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Ultra fine-grained metals obtained by severe plastic deformation exhibit higher specific strength that is useful for many applications and show promise for use as body implants. This ...work studied the microstructural evolution, mechanical and sliding wear behavior and corrosion behavior of 316L stainless steel warm multi axially forged at 600°C. Microstructural evolution studied using electron backscatter diffraction technique and transmission electron microscopy confirmed the formation of ultra fine-grained structure. Average grain size reduced from 30μm to 0.86μm after nine strain steps. A combination of Hall–Petch strengthening and strain hardening increased the hardness. Improved sliding wear resistance is attributed to a transition from micro cutting to wedge-forming mode of abrasive wear. Load-bearing orthopedic implants often fail from pitting initiated corrosion fatigue. Potentiodynamic tests, cyclic polarization, and FeCl3 immersion tests revealed enhanced pitting resistance of forged steel that is confirmed by Mott–Schottky analysis. This is ascribed to an increase in the grain boundary volume, and homogenization of pit inducing impurities and non-metallic phases due to severe deformation, which influenced the passive film properties. These model studies on 316L steel demonstrate that severely deformed ultra fine-grained metals have potential to deliver improved implant performance.
This model study on 316L steel demonstrates that severely deformed ultra fine-grained (UFG) metals have potential to deliver improved load-bearing implant performance. It is as interesting as is unclear as to how such severely deformed UFG material behaves electrochemically in the corrosive body fluids. This work is on studying the inter-relationship between structure, and mechanical, wear, and corrosion behavior of warm multiaxially forged (MAFed) UFG 316L stainless steel. Warm MAF is a bulk processing method capable of yielding large volume of UFG material and is an easily readily adaptable technique in industry. It can be a promising alternative to the expensive metallic alloys available for implant applications.
A feasibility study was performed to fabricate ITER In-Vessel components by Selective Laser Melting (SLM) supported by Fusion for Energy (F4E). Almost fully dense 316L stainless steel (SS316L) ...components were prepared from gas-atomized powder and with optimized SLM processing parameters. Tensile tests and Charpy-V tests were carried out at 22 °C and 250 °C and the results showed that SLM SS316L fulfill the RCC-MR code. Microstructure characterization reveals the presence of hierarchical macro-, micro- and nano-structures in as-built samples that were very different from SS316L microstructures prepared by other established methods. The formation of a characteristic intragranular cellular segregation network microstructure appears to contribute to the increase of yield strength without losing ductility. Silicon oxide nano-inclusions were formed during the SLM process that generated a micro-hardness fluctuation in the building direction. The combined influence of a cellular microstructure and the nano-inclusions constraints the size of ductile dimples to nano-scale. The crack propagation is hindered by a pinning effect that improves the defect-tolerance of the SLM SS316L. This work proves that it was possible to manufacture SS316L with properties suitable for ITER First Wall panels. Further studies on irradiation properties of SLM SS316L and manufacturing of larger real-size components are needed.
•The mechanical properties of SS316L made by selective laser melting fulfill RCC-MR.•SLM SS316L consists hierarchical structures of high heterogeneity.•Silicon rich oxide nano-inclusions are formed unexpectedly during SLM process.•Cellular structure and oxide nano-inclusions strengthen SLM SS316L.
