316L Stainless Steel Specimens Research Articles

Fatigue crack initiation and propagation in plain and notched PBF-LB/M, WAAM, and wrought 316L stainless steel specimens

Additively manufactured (AM) components—either made by laser-powder bed fusion or wire and arc additive manufacturing—typically contain process-related defects on and near surfaces that can be removed by machining. Various studies have shown that post-treatment, such as machining significantly improves the fatigue strength of AM parts. To this day, however, hardly any studies have investigated the fatigue strength of post-treated additively manufactured components with notches. In this study, fatigue tests were performed on plain and notched specimens to determine and compare the crack initiation and crack propagation behavior due to different manufacturing-related effects. Tests were performed on specimens produced by the two aforementioned AM processes and compared to specimens taken from wrought sheets. The fatigue strength of AM materials is influenced by microstructure, defects, residual stress, and notches. PBF-LB/M specimens exhibit the highest fatigue strength in plain, notch-free conditions, attributed to differences in microstructure and static strength affecting fatigue crack initiation. Notched specimens show larger differences among materials, with PBF-LB/M having shorter fatigue crack propagation life related to line-type defect clusters, while the plain PBF-LB/M specimens are less affected as their fatigue strength is primarily determined by fatigue crack initiation.

Strengthening mechanism of abnormally enhanced fatigue ductility of gradient nanostructured 316L stainless steel

Structural materials with a good combination of high fatigue strength and high fatigue ductility are highly desirable in engineering applications. However, it remains a challenge to “break” the exclusive relationship between fatigue strength and fatigue ductility. The cylindrical 316L stainless steel specimens with a gradient nanostructured surface layer were prepared using surface mechanical rolling treatment (SMRT). Fully reversed strain-controlled tension–compression fatigue experiments indicated that the SMRT processed 316L specimens achieved simultaneous improvement of fatigue strength and fatigue ductility. The abnormal increase in fatigue ductility is attributed to the significant secondary cyclic hardening caused by the martensitic phase transformation and the transition in fatigue crack initiation mode. The martensitic phase transformation enhances the fatigue strength while reducing the proportion of plastic deformation in the controlled strain amplitude. The competition between the driving force and resistance of fatigue nucleation in different structural units results in the migration of fatigue crack initiation sites from the surface to subsurface, thereby prolonging the fatigue crack initiation life. This work reveals the synergistic strengthening mechanism of fatigue strength and fatigue ductility of a gradient nanostructured 316L stainless steel.

Dissipative and thermal aspects in cyclic loading of additive manufactured AISI 316L

This study presents a comprehensive understanding of the fatigue behavior of 316L stainless steel specimens produced using the laser powder bed fusion (PBF-LB/M) method. The investigation has been conducted through a multifaceted approach that includes surface roughness analysis, density measurements, microstructural examination, fatigue testing, strain measurements, and thermographic analysis. Thermographic data processing models, i.e. the bi-power law (TCM-modified) model and the TCM method, are applied to fatigue test results of standard samples to evaluate fatigue limit values. The fatigue limit of the samples was estimated using the Murakami Method (MM), Constant Amplitude Loading (CAL) and Step Loading (SL) tests. The proposed methodology allows exploration of the entire stress level range within a single test, which could allow evaluation of the fatigue limit of components within only one test. This study is the starting point for a rapid evaluation method for estimating the fatigue limit using thermography, offering a cost-effective, time-efficient, and non-destructive means of assessing the fatigue performance of materials produced using additive manufacturing processes.

Peridynamic numerical investigation of asymmetric strain-controlled fatigue behaviour using the kinetic theory of fracture

