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Arc welding is a widely applied process in the fabrication of stainless steel structures. Various difficulties may arise in joining of these steels, particularly if high heat input is used. The main aim of this study is to evaluate the influence of heat input on microstructure of the joint region and thus on the mechanical behavior of gas metal arc welded AISI 304 L austenitic plates. To this end, AISI 304 L plates with a thickness of 5 mm were butt-welded by employing two different heat input values. Detailed microstructural and mechanical characterization were carried out. In addition, the influence of heat input on the microstructure and joint performance values was also determined. The higher heat input resulted in a complete recrystallization at the heat affected zone whereas the base metal microstructure was maintained in the lower heat input joint. Both joints displayed strength matching within the weld region, thus exhibited slightly higher tensile strength than that of the base plate (102 %).

The microstructure of solution-annealed and aged tensile properties and the stress rupture properties of 15Cr–15Ni titanium-modified austenitic stainless steel with different phosphorus contents were investigated using OM, SEM and TEM. The results showed that two phosphide morphologies were observed after long-term isothermal aging at 850 °C for 1000 h. One was the needle-like M2P distributed within the grain. The other was the blocky M3P distributed at the grain boundaries and twins. The tensile properties of the alloy were unaffected by the phosphorus content, but the stress rupture properties were significantly impacted. With the increase in the phosphorus content from 70 ppm to 250 ppm, the stress rupture life increased from 148 to 269.7 h. Since the strengthening effect of phosphides within the grain or at the grain boundary has been shown to improve the stress rupture properties of alloys, many nanosized granular precipitates, such as the sigma phase, carbides and phosphides, have been observed at the grain boundary, capable of alleviating the stress concentration and limit the crack propagation between two phases, improving the strength of the grain boundary. Intragranular needle-like phosphides can hinder dislocation movements effectively, which improves the intragranular strength of alloys.

The uniaxial tensile behaviors of Fe-19Cr-16Mn-2Mo-0.49N and Fe-18Cr-16Mn-2Mo-0.85N high-nitrogen and nickel-free austenitic stainless steels at two strain rates of 10−2 s−1 and 10−4 s−1 were comparatively investigated. The related deformation microstructure was characterized and fracture mechanism was analyzed. The results show that the nitrogen content and strain rate both have significant effects on the tensile properties of the tested steels. As the strain rate is the same, the tested steel containing a higher nitrogen content has higher Rm and Rp0.2. However, Rm is higher at a lower strain rate and Rp0.2 is higher at a higher strain rate in the case of the same nitrogen content. The tested steel with a lower nitrogen content (0.49 wt.%N) tensioned at a lower strain rate of 10−4 s−1 obtains the highest elongation, while the tested steel with a higher nitrogen content (0.85 wt.%N) tensioned at a higher strain rate of 10−2 s−1 has the lowest elongation. The tensile plastic deformation is mainly governed by slip and twinning, affected jointly by stacking fault energy and short-range order. Dislocation slip featured by planar slip bands and twin-like bands is the main deformation structure in the tested steel containing a higher nitrogen content (0.85 wt.%N) tensioned at a lower strain rate of 10−4 s−1, whereas twinning deformation becomes more prominent with decreasing nitrogen content and increasing strain rate.

The prediction of M-shaped δ-ferrite content along the thickness direction of continuously cast 304 austenitic stainless steel slab was performed using a diffusion-controlled phase transformation module of Thermo-Calc software. The surface and center of the slab, which solidified with different cooling rates and secondary dendrite arm spacing, were used for kinetic calculations. Comparison between observations and calculations for slab specimens demonstrated that the moving-boundary model could predict retained δ-ferrite content according to various thermal histories. After this validation, the effects of different cooling rates at liquid and solid-state were analyzed to understand the importance of each stage on the diffusion-controlled δ $$\to \gamma$$ phase transformation. Decrease in solid-state cooling rate generally had a larger effect on the reduction of δ-ferrite during cooling when the solidification rate was fast (27.75 °C/s) than when it was slow (0.075 °C/s). This model can be used to predict and control δ-ferrite behavior under different thermal histories during slab casting and welding practices.

In this study, room temperature tensile properties of type 304, a metastable austenitic stainless steel, prior rolled above Md temperature (200 °C and 300 °C) are compared with mill-annealed and material prior rolled at room temperature (25 °C). Strain-induced martensite that formed during the tensile tests, followed using in-situ using magnetic measurements, display kinetics that vary with prior deformation temperature and strain. Scanning Electron Microscopy and Electron Backscattered Diffraction studies provided direct evidence for shear band formation during prior deformation. Kinetics of strain-induced martensite is found to fit the Olson–Cohen equation modified to include the prior deformation strain and martensite formed during prior deformation. The systematic changes observed in the model parameters of the modified Olson–Cohen equation are explained. The strain hardening in the material is analyzed and correlated with changes in the rate of formation of strain-induced martensite with respect to strain.

