformation reduces the resistance to pitting and overall corrosion in both steels. It was found that the 301LN stainless steel has higher susceptibility to deformation-induced martensite and, despite the addition of nitrogen, it still has a lower performance relative to the 316L steel. The current work focused on evaluating the formation of pits and the dynamics of the microstructures of the AISI 301LN and 316L steels with their mechanical properties and corrosion resistance in a saline environment including chlorides. [ABSTRACT FROM AUTHOR]

This study involves data augmentation modeling using Generative Adversarial Networks (GAN) on the tensile test data of austenitic stainless steel, which encompasses chemical compositions, heat treatments, and mechanical properties. The synthetic data generated by GAN is subsequently used as the training dataset for six different algorithm models. The best-performing algorithm is selected based on the best evaluation metric values. The results of the Kolmogorov–Smirnov (KS) test indicate that the distribution of synthetic data does not significantly differ from the distribution of experimental data. Furthermore, the training results of predictive models employing synthetic data with the six machine learning algorithms demonstrate that the gradient boosting model exhibits superior performance in predicting the mechanical properties of austenitic stainless steel.

An alloy features‐based and chemical compositions‐based machine learning method was used to examine the low cycle fatigue life of austenitic stainless steels at different elevated temperatures employing one model. Furthermore, eight algorithms were used to examine the impact of algorithms on the precision of constructed models. As input, physicochemical features of elements were transformed from chemical compositions. After being conducted by the feature screening process, electronegativity deviation (E2.sd), ionization energy deviation (E6.sd), testing conditions, and tensile strength were chosen as input. The results show that algorithms affect accuracy and the models with the highest accuracy are SVR and ANN for alloy features and chemical compositions‐based method, respectively. Chemical composites‐based model demonstrates relatively lower precision than the alloy feature model. Almost all testing data distribute within two‐factor band lines predicted by alloying feature‐based model. The validation testing results indicate that 83% data plots distribute within two‐factor band lines. Highlights: Alloy features‐based models were constructed using eight algorithms.Chemical compositions‐based models were constructed using eight algorithms.Algorithms affected model's accuracy.Accuracy of chemical compositions model is lower than alloy feature model. [ABSTRACT FROM AUTHOR]

The creep response of AISI 316 and AISI 316 L was analysed to provide a coherent picture of the material behaviour, valid for both conventional wrought and additively manufactured steels. Literature evidences were considered. The analysis demonstrated that the presence of a fine distribution of particles, precipitated during creep exposure in both AISI 316 and 316 L(N), should be taken into account. A recent model, expressly developed for particle-strengthened alloys, was successfully used to describe the minimum creep rate dependence on applied stress for AISI 316 and 316 L(N). The same set of constitutive equations, in combination with the composite model, was then used for describing creep data obtained by testing the AISI 316 L produced by additive manufacturing. The main difference with wrought materials was that the microstructure of the additively manufactured alloy was composed by cells (soft zones) and thick walls heavily decorated with ultrafine oxide particles (hard zones). [ABSTRACT FROM AUTHOR]

• Transmission Electron Microscope investigation on austenitic stainless steels neutron-irradiated to low dpa at 380–400 °C. • SS 304 L(N) exhibited faulted dislocation loops of the highest number density at a given dpa level. • XRD - TEM data correlation. • Change in yield strength determined from number density and size of the faulted dislocation loop. Analysis of neutron irradiation-induced defects is carried out using Transmission Electron Microscope (TEM) on three variants of austenitic stainless steels, SS 304 L(N), SS 316 L(N), and SS 316 grid plate material irradiated in the Indian Fast Breeder Test Reactor at Kalpakkam to low displacement damage levels of 1.76–6.75 dpa at an irradiation temperature of 380 °C-400 °C and dpa rate in the order of 10−7 dpa /s. There is a significant variation in the faulted dislocation loop size distribution and number density as a function of irradiation dose among these stainless steels due to their different stacking fault energies which is evident from the quantified defects data presented. At ∼5.08 dpa , SS 304 L(N), SS 316 L(N) and SS 316 exhibited faulted dislocation loop number densities of 2.3 × 1022/m3, 4.8 × 1021/m3, and 1.2 × 1022/m3 with mean loop sizes of 15.5 ± 6.3 nm, 35.4 ± 13.7 nm, and 16.1 ± 5.6 nm respectively. Thus, at an identical dpa level, SS 304 L(N) and SS 316 L(N) have the highest and the lowest faulted loop number densities respectively. The general size-distribution behaviour, the increment in the loop size with enhanced size distribution as a function of dpa is observed only for SS 316 L(N). The dislocation density data previously determined using X-ray diffraction analysis is correlated to the number density of dislocation loops along with their size distribution determined using TEM analysis. Finally, the change in the yield strength due to the increment in the faulted dislocation loop number density is determined. [ABSTRACT FROM AUTHOR]