Selective laser melting (SLM) and laser cladding deposition (LCD) are two typical kinds of laser additive manufacturing techniques that have been developed for many years independently. Although they ...are based on the same principle of laser cladding, there are little comparison on the fundamental studies for metallurgical behavior (including melting and solidification behaviors) and the mechanical properties of these two techniques up to now. In this paper, the single-track formation and the deposition of block sample from 316L stainless steel powders have been carried out by both SLM and LCD techniques. A comparison on pool shape, cooling rate, columnar grain size and mechanical properties under different processing conditions by LCD and SLM respectively has been studied. It is found that, due to the increase of energy input and the decrease of depth-to-width ratio of melting pool (MP) from SLM to LCD, the primary cellular arm spacing (PCAS) of the sample increases from less than 1.0µm to more than 15.0µm, and thus the cooling rate of MP decreases from about 106K/s in SLM to about 102K/s in LCD. Furthermore, due to the decrease of cooling rate from SLM to LCD, the columnar grains of the as forming alloy are getting coarser. Especially, the relationship between gain size (λ) and the reciprocal of square root of cooling rate (Ṫ) in LCD significantly meets the classical linear function of λ=a+b/Ṫ (a and b are constants), while a new relationship of a cubic function is found in SLM, showing the different solidification characteristics between LCD and SLM. Lastly, the samples of 316L stainless steel by SLM have much stronger tensile strength but lower elongation than those by LCD, and the main reason is due to that the solidification behavior of the MPs by SLM can form much finer columnar grains than those by LCD.
We produce defect-free dissimilar friction stir welds between 1.86 mm-thick 2205 duplex stainless steel (DSS) and 1.95 mm-thick 304 austenitic stainless steel (ASS). The refined grain structure in ...the stirring zone (SZ) has three typical regions. The duplex phase region (DPR) and the austenite phase region (APR) have similar microstructures as the SZ of 2205 DSS welds and 304 ASS welds, respectively. The mixed region (MXR) has a smaller ferrite volume fraction (less than 25%) than the 2205 base metal (BM, 46.0%), whereas the average grain size of ferrite (0.82 μm) and austenite (0.83 μm) in the MXR is significantly smaller than that of austenite grains (2.32 μm) in the APR. The evolution of the corresponding microstructure and the diffusion of elements within the typical regions are interpreted according to experimental observations and phase diagrams. Due to the grain refining, the MXR (287.5 Hv) and the DPR (298.5 Hv) both have a higher microhardness than 2205 BM (252.7 Hv). The APR (226.5 Hv) also has a higher microhardness than 304 BM (189.7 Hv). The tensile tests indicate that the dissimilar welds have a good ductility and a higher tensile strength (905 MPa) than the 304 BM (783 MPa).
•The native oxide layers of SLM and wrought 316L SS were compared using EIS and XPS.•XPS analysis showed similar thickness and surface chemistry for both native oxides.•The SLM 316L SS showed better ...barrier properties than the wrought material.•The Young modulus proved to be the most appropriate model to fit the EIS results.•The results of this work infer structural differences between the native oxide films.
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.
Microbiologically influenced corrosion (MIC) of 2205 duplex stainless steel (DSS) in the presence of Pseudomonas aeruginosa was investigated through electrochemical and surface analyses. The ...electrochemical results showed that P. aeruginosa significantly reduced the corrosion resistance of 2205 DSS. Confocal laser scanning microscopy (CLSM) images showed that the depths of the largest pits on 2205 DSS with and without P. aeruginosa were 14.0 and 4.9μm, respectively, indicating that the pitting corrosion was accelerated by P. aeruginosa. X-ray photoelectron spectroscopy (XPS) results revealed that CrO3 and CrN formed on the 2205 DSS surface in the presence of P. aeruginosa.
•LPR and EIS showed that P. aeruginosa reduced the corrosion resistance of 2205 DSS.•The largest pit depth on 2205 DSS was 4.9μm after 14days incubation in sterile medium.•The largest pit depth on 2205 DSS was 14μm after 14days incubation with P. aeruginosa.•The accelerated pitting was due to the formation of CrO3 and CrN by P. aeruginosa.
This study aims to achieve a clear understanding of relationships between the anisotropy in the microstructure and corrosion behavior in a newly developed maraging stainless steel (Corrax® or SS CX) ...fabricated by the laser-powder bed fusion (L-PBF) technique. The focus was to compare the properties of the planes parallel (side) and perpendicular (top) to the building direction of the fabricated sample. Various electrochemical testing and multiscale electron microscopy techniques were used to investigate the microstructure and corrosion behavior of both planes. The results indicated that different thermal histories experienced on the top versus the side planes during the L-PBF lead to microstructural variations between the planes. A slight increase in the laths size, and the lower fractions of dislocation density and low angle grain boundaries, and plausibly the lower level of residual stresses on the side plane were found to contribute to the improved corrosion response of the side plane as compared to the top plane.
