Wrought Stainless Steel Research Articles

Element-resolved electrochemical analysis of the passivity of additively manufactured stainless steel 316L

The dissolution and passivation behavior of additively manufactured stainless steel 316L (AM-316L) prepared via selective laser melting was studied in quiescent 0.6 M NaCl. In a detailed comparison with the wrought stainless steel 316L (W-316L), it was determined via mass spectroelectrochemistry and x-ray photoelectron spectroscopy (XPS), that AM-316L exhibited a unique passive film relative to W-316L, resulting in a lower dissolution rate. The dissolution and enrichment of all alloying elements at open circuit potential and during polarisation were studied and elaborated.

Deuterium permeation and retention in 316L Stainless Steel Manufactured by Laser Powder Bed Fusion

Deployment of additively manufactured materials into nuclear energy systems requires investigation of the full range of the unique environmental effects on these materials. Hydrogen isotopes are common gaseous species in nuclear reactors and exhibit ample high mobility in most materials. While hydrogen isotopes mobility in wrought stainless steel is well understood, this property is not thoroughly studied for its additively manufactured variants. In this study, we investigated the deuterium permeation and retention in 316L stainless steel manufactured by laser powder bed fusion. The results showed that the deuterium permeability in the as-built additively manufactured 316L stainless steel (AM SS316L) is greater than that of the reference wrought 316L stainless steel by a factor of 2.8. However, the stress-relieved and solution-annealed AM SS316L samples exhibit lower deuterium permeability in comparison with the as-built condition. Following the solution annealing, the deuterium permeability of AM SS316L is comparable with that of the reference wrought SS316L. Deuterium retention in the as-built AM SS316L is 45% higher than that of the wrought 316L stainless steel and slightly higher than that of the two thermally annealed AM SS316L materials. Transmission electron microscopy and positron annihilation lifetime spectroscopy were used to obtain microstructural information used to determine deuterium permeation and retention behavior in the studied materials.

Mechanical behavior of additively manufactured and wrought 316L stainless steels before and after neutron irradiation

Fabrication of nuclear reactor components using additive manufacturing (AM) methods is now a practical option since the AM technologies have advanced to allow for building of complex parts with high quality materials. To assess the mechanical performance of printed components in reactor-relevant conditions and to build a property database for the AM 316L stainless steel (SS), mechanical testing and characterization were performed before and after neutron irradiation. Miniature tensile specimens were irradiated at the High Flux Isotope Reactor (HFIR) to 0.2 and 2 displacements per atom (dpa) at 300 and 600°C. The AM 316L SS was tested in the as-built, stress-relieved, and solution-annealed conditions, and the wrought (WT) 316L SS in solution-annealed condition as a reference alloy. The baseline test result showed that the AM 316L SS, regardless of the post-build heat treatment, had higher strength than the WT 316L SS, but similar ductility. Post-irradiation tensile testing was conducted at RT, 300°C, and 500°C for selected irradiation conditions. Neutron irradiation induced significant changes in the mechanical behavior of the AM stainless steels, including both hardening and softening. Although the as-built 316L steel after 300°C irradiation showed necking just after yielding, the overall property changes of the as-printed alloy became less significant after 600°C irradiation. Irradiation-induced ductilization was also observed after the higher temperature irradiation. In general, the strength change was smaller in the relatively stronger as-built and stress-relieved AM SSs than in the solution-annealed AM and WT SSs. These relatively lower strength 316L SSs overall retained higher ductility in the irradiation conditions tested, but the stronger 316L SSs demonstrated a similar level of ductility after the higher temperature (600°C) irradiation. It is a positive assessment for the AM 316L materials that no embrittlement was observed within the test and irradiation conditions of the experiment.

Microstructure Evolution of 316L Steel Prepared with the Use of Additive and Conventional Methods and Subjected to Dynamic Loads: A Comparative Study.

The mechanical properties and microstructure evolution caused by dynamic loads of 316L stainless steel, fabricated using the Laser Engineered Net Shaping (LENS) technique and hot forging method were studied. Full-density samples, without cracks made of 316L stainless steel alloy powder by using the LENS technique, are characterized by an untypical bi-modal microstructure consisting of macro-grains, which form sub-grains with a similar crystallographic orientation. Wrought stainless steel 316L has an initial equiaxed and one-phase structure, which is formed by austenite grains. The electron backscattered diffraction (EBSD) technique was used to illustrate changes in the microstructure of SS316L after it was subjected to dynamic loads, and it was revealed that for both samples, the grain refinement increases as the deformation rate increases. However, in the case of SS316L samples made by LENS, the share of low-angle boundaries (sub-grains) decreases, and the share of high-angle boundaries (grains of austenite) increases. Dynamically deformed wrought SS316L is characterized by the reverse trend: a decrease in the share of high-angle boundaries and an increase in the share of low-angle boundaries. Moreover, additively manufactured SS316L is characterized by lower plastic flow stresses compared with hot-forged steel, which is caused by the finer microstructure of wrought samples relative to that of additive samples. In the case of additively manufactured 316L steel samples subjected to a dynamic load, plastic deformation occurs predominantly through dislocation slip, in contrast to the wrought samples, in which the dominant mechanism of deformation is twinning, which is favored by a high deformation speed and low stacking fault energy (SFE) for austenite.

Study of the Effects of Hot Forging on the Additively Manufactured Stainless Steel Preforms

The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.

Effect of energy density on the interface evolution of stainless steel 316L deposited upon INC 625 via directed energy deposition

Directed energy deposition (DED) is an additive manufacturing technique that permits the manufacturing of complex multi-material components. In the present study, DED was used for the fabrication of Inconel 625 and stainless steel 316L couples. The effect of input energy density on the evolution of the dissimilar metal interface and its mechanical properties was explored by varying the laser power for each build. Columns that transitioned directly from Inconel 625 to stainless steel 316L were deposited onto mild steel substrates. The columns were cross-sectioned and characterised by coupling physical characterisation with microhardness. Scanning electron microscopy and compositional mapping were used to correlate the relationship between energy density and the produced functional gradient. It was seen that high energy deposition resulted in a measurable, remelted layer solute (RLS) fraction that comprised each deposited layer, creating a stepwise-graded interface. The RLS fraction was consistent across each layer of the interface and was influenced by the energy density of the build. Changes in the RLS affected the length of the graded interface but did not have a significant influence the mechanical properties. The yield strength and ultimate tensile strength of the through-interface compared well with the traditional wrought stainless steel 316L but suffered in ductility. Mix-mode fracture tended to occur in the near stainless steel composition regions.

Gas Gun Flyer Plate Impact Testing of Stainless Steel Deposited by Low Pressure Plasma Spraying and Cold Gas Dynamic Spraying

Understanding shock wave propagation behavior in coatings is valuable for predicting their performance in high-velocity impacts. Samples of stainless steel were deposited by low pressure plasma spray and cold gas dynamic spraying. The sound speeds of the deposits were measured, and the deposits were tested in flyer plate impact tests in a gas gun to determine the shock propagation and porosity compaction properties. The most porous sample was tested to determine its equation of state, while the other samples were tested to measure shock wave profiles as a function of deposition parameters. Comparison of the results between the coatings and to reference wrought stainless steel shows the effect of deposition conditions on the dynamic behavior of the deposits. Higher deposit density leads to higher sound speed. The shock speed for the highest porosity plasma sprayed deposit was below that of wrought stainless steel. The Hugoniot elastic limit stress for cold sprayed deposits is higher than that for plasma sprayed deposits due to the higher degree of cold work. Porosity crush up times were found to vary with the total porosity of the deposit. Better understanding of the behavior of these deposits under extreme impact conditions is demonstrated.

Technical Basis for Flaw Acceptance Criteria for Cast Austenitic Stainless Steel Piping

Abstract According to the current ASME Code Section XI, IWB-3640 and Appendix C flaw evaluation procedure, cast austenitic stainless steel (CASS) piping with ferrite content (FC) less than 20% is treated as wrought stainless steel. For CASS piping with FC equal or greater than 20%, there was no flaw evaluation procedure in the ASME Code prior to the 2019 Edition. In this paper, the technical basis for the recently approved Code change containing flaw acceptance criteria for CASS piping is presented. The procedure utilizes the current rules in ASME Code Section XI, IWB 3640/Appendix C and the existing elastic-plastic correction factors (i.e., Z-factors) for other materials in the Code. The appropriate Z-factor to use for the CASS piping is determined based on the FC (using Hull's equivalent factor). Experimentally measured fully saturated fracture toughness and tensile data of the three most common grades of CASS material in the U.S. (CF3, CF8, and CF8M) were used to determine the flaw acceptance criteria in the Appendix C Code method. As described here, the method is conservative since it utilizes the fully saturated condition of CASS materials. In addition, it is simple and consistent with the current regulatory guidance on aging management of CASS piping.

EIS comparative study and critical Equivalent Electrical Circuit (EEC) analysis of the native oxide layer of additive manufactured and wrought 316L stainless steel

In this work, a comparative electrochemical impedance spectroscopy (EIS) study of the native oxide layer of selective laser melted and wrought 316L stainless steel is conducted. A careful examination of the data is carried out in order to properly identify the appropriate model to fit the EIS response. From the parameters calculated by fitting the EIS data and a complementary XPS analysis, the electrical and dielectric characteristics of the passive oxide layers of the specimens were obtained. Clear differences were noticed between the two materials, which could definitely contribute to the overall understanding of the corrosion behavior of these materials.

Wire-Based Additive Manufacturing of Stainless Steel Components

Three different wire-fed additive manufacturing (AM) processes were employed to evaluate differences between laser, arc, and electron beam heat sources used for high-deposition-rate AM on the order of 1 kg/h. Optimum weld and build parameters were developed independently to match the characteristics of each heat source using 308L stainless steel welding wire as the feedstock. Laser-wire AM was made with the lowest energy per unit length of weld and had the best control of the melt pool and surface finish. Wire arc-based AM had an intermediate energy per unit length of weld of approximately 5× that of the laser process, while electron beam wire AM had the highest energy per unit length of weld at approximately 10× that of the laser process. Analysis of the parts that were built included evaluation of mechanical properties and microstructures, and these properties are discussed with respect to the difference in input energy and cooling rates. Results show that all three processes build parts with properties that exceed those of annealed 304L wrought stainless steel. How-ever, significant differences exist between the processes, and the results presented here can be used to help select the best wire-fed process for a given high-deposition-rate application.

Electrochemical Behavior, Microstructure, and Surface Chemistry of Thermal-Sprayed Stainless-Steel Coatings

Thermally sprayed stainless-steel coatings were produced with a wide range of deposition parameters. The electrochemical behavior of polished coatings was monitored for 3 weeks in 3.5 wt.% NaCl aqueous solution and compared to that of reference materials including a wrought stainless steel plate and a bulk ingot produced by arc melting of the spraying powder feedstock. Transitions in the polarization behavior are discussed based on the observed changes in coating microstructures as well as on the shifts in X-ray photoelectron spectra (XPS). Results show that the deposition parameters have a strong effect on the coating microstructures but the small differences in the polarization behavior of coatings mostly disappear after 1 week of testing. Microstructure evidence shows preferential corrosion at splats experiencing melting prior to deposition. Pitting and corrosion products between splat boundaries are also reported. XPS analysis shows that the coating surfaces are enriched in chromium oxides and hydroxides. Comparison between the coating and bulk stainless steels suggests that coating inherent defects play a major role on their impaired corrosion resistance.

Corrosion behaviour of 316L stainless steel manufactured by selective laser melting

AbstractThis work investigates the electrochemical behaviour of an AISI 316L stainless steel produced by selective laser melting (SLM) and compares its behaviour with that of wrought stainless steel with similar chemical composition. The SLM stainless steel specimens are tested in the as‐produced condition without stress relief or recrystallization heat treatments. The electrochemical tests are carried out in two electrolytes: 3.5 wt% NaCl solution with neutral pH and with pH of 1.8.At the macroscale, the microstructure of the SLM specimens is determined by the laser scanning pattern and displays an overlapping network of melt pools. At the microscale, the SLM specimens exhibit a cellular/columnar dendritic structure with submicrometric cell size. Electrochemical measurements highlight a more extended passive range for SLM stainless steel in both neutral and acid electrolytes indicating higher protective properties of the oxide film on SLM specimens. In contrast to the wrought material, the refined microstructure of the SLM specimens promotes a very shallow morphology of attack without deep penetration in the bulk.

Cavitation erosion of a spinodally decomposed wrought duplex stainless steel in a benign environment

This paper looks into the possibility of using spinodal decomposition (SD) to improve the cavitation erosion (CE) resistance of a wrought duplex stainless steel (DSS) in benign environments. The hardness increase of the ferrite associated with SD improves CE resistance of the DSS. Mechanical properties obtained from tensile curves (e.g., tensile toughness and strain-hardening capacity) do not appear to correlate with CE resistance. Grain boundaries are prime initiation sites of CE damage. Planar deformation features are also places where CE damage begins. In both the spinodally-decomposed and the solution-treated states, the ferrite of the DSS sustains more CE damage than does the austenite. The austenite actually acts as barrier to CE damage propagation.

Effect of heat treatments on microstructural evolution of additively manufactured and wrought 17-4PH stainless steel

Additively manufactured (AM) components usually have nonequilibrium microstructures. Post-built heat treatments are recommended for AM components to achieve homogenous microstructures. In this study, the effects were investigated of conventional solutionizing and precipitation hardening (H-900) heat treatments on the microstructure evolution of 17-4PH AM and wrought components. Microstructural characterization techniques including SEM, TEM and EBSD analysis were used on 17-4PH AM and wrought components to obtain quantitative information about the microstructure and phase evolution during these heat treatments. These microstructural studies demonstrate that 17-4PH AM components can achieve microstructures and hardnesses similar to those of wrought samples by post-built heat treatments.

A Comparative Tribocorrosion Study of Additive Manufactured and Wrought 316L Stainless Steel in Simulated Body Fluids

In this study, the corrosion and tribocorrosion behavior of additive manufactured (AM) 316L is investigated and compared to wrought 316L. The experiments were performed in both a 0.9 wt% NaCl solution and a simulated body fluid based on the protein albumin. The results are interpreted based on the analysis of the microstructure, inherent AM porosities and surface roughness. The results confirm that the AM samples present a substandard behavior compared to the wrought materials due to the higher surface area caused by the voids inherent to the AM process.

Early stage oxidation behavior of Al‐ and Si‐alloyed stainless steels as well as Ni‐based alloys in air at elevated temperatures

Several alloys including Al‐alloyed austenitic and ferritic stainless steels, Ni‐based alloy and Si‐alloyed ferritic stainless steel were annealed at 550, 750, and 950 °C for up to 700 h. AISI 347HFG and 10CrMo910 were used as reference materials. Weight gain was measured and the samples were analyzed with SEM/EDS and XRD. Materials having high enough Al‐content performed best due to ability to form a continuous protective Al2O3‐oxide layer on their surface at elevated temperatures. Lower Al‐content materials were not able to form a continuous protective Al‐oxide on their surface and suffered from accelerated internal Al‐oxide formation due to alloying elements. Si‐alloying improves the oxidation resistance of the studied materials and their performance was practically in the same level as that of the best Al‐alloyed materials. In Al‐alloyed materials the crystallinity of Al2O3 on ferritic stainless steel weld metal was higher than that on welded and wrought austenitic stainless steels.

Study of the effect of sigma phase precipitation on the sliding wear and corrosion behaviour of duplex stainless steel AISI 2205

The effects of σ-phase on the corrosion and unidirectional sliding wear behaviour of wrought duplex stainless steel (DSS) 2205 were investigated. The precipitation of σ-phase was induced by an isothermal aging at 850°C during 5, 15, 30 and 60min. Microstructural characterisation shows that the volume of δ-phase decreases as σ-phase increases. Dry sliding wear tests were carried out in a tribometer (model TRB, Anton Paar, Switzerland) with pin on disc technique at 7 and 15cm/s sliding rate with a load of 1N and an α-Al2O3 ball as counterpart. Results shown an important increase on wear resistance under dry sliding conditions after 5min of heat treatment due to the hardness provided by σ-phase on the DSS compared to base metal (BM). Nonetheless, cracks by Hertzian contact stress due to the difference of plastic deformation of matrix and σ-phase were observed at 15cm/s sliding rate in heat treated samples, induced by stress fatigue during the abrasion wear mechanism on the surface. Also there was observed that δ-phase is prone to dissolution in acidic media as σ-phase volume increases. Thus, results shown an improvement on wear resistance on samples aged for 5min in the starting critical intergranular corrosion susceptibility.

On the enhanced corrosion resistance of a selective laser melted austenitic stainless steel

Corrosion of stainless steels, including the austenitic Fe-Cr-Ni alloys, is associated with MnS inclusions that provide local sites for corrosion pits. Much research effort has been focused on the role of MnS in corrosion of conventional (cast and wrought) stainless steel to date, including microanalysis of MnS and the surrounding microchemistry. Herein, it was elucidated that an austenitic stainless steel (type 316L) when manufactured through selective laser melting was able to be produced without MnS inclusions and hence with no accompanying Cr-depletion in the vicinity of MnS, resulting in superior corrosion resistance relative to wrought form.

Compressive and Tensile Stress–Strain Responses of Additively Manufactured (AM) 304L Stainless Steel at High Strain Rates

Additively manufactured 304L stainless steel with different build orientations and post-manufacturing processes was dynamically characterized in compression and tension at various high strain rates (~500, 1500, and 3000 s−1) using Kolsky bar techniques. Wrought 304L stainless steel with a similar chemical composition as the additively manufactured material was also characterized at the same strain rates. The AM material exhibited higher yield and flow stresses in compression when the specimen strain is smaller than 0.3. In dynamic tension, the AM material also possessed higher yield and flow stresses but lower elongations to failure than the wrought material at all the strain rates tested. Depending on post processing and anisotropy, the high-rate yield strength of the AM 304L material is increased by 20–35%, compared to the yield strength of the wrought material.

Effects of Thermal Aging on Material Properties, Stress Corrosion Cracking, and Fracture Toughness of AISI 316L Weld Metal

Thermal aging and consequent embrittlement of materials are ongoing issues in cast stainless steels, as well as duplex, and high-Cr ferritic stainless steels. Spinodal decomposition is largely responsible for the well-known “748 K (475 °C) embrittlement” that results in drastic reductions in ductility and toughness in these materials. This process is also operative in welds of either cast or wrought stainless steels where δ-ferrite is present. While the embrittlement can occur after several hundred hours of aging at 748 K (475 °C), the process is also operative at lower temperatures, at the 561 K (288 °C) operating temperature of a boiling water reactor (BWR), for example, where ductility reductions have been observed after several tens of thousands of hours of exposure. An experimental program was carried out in order to understand how spinodal decomposition may affect changes in material properties in Type 316L BWR piping weld metals. The study included material characterization, nanoindentation hardness, double-loop electrochemical potentiokinetic reactivation (DL-EPR), Charpy-V, tensile, SCC crack growth, and in situ fracture toughness testing as a function of δ-ferrite content, aging time, and temperature. SCC crack growth rates of Type 316L stainless steel weld metal under simulated BWR conditions showed an approximate 2 times increase in crack growth rate over that of the unaged as-welded material. In situ fracture toughness measurements indicate that environmental exposure can result in a reduction of toughness by up to 40 pct over the corresponding at-temperature air-tested values. Material characterization results suggest that spinodal decomposition is responsible for the degradation of material properties measured in air, and that degradation of the in situ properties may be a result of hydrogen absorbed during exposure to the high-temperature water environment.