Austenitic Stainless Steel Specimens Research Articles

Role of grain size and anisotropy of neighboring grains in hydrogen‐assisted intergranular fatigue crack initiation in austenitic stainless steel

AbstractThis study explores the impact of microstructural features on fatigue crack initiation in poly‐crystalline materials, emphasizing hydrogen‐induced complexities. Grain anisotropy, misorientations, grain size variations, and elastic–plastic inhomogeneities concentrate stress at grain boundaries, making them susceptible to crack initiation during fatigue loading. The presence of hydrogen compounds this process, due to complications of characterization of local hydrogen content and activating embrittling mechanisms. Building upon a model for nickel, this research investigates 316L austenitic stainless steel specimens with varying grain sizes, both uncharged and hydrogen‐charged. In situ low‐cycle fatigue loading experiments establish correlations between fatigue crack initiation and microstructural features. The study reveals specific combinations of features crucial in the initiation process, undergoing alterations in the presence of hydrogen. A proposed qualitative model links microstructural features with accumulated plastic shear strain during fatigue and prevalent hydrogen embrittlement mechanisms like hydrogen‐enhanced local plasticity and hydrogen‐enhanced decohesion.

Constitutive modeling and tensile behavior of a fully austenitic gradient nanostructured stainless steel

The constitutive relationship can effectively describe and predict the deformation behavior of materials, and it is critical to establish appropriate constitutive relationships to obtain the mechanical response of gradient nanostructured (GNS) metallic materials during deformation. In this work, a general method for establishing the multi-layer constitutive relationship of the GNS metallic materials was proposed. A fully austenitic 310S stainless steel specimen with a thickness of 1.4 mm was fabricated by surface mechanical rolling treatment (SMRT) technology, and its depth-dependent microstructures and macroscopic mechanical properties were experimentally investigated. Based on the experimental results of uniaxial tensile and microhardness, using the quantitative relationship between microhardness and flow stress, the reverse analysis was conducted using a separation parameter method to establish the constitutive relationship of the 28 thin shells of the GNS310S specimen. Finite element analysis (FEA) confirmed that GNS310S was in a multiaxial stress state at low strains, and the evolution of stress and strain gradients during non-uniform deformation was analyzed. The method for establishing the constitutive relationship is independent of the microstructure and synergistic strengthening factors, and the experiment performs conveniently and has great generalizability, which can provide a useful guide for the mechanical modeling of the GNS metallic materials to obtain the intrinsic mechanical response.

Effect of grain boundary misorientation and carbide precipitation on damage initiation: A coupled crystal plasticity and phase field damage study

A coupled crystal plasticity phase field damage framework has been developed and applied to modelling damage initiation. A novel implementation of a grain misorientation angle dependent critical energy release rate has been used to determine a reduction in the local critical energy release rate resulting from the effects of intergranular carbide precipitates and grain boundary misorientation. When applied to a notched high temperature 316H austenitic stainless steel specimen, a good correlation between experimental results and void nucleation statistics for a misorientation dependent critical energy release rate was obtained. This has been evaluated through comparison with correlative electron microscopy experimental results, showing the potential of phase field models in the area of early damage formation. Additions to include plastic strain and creep deformation effects were made, and comparisons were drawn with experimental data to investigate the contributions of microstructural geometry properties such as the difference in and average values of Schmid factors across grain boundaries, as well as the loading direction stress and dislocation densities. The limitations to this approach and opportunities for further work in this area are discussed, with specific interest in the need for additional literature data characterising grain boundary carbide precipitation and cavity nucleation analysis.