Hydrogen is known to embrittle austenitic stainless steels, which are widely used in high-pressure hydrogen storage and delivery systems, but the mechanisms that lead to such material degradation are still being elucidated. The current work investigates the deformation behavior of single crystal austenitic stainless steel 316L through combined uniaxial tensile testing, characterization and atomistic simulations. Thermally precharged hydrogen is shown to increase the critical resolved shear stress (CRSS) without previously reported deviations from Schmid's law. Molecular dynamics simulations further expose the statistical nature of the hydrogen and vacancy contributions to the CRSS in the presence of alloying. Slip distribution quantification over large in-plane distances (> 1 mm), achieved via atomic force microscopy (AFM), highlights the role of hydrogen increasing the degree of slip localization in both single and multiple slip configurations. The most active slip bands accumulate significantly more deformation in hydrogen precharged specimens, with potential implications for damage nucleation. For 〈 110 〉 tensile loading, slip localization further enhances the activity of secondary slip, increases the density of geometrically necessary dislocations and leads to a distinct lattice rotation behavior compared to hydrogen-free specimens, as evidenced by electron backscatter diffraction (EBSD) maps. The results of this study provide a more comprehensive picture of the deformation aspect of hydrogen embrittlement in austenitic stainless steels. • Single crystal 316L stainless steel experiences hydrogen embrittlement. • Hydrogen increases the yield stress without deviations from Schmid's law. • Hydrogen and vacancy dislocation-pinning effects vary with local composition. • Hydrogen promotes slip localization for both single and multiple slip conditions. • Hydrogen affects the lattice rotation and slip distribution during 〈 110 〉 -tension. [ABSTRACT FROM AUTHOR]

Short-range order (SRO), the regular and predictable arrangement of atoms over short distances, alters the mechanical properties of technologically relevant structural materials such as medium/high entropy alloys and austenitic stainless steels. In this study, we present a generalized spin cluster expansion (CE) model and show that magnetism is a primary factor influencing the level of SRO present in austenitic Fe–Ni–Cr alloys. The spin CE consists of a chemical cluster expansion combined with an Ising model for Fe–Ni–Cr austenitic alloys. It explicitly accounts for local magnetic exchange interactions, thereby capturing the effects of finite temperature magnetism on SRO. Model parameters are obtained by fitting to a first-principles data set comprising both chemically and magnetically diverse face-centered cubic configurations. The magnitude of the magnetic exchange interactions are found to be comparable to the chemical interactions. Compared to a conventional implicit magnetism CE built from only magnetic ground state configurations, the spin CE shows improved performance on several experimental benchmarks over a broad spectrum of compositions, particularly at higher temperatures due to the explicit treatment of magnetic disorder. We find that SRO is strongly influenced by alloy Cr content, since Cr atoms prefer to align antiferromagnetically with nearest neighbors but become magnetically frustrated with increasing Cr concentration. Using the spin CE, we predict that increasing the Cr concentration in typical austenitic stainless steels promotes the formation of SRO and increases order–disorder transition temperatures. This study underscores the significance of considering magnetic interactions explicitly when exploring the thermodynamic properties of complex transition metal alloys. It also highlights guidelines for customizing SRO through adjustments of alloy composition. [Display omitted] [ABSTRACT FROM AUTHOR]

Heat input, a crucial factor in the optimization of high-temperature thermocouple laser welding, has a significant impact on the appearance and mechanical properties of dissimilar welded joints involving stainless-steel- and nickel-based alloys. This study focuses on laser overlay welding of austenitic stainless steels and nickel-based alloys. The findings indicate that an increase in heat input has a more pronounced effect on the penetration depth and dilution rate. Under high heat input, the weld has cracks, spatter, and other defects. Additionally, considerable amounts of chromium (Cr) and nickel (Ni) elements are observed outside the grain near the crack, and their presence increases with higher heat input levels. Phase analysis reveals the presence of numerous Cr2Fe14C and Fe3Ni2 phases within the weld. The heat input increases to the range of 30–35 J/mm, and the weld changes from shear fracture to tensile fracture. In the center of the molten pool, the Vickers hardness is greater than that of the base metal, while in the fusion zone, the Vickers hardness is lower than that of the base metal. The overall hardness is in a downward trend with the increase of heat input, and the minimum hardness is only 159 HV0.3 at 40 J/mm. The heat input falls within the range of 28–30 J/mm, and the temperature shock resistance is at its peak.