Numerical fatigue process modelling is complex and still an open task. Discontinuity caused by fatigue cracks requires special finite element techniques based on additional parameters, the selection of which has a strong effect on the simulation results. Moreover, the calculation of fatigue life according to empirical material coefficients (e.g., Paris law) does not explain the process, and coefficients should be set from experimental testing, which is not always possible. A new nonlocal continuum mechanics formulation without spatial derivative of coordinates, namely, peridynamics (PD), which was created 20 y ago, provides new opportunities for modelling discontinuities, such as fatigue cracks. The fatigue process can be better described by using the atomistic approach-based kinetic theory of fracture (KTF), which includes the process temperature, maximum and minimum stresses, and loading frequency in its differential fatigue damage equation. Standard 316L stainless steel specimens are tested, and then the KTF-PD fatigue simulation is run in this study. In-house MATLAB code, calibrated from the material S‒N curve, is used for the KTF-PD simulation. A novel KTF equation based on the cycle stress‒strain hysteresis loop is proposed and applied to predict fatigue life. The simulation results are compared with the experimental results, and good agreement is observed for both symmetric and asymmetric cyclic loading.

Damage behavior of additive manufacturing 316L stainless steel irradiated with Fe ions

The irradiation damage behavior of 316L stainless steel (SS) fabricated using selective laser melting (SLM) and traditional manufacturing (TM) was investigated in terms of microstructural changes in hardness and swelling at RT under irradiation with 3 MeV Fe ions at doses of 0.3 to 6 dpa. At a dose of 6 dpa, the swelling heights of the SLM and TM samples were 39.1 nm and 30.7 nm, respectively, corresponding to swelling rates of 3.0 % and 2.4 %. The irradiated SLM 316L SS exhibited a lower degree of hardening at continuous fluences than the TM 316L SS. At a dose of 6 dpa, the hardening rates of the SLM and TM 316L SS specimens were 102.1 % and 84.1 %, respectively. Furthermore, TEM results showed that the density of dislocation loops in the TM 316L SS was 8.36 × 1023/m3 while that in the SLM 316L SS was 6.15 × 1023/m3. The average sizes of the dislocation loops were 1.35 and 1.22 nm, respectively. According to calculations using the dispersed barrier hardening (DBH) model, the dislocation loops were the primary factor influencing the irradiation hardening behavior. Under Fe-ion irradiation at RT, SLM 316L SS showed superior resistance to irradiation hardening and swelling compared with TM 316L SS.

A new intelligent approach of surface roughness measurement in sustainable machining of AM-316L stainless steel with deep learning models

Due to the manufacturing sector's digitalization and ability to combine quality measurement and production data, machine learning and deep learning for quality assurance hold enormous potential. In this situation, industries may process data to inform data-driven estimates of product quality, thanks to predictive excellence. This research investigates the machinability of Laser Powder Bed Fusion (LPBF) − 316L stainless steel specimens, focusing on the impact of cutting parameters and cooling conditions (Dry, MQL, CO2 and CO2 + MQL) on surface roughness. The research employs advanced data augmentation techniques, incorporating TransGAN and multi-head attention (MHA) based Alexnet model for surface imperfection classification. The results highlight the effectiveness of the proposed methodology in accurately classifying surface conditions and underscore the superior performance of the MHA-Alexnet algorithm compared to alternative models (Alexnet and AE-Alexnet). Overall, the study contributes valuable insights into optimizing machining parameters and cooling strategies for enhanced surface finish in additively manufactured alloys.

Microstructural characterization and mechanical behaviour of laser powder Bed Fusion stainless steel 316L

Laser Powder Bed Fusion (L-PBF) is a highly precise and customizable additive manufacturing (AM) technique that uses a high-energy laser to selectively melt and fuse powdered material into a three-dimensional object. However, depending on the process parameters, the final components may have potential flaws that can affect their quality and mechanical properties, due to porosity, melting and incomplete fusion of powder particles; and because the process involves local heating and sometimes uneven heat transfer, the processed components may warp or crack due to residual stresses or thermal gradients. The manufacturing process itself reflects in the final component structure having a detrimental effect on the strength, durability, fatigue resistance, and corrosion.In this work, static tensile and fatigue tests were performed on traditional and L-PBF manufactured AISI 316L stainless steel specimens. The energetic release has been evaluated with an infrared camera during the static and fatigue tests aiming to identify material thermal response to the loading and to predict the failure in rapid way adopting Thermographic Methods. Differences were observed comparing the fatigue data of the L-PBF processed specimens with the traditional material. However, analysis of internal structure, porosity, and surface characteristics of the AM material in combination with fractographic analysis helped to explain the differences in the fatigue life. The observed energy release, different for both material types, was discussed based on the structural characteristics. The results show that the crack originates from a defect on the surface or just below the surface, with a transgranular propagation.