To study and understand the solidification behavior of super austenitic stainless steel under different cooling rates and segregation laws of alloying elements is of great significance to optimize the subsequent diffusion annealing homogenization treatment process and improve product quality. According to Thermo-Calc thermodynamic simulation results and combined with high temperature laser confocal scanning electron microscope (HT-CSLM), the tissue morphology of 904L super austenitic stainless steel was observed in-situ during solidification. The solidification path of the test steel was determined via calculation with the Scheil-Gulliver model. Microscopy techniques, including true color microscopy, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and electron probe microanalyzer (EPMA) were used to analyze the influence of different cooling rates (6 ℃/min, 50 ℃/min, and 100 ℃/min) on the solidification structure and determine the main distribution law of alloying elements. This analysis determined that the solute distribution coefficient (K) of Cr, Mn, Mo, Cu, and Si elements is less than 1 during the solidification process, which means that they will accumulate in the liquid phase. Among them, elemental Mo segregation is the most severe, while elemental Ni hardly segregates. As the cooling rate increases, the crystallization temperature of the test steel decreases, and the secondary dendrite arm spacing λ2 decreases, the concentration of Mo in the residual liquid phase increases.

The use of post-processing heat treatments is often considered a necessary approach to relax high-magnitude residual stresses (RS) formed during the layerwise additive manufacturing laser powder bed fusion (LPBF). In this work, three heat treatment strategies using temperatures of 450 °C, 800 °C, and 900 °C are applied to austenitic stainless steel 316L samples manufactured by LPBF. These temperatures encompass the suggested lower and upper bounds of heat treatment temperatures of conventionally processed 316L. The relaxation of the RS is characterized by neutron diffraction (ND), and the associated changes of the microstructure are analyzed using electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM). The lower bound heat treatment variant of 450 °C for 4 hours exhibited high tensile and compressive RS. When applying subsequent heat treatments, we show that stress gradients are still observed after applying 800 °C for 1 hour but almost completely vanish when applying 900 °C for 1 hour. The observed near complete relaxation of the RS appears to be closely related to the evolution of the characteristic subgrain solidification cellular microstructure.

Steel (St) and aluminum (Al) have a high affinity to each other and form intermetallic compounds (IMCs) when bonded by friction stir welding (FSW). Understanding the effect of alloying elements of steel on the formation of IMCs will help to design joints in which the formation of IMCs is controlled. In this study, two kinds of steel, carbon steel (CS) and austenitic stainless steel (SS) with 3 mm thickness, were selected to be welded to aluminum by FSW under similar conditions. The effect of the rotation speed of the FSW tool was also examined in the range of 850–1300 RPM. The morphology, thickness, and composition of IMCs in the interfaces were studied by various characterization techniques. It was observed that the IMC layers in the SS/Al joints were much thinner than the ones in CS/Al joints (0.1–0.7 µm in SS/Al and 2–6 µm in CS/Al). Moreover, the thickness of IMC layer in CS/Al joints increased with increasing the rotation speed while in SS/Al joints it began to decline by exceeding a certain rotation speed. In order to explain these differences, diffusion-based equations were used to calculate the interdiffusion coefficients in both SS/Al and CS/Al couples. It was found that the interdiffusion coefficient in SS/Al was lower than CS/Al. This was attributed to the alloying elements of SS such as Ni and Cr which can diffuse to IMC layer and hinder the growth rate of IMCs. It was also observed that some form of liquation occurred in SS/Al joints at high rotation speed of FSW due to the formation of a low-melting multielement compound of Al–Fe–Cr–Ni. The decline of IMC thickness in SS/Al joints at high rotation speed was attributed to this multielement compound which was melted during FSW and reduced the nucleation rate of Al–Fe IMCs. Finally, the fracture of the specimens and the effect of IMC formation on the joint establishment were elaborated. It was concluded that the alloying elements of steel have a beneficial effect on controlling the growth of IMCs.

Two different alloys were developed from the mixtures of pre-alloyed powders consisting of ferroalloys to get the required composition of austenitic stainless steel via the powder metallurgy techni...

