AISI 316L Austenitic Stainless Steel Research Articles

Microstructure analysis of novel LPBF-processed duplex stainless steels correlated to their mechanical and corrosion properties

The austenitic steel AISI 316L has recently become one of the most popular alloys for laser powder bed fusion as it shows good mechanical properties along with sufficient corrosion resistance. For duplex stainless steels the LPBF process does not seem to be as attractive as a solution annealing treatment is required to attain the desired austenite and ferrite microstructure. The goal of this study is to identify a chemical composition that shows enhanced corrosion resistance and tensile strength compared to AISI 316L without requiring a post-process heat treatment like the conventional duplex stainless steels. Based on thermodynamic simulations powder mixtures of austenitic stainless steel AISI 316L and duplex stainless steel AISI 318LN in three mixing ratios have been processed. The influence of the chemical composition and LPBF processing parameters on the resulting microstructure are investigated by light optical microscopy and electron backscatter diffraction. Tensile tests were conducted and polarization curves recorded to evaluate the potential of the most promising alloy. 30 wt.-% of AISI 318LN added to AISI 316L lead to an austenite/ferrite microstructure in the as-built condition. The phase fractions vary with the LPBF processing parameters. The strength as well as corrosion resistance of this new alloy is superior to that of pure AISI 316L, while the ductility is lower.

Dissimilar friction stir welding of AISI 430 ferritic and AISI 304L austenitic stainless steels

Dissimilar joints of AISI 430 ferritic and AISI 304L austenitic stainless steels were produced by friction stir welding process. A sound and defect-free joint was obtained at 1 mm tool offset towards the ferritic sample located in the advancing side, and at rotational and welding speeds of 560 rpm and 50 mm/min, respectively. The XRD measurements revealed the presence of approximately equal volume fractions of ferrite (51%) and austenite (49%) phases in the stir zone (SZ). The formation of low-angle grain boundaries through the occurrence of dynamic recovery along with the presence of shear texture components in both constituent phases of ferrite and austenite in the SZ approved the occurrence of continuous dynamic recrystallization throughout the evolved microstructure. Moreover, microstructural observations showed the formation of necklace structure through the microstructure of ferrite in the SZ. Taylor map approved the strain localization in the ferrite phase. Micro-hardness measurement indicated that the hardness value is increased in the SZ. The result of tensile test showed that fracture occurred from less ductile ferritic base metal.

Sintering Kinetics of Austenitic Stainless Steel AISI 316L Modified with Nanographite Particles with Highly Developed BET Specific Surface Area.

The subject of this work was the study of processes occurring during sintering of water atomized AISI 316L austenitic stainless steel powder modified by the addition of graphite nanoparticles. The main purpose of the work was to determine the effect of modification of the AISI 316L stainless steel austenitic powder by the addition of graphite nanopowder on the sintering kinetics and oxide reduction mechanism. The phenomena occurring during the sintering process and oxide reduction mechanisms were subjected to detailed characterizations. Mixtures with two types of nanopowder with a high BET (measurement technique of the specific surface area of materials based on Brunauer–Emmett–Teller theory) specific surface area of 350 and 400 m2/g and for comparison with graphite micropowder with a poorly developed BET specific surface area of 15 m2/g were tested. The conducted thermal analysis showed that the samples made of austenitic stainless steel doped with 0.2% and 0.3% by weight graphite nanopowder with a BET specific surface area of 400 m2/g, sintered best the oxide reduction reactions, with a more intensive participation of carbon, for these samples.

Plasma Nitrocarburizing of AISI 316L Austenitic Stainless Steel: a First Step for Treatment of Components with Complex Geometries

Abstract The surfaces of AISI 316L austenitic stainless steel components with complex shapes and geometries can be subjected to extreme loads with intensive wear stress resulting a short lifetime. Low temperature active screen plasma nitrocarburizing (ASPNC) can be applied to form a thin duplex layer known as expanded austenite, improving the hardness and wear resistance with acceptable corrosion resistance. However, ASPNC of shaped components and components with different aspect ratios may yield non-uniform expanded austenite layer and/or reproducibility problems limiting their applications for specific cases. In this study, first, ASPNC of AISI 316L treated at different temperatures was investigated using active screen made of carbon-fibre reinforced carbon (CFC) to determine a reliable treatment temperature without CrN precipitation. In addition, several non-uniformity effects related to geometry and shape of structured samples were investigated during ASPNC treatment at different biased conditions. It was shown that in case of structured samples, a weak bias power at the samples was an essential process parameter to guarantee the formation of thick and homogenous expanded austenite layer while in non-biased conditions, thin and inhomogeneous expanded austenite is formed.

