316L Austenitic Stainless Steel Research Articles

Influence of Prior Fatigue Damage on Tensile Flow Behaviour of Type 316L(N) Austenitic Stainless Steel

The present study aims to understand the influence of prior fatigue loading on the tensile flow behaviour of austenitic 316L(N) stainless steel. The steel specimens were subjected to prior fatigue damage up to various fatigue life fractions. i.e., 5, 10, 30 and 50% of the fatigue life at a strain amplitude of ±0.6% and a nominal strain rate of 3×10-3 s-1 and 300, 823 and 873 K. Subsequently the specimens were subjected to tensile deformation at the above temperature and strain rate and the tensile flow behaviour was evaluated in terms of Hollomon, Swift, Ludwigson and Voce equations. At 300 K, the flow curves of the specimens of as-received, and prior fatigue damaged were well described by the Ludwigson equation, whereas at 823 and 873 K, both Voce and Ludwigson equation could be used to describe the flow behaviour of the specimens. Increase of work hardening with the increase in prior fatigue damage at 823 and 873 K is attributed to the occurrence of dynamic strain ageing (DSA). At 300 K, a three-stage work hardening behaviour with a prominent stage II athermal hardening was observed for all the specimens, whereas at higher temperatures, only the as-received specimen exhibited the three-stage work hardening behaviour. At 823 and 873 K, a two-stage work hardening behaviour, comprising of a transient stage followed by stage III was observed for the specimens subjected to prior fatigue damage up to various fatigue life fractions. The inter relationship between the work hardening parameters of Ludwigson was rationalised in terms of the linearly inverse relationship between the work hardening rate and plastic strain rates.

Effect of ultrasonic surface impact on the microstructural characterization and mechanical properties of 316L austenitic stainless steel.

In the present study, effect of ultrasonic impact treatment (UIT) on the microstructural characterization and mechanical properties of 316L stainless steel (hereinafter referred to as 316L) was investigated experimentally. The fatigue fracture mechanism of 316L before and after UIT was revealed. The experimental results indicated that the martensitic grain size induced at the impact edge was about 2.00 Å. The surface modified 316L formed a gradient nanostructure and induced a martensitic phase transformation. The hardness of the surface layer of the modified 316L was twice the hardness of its matrix. The tensile strengths of 316L before and after UIT were 576 MPa and 703 MPa, respectively. The stretching stripes of 316L were more disordered after UIT. The fatigue strengths of 316L before and after UIT were 267 MPa and 327 MPa, respectively. The fatigue cracking of 316L started from the austenite grain boundaries. The fatigue fracture surface was relatively rough. The fatigue crack sources of the modified 316L came from internal inclusions. The inclusions were oxides dominated by SiO2. As the stress range increased, the crack initiation site migrated to the interior and the fatigue fracture surface became flatter.

Corrosion behavior of selective laser melted biomaterials in a physiological environment

Medical applications are today the most important areas that use additively manufactured parts. The choose of the convenient biomaterial during implant design depends on several criteria such as biocompatibility, osteointegration and processing cost. The purpose of this work is to compare the corrosion behavior of AISI 316L austenitic stainless steel and Titanium TiAl6V4 orthopedic implants manufactured by selective laser melting. For each material the process parameters were varied to obtain comparable surface states mainly evaluated by microgeometric quality and porosities distribution. The electrochemical interaction between biomaterial and the physiological environment was simulated by corrosion tests performed in a Ringer type solution. From the obtained results it can be seen that selective laser melted Titanium TiAl6V4, despite its higher roughness, exhibits the highest corrosion resistance compared to AISI.316L stainless steel samples. In fact, for comparable porosities distribution density, enhanced corrosion properties, such as corrosion rate, pitting potential (Epp) and corrosion potential (Ecorr) were achieved for Titanium alloy.

