AISI 316LN Austenitic Stainless Steel Research Articles

Effect of Defocusing Distance on Microstructural, Hardness, and Tribological Behavior of Ni-Cr-Si-B-C Powder Deposit on 316LN Austenitic Stainless Steel Substrate Using Laser Hard-Facing

The Ni-based alloy powder has been melted and bonded to an AISI 316LN austenitic stainless steel (ASS) base material (substrate) using a high-energy disc laser with a maximum power of up to 12 kW. The substrate’s hard-faced reservoir is highly diluted due to the significant difference in melting temperature between the substrate and the commonly used Ni-based alloy. The hardness, macrostructure, microstructure, and wear resistance effects of the defocusing distance were examined. The composition, microstructure, hardness, and wear resistance of phases have been examined with a pin-on-disk wear test, an energy dispersion spectroscopy (EDS), a scanning electron microscope (SEM), and X-ray diffraction (XRD). According to the findings, the height and diameter of the beads rose when the distance was increased from 17 mm to 37 mm. On the other hand, penetration and dilution decreased from 3.7 mm to 2.7 mm and from 9.7% to 3.1%, respectively. When the defocusing length is increased and the penetration profundity and dilution are limited, the density of energy per unit clad is accessible. The laser hard-faced surfaces contain microstructures of Ni-rich solid solution, boride, and carbide. These different factors cause higher hardness and resistance to wear.

Sliding wear behaviour of salt bath nitrided 316LN austenitic stainless steel

The wear behaviour of AISI 316LN austenitic stainless steel has been studied by varying the duration of nitriding. AISI 316LN steel was nitrided using salt bath nitriding which consist of a mixture of 70:30 ratio of alkaline cynates and carbonates. Nitriding was carried out for two time durations namely 60 min and 100 min. Pin-on-Disc wear test were carried out for both the untreated and treated sample at room temperature. The wear parameters were changed to understand the wear mechanism of AISI 316LN steel along with the nitrided sample by varying the sliding distance for 250 m, 500 m and 1000 m. Due to the presence of different wear mechanism the rate of wear varies as a function of sliding distance. For untreated sample the wear mechanism was dominant with adhesion, abrasion and plastic deformation. An optical micrograph, X-ray diffraction analysis, wear morphology, hardness measurement and surface roughness were carried out. Due to the presence of compound layer as a result of nitriding AISI 316LN austenitic stainless steel, the mechanism of wear was restricted to abrasive wear. Untreated specimens have more significant wear loss when compared to treated specimen as work hardening and an increase in hardness of wear track resulted in material pull out. Stability of the compound layer is achieved as a result of longer duration of nitriding. Abrasive wear is resisted to a greater extend due the availability of compound layer. AISI 316LN steel which was nitrided for 100 min exhibited a more stable compound layer when compared to the same nitrided for 60 min.

Optimizing the Characteristics of the Laser Hardfacing Process Parameters to Maximize the Wear Resistance of Ni-Based Hard-Faced Deposits Using the RSM Technique

The nickel-based Colmonoy-5 hardfacing alloy is used to hard-face 316LN austenitic stainless steel components in fast reactors. The nominal composition (in wt%) was listed as follows: 0.01 C, 0.49 Si, 0.87 Mn, 17.09 Cr, 14.04 Ni, 2.56 Mo, 0.14 N, and balance Fe. Hardfacing is a technique of applying hard and wear-resistant materials to substrates that need abrasion resistance. The thickness of hardfacing deposit varies between 0.8 mm and 2 mm based on parameter combinations. In this study, laser hardfacing process parameters including laser power, powder feed rate, travel speed, and defocusing distance were optimized to reduce weight loss of laser hard-faced Ni-based deposit. The tribological characteristics of reactor-grade NiCr-B hard-faced deposits were investigated. The RSM technique was used to identify the most important control variables resulting in the least weight loss of the nickel-based alloy placed on AISI 316LN austenitic stainless steel. Statistical techniques like DoE and ANOVA are utilized. Changing the laser settings may efficiently track the weight loss of laser hard-faced nickel alloy surfaces. These are created using the response surface technique. The deposit produced with a laser power of 1314 W, powder feed rate of 9 g/min, travel speed of 366 mm/min, and defocusing distance of 32 mm had the lowest weight loss of 16.4 mg. Based on the F value, the powder feed rate is the major influencing factor to predict the hardness followed by power, travel speed, and defocusing distance.

