316L Austenitic Stainless Steel Research Articles

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%.

Residual Stress Measurements on Rectangular Specimen SS316L using Numerical Computation Software and Selective Laser Melting

This research aimed to predict the residual stress of additively manufactured rectangular specimen using Selective Laser Melting (SLM) by means of non-linear numerical computation based on Thermo-mechanical method (TMM). The procedure starts with the geometrical and material modelling of rectangular specimen with regards to Austenitic Stainless Steel 316L (SS316L) in which the temperature dependent material data properties such as Young’s Modulus, Thermal Expansion Coefficient, Specific Heat Capacity and Thermal Conductivity are taken into account. The next phase consists of numerical computation procedure which is where the specimen is position 60° of inclination angle from the substrate plate. The support structure is to be generated within the lower surface of the specimen in order to avoid material to collapse during printing process. Laser heat source is to be modelled based on the laser beam width, power, efficiency and scanning speed in order for the numerical computation to accurately predict the thermal problem in SLM process. Furthermore, layer parameters used to fabricate the specimen such as hatch distance, hatch scan width and layer thickness are taken into TMM consideration. Similar set-up from numerical computation by means of laser and layer parameters to fabricate SS316L rectangular specimen is utilized in real fabrication process using SLM machine, Renishaw RenAM 500E. After fabrication of the specimen, electropolishing as for the sample preparation for X-Ray Diffraction (XRD) measurement are to be conducted by means of various depth on both sides of the specimen. For the validation procedure, residual stress on every depth is to be analysed and compared with the result from numerical computation. In conclusion, TMM simulation forecasting an acceptable residual stress on SLM product with relative error up to 14% and the computational time taken to predict the residual stress is only 56 minutes. This exploratory research using TMM simulation to predict residual stress on SLM product could benefit metal additive manufacturing (MAM) production as a whole by neglecting expensive trial and error approach.

Layer-level AISI 316L-18Ni (300) Maraging multi-material fabrication via Laser-Powder Bed Fusion

Overcoming most of the limitations of conventional subtractive techniques, Laser-Powder Bed Fusion (L-PBF) is a widely adopted technology that allows the fabrication of metallic components with complex geometries. Available machines can work with a large variety of materials; however, multi-material printing is limited to inter-layer fabrication, i.e., each layer is made out of a single material. In this work, we aim at expanding the multi-layer printing capabilities of L-PBF to intralayer manufacturing (i.e., two different materials in the same layer), overcoming the previous limitations. To investigate the effectiveness and validate the proposed method, two widely used steels are used as powders to manufacture bi-material specimens, i.e., the AISI 316L austenitic stainless steel and the 18Ni (300) Maraging steel. Based on metallurgical and mechanical testing evidence, the results show that the method is repeatable and capable of fabricating strong and tough intralayer interfaces. While weak interfaces between materials with rather different mechanical properties are common in multi-material printing techniques, our method allows us not only to produce bi-materials with stronger interfaces but also to reduce discontinuities to those normally found in additive manufactured components. These results are obtained without any modifications to the machine and with low-cost additional equipment, opening the way for easy and cost-effective implementation.

Effect of welding current on the microstructural and corrosion behavior of GTA welded austenitic stainless steel joints

In the present study, effect of welding current was analysed on the microstructural and corrosion properties of 316L austenitic stainless steel (ASS) joints welded by gas tungsten arc welding (GTAW). Three welding current were chosen from the GTAW process's operational range and welded joints fabricate using these currents were exposed to microstructural, microhardness as well as corrosion evaluations. Microstructural examination showed the lathy and vermicular morphology of δ-ferrite in the weld metal microstructures while different degrees of grain coarsening in the HAZ microstructures were seen. DLEPR method was used to assess the welds' resistance to corrosion, which depicted that high welding current specimen revealed relatively higher value of DOS (indicate higher amount of corrosion rate) as compared to low welding current specimen. This was due the fact that vermicular morphology of δ-ferrite was present in the high welding current specimens, which promoted the corrosion rate. However, lathy morphology of δ-ferrite was found in the low welding current specimens, which demotes the corrosion rate.