It is still a big challenge to calculate the probability of defect detection for inspecting pits on austenitic stainless steel welding using traditional POD models. Because we determine the decision threshold, the POD curve has a lot of changes as the decision threshold changes, there is no clear reason to insist which threshold is reasonable. This study proposes a new probability of detection (POD) model to quantitatively assess the detection capability of eddy current probes for inspecting pits on austenitic stainless steel welding. The experimental results show that the proposed model is more reasonable than traditional ones. The novel POD model was employed to analyze three eddy current probes, uniform, TR, plus-point probes on austenitic stainless steel welding. The results reveal that the uniform eddy current probe has the best detection capability for inspecting pits on among the three probes.

To resist large electromagnetic loads, the ITER correction coils (CCs) are protected by a 20-mm thickness 316LN austenitic stainless steel case. Due to the strict tolerance requirement of the case manufacture, tungsten inert gas (TIG) welding technology was used for subcase assembly welding, whereas laser beam welding (LBW) technology was applied to enclosure welding of the cases. The case shall be operated at 4.2 K and shall undergo considerably cyclic stress due to magnet electromagnetization as well as interaction with the toroidal field (TF) and poloidal field (PF). This article focuses on the assessment of the mechanical properties of the welding joints at cryogenic temperature. Results indicated that the values of the ultimate tensile strength of the TIG welding joint and the LBW welding joint were 1555 and 1522 MPa, respectively, which reach to 99.7 and 97.6% of that of the base material (BM). The elongation at the fracture of both the TIG and LBW weldments was larger than 35%. The plane strain fracture toughness K Ic( J ) of the TIG and the LBW weldments was 210 and 241 MPa·m1/2, respectively. The fractographic analysis indicated that the fracture surface was composed of fine equiaxed dimples, which indicated that both welding joints fractured in a ductile manner. The welding joints survived the minimum of 30 000 cycles fatigue test at stress level and load ratio specified by the ITER document. In addition, the fatigue crack growth rates of both welding joints were lower than that of the BM. All mechanical properties of both the welding joints satisfied the ITER specifications.

Pure commercial titanium was welded with two types of stainless steel, namely SUS 304 austenitic stainless steel and SUS 821L1 duplex stainless steel. The wavy interface of SUS 821L1 was smaller than that of SUS 304. The vortex zone was observed from both longitudinal and transverse directions, and its composition was analyzed. The interface of Ti/SUS 821L11 was able to bear 401–431 MPa shear load while that of Ti/SUS 304 could withstand 352–387 MPa. The weldability window was used to analyze experimental phenomenon. Furthermore, the smoothed particle hydrodynamics (SPH) numerical simulation method was used to simulate the wavy interface. The trend of wavelength and amplitude change with strength and the stand-offs was consistent with the experimental results.

AISI 316H austenitic stainless steel was subjected to equal channel angular pressing (ECAP) at 350°C up to eight passes and the resultant mechanical properties were compared with those obtained after cold rolling as representative for a cold worked state. A high density of thin parallel slip bands was observed inside coarse grains at the beginning of ECAP. Intense micro shear banding took place when pressing was continued by adding more passes. While no martensitic transformation was detected in the microstructure deformation twinning identified to occur at late passes of ECAP. The final microstructure after 8 passes is characterized by a tri-modal grain size distribution with equiaxed ultrafine grains of 176 nm and regions with larger grains of 217–1376 nm surrounded by shear bands containing nano-crystalline grains. Nano twins, 8 nm on average wide, were observed inside the nanocrystalline austenite grains. The observed microstructural features were explained by the stacking fault energy and temperature range for martensitic transformation of the studied material. In terms of mechanical properties, the processed material displayed a combination of very high yield strength exceeding 1550 MPa (close to four times the initial value) and good ductility with deformation ef = 11.6% at failure.

Intrinsic stacking fault energy (SFE) values of γ-Fe and AISI 304 austenitic stainless steels were determined as a function of carbon and nitrogen content using ab initio calculations. In contrast to previous investigations, the analysis was conducted incorporating the paramagnetic state to account for the magnetic constitution of real austenitic stainless steels. The effect of finite temperature was partially accounted for by performing ab initio calculations at the experimental volumes at room temperature. Including paramagnetism in γ-Fe increases the SFE of non-magnetic γ-Fe by ∼385 mJ.m−2. Interstitial alloying of non-magnetic γ-Fe causes a linear increase in intrinsic stacking fault energy wγith interstitial content. In comparison, interstitial alloying of paramagnetic γ-Fe increases the SFE at only about half the rate. The SFE of paramagnetic interstitial-free AISI 304 is within the range of -12 to 0 mJ.m−2 and only deviates slightly from the SFE of paramagnetic γ-Fe. It follows a similar, albeit flatter linear dependency on the interstitial content compared to γ-Fe. Both γ-Fe and γ-AISI 304 were found to be metastable in their interstitial-free condition and are stabilized by interstitial alloying. The possible effect of short range ordering between interstitials and Cr on the SFE was discussed. The calculated threshold nitrogen content necessary to stabilize austenite in AISI 304 is in good agreement with experimental investigations of deformation microstructures in dependence of the nitrogen content. Finally, the calculated negative SFE values of AISI 304 were reconciled with experimentally determined positive SFE values using a recent method that accounts for the kinetics of stacking fault formation.