The influence of Y addition on oxide layers of 15-15Ti-Y steel exposed to static Pb-Bi alloys with containing oxygen

The corrosion behavior was investigated in a static environment of liquid Pb-Bi eutectic (LBE) saturated with 10−6 wt.% oxygen at 500°C and 600 °C. The investigation focused on 15-15Ti and 15-15Ti-Y austenitic stainless steel specimens, exposing them for up to 5000 h. Additionally, both steel types were analysed after exposure to liquid Pb40Bi60 alloy with 10−6 wt.% oxygen concentration at 600 °C for 2000 h. The main objective was to investigate the structure and composition of oxide layers formed during corrosion, with specific attention to the role of yttrium (Y) in influencing corrosion behavior. The addition of Y resulted in promoting denser spinel oxide layer formation, which exhibited enhanced properties in reducing outward cation diffusion, inward oxygen diffusion, and PbBi penetration into the steels.

Prediction Technique of Surface Characteristics of Fatigue Specimens of Austenitic Stainless Steels by Controlling Finish Processing

In order to develop the technique to predict the surface characteristics of fatigue specimens of austenitic stainless steels by controlling finishing processing of lathe turning and shot peening, the applicability of the response surface models formerly created by the present authors using experimental design of experiments approaches to the fatigue specimens with different types and/or rolling conditions of austenitic stainless steels have been investigated. The conclusive remarks could be summarized as follows: (1) The predicted surface residual stress obtained by the response surface model of the lathe turning created for the round bars of SUS304 agreed with the observed surface residual stress of the fatigue specimen of SUS316. (2) The response surface model of the shot peening created for the flat plate of SUS316 was applicable to the prediction of the surface roughness RSm, the surface hardness and the surface residual stress of the fatigue specimen of SUS316 in spite of the different processing of the shot peening and the rolling conditions of the material.

Quantitative 3D Characterization for Kinetics of Corrosion Initiation and Propagation in Additively Manufactured Austenitic Stainless Steel.

In situ X-ray computed tomography (X-ray CT) is used to investigate the effects of characteristic microstructural features on the pitting initiation and propagation in austenitic stainless steel specimens prepared with laser powder bed fusion (LPBF) additive manufacturing. In situ X-ray CT in probing the mechanism and kinetics of localized corrosion is demonstrated by immersing two LPBF specimens with different porosities in an aggressive ferric chloride solution for the evaluation of corrosion. X-ray CT images are acquired from the specimens after every 8hours of immersion over an extended period of time (216hours). Corrosion pit growth is then quantitatively analyzed with a data-constrained modeling method. The pitting growth mechanism of LPBF stainless steel is found to be different from that of conventional stainless steels. More specifically, the mechanism of corrosion pit initiation is closely correlated with the original lack of fusion porosity (LOF) distribution on the surface of the specimens and preferential pit propagation through the LOF pores inside the specimens. Pit growth kinetics are derived from pit volume changes determined through 3D data analysis. The pit growth kinetics in LPBF specimens are found to vary in the initial pit formation, competitive pit propagation, and the dominant pit growth stages.

Quantitative study of the mechanical properties of micro-nanostructured 304 stainless steel

The austenitic stainless steel specimens with a micro-crystalline/nanocrystalline bimodal structure (micro-nanostructure) were produced by aluminothermic reaction casting method, and the tensile tests and in situ tensile tests were performed. It is found that the stress–strain curve displays the secondary yielding. The uniformly distributed mixed-grain microstructure of the micro-nanostructure is responsible for its strength and toughness. Based on the Hall–Petch relationship of a single-scale and considering the volume fraction of the micro-nanostructure, a mathematical model between the grain sizes and the yield strength is established. The model has a good accuracy when compared with the actual results.

Fracture surface topography investigation and fatigue life assessment of notched austenitic steel specimens

The objectives of this study were to investigate the fracture surface topography of X8CrNiS18-9 austenitic stainless-steel specimens for different loadings and notch radii and to supplement the knowledge about the fracture mechanisms for fatigue performance. Cases with three different values of the notch radius ρ and the stress amplitude σa were analysed. The fracture topographies were quantified by the areas over their entire surfaces with the aid of an optical confocal measurement system. The results showed a well-correlated root mean square height Sq and void volume Vv, identifying the characteristics of the entire fracture method. A fracture surface topography fatigue damage model was developed based on the product of the stress amplitude σa by the Sq to the Vv ratio. Overall, the predictions were close to the fatigue lives found in the experiments.