Novel Powder Feedstock towards Microstructure Engineering in Laser Powder Bed Fusion: A Case Study on Duplex/Super Duplex and Austenitic Stainless-Steel Alloys

Additive manufacturing of Duplex Stainless Steels (DSS) and Super Duplex Stainless Steels (SDSS) has been successfully demonstrated using LPBF in recent years, however, both alloys feature an almost fully ferritic microstructure in the as-built condition due to the fast cooling rates associated with the Laser Powder Bed Fusion (LPBF) process. Blends of DSS and SDSS powders were formulated with austenitic stainless-steel 316L powder, aiming to achieve increased austenite formation during in the LPBF as-built condition to potentially minimize the post heat treatments (solution annealing and quenching). Powder characteristics were investigated and process parameters were optimized to produce near fully dense parts. Nanoindentation (NI) tests were conducted to measure, not only the local mechanical properties and correlate them with the as-built microstructure, but also to gain a deeper understanding in the deformation behavior of individual phases that cannot be studied directly by macroscopic tensile tests. Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) were employed for microstructural analysis and phase quantification. The microstructural analysis and EBSD phase maps revealed an increase in austenite in the as-built microstructures. Blend 1 resulted in a duplex microstructure consisting of 10% austenite at the XY plane and 20% austenite at the XZ plane. The austenite content increased with increasing proportion of 316L stainless steel in the powder blends. The DSS blend required a much higher volumetric energy density for the fabrication of near fully dense parts. This imposed a slower solidification and a higher melt pool homogeneity, allowing for adequate diffusion of the austenite stabilizing elements. The presented workflow and findings from this study provide valuable insights into powder mixing for the development of custom alloys for rapid material screening in LPBF.

Additive manufacturing and spark plasma sintering as effective routes for manufacturing of AISI 316L austenitic stainless steel - WC composites

Metal matrix composites (MMCs) are lightweight, high-performance materials with continuous expansion in industrial applications. In this study, Laser Powder Bed Fusion (LPBF), Binder Jetting (BJ), and Spark Plasma Sintering (SPS), were used to produce AISI 316L austenitic stainless steel (SS316L)-WC composite with different compositions and powder preparations. In the present study, the products manufactured with these different technologies and material compositions were compared in terms of final density, microstructure, and mechanical property. Results show a significant difference in the microstructure of the parts produced by these different manufacturing techniques with microstructural analysis showing that samples processed by LPBF exhibit the finest grain size, while the BJ processed samples exhibit the coarsest grain size. Furthermore, analysis of the micrographs and the XRD patterns obtained for all processing routes suggest that there was some level of WC dissolution followed by inter atomic diffusion occurring between the WC inclusions and the SS316L matrix, involving dissolved W and C atoms from the inclusions and Cr and Fe atoms from the steel matrix, with the final structure and microhardness of the composite system depending on the kinetics of the manufacturing process.

Plastic deformation behavior and strengthening mechanism of SLM 316L reinforced by micro-TiC particles

In this work, the integrated study on the microstructure evolution, plastic deformation behavior, and strengthening mechanism of Selective Laser Melting (SLM) 316L austenitic stainless steel (ASS) reinforced with micro-TiC particles was investigated. The results indicated that there were lots of small angle grain boundaries in SLM 316L ASS, and the high dislocation density and component segregation of the cellular dislocation structure resulted in a good combination of high strength and good elongation in performance. The tensile strength, yield strength, and elongation of SLM 316L ASS were 682 MPa, 571 MPa, and 49.9%, respectively. The addition of TiC reinforced particles induced microstructure evolution, including grain refinement, reduction of epitaxial columnar dendrites, reduction of low angle grain boundary (LAGB) ratio and increase of Σ3 (60°/<111>) twin boundaries. Through the effects of grain refinement strengthening, Orowan strengthening, and dislocation strengthening, SLM 316L-2TiC showed a higher tensile strength of 1021 MPa and higher yield strength of 809 MPa, while maintaining an elongation of 30.8% due to the suppression of plasticity by twinning induced plasticity (TWIP) effect. Although nano-reinforced particles were commonly used to improve mechanical properties, in our work, we chose to add micrometer-sized TiC particles, and the strength of the SLM 316L-2TiC metallic matrix composites (MMCs) exceeded the previous reports of adding nanoparticles. The reason was that the small contact angle between TiC and 316L provides good wettability between the particles and the melt, and the selection of laser processing parameters ensures good bonding between TiC particles and the substrate.