The heterogeneous and homogeneous nanostructured 316LN austenitic stainless steel with tensile strengths of approximately 1.8 and 2.0 GPa were fabricated by 92% heavy cold rolling and high-pressure torsion straining, respectively. The influence of heterogeneous and homogenous nanostructures on the resistance to hydrogen embrittlement was investigated. Thermal desorption spectroscopy of the electrochemical hydrogen-charged specimen showed that both nanostructures exhibited almost the same concentration of diffusible hydrogen. While the tensile properties of the homogenous nanostructure were severely affected by the diffusible hydrogen, it had less impact on the tensile properties of the heterogeneous nanostructure. The difference in the embrittlement behavior between the heterogeneous and homogeneous nanostructures is attributed to the mechanism of crack growth observed by tensile fractography. Hydrogen embrittlement is suppressed in the heterogeneous nanostructure because crack growth is hindered by twin domains. The crack easily initiates in the homogeneous nanostructure and the resistance to crack growth is low.

International audience; Ti-stabilized austenitic stainless steel was carburized in sodium containing a high carbon activity at three different temperatures, 500 °C, 600 °C, and 650 °C during 1000 hours and 5000 hours. The carbon profile, the carbide volume fraction, and the lattice parameter evolution as function of depth were determined using high-energy X-ray diffraction and electron probe microanalysis. At 650 °C and 600 °C, the carbon precipitated as M23C6 and M7C3 carbides in the sample. The volume fraction of M7C3 carbides was lower than predicted by thermodynamic equilibrium using Thermo-Calc software®. At 500 °C, carbides almost did not form in the steel. Instead, high carbon supersaturation of the austenitic matrix occurred. Both results demonstrate that the carburization profile was strongly influenced by the kinetics of carbide formation at temperatures lower than 650 °C. High-energy X-ray diffraction measurements demonstrated that the austenite and carbide lattice parameters evolved along the carbon profile. Both measured lattice parameter profiles of austenite and M23C6 carbide were compared to the ones predicted from chemical changes of austenite and carbides.

Micro hot embossing is a well-established technology to produce polymer microdevices and parts. This technology on metal powder/binder feedstocks is an innovator challenge that fits into the sustai...

The article discusses the mechanical properties and cold resistance of austenitic stainless steel (analogue 07Cr25Ni13) obtained by 3D printing by electric arc surfacing from ER309LSI welding wire on a CNC machine. These properties were investigated in the process of physical tests of samples cut along and across the layers of 3D printing for tensile and impact bending. Using optical microscopy, the microstructures of steel sections were obtained for various temperature conditions of interlayer exposure, as well as the values ​​of the recommended microhardness. In the process of 3D printing, an intelligent system for monitoring the dynamic stability of the electric arc was applied, which made it possible to guarantee the stability of the structure and properties of the obtained samples throughout the entire process of surfacing. Additional heat treatment of experimental samples (austenitization) was considered as a way to improve mechanical properties and cold resistance. It has been established that the dynamic stability of an electric arc, modes of interlayer temperature holding and subsequent heat treatment largely determine the mechanical properties and cold resistance of ER309LSI steel obtained by 3D printing by electric arc surfacing.

Unstabilized nitrogen-containing silicon steel 03Kh18N13S2AM2VFBR-Sh (EP 302M-Sh), intended for heat exchanger tubes operating in contact with heavy liquid-metal coolants and water at temperatures of 350 to 550°С, is prone to prolonged thermal aging and sensitization, initiating local metal corrosion in water at 350°С. Changes in the chemical composition of steel and the production of pilot batches of heat exchanger tubes did not lead to an increase in their thermal stability. One of the reasons for this is the factor of nitrogen solubility in austenite, which was neglected during the development of chemical compositions of metal. The principles of alloying and the conceptual chemical composition of unstabilized austenitic steel (EP 302M1-Sh), not prone to thermal aging and sensitization at a temperature of 350°С, have been developed, which excludes the possibility of local corrosion of the heat exchanger tubes during long-term operation in water. A 25% increase in yield point at 350°С compared to steel 08Kh18N10T and the absence of susceptibility to pitting, crevice, and hideout corrosion make thermally stable steel the preferred candidate material for producing heat exchanger tubes for innovative water-cooled power reactors. A possibility of creating economically alloyed austenitic steel, characterized by thermal stability at 350 to 550°С and corrosion resistance in liquid-metal coolants and supercritical steam-water medium, is shown.