Study of corrosion properties of carbon steel, 304 and 316L stainless steels in sulfuric acid and their degradation products

In this study, corrosion properties of carbon steel and stainless steels, and degradation of sulfuric acid by the corrosion mechanism are presented. Carbon steel (CS), 304 stainless steel (SS304), and 316L stainless steel (SS316L) specimens were analyzed through their electrochemical response by using a potentiostat measurement. The specimens were submerged in concentrated 98 wt.% of H2SO4 acid for 0 day to 60 day. The degradation of H2SO4 was determined by its volume change, turbidity, and color due to the corrosion mechanism. Corrosion rate of CS, SS304 and SS316L specimens are 43.237 mm/year, 0.420 mm/year and 0.086 mm/year, respectively. After 60 days, the weight-loss of CS, SS304 and SS316L specimens are 48 wt%, 33 wt% and 0.1 wt%, respectively. Corrosion resistance of the materials are influenced by the passive oxide layer that forms on its surface and associated with electrochemical activity or semiconductive composition. The degradation of H2SO4 acid was observed due to the corrosion process of specimens and related to the turbidity and volume increase while the wt% concentration of H2SO4 acid decreases. In order to make material choices that enable continuous and safe operation of the process, it is important to understand the corrosion mechanism changes.

Performance assessment of carbon dots based nano cutting fluids in improving the machining characteristics of additively manufactured 316L stainless steel

The advantages of additive manufacturing (AM) over conventional manufacturing techniques, such as the ability to build parts with complicated geometry, have led to a rise in its applications in recent years. However, the mechanical and tribological qualities of an additively manufactured (AMed) parts are distinct from those of a conventionally made components because of the layer-by-layer nature of AM process. However, there is a need to machine these complex geometries of AMed components to achieve the desired final dimension and surface finish. Therefore, this study investigates the impact of four environmental regimes (dry, flood, MQL, and N-MQL) on the metal cutting of AMed 316L stainless steel specimen, with a focus on the efficacy of N-MQL lubrication containing rice bran oil + carbon dots (CDs) particles. Responses measured include cutting temperature, surface roughness (Ra), tool wear, microstructure, and micro-hardness. Outcomes show that N-MQL outperforms other cutting regimes, offering enhanced machining performance and surface quality, making it a crucial choice for AMed specimens.

CoxCrFeNiTi High-Entropy Alloys Prepared via Mechanical Alloying and Spark Plasma Sintering for Magnetron Sputtering Coatings.

The main objective of this study was to develop a high-entropy alloy (HEA) derived from the CoxCrFeNiTi HEA system (x = 0.5, 1) for protective coatings using the magnetron sputtering method. In order to produce the high-entropy alloy targets required for the magnetron sputtering process, mechanically alloyed metallic powders were consolidated via spark plasma sintering (SPS). The microstructural analysis results of the HEA mixture presented morphology changes after 30 h of alloying, with the particles presenting uniform polygonal shapes and dimensions. Subsequently, 316L stainless steel (SS) specimens were coated via magnetron sputtering, comparing their composition with that of the sputtering targets used for deposition to establish stoichiometry. Microstructural analyses of the SPSed HEAs revealed no defects and indicated a uniform elemental distribution across the surface. Furthermore, the CoCrFeNiTi equiatomic alloy exhibited a nearly stoichiometric composition, both in the coating and the sputtering target. The XRD analysis results indicated that amorphous coatings were obtained for both Co0.5CrFeNiTi and the CoCrFeNiTi HEA, and nanoindentation tests indicated that the CoCrFeNiTi HEA coating presented a hardness of 596 ± 22 HV, compared to the 570 ± 19 HV measured for Co0.5CrFeNiTi, suggesting an improved wear resistance.