Effect of Thermo-Mechanical Processing on Initiation and Propagation of Stress Corrosion Cracking in 304L Austenitic Stainless Steel

Despite the high corrosion resistance of austenitic stainless steels (SSs), a significant reduction of stress corrosion cracking (SCC) resistance has been reported in cases of high residual stress and metastable microstructural features. In this study, the effect of thermo-mechanical processing (TMP) on the initiation and propagation of SCC in 304L SS was studied. To better understand the SCC mechanisms, three TMPs conditions—welded, solution annealed at 1050 °C for tens of seconds, and straightened—were used. The research focused on analyzing the initial microstructure, residual stress, and hardness along the depth direction to assess SCC resistance and establish correlations with the observed SCC modes. Experimental results demonstrated that transgranular SCC was observed in regions exhibiting elevated residual stress induced by welding and straightening processes. Furthermore, the presence of strain-induced martensite transformation and slip bands formed during plastic deformation were identified as additional factors contributing to the susceptibility of SCC. The study findings highlighted that the magnitude and distribution of residual stresses, in conjunction with microstructural evolution, could be varied depending on the specific TMP condition, leading to different SCC susceptibilities, cracking modes, and directions.

Effect of nitrogen content on mechanical properties of 316L(N) austenitic stainless steel

Increasing the yield strength of 316L(N) stainless steel is both scientifically and commercially crucial. In this study, five 316L(N) stainless steels with different nitrogen contents were melted and specimens with grain size from 5 μm to 320 μm were obtained by cold rolling and heat treatment. The yield strength was examined through tensile tests. The microstructure was investigated using electron-backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The fracture morphology was studied by scanning electron microscopy. Nitrogen improved the coefficient in the Hall–Petch equation. As the nitrogen content increased from 0.008 wt% to 0.34 wt%, the coefficient K increased from 363 MPa μm0.5–1895 MPa μm0.5 and the coefficient σ0 increased from 168 MPa to 331 MPa, resulting in an improvement of yield strength from 249 MPa to 757 MPa (grain size of 20 μm). Planar slips and stacking faults were found in deformation structures of 316L(N) steel with higher nitrogen content, which is underlying factors for the significant improvement of yield strength. Strain-induced martensite near the fracture surface contributed to the plasticity of 316L(N) steel. This study discusses the contribution of nitrogen, including solution strengthening, stacking fault energy, and short-range ordered structure to strength enhancement. It provides a valuable reference for developing new austenitic stainless steel with an optimal strength–plasticity combination.

Prediction of Surface Microstructure, Grain Size, Martensite Content, and Microhardness of 316L Austenitic Stainless Steel in Surface Mechanical Grinding Treatment Process

Prediction of Surface Microstructure, Grain Size, Martensite Content, and Microhardness of 316L Austenitic Stainless Steel in Surface Mechanical Grinding Treatment Process

Fatigue crack growth behavior of SS316L deposition from wire arc additive manufacturing

Wire arc additive manufacturing has shown the potential to build large metal parts with high deposition rates and low cost of production compared to other additive manufacturing techniques. Austenitic stainless steel 316L is known for corrosion resistance and weldability, which makes it a strong candidate for building and repairing marine and offshore parts, with wire arc additive manufacturing. These parts experience fluctuating loads during service, which necessitates the fatigue performance study of wire arc additively build stainless steel. This work investigates the fatigue crack propagation behavior of stainless steel 316L depositions from wire arc additive manufacturing. Single beads of stainless steel 316L were deposited layer by layer to fabricate the wall structures. From the wall structures, the specimens were prepared for the tensile and fatigue crack growth rate tests along the traverse and build directions. In conjunction with mechanical testing, electron back-scattered diffraction and scanning electron microscopy techniques were employed to characterize the microstructure. Microstructural observations showed the presence of columnar dendrites along the build direction. The electron dispersive spectroscopy revealed the elemental diffusion at the layer interfaces due to the remelting of the layers. Differences in mechanical properties were observed across the two directions. Along build directions, the ultimate tensile strength was 8 % higher and yield strength was 7 % lower than the respective strengths along the traverse direction. The build direction also exhibited 46 % higher fatigue life cycle than the traverse direction. The difference in properties across different orientations was closely associated with the microstructural variations. Overall, the mechanical performance of the wire arc additively deposited SS316L is better than the previously reported selective laser melted SS316L.

