AISI 316L Austenitic Stainless Steel Research Articles

Comparison of conventional and severe shot peening effects on the microstructure, texture, roughness, hardness, and electrochemical behavior of austenitic stainless steel

In the present research, microstructure, texture, roughness, hardness, and electrochemical behavior of AISI 316L austenitic stainless steel before and after shot peening were studied to elucidate the effect of conventional and severe shot peening (CSP and SSP) processes. After the shot peening, the fraction of strain-induced martensite (SIM) and mechanical twins (MTs) in the sub-surface layer was increased. The fraction of SIM and MTs in the SSP sample was higher than in the CSP sample. The XRD patterns indicated that the SSP sample had a higher peak broadening compared to the CSP sample. In the CSP and SSP samples, a gradient microstructure was formed along the depth direction. The microstructure of the topmost layer of the CSP and SSP samples exhibited numerous ultrafine grains. The grain refining during severe shot peening was faster because of the accumulation of more strain. The CSP and SSP samples revealed a gradient distribution of elements. After the SSP, the intensity of ⟨110⟩‖ED fiber texture decreased from 12.7 to 11.6 × R and the average intensity of ⟨100⟩‖ED fiber texture increased from 1.7 to 2.0 × R, respectively, compared to the CSP sample. The surface roughness of the SSP sample (Rq = 73.6 nm and Ra = 45.2 nm) was lower than that of the CSP sample which represented the roughness decreased with surface coverage increasing from 100 % to 1500 %. Also, the wettability increased after the conventional and severe shot peening processes. In addition, the microhardness of the CSP and SSP samples showed a gradient distribution. The CSP sample had the lowest corrosion current density (0.13 μA/cm2) whereas the NP (non-peened) sample exhibited the highest current density (0.65 μA/cm2). The presence of ⟨100⟩-oriented grains in both CSP and SSP samples led to the higher corrosion resistance of shot-peened steels compared to the NP sample. The presence of favorable texture with higher intensity in the CSP sample was responsible for the higher corrosion resistance of the CSP sample compared to the SSP sample. Finally, the gradient distribution of elements along the depth direction in the CSP and SSP steels improved the corrosion resistance of the surface.

Tensile and micro-compression behaviour of AISI 316L austenitic stainless steel single crystals at 20 °C and 300 °C: Experiments, modelling and simulations

The mechanical behaviour of 316L stainless steel single crystal is characterised at room temperature and 300°C. Elasticity moduli at room temperature are obtained with resonant ultrasound spectroscopy. Their dependence on temperature is calibrated with molecular dynamics simulations. The plastic behaviour is characterised by tensile tests on millimetre-sized single crystal specimens and compression tests on micrometre-sized single crystal specimens. A constitutive model of crystal plasticity based on dislocation density hardening at finite strains is developed and implemented in an open-source material subroutine compatible with several finite element (FE) and fast Fourier transform (FFT) solvers. Tensile curves at room temperature and 300°C are used to calibrate the interaction coefficients for self and coplanar dislocation interactions. The dislocation mean free path for obstacle dislocations and the annihilation distance are also calibrated. The calibrated model predicts tensile curves in excellent agreement with experimental data. In addition, the predicted plastic strain fields are in good agreement with the experimental fields obtained by digital image correlation. Semi-quantitative agreement between simulation and experimental data is obtained for micro-compression tests without further calibration of the model. Finally, an extension to polycrystals with grain size effects is finally proposed. The predicted strain hardening behaviour is compared with experimental data on stainless steel polycrystals.

Influence of cutting edge microgeometry on the cutting forces when machining difficult-to-cut materials

The paper presents research investigating the influence of cutting tools microgeometry on the cutting forces when milling difficult-to-cut materials. Austenitic stainless steel AISI 316L and nickel alloy Inconel 718 were machined with cemented carbide tools with various cutting edge rounding size while measuring the cutting forces during the process. From the standpoint of milling difficult-to-cut materials lowering the cutting forces load on the tool can be difficult to achieve without significant reduction of cutting parameters. Previous research into the cutting edge microgeometry suggests that modification of the cutting edge of milling tools can substantially extend the effective tool life, reduce cutting forces in the process and ensure higher quality of the machined surface. Results of long term wear tests of tools with cutting edge rounding sizes of 15, 30 and 45 µm are compared to the results of a sharp unprepared cutting tool, and the results of each machined material are also compared. Possible influence of cutting edge radius on the process for both materials was tested for cutting conditions constituting finishing operation. The most effective cutting edge radius size differed between the materials, with 15 µm rounding performing the best for AISI 316L and the sharp unprepared tool performing the best for the Inconel 718 alloy.

Laser powder bed fusion in situ alloying of AISI 316L-2.5%Cu alloy: microstructure and mechanical properties evolution

AbstractThis work investigates the effects of copper addition on the microstructure and mechanical properties of AISI 316L austenitic stainless steel fabricated by the laser powder bed fusion (L-PBF) method. The outcomes reveal that the copper atom dissolves into iron and forms a complete austenitic structure under the condition of the L-PBF process. Microstructural observations demonstrate that the microstructure of the new alloy is characterised by columnar grains consisting of finer cellular structures, as compared to the as-built AISI 316L. The appearance of such a finer sub-structure could be originated from the effect of copper on the cooling rate during the L-PBF process. The energy-dispersive X-ray spectroscopy maps indicate that the distribution of copper in the AISI 316L matrix is homogeneous, and no significant segregation of elements in the matrix is revealed. The results of the tensile tests show that the ultimate tensile strength of AISI 316L-Cu alloy is 558 MPa, whereas the yield strength value and the tensile elongation are 510 MPa and 30.4%, respectively. Two mechanisms of solid solution strengthening, and refinement of cell sizes improve the mechanical properties of AISI316L-Cu alloy compared with AISI 316L one. The microscale fractography of the fracture surface shows ductile fracture with massive dimple networks and brittle fracture with a quasi-cleavage plane, which may indicate the melt pool boundary. All these results confirm that the development of new alloys following the in situ alloying approach is economical and reliable.

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.

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 %.

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.

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.

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.

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.

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.

Metallurgical investigation of leakage observed in welded interface of Pilot pressure distributor of liquid stage for aerospace application

Pilot Pressure Distributor (PPD) system is used in liquid fuel stage of launch vehicles for the pressurized flow of liquid propellant. The PPD system is fabricated out of AISI304L austenitic stainless steel. The system consisted of adaptor and plumbing lines is welded together using Gas Tungsten arc welding technique. The distributor system is subjected to hydro test at 120 bar pressure and then post- harnessing command line leak check is carried out after the hydro test. During the leak test at 60 bar with snoop solution, a frothing leak is observed at PPD distributor weld joint. A pressure drop of 6 bar was also observed over a duration of 10 min. The leaked PPD distributor was dismantled from the assembly and subjected to detailed metallurgical investigation to identify the root cause of leakage. The metallographic analysis revealed leakage in PPD distributor attributed to the lack of fusion in the weld joint.

Gradient self-organized dislocation in expanded austenite layer during low temperature nitriding

Abstract A typical nitrogen expanded austenite layer is formed by plasma-based low-energy nitrogen ion implantation (PBLEII) on AISI 304L austenitic stainless steel at a moderate temperature of 380° C. The dislocation self-organization structure in the nitrogen expanded austenite layer is characterized as an evolution from partial and Lomer-Cottrell dislocations in the inner layer near the interface to multilayer stacking faults in the outer nitrided layer. The self-organized dislocation density and forms are essentially dependent on the plastic deformation, strain-gradient, and nitrogen-related stacking fault energies, respectively, due to the constrained expansion in the nitrided layer. As the nitrogen concentration in the austenitic matrix increases, the stacking fault energy gradually decreases, resulting in the transformation of the defect from Lamer-Cottrell dislocations to a multilayer stacking faults. The appropriate stress, which is associated with orderly stress relief during dislocation self-organization, preserves the integrity of the nitrided layer with a combinedly improved in wear and corrosion resistance. Nitriding-induced dislocation self-organization is basically explored as the formation mechanism of the nitrogen expanded austenite layer, contributing to development of the specific low-temperature nitriding austenitic steel.

The Interplay Effects between Feed-Gas Composition and Bias Plasma Condition during Active-Screen Plasma Nitrocarburizing with a Solid Carbon Source

Recent technological development of utilizing an active screen made of solid carbon for plasma-assisted thermochemical diffusion treatments opens up new possibilities for control over the in situ generated treatment environment to guarantee reproducible treatment conditions and material responses. Until now, the investigations of active-screen plasma nitrocarburizing (ASPNC) using an active screen manufactured from solid carbon focused on the influence of a single treatment parameter variation on the material response. In this systematic study, experiments were conducted to vary the H2-N2 feed-gas composition while varying the bias plasma power. The experiments served to better understand a simultaneous variation in the mentioned parameters on the resulting treatment environment and material response during ASPNC of AISI 316L austenitic stainless steel. Therefore, nitriding and carburizing effects in the expanded austenite layer can be obtained. It is shown that an increased nitriding effect, i.e., nitrogen diffusion depth and content, was achieved in case of biased conditions and for H2-N2 feed-gas compositions with higher N2 amounts. On the contrary, an increased carburizing effect, i.e., carbon diffusion depth and content, was achieved in nonbiased conditions, independent from the H2-N2 feed-gas composition.

Investigating microstructural evolution and wear resistance of AISI 316L stainless steel cladding deposited over mild steel using constant current GMAW and pulsed current GMAW processes

Abstract The main objective of this study is to investigate the microstructure and wear resistance of AISI 316L austenitic stainless steel (ASS) cladding deposited over mild steel (MS) of grade IS 2062 using constant current (CC-GMAW) and pulsed current gas metal arc welding (PC-GMAW) processes. The PC-GMAW process was employed to overcome the problems of wider heat affected zone (HAZ), coarse grained deposited weld metal microstructure, lower penetration depth, higher dilution and reinforcement height encountered in CC-GMAW used for cladding of AISI 316L steel over mild steel. The microstructural features of cladded region were analyzed using optical microscopy (OM). The wear rate of cladded specimens was studied using pin-on-disc method and the morphology of wear surfaces was studied using scanning electron microscopy (SEM). The microhardness distribution of cladded region was analyzed and correlated to the wear performance of cladded specimens. The results showed that PC-GMAW cladding exhibit greater hardness and wear resistance compared to CC-GMAW cladding. It is correlated to the grain refinement offered by current pulsing in weld region of PC-GMAW cladding. The PC-GMAW cladding showed 5.22% increases in weld metal deposition compared to CC-GMAW cladding. The, PC-GMAW cladding showed 23.72% reduction in dilution of weld compared to CC-GMAW cladding. The PC-GMAW cladding disclosed 15.83 improvement in weld overlay hardness compared to CC-GAMW cladding. The PC-GMAW showed 20.18% average reduction in wear rate compared to CC-GMAW cladding.

The Mechanisms of Inhibition Effects on Bubble Growth in He-Irradiated 316L Stainless Steel Fabricated by Selective Laser Melting.

The AISI 316L austenitic stainless steel fabricated by selective laser melting (SLM) is considered to have great prospects for applications in nuclear systems. This study investigated the He-irradiation response of SLM 316L, and several possible reasons for the improved He-irradiation resistance of SLM 316L were systematically revealed and evaluated by using TEM and related techniques. The results show that the effects of unique sub-grain boundaries have primary contributions to the decreased bubble diameter in SLM 316L compared to that in the conventional 316L counterpart, while the effects of oxide particles on bubble growth are not the dominant factor in this study. Moreover, the He densities inside the bubbles were carefully measured using electron energy loss spectroscopy (EELS). The mechanism of stress-dominated He densities in bubbles was validated, and the corresponding reasons for the decrease in bubble diameter were freshly proposed in SLM 316L. These insights help to shed light on the evolution of He bubbles and contribute to the ongoing development of the steels fabricated by SLM for advanced nuclear applications.

Wear properties and microstructural characteristics of mild steel cladded with AISI 316L stainless steel using constant current gas metal arc welding process

The main objective of this study is to investigate the microstructure and wear resistance of AISI 316L austenitic stainless steel (ASS) cladding deposited over mild steel (MS) of grade IS 2062 using constant current (CC-GMAW) gas metal arc welding process. The microstructural features of cladded region were analyzed using optical microscopy (OM). The wear rate of cladded specimens was studied using pin-on-disc method and the morphology of wear surfaces was studied using scanning electron microscopy (SEM). The microhardness distribution of cladded region was analyzed and correlated to the wear performance of cladded specimens. The results showed that CC-GMAW cladded surface exhibited greater hardness and wear resistance compared to mild steel substrate. The overlay region of CC-GMAW clad showed the evolution of dendritic structure and columnar grains. The CC-GMAW cladding showed martensite laths and widmanstätten ferrite plates microstructure in the HAZ region closer to interface. The wear rate of CC-GMAW cladding increases with increase in load from 1.5 kg to 5.5 kg. It showed 422.85% and 423.58% increase in material weight loss and wear rate at higher loads. The materials weight loss of CC-GMAW cladding increases from 0.0082 g to 0.0517 g with increase in speed from 150 rpm to 550 rpm. The wear rate of CC-GMAW cladding increases from 3.66 × 10-5 g/m to 10.94 × 10-5 g/m with increase in temperature from 27 °C to 200 °C.

The Effect of Strain Rate on Hydrogen-Assisted Deformation Behavior and Microstructure in AISI 316L Austenitic Stainless Steel.

The influence of strain rate in the interval of (10-5-10-3) 1/s on room temperature tensile behavior, dislocation arrangement, deformation mechanisms, and fracture of austenitic stainless steel AISI 316L electrochemically charged with hydrogen was investigated. Independently on strain rate, hydrogen charging provides the increase in the yield strength of the specimens due to a solid solution hardening of austenite, but it slightly influences deformation behavior and strain hardening of the steel. Simultaneously, hydrogen charging assists surface embrittlement of the specimens during straining and reduces an elongation to failure, which both are strain rate-dependent parameters. Hydrogen embrittlement index decreases with increase in strain rate, which testifies the importance of hydrogen transport with dislocations during plastic deformation. The stress-relaxation tests directly confirm the hydrogen-enhanced increase in the dislocation dynamics at low strain rates. The interaction of the hydrogen atoms with dislocations and hydrogen-associated plastic flow are discussed.

Influence of nitrogen content on the corrosion fatigue behavior of additively manufactured AISI 316L stainless steel in chloride solution

The characterization of the corrosion fatigue behavior is detrimental for steels manufactured by laser beam powder bed fusion (PBF-LB). In this study, the corrosion and corrosion fatigue behavior of the austenitic stainless steel AISI 316L processed via PBF-LB with and without increased nitrogen content is investigated. The nitrogen alloyed steel shows improved pitting corrosion and fatigue behavior. A fracture mechanical approach was used for the fatigue data as surface defects lead to a high scatter in the results which not only allowed to separate the influence of defects but also to identify the improved defect tolerance due to nitrogen alloying.

Complex Interdependency of Microstructure, Mechanical Properties, Fatigue Resistance, and Residual Stress of Austenitic Stainless Steels AISI 304L.

Stainless steels are important in various industries due to their unique properties and durable life cycle. However, with increasing demands for prolonged life cycles, better mechanical properties, and improved residual stresses, new treatment techniques, such as deep cryogenic treatment (DCT), are on the rise to further push the improvement in stainless steels. This study focuses on the effect of DCT on austenitic stainless steel AISI 304L, while also considering the influence of solution annealing temperature on DCT effectiveness. Both aspects are assessed through the research of microstructure, selected mechanical properties (hardness, fracture and impact toughness, compressive and tensile strength, strain-hardening exponent, and fatigue resistance), and residual stresses by comparing the DCT state with conventionally treated counterparts. The results indicate the complex interdependency of investigated microstructural characteristics and residual stress states, which is the main reason for induced changes in mechanical properties. The results show both the significant and insignificant effects of DCT on individual properties of AISI 304L. Overall, solution annealing at a higher temperature (1080 °C) showed more prominent results in combination with DCT, which can be utilized for different manufacturing procedures of austenitic stainless steels for various applications.