Probing the stress corrosion cracking resistance of laser beam welded AISI 316LN austenitic stainless steel

The stress corrosion cracking (SCC) resistance of the laser beam welded (LBW) AISI 316LN austenitic stainless steel (SS) was assessed and compared to the base metal (BM). The weld joint was produced using a 2.5 kW laser power source at 1500 mm/min welding speed. Microstructural characterization of the base metal and weld joint were done by the following techniques: (i) Optical Microscopy (OM), (ii) Scanning Electron Microscopy (SEM) and (iii) Transmission Electron Microscopy (TEM). The primary mechanical properties such as strength, toughness and hardness of the welded joint were evaluated and compared with the base metal. Stress Corrosion Cracking (SCC) assessment was done in boiling 45 wt% MgCl2 solution at constant load condition as per American Society for Testing and Materials (ASTM) standard G36-94. From the SCC experiment data, steady-state elongation rate ([Formula: see text]), transition time ([Formula: see text]) and time to failure ([Formula: see text]) were found and generalized equations to predict the time to failure of the base metal and LBW joint were successfully derived. The passive film rupture mechanism majorly influenced the SCC failure for 316LN and welded joint. The formation of the discontinuous δ-ferrite network, residual stress and nitrogen pore nucleation at the fusion zone of the LBW joint deteriorated the SCC resistance. The metallographic and fractographic studies revealed brittle transgranular SCC failure of the base metal as well as the LBW joint in all the stress conditions.

Zinc diffusion induced precipitation of σ-phase in austenitic stainless steel

Zinc diffusion-induced degradation of AISI 316LN austenitic stainless steel pot equipment used in 55%Al-Zn and Zn-Al-Mg coating metal baths is described. SEM/EDS analyses results showed that the diffused zinc reacts with nickel from the austenite matrix and results in the formation of Ni-Zn intermetallic compounds. The Ni-Zn intermetallic phase and the nickel depleted zones form a periodic and alternating layered structure and a mechanism for its formation is proposed. The role of cavities and interconnected porosity in zinc vapour diffusion-induced degradation and formation of Ni-Zn intermediate phases is also discussed. The formation of Ni-Zn intermediate phases and the depletion of nickel in the austenite matrix results in the precipitation of σ-phase and α-ferrite in the nickel depleted regions of the matrix. This reaction will lead to increased susceptibility to intergranular cracking and accelerated corrosion of immersed pot equipment in the coating bath. Zinc diffusion induced precipitation of σ-phase in austenitic stainless steels that we are reporting in this work is a new insight with important implications for the performance of austenitic stainless steels in zinc containing metal coating baths and other process industries. This new insight will further lead to improved understanding of the role of substitutional diffusion and the redistribution of alloying elements in the precipitation of σ-phase in austenitic stainless steels.

Effect of Rolling Temperature on Microstructure Evolution and Mechanical Properties of AISI316LN Austenitic Stainless Steel.

The impacts of rolling temperature on phase transformations and mechanical properties were investigated for AISI 316LN austenitic stainless steel subjected to rolling at cryogenic and room temperatures. The microstructure evolution and the mechanical properties were investigated by means of optical, scanning, and transmission electron microscopy, an X-ray diffractometer, microhardness tester, and tensile testing system. Results showed that strain-induced martensitic transformation occurred at both deformation temperatures, and the martensite volume fraction increased with the deformation. Compared with room temperature rolling, cryorolling substantially enhanced the martensite transformation rate. At 50% deformation, it yielded the same fraction as the room temperature counterpart at 90% strain, while at 70%, it totally transformed the austenite to martensite. The strength and hardness of the stainless steel increased remarkably with the deformation, but the corresponding elongation decreased dramatically. Meanwhile, the tensile fracture morphology changed from a typical ductile rupture to a mixture of ductile and quasi-cleavage fracture. The phase transformation and deformation mechanisms differed at two temperatures, with the martensite deformation contributing to the former, and austenite deformation to the latter. Orientations between the transformed martensite and its parent phase followed the K–S (Kurdjumov–Sachs) relationship.