Plasma Nitrocarburizing of AISI 316L Austenitic Stainless Steel: a First Step for Treatment of Components with Complex Geometries

Abstract The surfaces of AISI 316L austenitic stainless steel components with complex shapes and geometries can be subjected to extreme loads with intensive wear stress resulting a short lifetime. Low temperature active screen plasma nitrocarburizing (ASPNC) can be applied to form a thin duplex layer known as expanded austenite, improving the hardness and wear resistance with acceptable corrosion resistance. However, ASPNC of shaped components and components with different aspect ratios may yield non-uniform expanded austenite layer and/or reproducibility problems limiting their applications for specific cases. In this study, first, ASPNC of AISI 316L treated at different temperatures was investigated using active screen made of carbon-fibre reinforced carbon (CFC) to determine a reliable treatment temperature without CrN precipitation. In addition, several non-uniformity effects related to geometry and shape of structured samples were investigated during ASPNC treatment at different biased conditions. It was shown that in case of structured samples, a weak bias power at the samples was an essential process parameter to guarantee the formation of thick and homogenous expanded austenite layer while in non-biased conditions, thin and inhomogeneous expanded austenite is formed.

Two-step annealing treatment for grain refinement of cold-worked AISI 316L stainless steel

Abstract Two-step annealing after cold rolling is proposed for the optimization of the annealing process of AISI 316L austenitic stainless steel toward more intense grain refinement. It was revealed that the first step of the annealing process at lower temperatures (i. e. 750 0C) can be used for the reversion of strain-induced martensite to austenite without uninhibited grain growth and recrystallization of the retained austenite. Afterwards, the primary recrystallization of the retained austenite takes place in the second step of the annealing process at higher temperatures (in the range of 850 to 950 0C). Carefully controlling the second annealing step resulted in a finer grain size and better mechanical response compared to those of a conventional one-step annealing approach.

Reducción de la fase de ferrita en aceros inoxidables austeníticos mediante el control del contenido de equivalentes de Cr-Ni

Los aceros inoxidables austeníticos AISI 316L son muy comunes en la industria. Grandes cantidades de este material se desechan como chatarra y pueden utilizarse posteriormente como material reciclado para la colada. En ambientes marinos (altamente corrosivos), este tipo de acero presenta corrosión intragranular, específicamente en una de sus fases, en la ferrita, por lo que se prefiere mantenerla en la menor proporción posible (<1%), pero normalmente se encuentra alrededor del 12%. Si se requiere que una pieza de fundición de acero AISI 316L transporte un fluido presurizado, debe realizarse de acuerdo con las consideraciones descritas en la norma ASTM A351. En este trabajo, se encontró que el acero CF3M tiene mayor solicitud en términos de proporciones químicas elementales de sus componentes. Por lo tanto, se utilizaron los diagramas de Schaeffler y Schoefer para reducir el contenido de "ferrita equivalente" en al menos el 90%, haciendo ajustes en la fundición modificando los porcentajes de Ni y Cr.

The investigation of microstructure and mechanical properties of resistance spot welded AISI 316L austenitic stainless steel

Today, the resistance spot welding (RSW) has become one of the most deployed welding techniques in different industries, with the bulk of automobile industries deploying it due to the simplicity, automation, and higher process speed. Past studies are a deficit of findings involving the use of 3 mm thick sheets. The bulk of the studies focused on the spot welding of 316L with a thickness below 2 mm. The lack of studies on resistance spot welding of 316L stainless steel with a thickness of more than 2 mm is evidenced. This paper aims to investigate the mechanical and microstructure properties of resistance spot welded 316L stainless steel with 3 mm thickness. The effect of constant welding parameters on the weld properties such as weld nugget size, tensile–shear load of welded materials, failure modes, Microhardness, and microstructure of weld nuggets were investigated. The results show that the maximum tensile- shear load was 21.549 KN. The Microhardness of the Fusion Zone (FZ) was 230 HV which was higher than that of both Base Metal (BM) (198.8 HV) and Heat Affect Zone (HAZ) (184 HV) respectively. The optical microscopic images showed phase transformation in the FZ.