An Nd:YAG laser device is used to conduct laser welding for a 1 mm austenitic stainless steel plate and a ferritic stainless steel plate. A finite element model of the shear punching test is constructed to generate the maximum shear strength in the weld, and the finite element model of laser welding is created using the welding temperature field. The hole drilling test result and the residual stress generated by two algorithms (Nonuniform-material and uniform-material) are compared. Results show that a drop-off of residual stress in the central area of the welded joint is observed when materials heterogeneity is disregarded. When materials heterogeneity is considered, the residual stress curve indicates smooth transition. The value of the latter appears reasonably similar to the experimental value. Therefore, this solution is advantageous in terms of residual stress simulation in dissimilar welded joints and does not critically affect residual deformation.

δ-ferrite was formed in the weld metal when the melted metal solidified to the room temperature. The δ-ferrite morphology depended on the composition, temperature gradient and growth rate. Research on the influence of heat treatment temperature (400°C, 600°C, 900°C) on the morphology changes and the δ-ferrite content is presented in this paper using optical microscopy, SEM, TEM. The δ-ferrite concentration reduced continuously in increasing temperature (from 23.5% after welding to 11% at 900°C for 10 hours). Besides, the formation of sigma phase and carbides at 600°C were the main cause of increasing hardness values in the fusion zone. However, the heat treatment at a temperature of 900°C eliminated both the sigma phase and brittleness.

The effect of cold rolling on hydrogen embrittlement in stable 18Cr–1Mn–11Ni-0.15 N austenitic stainless steels was investigated. Alloy plates were cold-rolled to 15% or 30% reduction, then pre-charged with hydrogen and subjected to tensile testing with slow strain rate. Hydrogen-induced degradation of tensile elongation became increasingly severe with the increase in the degree of cold rolling. During cold rolling, deformation twins with various orientations were actively generated, and twins with specific orientations were vulnerable to hydrogen-induced cracking. Cold rolling also increased the density of defects, and thereby facilitated penetration of hydrogen into the steels. The combination of cracks generated at the twin boundaries, and the promoted hydrogen diffusion caused severe hydrogen embrittlement in the cold-rolled steels.

During the U-type hot cracking test of gas tungsten arc welding (GTAW), the back side stainless steel was subjected to a continuous water flow (water-cooled GTAW) to prevent solidification cracking...

In this study, AISI 316L stainless steel was passivated under galvanostatic conditions in citric acid solutions in the absence and presence of hydrogen peroxide (H2O2). The electronic-semiconducting properties of the resulting passive layers were assessed using Mott–Schottky (MS) analysis. Moreover, the corrosion resistance of the passivated alloy produced under different experimental conditions was examined using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy test methods. The passive films were also characterized using grazing incidence X-ray diffraction. The MS results revealed that higher passivating current densities foster film formation, leading to lower concentration of charge carriers, which beneficially influences the corrosion performance of the alloy. Besides, the addition of H2O2 to the passivating electrolyte was accompanied by deterioration of passive film properties and inferior corrosion resistance. The DC and AC corrosion test results were also consistent with those of the MS approach. Last, but not least, the samples that were surface treated in H2O2-containing baths, especially those passivated at higher current densities, were more prone to localized attack compared to their counterparts produced in H2O2-free electrolyte.

The microstructure and mechanical properties of chromium–nickel austenitic stainless steel fabricated by selective laser melting using a Realizer SLM100 3D printer have been investigated in this work. The structure of the studied specimens has been formed by the complete melting of the initial powder and high-speed cooling of the melt. Cooling of the melt initially leads to the formation of δ ferrite, and then, polymorphic δ → γ transformation results in the formation of the final austenitic structure. The structure of δ ferrite which formed during melt crystallization has been found to exhibit a clear pattern of periodicity. The periodicity depends on the parameters of the melting process, such as the distance between neighboring bands formed during laser-beam traveling (intertrack distance) and the step of platform feeding (distance between layers). The polymorphic δ → γ transformation takes place by a disordered mechanism and no austenite texture forms. However, some structural heredity remains. It can be seen in the orientational relationship between some austenite grains and δ-ferrite grains. The steel fabricated by laser melting is shown to have high mechanical properties such as the yield strength, the ultimate tensile strength, and the tensile elongation at a strain rate of 10–2 s–1, which are 320, 765 MPa, and 50%, respectively. The yield strength and ultimate tensile strength of the specimens under dynamic compression by the Hopkinson–Kolskii technique at an average strain rate of 103 s–1 are 550 and 945 MPa, respectively.