A brief note on monotonic and fatigue fracture events investigation of thin-walled tubular austenitic steel specimens via fracture surface topography analysis (FRASTA)

The main objective of this short communication is to show the fracture progression in each loading case and complement knowledge about fracture mechanisms underpinning the tensile and fatigue performance of thin-walled tubes. For this purpose, the fracture surface topography analysis (FRASTA) method was used in the thin-walled tubular austenitic stainless-steel specimens. Two cases were analyzed: monotonic tension, and uniaxial fully-reversed fatigue. Furthermore, the fractures topographies were quantified through the profiles over their entire surfaces with the support of an optical confocal measurement system. The results showed the usefulness of the FRASTA method in identifying characteristic zones in the cracking process for the analyzed cases and motivates its development for other materials and complex loading cases.

Material behaviour of austenitic stainless steel subjected to cyclic and arbitrary loading

In this paper, an experimental program on cyclic loading of austenitic stainless steel specimens is presented. The program encompasses a series of forty specimens subjected both cyclic (low and extremely low) and arbitrary loading (following certain rules). These protocols include companion, multiple-step and a set of arbitrary earthquake-like strain history, which represents a novelty in the material understanding. Stainless steel exhibits strain hardening as a key feature for structural applications. This feature is fundamental in the definition of cross-sectional resistance, cross-section classification, ductility and energy dissipation. In recent years, the strategic use of stainless steel as a potential structural material in dissipative zones of earthquake-resistant elements is under consideration. When it comes to seismic design, strain-hardening must be known precisely for further characterization of the seismic structural behaviour of the actual system in which stainless steel is used strategically. The use of stainless steel in dissipative zones of earthquake-resistant structures requires research related to cyclic loading at many levels. Thus, its potential use in seismic areas, as well as the performance of existing structures, can be considered. A systematic analysis of the results, which include cyclic hardening, stabilisation and material degradation, is presented. In addition, these results are used for the further numerical implementation of cyclic hardening in non-linear models.

Production and Characterization of Austenitic Stainless Steel Cast Parts Reinforced with WC Particles Fabricated by Ex Situ Technique.

In this work, austenitic stainless steel specimens were locally reinforced with WC particles. The reinforcements were fabricated via an ex situ technique based on powder technology. Mixtures of WC, Fe, and M0101 binder were cold-pressed to obtain powder compacts. After debinding and sintering, the porous WC–Fe inserts were fixed in a mold cavity, where they reacted with liquid metal. Microstructural analysis was conducted for characterization of the phases constituting the produced reinforcement zone and the bonding interface. The results revealed that the reinforcement is a graded material with compositional and microstructural gradients throughout its thickness. The zone nearest to the surface has a ferrous matrix with homogeneously distributed WC particles and (Fe,W,Cr)6C and (Fe,W,Cr)3C carbides, formed from the liquid metal reaction with the insert. This precipitation leads to austenite destabilization, which transforms into martensite during cooling. A vast dissolution of the WC particles occurred in the inner zones, resulting in more intense carbides formation. Cr-rich carbides ((Fe,Cr,W)7C3, and (Fe,Cr,W)23C6) formed in the interdendritic regions of austenite; this zone is characterized by coarse dendrites of austenite and a multi-phase interdendritic network composed of carbides. An interface free of discontinuities and porosities indicates good bonding of the reinforcement zone to stainless steel.

Mechanical performance of additively manufactured austenitic 316L stainless steel

For tensile tests, Vickers hardness tests and microstructure tests, plate-type and box-type specimens of austenitic 316L stainless steels were produced by a conventional machining (CM) process as well as two additive manufacturing processes such as direct metal laser sintering (DMLS) and direct metal tooling (DMT). The specimens were irradiated up to a fast neutron fluence of 3.3 × 109 n/cm2 at a neutron irradiation facility. Mechanical performance of the unirradiated and irradiated specimens were investigated at room temperature and 300 °C, respectively. The tensile strengths of the DMLS, DMT and CM 316L specimens are in descending order but the elongations are in reverse order, regardless of irradiation and temperature. The ratio of Vickers hardness to ultimate tensile strength was derived to be between 3.21 and 4.01. The additive manufacturing processes exhibit suitable mechanical performance, comparing the tensile strengths and elongations of the conventional machining process.