Usporedba topitelja za ATIG zavarivanje i njihov uticaj na mehaničke osobine

The application of conventional electric arc welding in argon protection (TIG welding process) has limited possibilities. These limitations are primarily related to the thickness of the welded material and the need to prepare the groove for their welding. Overcoming these limitations in TIG welding was done by using activating flux, so the procedure was named ATIG (A-TIG) welding. The paper presents the results of research into the application of activating fluxes in welding austenitic stainless steel 316L with a thickness of 6 mm. By using the BC-31 and QuickTIG melters of different chemical bases, we compare the mechanical properties of the welded joints, as well as with the welded joints of the same material obtained using the conventional TIG procedure. The results show that the best mechanical properties are possessed by welded joints realized by the ATIG process using a mixture of activating fluxes BC - 31 (50%) and QuickTIG (50%) (green + white flux).

Influence of Cu interlayer on the interfacial strengthening of the diffusion-bonded SS-304L/IN718

Diffusion bonding (DB) of AISI 304L austenitic stainless steel (SS-304L) and Inconel 718 with and without Cu interlayer was performed at 950 °C for 90 min under vacuum of 8x10−2 MPa. The resulting mechanical, microstructural, and compositional analyses of the bonded interfaces were analyzed using lap shear and microhardness tests, optical and scanning electron microscopes (OM and SEM) equipped with energy-dispersive spectroscope (EDS) and X-ray diffraction (XRD). It was observed that the bonding interface without Cu interlayer was comprised of TiC, NbC, Mn23C7, Cr23C6, Cr3C2, μ-phase (Fe2Nb), NiTi2, and FeTi. However, these carbides and intermetallic compounds (IMCs) were eliminated with the insertion of the Cu interlayer. This enhanced the joint strength from 143 to 334 MPa and reduced the interfacial hardness by 17 %.

Microstructure Evolution and Mechanical Properties of Ferrite–Austenite Stainless Steel Bimetals Fabricated via Wire Arc Additive Manufacturing

A bimetallic structure is implemented by wire arc additive manufacturing from 316L austenitic stainless steel to 430 ferritic stainless steel. Microstructure, interface characteristics, and tensile properties of 316L‐430 bimetals are investigated. In 316L/430 bimetals, defects like porosity and cracks are not present. The compositional distribution and microstructure evolution of 316L‐430 bimetallic deposition are significantly impacted by the deposition sequence and heat input. As heat input increases, microstructure evolution of 430 deposited layer is single F → F+ diffusely distributed (Cr, Fe)23C6 → F+ grain internal (Cr, Fe)23C6 → F + M, and 316L deposited layer presents typical austenite + skeletal δ ferrite. Microstructure of 430 deposited layers presents heterogeneous, grain coarsening, and martensite along grain boundaries, resulting in high‐temperature embrittlement. The highest hardness of 316L‐430 bimetals appears on 430 sides near the interface. The tensile strength of 430/316L bimetals is greater than that of 316L/430, and the tensile specimens exhibit brittle fracture.

Influence of ultrafine surface grain size on the passive film chemistry and critical pitting temperature (CPT) in AISI 316L austenitic stainless steel

ABSTRACT Severe shot peening (SSP) was applied to induce a profound surface deformation characterised by the formation of ultrafine-grained structures with an average grain size of 99 ± 5 nm on AISI 316L. The impact of these ultrafine-grains on the critical pitting temperature (CPT) was investigated in 3.5 wt.% NaCl solution. Additionally, this study comprehensively explored pit chemistry using electrochemical techniques and mechanistic analyses. Moreover, a comparative investigation of the chemical composition of passive films, developed on severe shot peened (SSPed) samples, was undertaken in contrast to their solution-annealed (SAed) counterparts, employing X-ray photoelectron spectroscopy (XPS). Potentiostatic measurements showed that the mean CPT value was 8.8 ± 0.3 °C for the SAed samples, whereas the SSPed counterparts demonstrated a notably higher CPT, averaging at 13.6 ± 0.5 °C. Insights from pencil electrode measurements revealed shallower pit depths and lower critical current density requirements for passivation in the SSPed sample. Furthermore, the passive films formed on the SSPed samples showed a higher concentration of Cr and Mo. These observations were comprehensively elucidated through effective diffusion equations, leading to a discussion regarding the improved resistance to pitting corrosion exhibited by the SSPed samples, thus shedding light on the pivotal role of surface grain refinement in corrosion behaviour improvements.