PERFORMANCE OF THERMAL SPRAY COATED METALLIC MATERIALS FOR BIO-IMPLANT APPLICATIONS

Metallic biomaterials are used to replace and rebuild human structural components due to their biocompatibility, corrosion resistance, and good mechanical properties. Biomedical metals have been used for 30 years. Medical implants use metallic biomaterials. Metallic biomaterials are needed to cure failed hard tissue, bone, and fractures. Because they’re strong, tough, and long-lasting. As the world population ages and seniors are more prone to experience hard tissue disintegration, there is a huge demand for improved metallic biomaterials. Titanium-based alloys, cobalt-based alloys, and stainless steel are feasible metallic biomaterials (316L). These biomaterials’ Young’s modulus should match that of human bones, reducing stress shielding. Implant designs include plates, rods, screws, and pins. Since the FDA approved these biocompatible metallic implants, orthopedic practices often employ them. Metals aren’t believable as biomaterials because they’re synthetic and have insufficient bio-functionality. The biocompatibility of metallic biomaterials must be considerably enhanced. Metallic biomaterials are often synthetic materials with no biological activity. The key issue is coating-to-substrate adhesion. Cover spalling from the substrate causes implant and tissue responses. Due to inadequate crystallization, the hydroxyapatite (HA) coating degrades, increasing implant failure risk by lowering titanium adhesion. Coated 316L stainless steel specimens have better adherence than untreated ones. The coating/substrate material, coating process, and coating thickness are thoroughly identified and discussed. The surface structure and microstructure of HA-based coating are explored to support the conclusions.

Tensile properties of SS316L produced by LPBF: Influence of specimen dimensions and surface condition

This study is part of the global effort to develop precise and accurate mechanical testing methodologies for metallic parts in as-built condition produced by powder-bed additive manufacturing processes. Miniature 316L stainless steel specimens with a 7 × 1 × 1.5 mm3 reduced section were produced by laser powder bed fusion. Three distinct surface conditions were considered. Standard specimens were also produced for comparison. Special attention was given to the non-destructive measurement of the cross-sectional area of small as-built specimens. The ultimate tensile strength of the as-built miniature specimens was 12% below standard-sized or machined specimens. Using caliper measurements lead to an overestimation of the section by as much as 20% compared to CT-scan values, resulting in an underestimation of UTS. Point micrometer measurements yielded more accurate estimations of cross-sectional area. With the experimental error taken into account, the specimen dimensions and surface condition influenced UTS. A coarser average microstructure for miniature as-built specimens was linked to a reduction of UTS by about 4% compared to standard specimens produced in the same conditions. Despite a larger scatter obtained with miniature specimens, no effect of specimen dimensions on the average value of elongation at break could be observed. Higher surface roughness of the order of Ra = 15 μm resulted in an elongation at break reduced from 52.7 ± 5.2 to 39.2 ± 5.0% for as-built miniature specimens. It was concluded that using large-scale or machined specimens may produce less accurate tensile characterization of thin 316L sections produced by laser powder bed fusion. Mechanical coupons with dimensions comparable with the structural part of interest should be employed. Accurate measurement of the cross-sectional area of small as-built specimens is critical for accurate measurements of mechanical strength. These results rationalize the scatter on tensile properties of LPBF 316L available in the literature.

A Novel Method for Early Fatigue Damage Diagnosis in 316L Stainless Steel Formed by Selective Laser Melting Technology.

Early fatigue damage is an important factor affecting the service safety of 316L stainless steel parts formed by selective laser melting (SLM) technology. Nonlinear ultrasonic testing for early fatigue damage in SLM 316L stainless steel specimens was carried out. A new method for evaluation of early fatigue damage based on nonlinear ultrasonic testing was proposed. Empirical mode decomposition (EMD) was applied to the unsteady ultrasonic testing signal, and the signal was decomposed into multiple intrinsic mode functions (IMFs) that meet certain conditions; then, the specific IMF (ESI) containing the effective fatigue damage information was extracted. Lastly, fast Fourier transform (FFT) was applied to the specific IMF signal to obtain the required information to evaluate the damage in the measured part caused by fatigue. The results of nonlinear ultrasonic testing agreed well with transmission electron microscope experimental analysis and theoretical model of acoustic nonlinearity caused by dislocations. The change in nonlinear ultrasonic testing results reflected the generation and evolution of dislocation structure during the low-cycle fatigue regime of the SLM 316L stainless steel specimen and revealed the early fatigue damage mechanism of this metal part. Compared with the classical FFT method, the EMD-ESI-FFT method is more sensitive in identifying the early damage in SLM 316L stainless parts induced by fatigue loading, which is equivalent to improving the early fatigue damage identification and diagnosis ability and can better ensure the service safety of important metal parts.