Tensile Deformation Temperature Impact on Microstructure and Mechanical Properties of AISI 316LN Austenitic Stainless Steel

Uniaxial tensile tests were conducted on AISI 316LN austenitic stainless steel from − 40 to 300 °C at a rate of 0.5 mm/min. Microstructure and mechanical properties of the deformed steel were investigated by optical, scanning and transmission electron microscopies, x-ray diffraction, and microhardness testing. The yield strength, ultimate tensile strength, elongation, and microhardness increase with the decrease in the test temperature. The tensile fracture morphology has the dimple rupture feature after low-temperature deformations and turns to a mixture of transgranular fracture and dimple fracture after high-temperature ones. The dominating deformation microstructure evolves from dislocation tangle/slip bands to large deformation twins/slip bands with temperature decrease. The deformation-induced martensite transformation can only be realized at low temperature, and its quantity increases with the decrease in the temperature.

Experimental Evaluation of Mechanical and Tribological Behaviours of Gas Nitride treated AISI 316LN Austenitic Stainless Steel

Austenitic stainless steel is limited to utilize directly in numerous industries on account of its low surface hardness. Hence, in this work, an effort has taken to enhance the surface hardness of austenitic stainless steel type AISI 316LN Stainless Steel (316LN SS) by Gas Nitride (GN) technique. The GN was done on 316LN SS at 520°C for 42 hours. The GN was done in two phases to limit the white nitride layer. The GN treatment enhances the surface hardness from 210 HV25g (untreated 316LN SS sample) to 1230 HV25g (GN treated sample). The GN treatment produces a thick and uniform nitride layer on the 316LN SS. The tribolgical test conclude that the GN layer on 316LN SS eliminates adhesion bonds at the contact points and show smooth surface even after 1000m sliding distance. Consequently, the GN treatment was not just reduce the friction and also formed an excellent wear resistance GN layer on the 316LN SS substrate.

Microstructural Characteristics and Mechanical Properties of Dissimilar Joints of Aisi 316LN Austenitic Stainless Steel and Modified 9Cr-1Mo Steel

In liquid metal cooled fast breeder reactors, the dissimilar joint between grade 91 ferritic steel and 316LN stainless steel is frequently encountered. For better integrity assessment, mechanical properties of each region need be evaluated. In the present investigation, dissimilar joints between grade 91 to 316LN SS were fabricated by shielded metal arc welding process using nickel based electrodes. Mechanical properties (Tensile and impact toughness) of different regions were evaluated by placing the notch at each location. Microhardness variation across the dissimilar joint was recorded. Microstructural analyses of various regions were done by optical and scanning electron microscopy. From this investigation, it is understood that the in-homogeneous mechanical properties were observed across the dissimilar joint. The development of complex microstructure at the fusion interfaces will alter the mechanical properties across the dissimilar joint.

Microstructural Characteristics and Mechanical Properties of Dissimilar Joints of Aisi 316LN Austenitic Stainless Steel and Modified 9Cr-1Mo Steel

In liquid metal cooled fast breeder reactors, the dissimilar joint between grade 91 ferritic steel and 316LN stainless steel is frequently encountered. For better integrity assessment, mechanical properties of each region need be evaluated. In the present investigation, dissimilar joints between grade 91 to 316LN SS were fabricated by shielded metal arc welding process using nickel based electrodes. Mechanical properties (Tensile and impact toughness) of different regions were evaluated by placing the notch at each location. Microhardness variation across the dissimilar joint was recorded. Microstructural analyses of various regions were done by optical and scanning electron microscopy. From this investigation, it is understood that the in-homogeneous mechanical properties were observed across the dissimilar joint. The development of complex microstructure at the fusion interfaces will alter the mechanical properties across the dissimilar joint.

Effect of brazing temperature on the microstructure of martensitic–austenitic steel joints

ABSTRACTDissimilar joining of reduced activation ferritic–martensitic steel to AISI 316LN austenitic stainless steel is carried out by brazing in inert atmosphere at three different temperatures, i.e. 980, 1020 and 1040°C using AWS BNi-2 powder. The braze joints are characterised by scanning electron microscopy, X-ray diffraction, micro-hardness measurement. With increasing brazing temperature from 980 to 1040°C, the approximate width of the braze layer decreases from 350 to 80 µm and hardness reduces from 600 to 410 VHN. However, not much difference is found in microstructure and hardness between braze joints produced at 1020 and 1040°C. With increasing brazing temperature, morphology and volume fraction of intermetallics formed in the braze layer change, thereby reducing the hardness variation between the braze layer and the base metal.