The corrosion behavior of austenitic stainless steel after low-temperature liquid oxy-nitriding (LON) was investigated by exposing in H2S/CO2-saturated liquid and vapor environments up to 720 h at 60 °C. The corrosion rates before and after LON were compared by the weightlessness method, and the microstructure as well as the corrosion scales were characterized using surface analysis methods. The results indicated that the composite S-phase layer with the outer Fe3O4 layer and the inner nitrogen-rich sublayer could improve the corrosion performance in H2S/CO2-saturated environment. The base material (BM) suffered local corrosion first, which then transformed into uniform corrosion. As a comparison, The LON sample, covered with a thin corrosion product layer, indicated slight local corrosion. The excellent corrosion resistance of the S-phase should be attributed to the blocking effect of the continuous Fe3O4 film as well as the suppression of the atomic mobility by the nitrogen-containing supersaturated solid solution.

Recently, it has been reported that the wear resistance of different stainless steel grades can be substantially improved by addition of boron as alloying element, however, with a trade-off in the corrosion resistance. In this work, CALPHAD method was employed to design a ferritic and an austenitic stainless steel modified with 1 wt pct B with high corrosion resistance. The designed alloys were produced by conventional casting in graphite mold and characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Structural characterization revealed that the target microstructures composed of ferrite and austenite matrixes with hard borides were successfully achieved. Their corrosion resistance was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in synthetic seawater and compared to conventional 316 L stainless steel. It was shown that designing the chemical composition of the alloys’ matrixes is the key to obtain corrosion resistant boron-modified stainless steels.

Orientation microscopy (EBSD) has been used to investigate the structure of austenitic stainless steel containing 18% Cr and 9 wt % Ni after its long-term operation at high temperatures and neutron irradiation. The γ → α shear transformation has been implemented partially in the samples during deformation-free cutting. Austenite has decomposed due to isotropic stresses oriented normally to the surface under study. The precision of orientation relationships (ORs), such as Kurdjumov–Sachs, Nishiyama–Wassermann, Greninger–Troiano, and others during γ → α transformation has been comparatively analyzed. The Greninger–Troiano OR has been found to be the best possible OR describing the transformation in this case. The α phase has been shown to nucleate at coherent twin boundaries (Σ3 in the coincidence site lattice model) between austenite grains. This has determined the variants of the current ORs and the unambiguous crystallographic orientation of α-phase grains.

One of ways to increase the strength of a structural material is to grind its microstructure to an ultra-fine-grained state. But with the simultaneous increase in strength properties in ultrafine materials, there is an inevitable decrease in its plastic properties. The use of metal materials with a gradient structure is an effective way to solve the problem of increasing the plasticity of metal products in general. This work is devoted to the study of the possibility of a gradient structure forming in austenitic stainless steel AISI-321 using radial-shear rolling. The results of the studies showed that the UFG-structure extends in the bar from its surface to a depth of at least a quarter of the radius of the bar. The transition zone is in the region between 0.5 R and 0.25 R of the bar section. Anything deeper is rolling texture. Due to the structural heterogeneity of the cross-section of the bar, there is a smooth drop in the micro-hardness of the central zone of the bar by 10.2 %. All this testifies to the gradient character of the structure formed in bars of AISI-321 steel deformed on the radial-shear rolling mill.

Vacuum brazing of austenitic stainless steel (SS) under stress with copper-based filler was carried out to assess the susceptibility of liquid metal embrittlement (LME) on stress. Dipping of the SS...

Heterogeneous microstructures have been proposed to optimize the mechanical properties in the respect of strength and ductility synergy in many alloy systems. In this work, the multi-phase hetero-structured microstructure consisting of fine grains and coarse grains where the ferrite is embedded in the vicinity of the nano-grains was obtained by regulating the thermomechanical processing parameters. A yield strength of 1.2 GPa which is more than two times higher than the coarse grained counterpart with less uniform elongation loss is achieved in the multi-phase hetero-structured steel. The microscopic load transfer in this complex microstructure is investigated by the in situ high energy synchrotron X-ray diffraction. The persistent strain hardening is ascribed to the hetero-deformation induced stress associated with the joint activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects. As a comparison, the single-phase hetero-structured steel exhibits little lower strength but higher uniform ductility.