Austenitic stainless steel with a bimodal harmonic structure (HS) is a recently developed material that consists of a periodic arrangement of coarse-grained (CG) structures surrounded by a network of ultra-fine grains (UFG). This material is characterised by high strength and good ductility. In this study, we analysed the fatigue performance of HS materials by modifying our previous multiscale model to extend its applicability from only homogeneous materials to HS materials. A novel method was proposed to characterise the microstructural features of HS materials considering the distributions of CG and UFG. A microstructure model was developed to reproduce grain maps using these characterised microstructures. The multiscale model was successfully validated by experimental results for two types of austenitic stainless steels. The model was used to elucidate the governing microstructural factors of fatigue performance in HS materials through systematic simulations. Consequently, a higher UFG area fraction was confirmed to improve fatigue life, and a UFG area fraction of approximately 40% is suggested for optimal microstructural design to balance fatigue performance and fabricating costs. This study presents the first step in the qualitative design of the fatigue performance of HS materials to replace the expensive and inefficient experimental analyses used in previous studies.

The fatigue behavior of the two austenitic stainless steels AISI 904L and AISI 347 with different surface morphologies, (i) conventionally turned and finally polished, (ii) cryogenic turned using CO2 snow, as well as (iii) cryogenic turned and finally polished, was investigated using an ultrasonic fatigue testing system up to the very high cycle fatigue regime using an ultrasonic fatigue testing system. The AISI 904L is stable against deformation-induced phase formation while the AISI 347 is in the metastable state and shows martensite formation induced by cryogenic turning as well as mechanical loading. For the detailed characterization of the surface morphology, confocal microscopy, scanning electron microscopy, and X-ray diffraction methods were used. The specimens from stable austenite failed in the high cycle fatigue and very high cycle fatigue regime. Opposed to this, the metastable austenite achieved true fatigue limits up to load cycle N = 1 × 109 and failed only in the high cycle fatigue regime. Furthermore, due to surface modification, an increase of fatigue strength of metastable AISI 347 was observed. [ABSTRACT FROM AUTHOR]

The current study applied thermomechanical processing (TMP) on 301 austenitic stainless steel to produce an ultrafine-grained austenitic structure, examining the dual effects of deformation at subzero temperature and TMP cycles on the strain-induced α′-martensitic transformation and austenite reversion occurring upon subsequent annealing. Three TMP schemes were adopted: (1) one cycle using a strain of 0.30, (2) two cycles using a strain of 0.20, and (3) three cycles using a strain of 0.15. Each cycle consisted of tensile deformation at −50 °C followed by annealing at 850 °C for 5 min. Compared to other schemes, the use of three cycles of the 0.15 strain scheme resulted in a significant formation of the martensitic phase to about 99 vol.%. Consequently, the austenite reversion occurred strongly, providing a mixture of the austenitic structure of reverted ultra-fine grains and retained coarse grains with an average grain size of 1.9 µm. The development of a mixed austenitic structure was found to lower the austenite stability and thus enhance the α′-martensitic transformation upon deformation in subsequent cycles. Moderate growth of high-angle grain boundaries occurred in the austenitic phase for all schemes, reaching a maximum of 64% in cycle 3 of the 0.15 strain scheme. The tensile behavior during subzero deformation was generally characterized by an initial strain hardening by slip (stage I), followed by a remarkable increase in strain hardening rate due to the strain-induced α′-martensitic transformation (stage II). Further straining promoted breakage of the α′-martensite banded lath structure for forming dislocation cell-type martensite, which was marked by a decline in strain hardening rate (stage III). Accordingly, the latter hardening stage had a lesser hardness enhancement of deformed samples with an increasing number of cycles. Nevertheless, the yield strength for samples processed by the 0.15 strain scheme improved from 450 MPa in cycle 1 to 515 MPa in cycle 3. [ABSTRACT FROM AUTHOR]

A combination of fractographic and metallographic analysis during tensile tests over the temperature ranging from 20 °C to 750 °C were carried out to investigate the fracture behaviors and deformation modes so as to clarify the temperature dependence of mechanical properties of AISI 316 austenitic stainless steel. Planar slip mode of deformation was observed during tensile tests at 20 °C due to a relatively low SFE (stacking fault energies). Pronounced planar slip characteristics were observed in the range of 350–550 °C, and the resultant localized deformation led to the formation of shear bands. The dislocation cross-slip was much easier above 550 °C, leading to the formation of cell/subgrain structures. The preferential microvoid initiation and subsequent anisotropic growth behavior in the shear bands led to large-size and shallow dimples on the fracture surfaces in the range of 350–550 °C. However, the microvoid tended to elongate along the tensile direction in the localized necking region above 550 °C, resulting in small-size and deep dimples. The shear localization reduced the uniform deformation ability and accelerated the fracture process along shear bands, leading to a plateau in uniform elongation and total elongation in the range of 350–550 °C. The higher capability to tolerate the localized deformation through sustained necking resulted in a significant increase in the total elongation above 550 °C. [ABSTRACT FROM AUTHOR]