Statistical analysis of the reproducibility of residual stress measurements in cold extruded parts

Cold extruded components are characterized by residual stresses, which originate from the experienced manufacturing process. For industrial applications, reproducibility and homogeneity of the final components are key aspects for an optimized quality control. Although striving to obtain identical deformation and surface conditions, fluctuation in the manufacturing parameters and contact shear conditions during the forming process may lead to variations of the spatial residual stress distribution in the final product. This could lead to a dependency of the residual stress measurement results on the relative axial and circumferential position on the sample. An attempt to examine this problem is made by the employment of design of experiments (DoE) methods. A statistical analysis of the residual stress results generated through X-Ray diffraction is performed. Additionally, the ability of cold extrusion processes to generate uniform stress states is analyzed on specimens of austenitic stainless steel 1.4404 and possible correlations with the pre-deformed condition are statistically examined. Moreover, the influence of the coating, consisting of oxalate and a MoS2 based lubricant, on the X-Ray diffraction measurements of the surface is investigated.

Hysteretic behavior comparison of austenitic and lean duplex stainless steel square hollow section members under cyclic axial loading

In this study, cyclic loading tests for a total of 10 specimens were performed in order to study the cyclic response of square hollow section bracing members with three types of cold-formed stainless steel materials; two austenitic stainless steels and one lean duplex stainless steel. The buckling behaviors, tensile strength, and energy dissipation capacity of the specimens were investigated according to the difference of material properties, width-thickness ratio and slenderness ratio. This study indicated that the number of cycles for grade 304 stainless steel tubular specimens at ultimate state was lower than that of grade 316 stainless steel specimens due to tensile fracture in the weld heat-affected zone. The ultimate strength and energy dissipation capacity of the lean duplex stainless steel specimens was also lower than those of austenitic stainless steel specimens due to initial local buckling. It is found that the grade 316 specimens showed the higher energy dissipation capacity and energy index by 1.78, 2.72 times on average, respectively, compared to the grade 304 members despite their higher material strength. This study also showed that European code tended to underestimate the compressive buckling strength of the bracing members, while Specification for the Design of Cold-formed Stainless Steel Structural Members by American Society of Civil Engineers provided unconservative the buckling strength prediction in the long columns and duplex stainless steel members.

Influence of activated flux on weld bead hardness of MIG welded austenitic stainless steel

Welding is one of the ancient manufacturing technologies used for joining of parts. The technology is still applicable to meet various needs of industries, but of course with time-to-time amendments. With the passage of time, numerous scientific investigations has been carried out across the globe on welding technology to keep pace with the modern day requirements. Accordingly, numerous technological modifications and advancements have been suggested & incorporated into almost all the different types of existing welding techniques available, besides developing new techniques also. Metal inert gas (MIG) welding is one such type, which primarily falls under the domain of arc welding. In the present work, bead-on-plate weld beads have been prepared using MIG over austenitic stainless steel specimen (SS-202) using three different types of flux materials and the bead geometry. Results reflected that activated flux material has significant impact over depth of penetration, bead width, weld quality and hardness of welds.

ВЛИЯНИЕ МЕТОДА И ТЕМПЕРАТУРЫ ИОННО-ПЛАЗМЕННОЙ ОБРАБОТКИ НА ФИЗИКО-МЕХАНИЧЕСКИЕ СВОЙСТВА ПОВЕРХНОСТНЫХ СЛОЕВ В АУСТЕНИТНОЙ НЕРЖАВЕЮЩЕЙ СТАЛИ