•Martensite packets with a slight content of retained austenite were observed in L-PBF SS CX.•Nanometric round regions of inclusions containing Al and O were detected in L-PBF SS CX.•Anisotropic corrosion resistance of L-PBF SS CX was related to the differences in the level of residual stresses and the LAGBs density.•The plane parallel to building direction with larger grain size revealed higher corrosion resistance.
•The α and γ phases of duplex stainless steels were electrochemically characterized.•Duplex stainless steels suffer selective corrosion in acid solutions at elevated T.•Impedance measurements showed ...a corrosion mechanism through adsorbed species.•Micro-galvanic couplings on stainless steels impair their tendency to passivate.•AISI 904L and Sanicro 28 are the most reliable alternative to AISI 316L.
The corrosion resistance of austenitic stainless steels AISI 316L, AISI 904L and Sanicro 28 and duplex stainless steels SAF 2205, SAF 2507 and SAF 2707, was studied at 40 and 60°C in a strong acid mixture containing tartaric acid saturated solution, H2SO4 and HCl. Anodic selective dissolution, weight loss and electrochemical impedance tests were performed. The results show that all stainless steels corrode actively and the micro-galvanic coupling, present on duplex stainless steels and 316L, determines severe selective corrosion and impairs their tendency to passivate. 904L and Sanicro 28 showed this tendency and the lowest corrosion rates.
The tensile properties of additively manufactured (AM) metals and alloys are among the most important variables that impact the potential applications of these materials. Here we examine and report ...on the tensile properties of AM 316L stainless steels fabricated by the laser powder-bed-fusion (L-PBF) technique, via twelve sets of optimized laser processing parameters that produce materials with density >98.8 ± 0.10%. A heterogeneous microstructure is observed in all L-PBF samples, including microscopic features such as dislocations, cellular walls, elemental segregations, local misorientations, impurities, precipitates, and a large fraction of low-angle grain boundaries (2-10°, ∼40–60%). The derived average grain size defined by high-angle grain boundaries (>10°) is ∼30–50 μm. Tensile testing reveals a yield strength ranging from 552 to 635 MPa and a tensile-elongation-to-failure (TEF) of 0.09–0.42 for directly-printed samples, whereas these values are 592–690 MPa and 0.29–0.50 for samples machined from the as-built rectangular thin plates. In all samples, we observe a variation of tensile yield strength within ∼15% but not the TEF, suggesting marginal microstructural changes despite a wide range of laser processing parameters. The large scatter of TEF in directly-printed samples originates from the sensitivity of thin gauge geometry (∼2 mm2 cross-section area) to the built-in flaws. We measured a substantially higher strain rate sensitivity (m∼0.02–0.03) of L-PBF 316L compared to the coarse-grained counterparts (∼0.006), together with a small activation volume of ∼20–30b3 (where b is the Burgers vector of 316L). These deformation kinetics parameters suggest that the tensile plasticity of L-PBF 316L is controlled by a much finer microstructural length scale than the measured grain size, consistent with the high strength and juxtaposed nano- to macro-structures seen in these materials. Strategies to optimize the tensile properties of AM materials are discussed.
•The yield strength of L-PBF 316L from a single build machine exhibits a variation within ∼15% but not the tensile elongation-to-failure (TEF).•The large variation of TEF in L-PBF 316L is due to its sensitivity to sample geometry and built-in flaws.•An m∼0.02–0.03 is measured in L-PBF 316L compared to coarse-grained counterpart (∼0.0061), together with a small V∼20–30b3.•Nanoinclusions in L-PBF 316L SS were found located in the dimple center and may impact its fracture strength.
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