Effect of Isothermal Annealing on Residual Stresses and Fatigue Properties of LPBF 316L Steel

This study aims to investigate the influence of isothermal annealing on the residual stresses and fatigue properties of a 316L austenitic stainless steel, manufactured by the laser powder-bed fusion (LPBF), possessing a high density of 99.98%. Residual stresses were evaluated using the X-ray diffraction techniques. High-cycle fatigue tests were performed on cylindrical samples manufactured in both horizontal and vertical orientations, subjected to force-controlled axial fully reversed loading. Following fabrication, the samples underwent isothermal annealing in a furnace either at 600 °C for 120 minutes or at 900 °C for 30 minutes. Subsequently, the samples were machined to their final dimensions and electropolished to a mirror surface finish. Preliminary findings revealed that increasing the annealing temperature effectively reduced the surface residual stresses. However, this reduction did not lead to an improvement in the fatigue resistance of this nearly fully dense material in the high-cycle fatigue regime. Interestingly, the structure annealed at 600 °C exhibited a higher fatigue strength compared to the structure annealed at 900 °C, with no discernible difference between the printing directions. Fracture surfaces and microstructural features examined using light and electron microscopy revealed that cracking was primarily initiated at surface defects or slip bands. These results highlight the complex interplay between residual stresses, microstructure, strength, and fatigue behaviour of LPBF 316L austenitic stainless steel. Further analysis and investigations are required to fully understand the underlying mechanisms and develop strategies for enhancing the fatigue performance of additive manufactured components.

Early stages of liquid-metal corrosion on pre-oxidized surfaces of austenitic stainless steel 316L exposed to static Pb-Bi eutectic at 400 °C

The effect of pre-oxidation in air at 300–500 °C on the initiation and development of liquid metal corrosion attack on 316L austenitic steel in static lead-bismuth eutectic (LBE) with ∼10−10 – 10−9 mass% dissolved oxygen at 400 °C for 1200 h is investigated. General dissolution with preferential attack along twin boundaries was observed on polished surfaces with native oxide films. Fe/Cr-based oxide films pre-formed in air at 300 and 400 °C protect surfaces against dissolution. Localized corrosion was observed on surfaces pre-oxidized at 500 °C. Thermal oxide degradation rate depends on its composition rather than thickness.

Mitigating Stress Corrosion Cracking of 304L and 316L Laser Welds in a Salt Spray through Micro-Shot Peening

Two austenitic stainless steel (ASS) plates, 304L and 316L, were cold-rolled (304R and 316R) with a 10% reduction in thickness and then subjected to laser welding. Cold rolling caused slight surface hardening and introduced residual tensile stress into the ASS plates. The susceptibility to stress corrosion cracking (SCC) of the welds (304RW and 316RW) was determined using the U-bend test pieces in a salt spray. To highlight the stress concentration at the weld’s fusion boundary (FB), the top weld reinforcement was not ground off before bending. Moreover, micro-shot peening (MSP) was performed to mitigate the SCC of the welds by imposing high residual compressive stress and forming a fine-grained structure. Cold rolling increased the susceptibility of the 304R specimen to pitting corrosion and intergranular (IG) microcracking. Moreover, pitting corrosion and SCC were found more often at the FBs of the 304RW. The corrosion pits of the peened 304RW (304RWSP) were finer but greater in amount than the those of the un-peened one. The results also indicated that the 316L ASS welds with MSP were resistant to the incidence of pitting corrosion and SCC in a salt spray. The better reliability and longer service life of dry storage canisters could be achieved by using 316L ASS for the construction and application of MSP on it.