This work is devoted to studying the possibility of forming a graded structure in 08Kh18N10T stainless austenitic steel using radial-shear rolling. Experimental studies show that an ultrafine-grained structure extends into the bar from its surface to a depth of at least a quarter of the bar radius. A transition zone is located in the area between 0.5R and 0.25R of the bar cross section. Due to structural heterogeneity in the bar cross section there is an increase in microhardness by more than a factor of two from the initial value of 152 to 320HV in surface layers and a smooth drop by 10.2% in microhardness within the central area of a bar. This points to a graded nature of the structure formed in the bars of 08X18N10T steel after radial-shear rolling. A study of mechanical properties shows an increase in the tensile strength by about a factor of two to 938 MPa and a reduction in elongation to 24% in the peripheral zone and to 19% in the axial zone.

The fatigue crack growth rate of warm-rolled AISI 316 austenitic stainless steel was investigated by controlling rolling strain and temperature in argon and hydrogen gas atmospheres. The fatigue crack growth rates of warm-rolled 316 specimens tested in hydrogen decreased with increasing rolling temperature, especially 400 °C. By controlling the deformation temperature and strain, the influences of microstructure (including dislocation structure, deformation twins and α′ martensite) and its evolution on hydrogen-induced degradation of mechanical properties were separately discussed. Deformation twins deceased and dislocations became more uniform with the increase in rolling temperature, inhibiting the formation of dynamic α′ martensite during the crack propagation. In the cold-rolled 316 specimens, deformation twins accelerated hydrogen-induced crack growth due to the α′ martensitic transformation at the crack tip. In the warm-rolled specimens, the formation of α′ martensite around the crack tip was completely inhibited, which greatly reduced the fatigue crack growth rate in hydrogen atmosphere.

The effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in specimens of austenitic stainless steel 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. After the helium implantation, the microstructures were examined by a transmission electron microscopy and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels at 823 K implanted with He ion up to 100 appm with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behaviors were examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the results of TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions.

Material choices for liquid lead bismuth spallation target are some of austenitic stainless steel, ferrite martensitic steel and cold-worked austenitic stainless steel. In order to ensure materials resistance to irradiation and corrosion as well as compatibility with lead bismuth, it is appropriate to lower the incident proton current density and the process temperature, in which temperature range engineering design can control to work, especially in ADS (Accelerator-Driven nuclear transmutation System) concept. The lower limit temperature is determined from the physical melting temperature and the engineering efficiency of the steam generator involved in process control. The material related issues for liquid lead bismuth are mass loss by impinging secondary flow, wettability at the device interface for ultrasonic waves application, detachable control of the slag in the flowing system, stabilized electrical resistance between the material and the liquid lead bismuth interface. Electromagnetic fluid analyses show how flow rate relates electrical resistivity of flow channel material.

The effects of internal hydrogen on the deformation microstructures of 304L austenitic stainless steel have been characterized using electron backscattered diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution scanning transmission electron microscopy (HRSTEM), and nanoprobe diffraction. Samples, both thermally precharged with hydrogen and without thermal precharging, were subjected to tensile deformation of 5 and 20 pct true strain followed by multiple microscopic interrogations. Internal hydrogen produced widespread stacking faults within the as-forged initially unstrained material. While planar deformation bands developed with tensile strain in both the hydrogen-precharged and non-precharged material, the character of these bands changed with the presence of internal hydrogen. As shown by nanobeam diffraction and HRSTEM observations, in the absence of internal hydrogen, the bands were predominantly composed of twins, whereas for samples deformed in the presence of internal hydrogen, ε-martensite became more pronounced and the density of deformation bands increased. For the 20 pct strain condition, α′-martensite was observed at the intersection of ε-martensite bands in hydrogen-precharged samples, whereas in non-precharged samples α′-martensite was only observed along grain boundaries. We hypothesize that the increased prevalence of α′-martensite is a secondary effect of increased ε-martensite and deformation band density due to internal hydrogen and is not a signature of internal hydrogen itself.

A previous work has shown that a modified HP-NbTi heat-resistant stainless steel exhibited a different magnetic behavior after 90,000 hours of operation in a steam reformer furnace exposed to temperatures between 500 °C and 550 °C or between 950 °C and 1000 °C. In this work, in the sample exposed to the lower temperature range, a thin ferromagnetic layer was detected by magnetic force microscopy in the matrix around the chromium and niobium carbides. However, for the sample exposed to the higher temperature range, no ferromagnetic response was detected whatsoever. The understanding of the influence of microstructure on magnetic behavior is important for the development of non-destructive inspection test methodologies. For this reason, the aim of this study is to perform a microstructural analysis to assess the different magnetic responses. Transmission electron microscopy allowed identifying a Cr-depleted zone in austenite at the Cr23C6-matrix and (NbTi)C-matrix interfaces in the sample exposed to the lower temperature range. Therefore, the austenitic region, with a lower Cr content, becomes ferromagnetic. In the sample exposed to the higher temperature range, the chemical composition of the austenitic matrix was homogenous, and the sample was completely paramagnetic.