Influence of viscoplasticity, hardening, and annealing effects during the welding of a three-pass slot weld (NET-TG4 round robin)

In this paper, numerical simulations of a three-pass bead-in-slot weld in an AISI 316LN austenitic stainless steel plate are performed for different material constitutive behaviors, including viscoplasticity. This work has been undertaken as a part of the Task Group 4 (TG4) measurement and simulation round robin of the NeT European network. Finite Element (FE) Welding simulations are performed assuming a moving 3D heat source, accurately calibrated against thermocouple measurements and metallographic analysis. Different constitutive equations, calibrated from cyclic mechanical tests and creep data at different temperatures, are considered for the mechanical computations of residual stresses. The respective influence of cyclic hardening, annealing effects, and viscoplasticity is discussed, considering simple 1D models (fixed end bar analogy, subjected to thermal cycles identical to those experienced during welding) or the more complex 3D 3-pass slot weld FE model, representative of the NET-TG4 mock-up. Simulations results are also compared with residual stresses measurements provided within the NET-TG4 measurement round robin. The simulations emphasize the strong influence of hardening and annealing modelling, and the rather weak effect of creep on residual stresses for this special test case.

Tensile properties of shielded metal arc welded dissimilar joints of nuclear grade ferritic steel and austenitic stainless steel

AbstractIn nuclear power plants, modified 9Cr-1Mo ferritic steel (Grade 91 or P91) is used for constructing steam generators (SG’s) whereas austenitic stainless steel (AISI 316LN) is a major structural member for intermediate heat exchanger (IHX). Therefore, a dissimilar joint between these materials is unavoidable. In this investigation, dissimilar joints were fabricated by Shielded Metal Arc Welding (SMAW) process with Inconel 82/182 filler metals. Transverse tensile properties and Charpy V-notch impact toughness for different regions of dissimilar joints of modified 9Cr-1Mo ferritic steel and AISI 316LN austenitic stainless steel were evaluated as per the standards. Microhardness distribution across the dissimilar joint was recorded. Microstructural features of different regions were characterized by optical and scanning electron microscopy. The transverse tensile properties of the joint is found to be inferior to base metals. Impact toughness values of different regions of dissimilar metal weld joint (DMWJ) is slightly higher than the prescribed value. Formation of a soft zone at the outer edge of the HAZ will reduce the tensile properties of DMWJ. The complex microstructure developed at the interfaces of DMWJ will reduce the impact toughness values.

Application of Taguchi method in optimizing the microstructure of 316LN austenitic stainless steel

The hot deformation behavior of AISI 316LN austenitic stainless steel was studied through isothermal hot deformation tests conducted on Gleeble 1500 thermo‐simulation machine. By using Taguchi method with an L16 (43) orthogonal array, the hot deformation conditions including deformation temperature, strain rate and strain were optimized for obtaining minimum mean grain size of the steel. Analysis of variance was used for determining the effect of the hot deformation parameters on the mean grain size. The results suggested that the strain had the most significant effect on the mean grain size among the three factors. The percent contributions of deformation temperature, strain rate and strain to the mean grain size were 0.80 %, 2.45 % and 93.32 %, respectively. Finally, the confirmation experiment under the optimum conditions, i.e., 1050 °C temperature, 0.01 s–1 strain rate and 0.7 strain was carried out. The observational mean grain size (11.4 μm) is approximately twelve times smaller than that of the initial microstructure.

Numerical Analysis of Gas Tungsten Arc Welded AISI 316LN Austenitic Stainless Steel Joints using Response Surface Methodology and Finite Element Anlysis

Numerical Analysis of Gas Tungsten Arc Welded AISI 316LN Austenitic Stainless Steel Joints using Response Surface Methodology and Finite Element Anlysis

Identification of crystal plasticity parameters using DIC measurements and weighted FEMU

An inverse method for the identification of a set of crystal plasticity parameters is introduced and applied to in situ tensile tests on steel polycrystals. Various mean grain sizes are obtained with different conditioning of AISI 316LN austenitic stainless steel. Identification is based on a weighted Finite Element Model Updating (FEMU) using both displacement fields at the microscale and macroscopic load levels. The values of the identified parameters depend on the factor weighing the contributions of the microscopic and macroscopic quantities. On the one hand, surface displacement fields are measured by Digital Image Correlation (DIC). On the other hand, they are simulated by resorting to Finite Element calculations with a local crystal plasticity law (i.e.,Méric-Cailletaud’s model) and a 2D simulation of the microstructure. The parameters associated with isotropic hardening are calibrated for the different mean grain sizes. The benefits of considering full-field measurements are manifest for instance in the excellent Hall-Petch trend captured at the microstructural scale. The identification procedure is also applied to the estimation of hardening parameters describing the interaction between slip systems.