Effect of Fiber Laser Surface Modification on the Corrosion Behavior of 316L Stainless Steel

This paper investigates the effect of fiber laser surface modification of AISI 316L austenitic stainless steel on corrosion behavior. In the experiments, the fiber laser with center wavelength of 1062 nm was employed with various laser parameters of beam velocity and laser frequency. The laser-treated has performed on the specimen surface in order to form the melted layer with an argon gas shielding. The electrochemical tested results showed that the laser-treated increases 40% pitting potential. Moreover, the results also exhibited corrosion potential shift to more positive potential. On the basis of the findings on the corrosion improvement, it can be concluded that the pitting potential of the material can be improved by a corrosion protective layer from the new laser-treated surface.

Two-step annealing treatment for grain refinement of cold-worked AISI 316L stainless steel

Abstract Two-step annealing after cold rolling is proposed for the optimization of the annealing process of AISI 316L austenitic stainless steel toward more intense grain refinement. It was revealed that the first step of the annealing process at lower temperatures (i. e. 750 0C) can be used for the reversion of strain-induced martensite to austenite without uninhibited grain growth and recrystallization of the retained austenite. Afterwards, the primary recrystallization of the retained austenite takes place in the second step of the annealing process at higher temperatures (in the range of 850 to 950 0C). Carefully controlling the second annealing step resulted in a finer grain size and better mechanical response compared to those of a conventional one-step annealing approach.

Influence of Ethanol on Pitting Corrosion Behavior of Stainless Steel for Bioethanol Fermentation Tanks.

The role of ethanol (C2H5OH) in pitting corrosion behavior of AISI 316L austenitic stainless steel was investigated in aqueous ethanolic solution with chloride. The pitting susceptibility and surface morphology of 316L in a series of ethanol-containing solutions were examined using X-ray photoelectron spectroscopy (XPS), optical microscopy with 3D stitching, immersion tests, and potentiodynamic polarization measurements. Results demonstrated that the ethanol concentration impacted little on the passive film stability while it dramatically influenced the pitting corrosion susceptibility. Corrosion rate of 316L after immersion tests first increased and then decreased as the concentration of ethanol increased from 0 to 10 M in ferric chloride solution. This, however, did not correspond to the breakdown potential which directly decreased from 489 to 249 mV as the water concentration decreased in ethanolic NaCl solutions. The pits density after both immersion and electrochemical tests showed that the initiation of pitting in ethanolic solution tended to occur at multiple points at the same time. The synergy effect on pitting behavior of hydrolysis enhancement and solubility reduction of metal cations due to the introduction of ethanol has also been discussed.

The passivity of low-temperature carburized austenitic stainless steel AISI-316L in a simulated boiling-water-reactor environment

The passive film on low-temperature-carburized AISI-316L austenitic stainless steel was studied in an oxygenated, simulated boiling-water-reactor environment with normal water chemistry. The microstructure was studied using scanning electron microscopy, grazing-incidence X-ray diffractometry, Auger electron spectroscopy, X-ray energy-dispersive spectroscopy, and transmission electron microscopy. For non-surface engineered AISI-316L, three distinct layers were observed after exposure: an outer layer of larger nickel-oxide enriched particles with the spinel structure, an intermediate layer of small Cr-oxide-enriched hematite particles, and a compact inner layer of Cr-enriched oxide also having the spinel structure. Exposure of low-temperature-carburized AISI-316L, in contrast, resulted in only two distinct layers. These layers did not include a compact Cr-rich inner layer. Instead, the innermost layer consisted of an oxide with the spinel structure that was a mixture of Fe, Cr and Ni oxide. The outer layer on the low-temperature-carburized specimen consisted of large loosely stacked particles with the spinel structure of mixed composition. In addition to passive-film characterization, the corrosion rate was studied by monitoring mass loss. After 500 h of immersion, the mass changes measured were relatively low for all specimens indicating good adhesion and low oxidation rates.