Influence of Gas Mixture Ratio on Wear and Corrosion Properties of Oxynitrided Films for 316L (03Kh17N14M3) Austenitic Stainless Steel

Influence of Gas Mixture Ratio on Wear and Corrosion Properties of Oxynitrided Films for 316L (03Kh17N14M3) Austenitic Stainless Steel

A model to predict the s-phase thickness and the change in corrosion behavior toward H2SO4 of 316L austenitic stainless steel after plasma nitriding

Austenitic steels are known for their high corrosion resistance, but at the same time exhibit low hardness and wear resistance. While plasma nitriding improves tribological properties, it also affects corrosion behavior depending on the electrolyte. In this paper, a model should be present, that allows to predict the thickness of the s-Phase and the change of the corrosion behavior in plasma nitriding processes for austenitic stainless steels. For this purpose, different processes under specific variation of the temperature ranging from 360 °C to 450 °C and duration of 10 h to 24 h were performed. Other process parameters such as the voltage or gas mixture remained constant. There are two stages in the developed model: the first one allows the prediction of the s-Phase thickness by isothickness curves. The second stage implements an evaluation system of the corrosion resistance. By assigning numerical values for the observed corrosion, a quantitative comparison is allowed, which is represented by a color scaling.

Laser powder bed fused 304L steel shot-peened with various ceramic shot sizes: Surface/subsurface characteristics, tensile behavior, and fatigue behavior

Shot peening is an efficient post-processing technique for eliminating surface manufacturing defects, inducing compressive residual stress, and improving the mechanical properties of laser powder bed fused (L-PBFed) metallic materials. With a series of microstructural characterizations, this work comprehensively evaluated the influence of ceramic shot size on the surface characteristics, subsurface characteristics, and mechanical properties, especially fatigue performance, of L-PBF manufactured 304L austenitic stainless steel (SS). The mechanical testing process was in situ observed using an infrared thermal camera, and the tensile-fractured and fatigue-fractured microstructures were systematically examined, aiming to explore the life improvement mechanisms caused by different ceramic shot sizes. The results revealed that when compared to small-sized AZB 100 shots, the large-sized AZB 300 shots could result in higher surface roughness, larger thickness reduction, more apparent surface gradient structure, higher strength, and longer fatigue lifetimes. The strength improvement could be associated with the size reduction of L-PBF induced subsurface defects, the grain refinement at the surface layer, and the α'-martensitic transformation during tensile deformation. The life improvement could be primarily attributed to the subsurface defect strengthening, which resulted in the fatigue crack initiation transition from subsurface defects to surface defects. During the fatigue process, the energy dissipation originating from the core materials dominated the thermal equilibrium temperature, whereas the density of micro-defects (crystalline defects) at the surface layer dominated the crack tip temperature. These findings provide a basic understanding of how to advance the shot peening process for L-PBFed materials.

Comprehensive microstructural and mechanical characterization of transient liquid phase bonded austenitic stainless steel by Ni–Cr–Si–B amorphous interlayer

In this research, along with the joining of the gas turbine fuel nozzle part from austenitic stainless steel 316L alloy with BNi-2 interlayer by the transient liquid phase bonding process, microstructure test, shear strength and tensile strength tests have been placed. Bonding at a temperature of 1080 °C and a holding time of 45 min has led to the completion of isothermal solidification in the bonding zone. Microstructural and phase investigations showed that austenitic stainless steel 316L undergoes phase transformation after exposure to temperature conditions and the δ ferrite phase is formed as separate areas due to the diffusion of the solid state of ferrite-forming elements, and the grains of the base metal are 32 μm have reached 43 μm, which has caused a decrease in the shear strength of the base metal from 362 MPa to 347 MPa. Despite the completion of isothermal solidification in the joint centerline due to the boron diffusion of the solid state into the base metal and achieving the maximum shear strength of the joint (402 MPa), two generations of boride deposits have been formed. The first-generation borides based on nickel-iron-chromium-molybdenum are formed during heating up to the bonding temperature, and the second-generation boride deposits are based on iron-chromium-molybdenum, which were deposited in situ during the isothermal solidification stage. The microhardness test confirms the uniform distribution of hardness in each of the bonding zones, which feedback was determined in the tensile strength test. The tensile strength of the bonded sample was 485 MPa, which is approximately 82 % of stainless steel 316L. In both shear and tensile tests, the presence of small dimples indicates the ductile failure of the joint samples in base metal.

Comparison of Grain-Growth Mean-Field Models Regarding Predicted Grain Size Distributions.