Tensile and fatigue properties of the binder jet printed and hot isostatically pressed 316L austenitic stainless steel

The binder jet printed (BJP) metallic components are not yet considered for widespread structural applications that involve cyclic loading conditions due to the high levels of porosity typical to them. In the present study, the porosity in BJP 316L stainless steel (SS) was reduced from 3.5-5.6% to 1–1.2% by improving process parameters first and then employing the post-processing hot iso-static pressing (HIP) treatment. The reduced porosity in the as-sintered conditions resulted in ⁓15 and 5% improvement in tensile yield and ultimate tensile strengths, respectively. However, the high cycle fatigue strength (⁓250 MPa) did not improve even after a ⁓fivefold reduction in the porosity. HIP resulted in a further reduction in the porosity, which improved the elongation to failure by 30% while reduced the yield strength by ∼10% (due to grain coarsening that occurred during HIP). It, however, did not improve the unnotched fatigue strength. The microstructural coarsening during HIP also affected the short fatigue crack growth resistance of the BJP 316L SS specimens. Moreover, after reduction in overall porosity and the size of the pores, the void and fatigue crack nucleation mechanisms during tensile and fatigue loadings changed from pore assisted to inclusion assisted.

High-speed X-ray study of process dynamics caused by surface features during continuous-wave laser polishing

During high-speed X-ray imaging of laser surface polishing experiments of specimens of 316L stainless steel at Argonne National Lab's Advanced Photon Source, it was discovered that the induced keyhole changes shape and dimensions while crossing an engineered surface feature without altering process parameters. It was observed that the post-surface feature keyhole was deeper than that of the pre surface feature keyhole. This work reports on the first in-situ observation of the effect of localized surface geometry on underlying melt pool behavior. This has implications for defect formation mechanisms during laser melting processes that rely on melt pool geometry.

Hybrid Polyurethane/Polypyrrole Composite Coatings on Passivated 316L SS for Surface Protective Action against Corrosion in Saline Medium

Hybrid treatments consisting of surface modification and subsequent protective coatings have gained extensive attention among corrosion mitigation approaches for a wide variety of structural metallic materials. This study aims to review the enhancement of the corrosion protection performance of polyurethane (PU) coatings on 316L stainless steel (SS) specimens. This was achieved via a two-step strategic treatment, primarily by electrochemical passivation and subsequent deposition of PU composite coatings with the different feed ratio of synthesized polypyrrole (PPy) nanoparticles. The effect of different applied voltage on the surface features and the corrosion behavior of the passivated SS surfaces was systematically investigated using surface characterization techniques and a potentiodynamic polarization test in a NaCl solution. Surface morphological images revealed the porous structure on the passivated surface. It is inferred from the topographical surface results that homogeneous surface roughness was achieved with the applied voltage of 5 V. Infra-red spectroscopic results validate the formation of PU/PPy composite coatings and the intermolecular chemical interaction between the PU and PPy moieties. Furthermore, corrosion measurements corroborate the improved corrosion resistance of PU/30PPy coatings with higher values of charge transfer resistance, Rct (1.0869 × 107 Ω cm2), and film resistance, Rf (2.258 × 105 Ω cm2), with the lowest values of corrosion, icorr (4.7 × 10−3 µA cm−2) compared to that of the PU/Bare specimen. In conclusion, it is confirmed that the passivated surface enhances the corrosion resistance performance of PU coated SS, and this performance is further increased with the incorporation of PPy particles.