Improving Wear Resistance of AISI 316LN Austenitic Stainless Steel Using Friction Stir Processing

An attempt is made to modify the surface metallurgically and enhance the wear resistance of AISI 316LN austenitic stainless steel using friction stir processing. Friction stir welding tools made up of tungsten based alloy with pin and pinless configuration was used. Fine equiaxed grains were observed in the friction stir processed zone irrespective of tool configuration used. Dry sliding wear resistance was evaluated using pin-on-disc wear tester and it is found that, the friction stir processed zone showed superior wear resistance compared to the base metal. Microstructure, micro hardness, and worn surfaces were used to correlate the results obtained.

Comprehensive numerical analysis of a three-pass bead-in-slot weld and its critical validation using neutron and synchrotron diffraction residual stress measurements

The current paper presents a finite element simulation of the residual stress field associated with a three pass slot weld in an AISI 316LN austenitic stainless steel plate. The simulation is split into uncoupled thermal and mechanical analyses which enable a computationally less expensive solution. A dedicated welding heat source modelling tool is employed to calibrate the ellipsoidal Gaussian volumetric heat source by making use of extensive thermocouple measurements and metallographic analyses made during and after welding. The mechanical analysis employs the Lemaitre–Chaboche mixed hardening model. This captures the cyclic mechanical response which a material undergoes during the thermo-mechanical cycles imposed by the welding process. A close examination of the material behaviour at various locations in the sample during the welding process, clearly demonstrates the importance of defining the correct hardening and high temperature softening behaviour. The simulation is validated by two independent diffraction techniques. The well-established neutron diffraction technique and a very novel spiral slit X-ray synchrotron technique were used to measure the residual stress–strain field associated with the three-pass weld. The comparison between the model and the experiment reveals close agreement with no adjustable parameters and clearly validates the used modelling procedure.

Carburization behavior of AISI 316LN austenitic stainless steel – Experimental studies and modeling

AISI type 316LN austenitic stainless steel was exposed to flowing sodium at 798 K for 16,000 h in the bi-metallic (BIM) sodium loop. A modified surface layer of 10 μm width having a ferrite structure was detected from X-ray diffraction and electron micro probe based analysis. Beneath the modified surface layer a carburized zone of 60 μm width was identified which was found to consist of M 23C 6 carbides. A mathematical model based on finite difference technique was developed to predict the carburization profiles in sodium exposed austenitic stainless steel. In the computation, effect of only chromium on carbon diffusion was considered. Amount of carbon remaining in solution was determined from the solubility parameter. The predicted profile showed a reasonably good match with the experimental profile. Calculations were extended to simulate the thickness of the carburized layer after exposure to sodium for a period of 40 years. Attempt was also made to predict the carburization profiles based on equilibrium calculations using Dictra and Thermocalc which contain both thermodynamic and kinetic databases for the system under consideration.

Effect of grain size on austenite stability and room temperature low cycle fatigue behaviour of solution annealed AISI 316LN austenitic stainless steel

The present paper investigates completely reversed room temperature low cycle fatigue (LCF) behaviour of solution annealed austenitic stainless steel AISI 316L with two different grain sizes of 90 and 139 μm developed by solution annealing treatment at 1050 and 1150°C respectively and at six strain amplitudes ranging between ± 0·375 and ± 1·00%. Complete cyclic hardening has been observed for both the grain sizes. While fine grained steel shows an improvement in cyclic life compared with that of coarse grained steel for strain amplitudes ± 0·375 and ± 0·50%, and perfectly follows the Coffin–Manson (C–M) behaviour within the experimental domain, higher cyclic life with bilinear C–M behaviour is observed in the case of coarse grained steel at ± 0·625% strain amplitude and above. Optical microscopy of fatigue fracture surfaces reveals the formation of martensite on cyclic straining predominantly at higher strain amplitudes.