Running-in period for the abrasive wear of austenitic steels

The initial transient, or running-in, period during abrasive wear has been largely overlooked in the literature, with the focus instead being on steady-state conditions. However, the running-in period of abrasion might determine the evolution to steady-state wear behaviour, as is known to occur in sliding wear conditions. To this end, the running-in period during the abrasive wear of the austenitic stainless steel AISI 316L and Hadfield (15%Mn-1.5%C) steel was analysed through the testing pin (flat-ended)-abrasive paper wear configuration. The effects of the normal load, the size of the abrasive, and the type of material were evaluated, and the wear rate and the strain hardening of the matrix were recorded. The steady-state wear might be influenced by the phenomena occurring during the running-in period. Thus, the hardening, roughness, friction coefficient and wear micromechanisms were correlated with the wear rate to elucidate these possible relationships.

In-situ ToF-SIMS analyses of deuterium re-distribution in austenitic steel AISI 304L under mechanical load

Hydrocarbons fuel our economy. Furthermore, intermediate goods and consumer products are often hydrocarbon-based. Beside all the progress they made possible, hydrogen-containing substances can have severe detrimental effects on materials exposed to them. Hydrogen-assisted failure of iron alloys has been recognised more than a century ago. The present study aims to providing further insight into the degradation of the austenitic stainless steel AISI 304L (EN 1.4307) exposed to hydrogen. To this end, samples were electrochemically charged with the hydrogen isotope deuterium (2H, D) and analysed by scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). It was found that deuterium caused a phase transformation from the original γ austenite into ε- and α’-martensite. Despite their low solubility for hydrogen, viz. deuterium, the newly formed phases showed high deuterium concentration which was attributed to the increased density of traps. Information about the behaviour of deuterium in the material subjected to external mechanical load was gathered. A four-point-bending device was developed for this purpose. This allowed to analyse in-situ pre-charged samples in the ToF-SIMS during the application of external mechanical load. The results indicate a movement of deuterium towards the regions of highest stress.

Metallurgical Characterization of the Interfaces in Steel Plates Clad with Austenitic Steel or High Ni Alloys by Hot Rolling

An integrated experimental-theoretical approach to the metallurgical characterization of the interfaces in steel plates clad by hot rolling is proposed. Three different couplings of materials have been studied: ASTM A 515 Gr.60 low carbon steel clad with austenitic stainless steel AISI 304L; extra low carbon steel ASTM A283 clad with high Ni content Alloy 59; and, low carbon steel AISI 1010 clad with Cu-Ni Monel 400. Experimental investigations, which are addressed to analyse the microstructural changes near the interfaces and identify the present phases, have been carried out through scanning electron microscopy (SEM) observations, microanalytical measurements by energy dispersive spectroscopy (EDS), and Vickers microhardness tests. In all of the cases examined, the zones that are affected by detrimental microstructural changes results in being considerably less thick than the overall cladding layer. Simulations that are based on theoretical diffusion modelling have been integrated to the experimental characterization by introducing a cladding parameter that acts on the diffusion bonding efficiency, in order to evaluate the effects of process temperature and time variations on diffusion bonding efficiency and stability. In particular, this analytical investigation has shown how the shorter is the duration of the diffusion transient and the higher the temperature, the lower results the sensitivity of the diffusion processes to temperature fluctuations.

Unraveling the effects of surface preparation on the pitting corrosion resistance of austenitic stainless steel

The effects of surface preparation on the corrosion resistance of AISI 316L austenitic stainless steel were studied using the cyclic potentiodynamic polarization method. Grinding, mechanical polishing, and electropolishing were considered as the surface modifier methods. Regarding the surface roughness parameters, besides the conventional height parameter (Ra), the kurtosis (Rku) as the shape parameter was also considered to rationalize the pitting resistance for the first time. Based on the results of the Tafel extrapolation method, it was revealed that the uniform corrosion can be adequately correlated to Ra. However, the pitting resistance was found to mainly relate to the kurtosis, where by decreasing Rku (increased bluntness of topographic features), the pitting resistance enhanced. It was also found that a surface with Rku less than three (platykurtic) is resistant to pitting attack, where this surface can be obtained via electropolishing performed for an optimum time. The effect of electropolishing on the chromium content at the surface and its relation to the corrosion properties were also discussed.