Mean-field models have the ability to predict the evolution of grain size distribution that occurs through thermomechanical solicitations. This article focuses on a comparison of mean-field models under grain-growth conditions. Different microstructure representations are considered and discussed, especially regarding the consideration of topology in the neighborhood construction. Experimental data obtained with a heat treatment campaign on 316L austenitic stainless steel are used for the identification of material parameters and as a reference for model comparisons. Mean-field models are also applied to both mono- and bimodal initial grain size distributions to investigate the potential benefits of introducing neighborhood topology in microstructure prediction models. This article demonstrates that improvements in the predictions can be obtained in monomodal cases for topological models. In the bimodal test, no comparison with experimental data was performed as no data were available. But relative comparisons between models indicated few differences in the predictions. Although of interest, the consideration of neighborhood topology in grain-growth mean-field models generally results in only small improvements compared to classical mean-field models, especially in terms of implementation complexity.

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

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

A novel pathway for joining power plant dissimilar steels by diffusion bonding in as-machined surface condition

Surface preparation, which includes polishing and cleaning, has become a crucial stage in diffusion bonding for creating excellent quality durable joints. Polishing is a time-consuming and labor-intensive task that reduces process productivity. To increase process productivity, the joints must be established under as-machined surface conditions. Pressure impulses were used to conduct diffusion bonding of P92 steel and 316L austenitic stainless steel under as-machined surface conditions. At 1000 °C, 30 min bonding time and 20 MPa Maximum impulse pressure, tensile strength and elongation of the joint was found to be 628.5 MPa and 38.6 % respectively.

Effect of Welding Microstructure on Stress Relaxation Cracking Studied by Controlled Residual Stress Generation in a 316L(N) Austenitic Stainless Steel

Effect of Welding Microstructure on Stress Relaxation Cracking Studied by Controlled Residual Stress Generation in a 316L(N) Austenitic Stainless Steel

High-Efficient Gas Nitridation of AISI 316L Austenitic Stainless Steel by a Novel Critical Temperature Nitriding Process

To improve the surface properties of austenitic stainless steels, a thick S-phase layer was prepared by using a novel critical temperature nitriding (CTN) process. The properties of the thick S-phase layer were optimized by controlling the process parameters. The microstructures and phase compositions of CTN-treated layers were characterized by the optical microscope, scanning electron microscope and X-ray diffraction, respectively. The surface properties, including corrosion and wear resistance, were systematically investigated by the electrochemical workstation, micro-hardness tester and ball-on-disk tribometer, respectively. The results showed that a thick S-phase layer with a thickness of 18 to 25 μm can be fabricated in a short time by critical temperature nitriding, which represented higher efficiency than conventional low-temperature nitriding. Although the most top surfaces of CTN-treated layers contain massive iron nitrides, there are no precipitates in the inner nitrided layer. The electronic work function calculated by first-principles method has confirmed that those iron nitrides had a slight influence on the corrosion resistance of nitrided layers. The optimized CTN-treated layer exhibited a comparable corrosion resistance and wear resistance as the low-temperature nitrided layer. The CTN process is considered a potentially highly efficient surface modification method for austenitic stainless steels.

Localised Web Bearing Behaviour of Cold-Formed Austenitic Stainless-Steel Channels: Review of Design Rules and New Insight under Interior Loading

Stainless steels are modern high-performance construction materials exhibiting excellent corrosion resistance, recyclability, ductility, and durability which make them appealing to use in the construction industry. However, when used as structural sections, they are subjected to localised failure in the web. This study aims to examine the structural behaviour of cold-formed low-carbon content standard austenitic 304L and 316L stainless steel channels under localised interior bearing loads. The results of 21 tests on unlipped channels with different cross-section sizes and thicknesses are presented. A nonlinear quasi-static Finite Element (FE) model is then developed. The FE model is validated against experimental test results and demonstrated good agreement in terms of bearing strength and failure modes. In addition, the experimental and FE results are used to compare the results against the results predicted in accordance with the American specification SEI/ASCE 8:2002 and European Standard EN 1993-1-4:2006. It is found that the current design equations are unreliable and too unconservative to use for cold-formed austenitic stainless steel unlipped channels, especially when compared to SEI/ASCE 8:2002, as much as 41%.