Ion-plasma saturation with interstitial atoms (nitrogen or carbon) is a promising method for enhancing the surface strength and wear resistance of austenitic stainless steel parts and products. The paper considers the influence of method and temperature of ion-plasma treatment (IPT) on phase composition, thickness, and strength properties (microhardness) of the surface layers in 01H17N13M3 austenitic stainless steel specimens. Steel specimens with a coarse-grained structure were nitrided in the arc and glow discharge plasma at different temperatures (400 °C, 550 °C, and 700 °C). Regardless of temperature and IPT-method, ion-plasma nitriding leads to the formation of hardened surface layers in steel specimens. In this case, the thickness and phase composition of IPT-hardened layers depend on both the method and temperature of nitriding. Nitrogen saturation of specimen surfaces in the glow discharge at a temperature of 400 °C promotes the formation of a thin S-phase layer (nitrogen-expanded austenite, 4 μm in thickness). At the same IPT temperature in the arc discharge plasma, the authors observed the formation of a heterophase (Fe-γN, Fe4N, CrN, and Fe-α) surface layer with a significantly greater thickness (40–45 μm). Regardless of the IPT-method, a saturation of specimens at temperatures of 550 °C and 700 °C is accompanied by the formation of thick heterophase hardened layers (40–60 μm). In this case, the IPT method has a negligible effect on the phase composition of layers but significantly affects the ratio of the volume content of the hardened phases. After being IPT-processed in different modes, the microhardness distribution profile for all specimens has three typical zones: a composite layer (or S-phase at the IPT in a glow discharge at Ta=400 °C), a diffusion zone, and a matrix. With an increase in the saturation temperature, the thickness of the transition diffusion zone increases regardless of the IPT method.

Topographic evolution of balls used in microabrasion tests

This work aims to contribute to the understanding of the topographic modification of the surface of the counter-body during micro-abrasion and its complex relationship with specimen wear. The topographic evolution of the surface of five different balls (Ø25.4 mm) was monitored: one ceramic (Si3N4), one metal (AISI 52100 steel), and three thermoplastic polymers (polypropylene, polyacetal and polyamide 6.6). AISI 304 austenitic stainless steel specimens were tested in a fixed-ball micro-abrasion test rig using a silica slurry (10%wt. in water), rotation speed of 150 rpm, and total test time of 90 min (in 15 min intervals). The topographic evolution of the ball topography was monitored by laser interferometry of the abrasive tracks at each measurement interval, always in the same region. The parameters Sq, Sz, Ssk, Sku, λq, Sdq and Str were computed. Significant wear and changes of the surface topography of the balls were observed along the tests. Harder balls underwent evident grooving, while the softer polymeric balls showed fibrillation characteristics, alignment of polymeric chains and transfer film. For the initial 15 min of test, before reaching a steady-state wear regime, the softer balls (polypropylene, polyamide 6.6 and polyacetal) showed, in general, an increase in Sq, λq and Sdq, possibly by the generation of a transfer film on the surface, which was not observed for the harder balls. After the steady-state regime was achieved, all balls reduced their values of Sq, λq and Sdq. Plots of Sku x Ssk identified non-Gaussian height distribution curves (mostly with high kurtosis and negative asymmetry) for all balls. Sq and Sdq presented a linear relationship with the wear coefficients of the specimens (k) dependent on the material of the ball, whereas an overall linear relationship between λq and k was observed independently of the ball material. Str was a good indicator of aligned grooving abrasion, but showed a linear relationship with k only for the steel ball.