Improving the electrochemical corrosion behavior of stainless steel (316L) through the deposition of tantalum-based thin films

Austenitic 316L stainless steel (SS) is commonly used as a biomaterial for human implant applications. However, its clinical uses are limited due to its relatively high corrosion rate in physiological conditions. Various surface modifications, including coatings, have been developed to enhance its performance. In this study, a thin layer of Tantalum (Ta) was applied to the surface of 316L stainless steel using a DC Magnetron Sputtering (DCMS) system; the thickness of the coating varied from 1.504 to 6.083 µm, achieved by deposition times ranging from 15 to 60 min. The aim was to examine the electrochemical corrosion characteristics of the uncoated 316L SS and the Ta-coated 316L SS, followed by Potentiodynamic Polarization (PDP), Electrochemical Impedance Spectroscopy, and Open Circuit Potential tests. These tests were conducted for 10 min, 60 min, and 90 min to assess their performance over time. The PDP measurements took place in a simulated body fluid (SBF) at a pH of 7.8 ± 0.25 and a temperature of 40 °C, with varying coating durations. The PDP test outcomes revealed that the Ta-coated 316L stainless steel exhibited enhanced resistance to corrosion compared to the uncoated 316L stainless steel sample. The efficacy of Ta coatings on the 316L stainless steel samples was assessed through several techniques, including Optical microscopes, Energy Dispersive X-ray Spectroscopy, Scanning Electron Microscopy, and X-ray Photoelectron Spectroscopy. The findings from SEM and EDS verified the successful application of the Ta coating onto the stainless steel. The composition of the surface oxide film of Ta, the substrate, and the thickness of the coating were determined using the XPS technique, which confirms the effectiveness of the Ta coating in safeguarding 316L stainless steel against electrochemical corrosion, particularly for biomedical applications.

Effect of Strain Hardening on Wear and Corrosion Resistance of 316L Austenitic Stainless Steel

Effect of Strain Hardening on Wear and Corrosion Resistance of 316L Austenitic Stainless Steel

The influence of printing strategies on fatigue crack growth behavior of an additively manufactured AISI 316L stainless steel

AbstractSelective laser melting (SLM) is an additive manufacturing powder‐bed fusion process that allows producing complex metallic parts with a relative density of up to 99.9%. Nevertheless, the mechanical properties of these components depend on numerous process variables, and there is a lack of systematic studies focused on SLM stainless steels. In the work herein presented, three specific printing strategies were X‐Y patterned to manufacture Compact Tension specimens transversally, longitudinally, or chess‐oriented, without any post‐processing heat treatment. The specimens were made of AISI 316L austenitic stainless steel. Thereafter, fatigue crack growth rate (FCGR) tests were carried out at room temperature, at constant amplitude loading (R = 0.2), to determine the fatigue crack propagation constants of the Paris Law associated with each print strategy. It was possible to conclude that transversal additively manufactured specimens showed the lowest FCGRs, and all results were well below the fatigue design curve given in ASME BPVC, Section XI.

High Temperature Heat Treatment and Severe Shot Peening of PBF-LB Manufactured 316L Stainless Steel

Laser powder bed fusion manufactured (PBF-LB) austenitic stainless steel 316L offers higher strength than traditionally manufactured counterparts. Further improvement can be obtained with suitable surface modification. This work focuses on improving the material qualities with the aid of severe shot peening (SSP), which can increase the surface hardness, reduce roughness and produce grain refinement and compressive residual stresses on the surface. These qualities are all beneficial for the fatigue life of the material. Material was studied in two conditions: as built and heat treated (HT) at 1100 °C and the effect of SSP on both. The results showed clear microstructural changes on both structures leading to increased strength. The fatigue strength of as built material benefits greatly from the SSP treatment, but when performed on a high temperature HT material the benefits are negligible. However, in applications where the parts are subjected to bending forces the surface modification plays a role also with the HT material.

Micromechanical analysis and finite element modelling of laser-welded 5-mm-thick dissimilar joints between 316L stainless steel and low-alloyed ultra-high-strength steel

As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (HIT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The HIT values displayed a distinct variation in strength between different zones. The HIT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity.