316L Austenitic Stainless Steel Research Articles

EBSD study on magnetic field altering crystal texture and grain growth during laser-hybrid welding

In this study, evolutions of austenite and ferrite microstructures and relevant variations of performances were analyzed during laser-arc welding 316L austenitic stainless steel with a magnetic field. For austenite grains, the magnetic field gave rise to grain refinement in columnar zones, grain coarsening in central zones and discontinuous grain growth at interfacial zones. For ferrite dendrites, the magnetic field suppressed the concentrated growth, side-branched irregular morphology as well as orientation deflections across the weld. Results indicated that grain refinement, increased boundary disorientation and reduced residual strain enhanced the tensile strength, lowered brittleness and cracking susceptibility. Furthermore, variations of current, supercooling and electromagnetic stirring were analyzed at 0mT and 18mT magnetic intensities. At 0mT, current first increased and then decreased, attaining the highest in the columnar zone while supercooling rose in a steady form. At 18mT, the enhanced heat diffusion induced by fusion line variation lowered the supercooling and inhibited the columnar-to-equiaxed behavior while electromagnetic stirring reached the highest in columnar zones. Therefore, grain refinement was observed in columnar zones while coarsening in central zones at 18mT. In the interfacial zone, continuous and directional growth was achieved by the current at 0mT while grain refinement and growth deflection by electromagnetic stirring.

Effect of MIG Welding Parameters on the Mechanical Properties of AISI 304 Austenitic Stainless Steels

The present research investigates joints welding of 304L austenitic stainless steel using metal inert gas (MIG) welding method. The research explores the effect of process parameters (arc voltage, wire feed rate, and electrode wire diameter) on the mechanical properties of stainless steel. The above variables are varied respectively with 18.5, 19, 19.5 V, 116, 127, 137 mm/s, and 0.8, 1, 1.2 mm, with E308L as a filler electrode. The design matrix of the experiments was determined using the design of experiment (DOE) program Minitab 17 based on the levels of input elements used. The Taguchi orthogonal matrix methodology (Taguchi) technique was used to develop some empirical analysis for the maximum tensile strength and proper surface hardness as a function of the welding parameters mentioned above. The ( ) analysis was used to statistically verify the model's adequacy. The results show that the wire feed rate had the greatest effect on the tensile strength of the welded joints when choosing the medium level of arc voltage (19 V), while the arc voltage had a substantial influence on the micro-hardness of all welded joints. The weld-joint zone is given a higher micro-hardness value compared to the base metal zone.

Laser Welding of 316L Austenitic Stainless Steel in an Air and a Water Environment.

Laser welding is an innovative method that is frequently used and required by different disciplines and represents a technique of choice in a wide range of applications due to important advantages such as precision, speed, and flexibility. However, the welding method must be used properly otherwise it may deteriorate the mechanical properties of the welded metal and its environment. Therefore, the laser parameters should be precisely determined and carefully applied to the sample. The primary objective of this study was to investigate and propose optimal welding parameters that should be adjusted during the neodymium-doped yttrium aluminum garnet (Nd: YAG)-pulsed laser welding of austenitic stainless steel 316L in an air welding environment by using Argon shielding gas and in wet welding settings in serum medium. The investigation of the welding process in serum medium was conducted in order to propose the most suitable welding parameters being important for future possible medical applications of laser welding in in-vivo settings and thus to investigate the possibilities of the welding process inside the human body. In order to evaluate the quality of welding in air and of wet welding (in serum), a detailed parameter study has been conducted by variation of the laser energy, the welding speed and the focal position. The relationship between the depth of penetration and specific point energy (SPE) was also evaluated. The microstructure of the welded metal was examined by an optical microscope and scanning electron microscope (SEM). Based on the microscopy results, it was found that the largest depth of penetration (1380 µm) was achieved with 19 J laser energy in air medium, while the depth reached the largest value (1240 µm) in serum medium at 28 J laser energy. The increasing energy level showed opposite behavior for air and serum. The results of our study imply that when welding of 316L stainless steel is implemented properly in the body fluid, it would be a promising start for future in-vivo studies.

Residual stress relaxation in the carburized case of austenitic stainless steel under alternating loading

Low-temperature gaseous carburization is an effective thermochemical surface modification process for austenitic stainless steels. The treatment results in the development of a carbon-enriched case with high hardness, improved localized corrosion performance and high compressive residual stress, that in principle improves the fatigue performance. Residual stress relaxation occurs during cyclic loading and the residual stress relaxation in carburized case on 316L austenitic stainless steel under cyclic loading was investigated with rotating bending fatigue testing. Two stages of residual stress relaxation in the surface region were identified: an instantaneous and large relaxation in the first cycle followed by gradual relaxation with the number of fatigue cycles. Depth-resolved analysis of residual stress showed that relaxation mainly occurs in the surface-near region of the carburized case. The rate of stress relaxations depends on the amplitude of the alternating load. The occurrence of plastic accommodation provoked by lattice expansion and the additional plastic deformation imposed during loading is decisive for stress relaxation. A model of surface residual stress evolution for carburized case under cyclic loading was established, which could accurately describe the relaxation.

Chaboche Model for Fatigue by Ratcheting Phenomena of Austenitic Stainless Steel under Biaxial Sinusoidal Loading

This study deals with the investigation of the cyclic behaviour of 316L and 304L austenitic stainless steels in oligocyclic fatigue under biaxial loading. As a first step, we investigated the prediction of the character of 316L steel under imposed stress, by the fixation of a stress and the evolution of another, forming a cross-proportional loading path in a range of stresses. In addition, the analysis of the behavior of steel 304L with respect to the bi-axial union (primary and secondary loadings) was studied in order to produce the ratcheting phenomenon induced by the non-zero mean stress, governing the structure to damage in two opposite directions, diagonally symmetrical. An appreciable confrontation of the intrinsic characters of the two steels under the same loading conditions was discussed in the last intervention, controlled in strain, generating the phenomenon of cross-hardening and imposed stress. Producing the progressive strain that manifests itself at each loading cycle will make it possible to quantify the degree of plasticity of each material and optimize the most relevant steel. In this numerical study, the Chaboche model is selected, which is based mainly on perfect predictions and robust constitutive laws capable of reproducing observed macroscopic phenomena. All the simulations were carried out using the ZéBulon computation code. A lot of work on the behavior of 304L and 316L stainless steel has been carried out by several researchers in recent years. The results of previous experiments and numerical simulations have been compared to the results of this study, and a good match has been found.

Study of the influence of wire metals on dissimilar welds

The aim of this study is to investigate the effect wire metal on microstructure and mechanical properties of dissimilar welds between HSLA-X70 high strength steel alloy and 304L austenitic stainless steel produced by automatic tungsten arc welding (TIG). The weld joints were prepared using E304L, E316L, E2209L, and E7010 wire metal. The mechanical characteristics obtained from hardness, tensile and impact testing, were correlated to the optical and SEM microscopy, to establish a relationship between wire metal composition and the microstructures in different weld regions. It is concluded that E2209 wire metal lead to improve in the resilience characteristics and tenacity with a slight reduction in the ultimate tensile strength and hardness.

Regulating the Tensile Properties of a Nano‐Lamellar Austenitic Stainless Steel via Cryogenic Electropulsing

Electropulsing treatment (EPT) in the liquid nitrogen (LN) circumstances, referred to as LN‐EPT, is utilized to regulate the microstructure and tensile properties of a cold‐rolled 316L austenitic stainless steel. The electropulsing effects between the samples treated in the ambient environment at room temperature (RT) and in the LN temperature are compared. The results confirm that the LN‐EPT‐processed samples can also realize a fully recrystallized microstructure, and even the Joule heat is significantly suppressed compared with the RT‐EPT samples. The tensile properties of the LN‐EPT samples show both higher yield strength and higher ultimate strength while keeping considerable elongations compared with the RT‐EPT samples because of the finer recrystallized structure in the LN‐EPT samples. This attempt of LN‐EPT may shed some new light on developing the cryogenic electropulsing‐assisted forming methods for metals in the future.

An approach for obtaining surface residual stress based on indentation test and strain measurement

Abstract During the welding process of welded joints in nuclear power plants, uneven deformation caused by temperature changes will lead to the generation of residual stress. Aiming at the problem that the demarcate experiment is easy to cause errors when the traditional indentation strain method is used to measure the residual stress. Using 316L austenitic stainless steel as an example, ABAQUS finite element simulation is combined with physical experiments to study the effects of different indentation depths on the strain increment after unloading under a state of residual stress. The results show that the indentation depth and strain increment are linearly related. Based on the linear relationship, an independent parameter that characterizes the residual stress is obtained and a model between the parameter and residual stress established. It provides a certain basis for the convenience and precise detection of residual stress in nuclear power plants machinery structures in service.

Study on microstructure and mechanical behavior relationship for laser-welded dissimilar joint of P92 martensitic and 304L austenitic steel

A dissimilar laser beam welded joint of ferritic/martensitic P92 steel, and 304L austenitic stainless steel (ASS) has been examined in as-weld and post-weld heat treatment (PWHT) at 760 °C for 120 min for microstructure feature and mechanical aspects. The microstructure of the weld metal, interface, and heat-affected zone (HAZ) was characterized in detail, and it was observed that a great level of inhomogeneity exists in a dissimilar joint. In addition, the effect of the inhomogeneity in microstructure on mechanical properties, microhardness, and impact strength of the welded joint was discussed. The welded joint's mechanical properties met the USC boiler requirement for both as-weld and PWHT situations. The welded joint showed a decent combination of tensile strength and impact strength for both as-weld and PWHT conditions. The tensile results showed the failure from the 304L SS base metal, which ensured the acceptable strength of the weld metal for the laser welded joints. The part of the welded joint with relatively poor impact strength was the weld metal. Though, the Charpy impact toughness of the weld metal (89 J) was measured higher than the ASME standard value (>41 J) and EN 1599:1997 standard value (>47 J) in the as-weld situation. The impact strength of the weld fusion zone was 138 J in PWHT condition.

Structural Characterization of Ion Nitrided 316L Austenitic Stainless Steel: Influence of Treatment Temperature and Time

The ion nitriding behavior of AISI 316L austenite stainless steel was investigated at different nitriding times (2 h, 4 h, and 9 h) and temperatures (450 °C, 500 °C, and 550 °C). The structural characterization has been assessed by several considerations which can be listed: (i) the evaluation of phase distribution through Rietveld analysis of X-ray diffraction patterns and accompanying peak fitting process, (ii) hardness profile and related nitride layer thickness by microhardness and microscopic measurements, and (iii) displacement measurements to assess the residual stress accumulation. The diffusion of nitrogen atomic species into the sample surface caused a transformation of the γ phase matrix into an expanded austenite (γN) phase, which is recognized with its high hardness and wear resistance. Furthermore, depending on the nitriding condition, chromium nitride (Cr1-2N) and iron nitride (ε-Fe2-3N and γ′-Fe4N) phases were detected, which can be detrimental to the corrosion resistance of the 316L austenite stainless steel. The γN phase was observed in all nitriding conditions, resulting in a significant increase in the surface hardness. However, decomposition of the γN phase with an increase in nitriding temperature eventually altered the surface hardness distribution in the nitriding layer. Considering the phase-type and distribution with the consequent hardness characteristics in the nitride layer, to our best knowledge, this is the first report in which an ion-nitriding temperature of 500 °C (higher than 450 °C) and time of 9 h can be proposed as ideal processing parameters leading to optimal phase composition and hardness distribution for 316L austenite stainless steels particularly for the applications requiring a combination of both wear and corrosion resistance.

Investigation of Material Properties of Wall Structures from Stainless Steel 316L Manufactured by Laser Powder Bed Fusion

To make powder bed fusion (PBF) via laser beam (-LB) for metals (/M) available for highly regulated components such as pressure equipment according to the Pressure Equipment Directive, system-specific qualification methods need to be established to deal with process- and geometry-dependent inhomogeneous material behavior. Therefore, the material properties of austenitic stainless steel (316L) and their influences on normative acceptable qualification strategies were investigated in this study. Flat tensile test specimens were produced by two manufacturing systems identical in construction and were compared to specimens produced from conventionally rolled sheet material. Specimens were compared in the horizontal and vertical building directions in relation to different slope angles, wall thicknesses and cross-sectional areas. Despite identical process setups, parameters and powder feedstock, differences in mechanical behavior could be seen. Furthermore, the mechanical properties, surface roughness and density showed dependencies on the wall thickness and slope angle. In particular, the influence of wall thickness has not been covered in publications about PBF-LB/M before. These results suggest that geometry- and system-dependent components can be designed based on associated data from qualification processes. Therefore, a new qualification method based on wall structure properties is suggested for standard qualification processes of components with wall structures, such as pressure equipment.

Fundamental aspects of functional grading via powder hot isostatic pressing – Development of microstructure and diffusional processes

Functional grading of a ferritic steel (SA508 grade 3) and austenitic stainless steel (316L) powder via hot isostatic pressing (HIP) consolidation is investigated as an alternative to dissimilar welding. Fundamental aspects of this method such as the development of microstructure and diffusional processes during the HIP process of such mixed powder are studied in this work. A change of the post-HIP α to γ ratio with HIP temperature was observed in mixed powder specimens, combined with the formation of a ferritic lath zone which affects the properties of the specimens (throughout this work the term ferritic laths refers to a ferritic phase of unknown structure which is likely to consist of martensite, bainite or a mixture of both). These effects were attributed to extensive diffusion of alloying elements from 316L into SA508 grade 3. Simulation and experimental quantification of diffusion in diffusion couple experiments were applied to derive an empirical model for estimating α-Fe boundary migration and the formation of the ferritic lath zone and phase fractions for various temperatures and dwell times of the HIP process.

Investigating the Microstructural Properties and Phase Distributions of TIG Welded AISI/SAE 304L Stainless Steel Runners in TNT Filled Ammunutions

Among the stainless steels, the most widely used group is austenitic stainless steels. The prominent reason why austenitic stainless steels are preferred in welded stainless steel fabrications is their good weldability. AISI/SAE 304L austenitic stainless steels exhibit adequate corrosion resistance in oxidizing environments. AISI/SAE 304L austenitic stainless steels are also used in filling interfaces of runners between the Tri-Nitro-Toluen filling vessel and ammunitions in defense industry. For the manufacture of the runners, the stainless steel plates and the stainless steel pipes are mainly joined by fusion welding. These runners are cleaned with hot tapping water at 70°C after each filling of ammunitions by TNT so that cleaning operation can deteriorate the corrosion properties of runners especially weld zones. Austenitic stainless steel weldments include basically delta-ferrite and austenite phases on weld regions. Delta ferrite phase exhibits resistance to chloride bearing medias while austenite phase is more resistive to high temperature oxidizing ambients In this study; 3 mm thick 304L austenitic stainless steel plates and rings are welded to each other with two different filler weld wires of ER316L and ER2209 by Tungsten Inert Gas welding method under pure argon shielding gas. Weld metals and heat-affected zones of welded joints were examined using optical metallurgical microscope. It has been determined that the additional filler weld wires have changed the weld metal and the heat affected zone microstructures and phase distributions dominantly. Microstructures of all welded joints have been compared. Besides, phase distributions of weld metals and heat effected zones are made by image analysis software and Schaefflers diagram. ER2209 welding filler metal increased the phase fraction of delta-ferrite in weld metals while it decreased the austenite phase fraction as compared to ER316L filler metal.

Production and Properties of the Porous Layer Obtained by the Electrochemical Method on the Surface of Austenitic Steel.

The growing demand for implants has seen increasing interest in the introduction of new technologies and surface modification methods of metal biomaterials. This research aimed to produce and characterize a porous layer grown on austenitic stainless steel 316L, obtained via the anodization process near the micro-arc oxidation, i.e., low voltage micro-arc oxidation (LVMAO). The discussed layer significantly influences the properties of metallic biomedical materials. The surface topography, layer thickness, surface roughness, pore diameter, elemental composition, crystal structure, and surface wettability were assessed for all anodized layers, together with the resultant corrosion resistance. Attention was paid to the influence of the process parameters that affect the specification of the produced layer. The obtained results showed surface development and different sized pores in the modified layers, as well as an increase in corrosion resistance in the Ringer’s solution.

Anisotropic yield surfaces of additively manufactured metals simulated with crystal plasticity

The mechanical anisotropy created by additive manufacturing (AM) is not yet fully understood and can depend on many factors, such as powder material, manufacturing technology and printing parameters. In this work, the anisotropic mechanical properties of as-built, laser powder bed fusion (LPBF) austenitic stainless steel 316L and titanium alloy Ti-6Al-4V are investigated through crystal plasticity simulations. Periodic representative volume elements (RVEs) are used that are specific to each material. The RVE for austenitic stainless steel consists of FCC crystals with a crystallographic texture measured by X-ray diffraction. The α′ martensite microstructure of Ti-6Al-4V is captured with a multi-scale RVE, including internal lamellar structures, using HCP crystals and a synthetically generated texture. For both materials, the crystal plasticity parameters are calibrated against tensile tests carried out on dog-bone specimens printed in different orientations. The RVEs, calibrated to experiments, are applied in virtual material testing and subjected to multiple load cases to generate the Hill-48 and Yld2004-18p yield surfaces of the materials.

Mechanical and Microstructural Anisotropy of Laser Powder Bed Fusion 316L Stainless Steel.

This paper aims at an in-depth and comprehensive analysis of mechanical and microstructural properties of AISI 316L austenitic stainless steel (W. Nr. 1.4404, CL20ES) produced by laser powder bed fusion (LPBF) additive manufacturing (AM) technology. The experiment in its first part includes an extensive study of the anisotropy of mechanical and microstructural properties in relation to the built orientation and the direction of loading, which showed significant differences in tensile properties among samples. The second part of the experiment is devoted to the influence of the process parameter focus level (FL) on mechanical properties, where a 48% increase in notched toughness was recorded when the level of laser focus was identical to the level of melting. The FL parameter is not normally considered a process parameter; however, it can be intentionally changed in the service settings of the machine or by incorrect machine repair and maintenance. Evaluation of mechanical and microstructural properties was performed using the tensile test, Charpy impact test, Brinell hardness measurement, microhardness matrix measurement, porosity analysis, scanning electron microscopy (SEM), and optical microscopy. Across the whole spectrum of samples, performed analysis confirmed the high quality of LPBF additive manufactured material, which can be compared with conventionally produced material. A very low level of porosity in the range of 0.036 to 0.103% was found. Microstructural investigation of solution annealed (1070 °C) tensile test samples showed an outstanding tendency to recrystallization, grain polygonization, annealing twins formation, and even distribution of carbides in solid solution.

Evaluation of thermal aging activation energies based on multi-scale mechanical property tests for an austenitic stainless steel weld beads

The long-term thermal aging behavior of a 316L austenitic stainless steel welds containing ∼12% of δ-ferrite was investigated using the nano-scale (nanopillar compression), micro-scale (micro-hardness and small punch), and macro-scale (tensile and J-R) mechanical property testing methods. Specimens were aged at 343, 375, and 400 °C for up to 20,000 h. The thermal aging activation energies were estimated based on the various mechanical test results using two fitting methods. The activation energies vary from 124 to 300 kJ/mol, depending on fitting and mechanical testing methods. Among the mechanical properties, the nanopillar and J-R test results showed similar activation energies lower than those from other mechanical properties. The similarities and discrepancies among the estimated activation energies were discussed in view of the contribution of embrittled δ-ferrite to the fracture and deformation of test specimens.

3D Numerical modelling of turning-induced residual stresses in 316L stainless steel

This paper lies in the framework of turning-induced residual stress prediction by presenting an up-dated method based on a two-scale approach. Previous works aimed at building a 3D hybrid method consisting in modelling the turning consequences through equivalent thermo-mechanical loadings combining a finite element model with experimental tests. The new proposed global hybrid strategy keeps the same concept but has the advantage to define the thermo-mechanical shapes more accurately thanks to local 2D orthogonal cutting models. After carefully detailing each step of the numerical method, a validation case study concerning a longitudinal turning operation on a 316L austenitic stainless steel is proposed. By comparing numerical results with experimental residual stresses coming from X-Ray diffraction, the new numerical method shows its superiority to predict, within few hours, much more accurately the residual stress state induced by a real longitudinal turning operation.

Tribological investigation of HVOF sprayed Ni-based coatings on SS 316L in water-cooled nuclear reactor

The sodium and water-cooled nuclear reactors play a significant role in energy producing sectors. However, the maintenance of these reactors is more challenging. A 316L austenitic stainless steel are a major construction material used in nuclear reactors due to its high corrosion resistance. However, these components are subjected to wear due to its low hardness. The cobalt-based coatings were employed in some reactors for improving the wear properties. However, it has been observed that these coatings lead to the formation of Co-60 isotopes in the reactor and emits gamma rays, which are harmful to the maintenance personnel. Nickel based coating can be used as a substitute to the Co-based coating on SS 316L to avoid the risk involved. This paper presents the characterization of the HVOF sprayed Ni-based coating over SS 316L substrate and the wear and friction behavior of SS 316L and coating against silicon nitride at nuclear environment condition. The coating properties were obtained and presented. The friction and unidirectional sliding wear tests have been carried out using a pin on disc tribometer. The estimated mass wear rate for the coating has been found as 4.15 × 10-5g/Nm. The coating offers high wear resistance as compared to SS 316L material and hence recommended for the components of the water-cooled nuclear reactor.

Comparative Study on the Oxidation Behavior of Modified 20cr-25ni-Nb with Hr3c and 316l Austenitic Stainless Steel in Co2 Environment at 650 °C

Comparative Study on the Oxidation Behavior of Modified 20cr-25ni-Nb with Hr3c and 316l Austenitic Stainless Steel in Co2 Environment at 650 °C