Effect of surface nanocrystallization on low-temperature gas carburization for AISI 316L austenitic stainless steel

In this paper, the effect of surface nanocrystallization on low-temperature gas carburization (LTGC) for AISI316L austenitic stainless steel was studied. Surface ultrasonic rolling processing (SURP) was used to prepare the nanostructured surface layers and then the un-SURP and SURP specimens were treated by LTGC at 430 and 470 for 10 h, 20 h and 30 h. After the SURP treatment, the thickness of plastic deformation layer was about 350 μm and the compressive residual stress generated on the surface was −857.67 ± 55.54 MPa. In addition, the surface roughness decreased obviously. The SURP treatment has a negative effect on the thickness of the carburized layer but it significantly increases the carbon concentration, nanohardness, elastic modulus and compressive residual stress of the outer surface of the carburized layer. However, these parameters then rapidly decreased along the depth of the carburized layer. Both SURP and LTGC could significantly strengthen the surface of AISI316L steel and the combination of the two would achieve better surface properties.

Tensile properties and microstructural features of 304L austenitic stainless steel produced by wire-and-arc additive manufacturing

Additive manufacturing (AM) has gained great importance in the recent development to produce metallic structural elements for civil engineering applications. However, research effort has been focused mainly on powder-based processes, while there is still limited knowledge concerning the structural response of wire-and-arc additive manufactured (WAAM) metallic elements, and very few experimental data concerning their mechanical properties. This paper presents the first results of a wide experimental campaign aimed at assessing the mechanical properties of WAAM plates produced using a commercial ER308LSi stainless steel welding wire. The aim is to evaluate the effect of the orientation in the tensile behavior of planar elements considering specimens extracted along three different directions with respect to the deposition layer: transversal direction (T), longitudinal direction (L), and diagonal direction (D). Compositional, microstructural, and fractographic analyses were carried out to relate the specific microstructural features induced by WAAM to the mechanical properties. The results show that the chemical composition of the plates meets the requirements of UNS-S-30403 for an AISI 304L austenitic stainless steel. The as-built samples were substantially defect-free and characterized by a very fine microstructure of γ and δ phases The fineness of the microstructure and the negligible defect content led to values of tensile strength and elongation to failure in line with the traditionally manufactured stainless steel elements. Anisotropy in the tensile properties between T, L, and D specimens was observed, and the highest elastic and plastic properties were measured in D specimens. This result is related to the crystallographic and mechanical fibering induced by the additive process, that led also, in case of D samples, to the highest density of cell boundaries, obstacles to the dislocation slip, located at 45° with respect to the loading direction, where plastic deformation preferentially occurs.

Improvement of mechanical properties and corrosion resistance of 316L and 304 stainless steel by low temperature plasma cementation

Low temperature plasma carburizing treatment of austenitic stainless steels is a carbon surface diffusion process for a surface hardness and corrosion and wear resistance. The process is carried out by introducing a mixture of carbon-containing gases and through the use of low temperatures the resulting cemented layer usually contains a single phase of supersaturated austenite with carbon – S-phase. For the present investigation, austenitic stainless steels AISI 316L and 304 were plasma cemented for 8 hours in the gas mixture containing 7.5% CH4 in H2, with a pressure of 500 Pa, at temperatures of 375 oC and 450 oC. The phases formed were determined by X-ray diffraction. The corrosion resistance was evaluated through immersion tests over time and cyclic voltammetry. The results indicate that there was no formation of compounds (carbides) in the cemented layer for both steels at any of the temperatures and there was a corrosion resistance improvement. Keywords: plasma carburizing, AISI 304, AISI 316L, corrosion.

Fatigue Strength Evaluation of Notched Ductile Steel Specimens Using Critical Distances

In the case of AISI 304L austenitic stainless steel it was found that the fatigue strength of specimens characterized by the presence of blunt notches cannot be rationalized either in terms of linear elastic local stress or nominal stress amplitude. This fact was related to the high level of ductility shown by this material also in the case of high cycle fatigue regime. Therefore, in this paper some classical fatigue design methodologies are extended to account for this aspect also in the case of severe notches. On this basis a FEM oriented extension of the concept of geometric elastic stress concentration factor is proposed.