Characterization of porous biomaterials of 316L stainless steel made from polyethylene waxes as pore-forming agents

For biomedical purposes, the 316L stainless steel has been used due to its high mechanical strength and its biotolerability by the human body. Its porosity allows the growth of living tissue within the implant, which improves fixation to hard tissues. Porous 316L stainless steel specimens were produced by powder metallurgy with controlled porosity by the addition of different polyethylene waxes granulometries (50 and 100 mm) and concentrations (0.0, 1.0, 2.0, 3.0 and 4.0%). The elastic modulus, porous morphologies and densities of the steel samples were determined. Thus, the densities and the elastic modulus were evaluated statistically through Tukey’s test. The results showed that the porosity increases proportionally to polyethylene wax concentration, but the elastic modulus decreases. The Tukey’s Test showed that there are no significant differences in most samples using the two waxes at most concentrations. Range values of 1.89 to 1.51 GPa using wax with 50 mm, and 1.72 to 1.19 GPa were respectively obtained using 100 mm particle size. The highest pore size (45 mm) and the lowest elastic modulus (1.19 GPa) were obtained by use of 4.0% of wax.

Mechanism of localized corrosion issues of austenitic steels exposed to flowing lead with 10−7 wt.% oxygen at 480°C up to 16,000 h

As part of research into resistant materials for the construction of lead-cooled fast reactors, specimens of 316L and 15-15Ti stainless steel were exposed to flowing lead with 10−7 wt.% oxygen at 480 °C for 16,000 h. Post-test microscopic analyses revealed a possible mechanism of long-term susceptibility to localized solution-based attack. The corrosion of both austenitic steels was low, a thin oxide layer (80–240 nm) thin was formed, and a large-scale dissolution attack was not observed. The surface was mostly covered by a protective thin spinel film with variable Cr, Fe, and Mn ratios. With increasing exposure time, the proportion of Mn was increased replacing Fe, and the probability of localized deeper failures increased. Maintaining the spinel film depleted the subsurface layer and triggered the precipitation of new phases. The corrosion resistance was enhanced by the presence of a work-hardened surface layer formed by lathe turning. The thin film is sometimes exfoliated and then regenerated without any consequences or allows deep oxidation to start.

Space Node Topology Optimization Design Considering Anisotropy of Additive Manufacturing

At present, a large number of scholars have conducted related research on topology optimization for additive manufacturing (AM). However, there are few relevant research reports on the impact of different directions of additive manufacturing on the optimal design and manufacturing results. In this paper, using the bidirectional evolutionary optimization (BESO) method, anisotropic optimization analysis was carried out on space nodes that are currently popular in the field of additive manufacturing and topology optimization. The elastic constants in different directions were used as anisotropic material properties for optimization research in this paper through tensile testing, which was carried out on 316L stainless-steel specimens fabricated using Selective Laser Melting (SLM) technology. In addition, SEM analyses were performed to explore the microscopic appearance of the material. The study found that additive manufacturing is affected by the printing direction in terms of both macroscopic mechanical properties and microscopic material structure; the deformation obtained by anisotropic optimization was about 1.1–2.3% smaller than that obtained by isotropic optimization.

The Effect Of The Building Direction And Surface Finish On The Mechanical Properties Of The Direct Energy Deposited AISI 316L Stainless Steel

Additive Manufacturing (AM) technologies, such as Direct Energy Deposition (DED) ones, offer new design possibilities for a wide range of industrial applications in different sectors. However, in order to widen the application fields, the knowledge of the mechanical behavior of AM parts is essential. The purpose of this paper is to investigate the effect of the building direction and surface finish on the static and dynamic mechanical properties of AISI 316L stainless steel specimens fabricated via DED. Samples were fabricated in their final shape through DED with the axis parallel to the building direction as well as prisms were fabricated from which samples with the axis oriented both vertically and horizontally with respect to the building direction were machined. Then, both the as-built and machined samples were characterized in terms of surface topography and microstructure as well as tensile and fatigue tested. Finally, the fracture surfaces were investigated.Results showed that the horizontally machined samples were characterized by higher tensile strength than the vertically machined samples, since the load is parallel to the slide layers, which represent weakness points. On the contrary, it was proved that the surface finish plays the major role in increasing the fatigue resistance, as the fatigue limit of the machined samples was more than double than that of the as-built samples.