The effects of cold unidirectional/cross-rolling on the development of substructure, strain-induced martensitic transformation, crystallographic texture (preferred orientation), and mechanical properties, as well as the microstructure after subsequent annealing of metastable austenitic stainless steels were overviewed. First, the deformed state was discussed. Compared to unidirectional rolling, it was revealed that cross-rolling leads to the formation of a greater amount of deformation-induced martensite, which is related to the generation of numerous intersecting shear bands and nano-twins, as well as a higher dislocation density in the austenite phase (activation of higher number of slip/twinning systems). It was concluded that these effects are more pronounced at low reductions in thickness. Regarding texture evolution, cross-rolling tends to strengthen the Brass component in the retained austenite phase. Subsequently, the mechanical properties were reviewed, where it was concluded that a more rapid work-hardening and higher strength/hardness at low strains can be obtained due to the effects of cross-rolling on the microstructure. Moreover, while the effects of cross-rolling on the strength at high rolling reductions might not be significant, it is possible to decrease the anisotropy of the sheet due to the alternate change in the rolling direction. Afterward, the annealing of cold-rolled sheets was discussed. It was deduced that cross-rolling might be used for more intense grain refinement based on the thermomechanical processing of cold-rolling and reversion/recrystallization annealing, where the activation of greater number of slip systems, higher dislocation density, and greater martensite content in the deformed state are responsible in this regard. Finally, the suggestions for future works were proposed. [ABSTRACT FROM AUTHOR]

The disadvantages of widely used austenitic stainless steels are their low hardness and relatively low fatigue strength. Conventional chemical-thermal surface treatments are unsuitable for these steels since they create conditions for inter-granular corrosion. An effective alternative is a low-temperature surface treatment, creating an S-phase within the surface layer, but it has a high cost/quality ratio. Austenitic steels can increase their surface micro-hardness and fatigue strength via surface cold working. When the goal is to increase the rotating bending fatigue strength of austenitic chromium-nickel steels, and the requirements for significant wear resistance are not paramount, diamond burnishing (DB) has significant potential to increase the fatigue strength and, based on the cost/quality ratio, can successfully compete with low-temperature chemical-thermal treatments. The main objective of this study is to establish the effect of DB on the rotating fatigue strength of AISI 304 L chromium-nickel austenitic steel. The influence of DB parameters on the surface integrity (SI) characteristics was studied. Optimal DB parameters under minimum roughness and maximum micro-hardness criteria were obtained. Rotating bending fatigue tests of the diamond burnished (in a different manner) and untreated specimens were performed. DB implemented via parameters providing maximum micro-hardness increased fatigue limit by 38% compared to untreated specimens. [ABSTRACT FROM AUTHOR]

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Industries developing cold spray aim at dense and resistant coatings for component repair. However, as-sprayed 316L coatings display non-equilibrium microstructure and brittle fracture behavior. Improving their mechanical properties requires controlling their microstructure; post-spraying heat treatment is a promising approach. The recovery and recrystallization of coatings were little studied, and heat treatments reported in literature mostly used holding for long time in furnaces, not adapted to on-site repairs. This study aimed at gaining insights into recovery and recrystallization mechanisms of 316L coatings, for a broader range of heat treatment kinetics. A study of powders and as-sprayed coatings was conducted to characterize the initial state. In situ XRD measurements provided input for heat treatment definition. Microscopy, room temperature XRD and hardness measurements allowed to better understand the microstructural evolutions and to select treatments leading to original microstructures. In this work, a variety of microstructures were produced by adapting heat treatment conditions for a given set of spraying parameters. The recrystallization path of the heterogeneous skin-core microstructure of deposited particles, as well as the interaction between grain growth and precipitation was revealed. A novel, optimized fast heat treatment led to a fully recrystallized, fine-grained coating and significantly reduced hardness. [ABSTRACT FROM AUTHOR]

The potential of the concept of phase reversion process to obtain nanograined (NG) structure with high strength–high ductility combination and to fundamentally understand the deformation and fracture mechanisms is described. Deformation studies conducted as a function of grain size suggested that there was a clear distinction and fundamental transition in the deformation behaviour of NG and CG austenitic alloy such that 'deformation twinning' contributed to the excellent ductility of 'high strength' NG alloy, while in the 'low strength' CG alloy, ductility was also good but because of strain-induced martensite nucleation at shear bands. It was postulated that in a given austenitic stainless steel used for biomedical devices, the decrease in grain size from the CG to NG regime increases the stability of austenite. The change in the deformation mechanism impacted the mode of fracture such that it was striated fracture in NG alloy and microvoid coalescence in CG counterpart. [ABSTRACT FROM AUTHOR]