Molecular Dynamics simulations of the diffusion process along a grain boundary in a model austenitic stainless steel are presented. We address diffusion in a series of general grain boundaries of tilt character with random misorientations around the axis. The diffusion process is quantified by analyzing individual atomic displacements. The results of simulations are verified by comparison with available experimental data. The results show that Cr diffuses about 25 pct faster than Fe or Ni. Most importantly, the simulations show that the diffusion kinetics varies widely among the set of general high angle boundaries studied, with diffusivities that span more than an order of magnitude. The variation is analyzed for correlations with grain boundary structure and energy. Faster diffusion kinetics are associated with higher energy grain boundaries and boundaries that are narrower, with more extra volume and are less centrosymmetric in their structure.

Steel containers and instruments are used to hold and transport Mg melts. Thus, quantitative analysis of the dissolution of metallic elements from the steel materials into the liquid Mg is important for controlling the impurities in processes such as Mg alloy production and Ti smelting. When austenitic stainless steels containing Cr and Ni come in contact with a Mg or Mg alloy melt, a large amount of Ni and limited amounts of Fe and Cr dissolve into the melt. In this study, the composition of a Mg-Ni-based melt in equilibrium with SUS316 was investigated to determine the maximum amounts of impurities dissolved from SUS316 into the liquid Mg. Mg-Ni melts of different compositions were held in a closed crucible of SUS316 at 1073 K (800 °C) to 1273 K (1000 °C), and the compositions of the inner wall of the crucible and the Mg-Ni alloy were evaluated after quenching. Subsequently, the relationships between the solubility limit of element i (i: Ni, Fe, and Cr) in liquid Mg with the coexistence of SUS316, $$ C_{{{\text{sol}},{i}}}^{*} $$ (mass pct), and the temperature, T (K), were determined: $$ \log \,(C_{{{\text{sol}},{\text{Ni}}}}^{*} ) = {{ - 1.40 \times 10^{3} }}/{T} + 2.57 \, ( \pm \,0.12), $$ $$ \log \,(C_{{{\text{sol}},{\text{Fe}}}}^{*} ) = - {{5.20 \times 10^{3} }}/{T} + 3.93 \, ( \pm \,0.16),\;{\text{and}} $$ $$ \log \,(C_{{{\text{sol}},{\text{Cr}}}}^{*} ) = - {{5.96 \times 10^{3} }}/{T} + 3.80 \, ( \pm \,0.05). $$ These solubility limits were compared with those estimated based on the available thermodynamic data for the SUS316 and Mg-i binary systems. The validity of the obtained data and the reliability of the previously reported thermodynamic data were also discussed.

The vacancy behavior in austenitic stainless steel 316L with high γ-phase stability in a hydrogen environment was investigated to clarify the critical defects of hydrogen embrittlement. Hydrogen was introduced in the samples by the high-pressure gas method and tensile straining was conducted at variable low temperatures. Characterization of the strain distribution and vacancy defects was performed by SEM-KAM and low-temperature positron annihilation lifetime spectroscopy measurements, respectively. Upon straining at low temperatures (

Static strain aging behavior of cold-rolled 16Cr-5Ni supermartensitic stainless steel was investigated after it had been aged for 30 min at 400°C. In uniaxial tensile tests of 20% cold-rolled specimens, increase of yield strength to ~300 MPa and decrease of strain hardening rate were observed in the aged specimen, compared to the as-rolled specimen. In interrupted tensile tests, delayed martensitic transformation was observed in the aged specimen. Dilatometry analysis detected volume shrinkage during the 30 min holding period at 400°C indicating partitioning of carbon (C) from α’ martensite to austenite. The clustering of C in α’ martensite was confirmed by impulse internal friction technique with observation of Cottrell atmosphere formation. 3D atom probe tomography analysis revealed partitioning of C atoms into austenite and clustering of C atoms in the α’ martensite. The remarkable increase of yield strength was attributed to redistribution of C atoms during the aging treatment.

The grain boundary character distribution (GBCD) in an austenitic stainless steel produced by additive manufacturing (AM) in both as-built and annealed conditions was studied. Relatively fine grains and a non-fibre texture was achieved by AM, and as-built structure showed a high population of Σ3 boundaries. A five-parameter GBCD analysis revealed that the microstructure is mostly dominated by highly incoherent Σ3 boundaries. The grain boundary network also consisted of random high angle, coherent Σ3s terminating on (111) planes with a pure twist character, and tilt Σ9 boundaries. The findings show prospects for the possibility of engineering the grain boundary network of materials in-situ, via the stress and heat induced by the thermal cycles during AM.