Optimization of A-TIG Welding Process Using Simulated Annealing Algorithm

Flux-assisted tungsten inert gas welding process, also known as activated tungsten inert gas (A-TIG) welding, is extensively used in order to improve the performance of the conventional TIG welding process. In this study, the orthogonal array Taguchi (OA-Taguchi) method, regression modeling, analysis of variance (ANOVA) and simulated annealing (SA) algorithm have been used to model and optimize the process responses in A-TIG welding process. Welding current (I), welding speed (S) and welding gap (G) have been considered as process input variables for fabricating AISI316L austenitic stainless steel specimens. Depth of penetration (DOP) and weld bead width (WBW) have been taken into account as the process responses. In this study, SiO2, nano-particle has been considered as an activating flux. To gather required data for modeling, statistical analysis and optimization purposes, OA-Taguchi based on the design of experiments (DOE) has been employed. Then the process responses have been measured and their corresponding signal-to-noise (S/N) ratio values have been calculated. Different regression equations have been applied to model the responses. Based on the ANOVA results, the most fitted models have been selected as an authentic representative of the process responses. Furthermore, the welding current has been determined as the most important variable affecting DOP and WBW with 68% and 88% contributions, respectively. Next, the SA algorithm has been used to optimize the developed models in such a way that WBW is minimized and DOP is maximized. Finally, experimental performance evaluation tests have been carried out, based on which it can be concluded that the proposed procedure is quite efficient (with less than 4% error) in modeling and optimization of the A-TIG welding process.

The Influence of Phase Composition and Phase Distribution on Crack Formation and Fracture Mechanisms of Cr–Ni Steels Produced by the Method of 3D Electron-Beam Printing

The features of crack formation near the fracture region and the fracture micromechanism are investigated during uniaxial tension of specimens of Ni–Cr stainless steels, Fe–19Cr–9Ni–0.1C and Fe–19Cr–9Ni–1.4Nb–0.1C, produced by the method of 3D wire-feed electron beam additive manufacturing (EBAM) as a function of the phase composition and phase distribution in the structure. It is found out that in the specimens with a twophase (austenite/δ-ferrite) microstructure the ferrite morphology and its volume content (up 25%) affect the plastic shear distribution in austenite and ferrite and exert a slight influence on the formation of strain localization micro- and macrobands in the pre-fracture stage, and the crack-formation mechanism is similar to that observed in cast stainless steels of similar compositions, which possess single-phase austenitic structure. In additively-manufactured steel containing niobium the formation of brittle niobium- and iron-based intermetallic phases favors the formation of pores and microcracks at the austenite/NbFeCrNi-phase or the δ- ferrite/NbFeCrNi-phase interfaces, while the processes of cracking in the strain localization bands (and their formation) turn out to be suppressed. Irrespective of the elemental and phase compositions of steel specimensformed by the method of 3D additive manufacturing, the principal micromechanism of their fracture, transgranular dimple fracture, is similar to that observed in the cast specimens of austenitic stainless steels.

Study on Deformation Behavior of Non–Hardenable Austenitic Stainless Steel (Grade X5CrNi18–10) by Hot Torsion Tests

The steel’s deformation resistance, in which high strain rates have an important influence on the mechanism of failure, might be obtained from a suitably instrumented torsion test. Determination of stainless steel deformability by hot torsion test is the only method that allows obtaining large deformations along the length of the test specimen, so it is mainly used to determine the characteristics at large plastic deformations. By this method, the hot deformability of stainless steel is determined by subjecting to hot torsion the cylindrical stainless steel specimens maintained at the deformation temperature in a tubular oven belonging to the Laboratory of Metal Rolling and Plastic Deformation, at the Faculty of Engineering – Hunedoara, University Politehnica Timişoara. For the experimental hot torsion tests, several stainless steel grades were used and included in a large series of studies destined to determining the deformation behavior of steel. Having in view the previous results obtained in the study of deformability characteristics of two stainless steels (hardenable martensitic stainless steel, grade X46Cr13 and non–hardenable ferritic stainless steel, grade X6Cr17), this paper includes the results of the hot torsion tests conducted to find the deformation behavior of the non–hardenable austenitic stainless steel (grade X5CrNi18–10). For analysis of laboratory hot torsion tests results the univariate and multivariate regression analysis was used, estimating the relationships among the hot–testing temperature, torque moment and number of torsions up to the breaking point of the specimens of austenitic stainless steel. Therefore, the optimum range of heating temperatures applied for deforming the studied steels results clearly from the deformability – temperature (plasticity – temperature and deformation resistance – temperature) diagrams. Correlations are useful because they can indicate a predictive relationship that can be exploited in the laboratory or industrial practice.