Behavior Of 316L Stainless Steel Research Articles

Understanding the strain localization in additively manufactured materials: Micro-scale tensile tests and crystal plasticity modeling

Metallic parts fabricated by additive manufacturing (AM) usually exhibit unique microstructures and non-negligible residual stresses compared with the counterparts produced by conventional manufacturing. These inherent microstructural factors strongly affect the mechanical response of the as-built AM parts. In this study, we focus on the strain localization behavior of 316L stainless steel produced by laser powder-bed-fusion. In-situ tensile tests under a scanning electron microscope are performed, and the digital image correlation method is used to measure the strain distribution combined with electron backscatter diffraction. Meanwhile, a dislocation-based crystal plasticity finite element model incorporating residual stresses is developed to study the origins of the strain localization in the AM material. The results indicate that strain localization in AM materials is closely associated with microstructural features, encompassing behaviors related to slip activities, interactions with neighboring grains and dislocation evolutions. Additionally, the columnar grain features also render the strain distribution sensitive to the loading direction. The strain localization is serious in some small grains with high residual stresses, while in large grains the effect is less significant. These factors collectively contribute to the increasing likelihood of strain localization occurring in the AM microstructures with heterogeneous grain size and texture distribution. This work provides detailed insights into the strain localization in AM materials and would facilitate the manufacturing parameter optimization of AM materials by tuning the microstructure to reduce deformation inhomogeneity.

The Gradient Effect on Cyclic Behavior of 316L Stainless Steel in the Ultrasonic Bending Test.

Nanoindentation measurements were conducted to investigate the high-cycle response of 316L stainless steel in bending fatigue. Hardness variation owing to the gradient flexure stress amplitude for different curvatures was plotted along with the thickness and length, respectively. Scanning electron microscopy (SEM) was subsequently conducted to explore the deformation characteristics in multiple layers, which had cyclic gradient stress, on the cross-section of specimens. The nanoindentation results indicated that the cyclic hardening response of 316L stainless steel is correlated with the level of stress amplitude in the high-cycle fatigue (HCF) regime. Furthermore, an analytical model was proposed to clarify the relationship between nanohardness and stress amplitude. Finally, the evolution of damage accumulation due to irreversible plastic deformation is continuous during stress reduction up to the neighboring zone at the neutral surface of the flexure beam in some individual grains.

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.

Effect of Laser Shock Peening on Electrochemistry and Wettability of Additively Manufactured Stainless Steel

Abstract Laser shock peening (LSP) is investigated for its use in altering the electrochemical and wetting behavior of 316L stainless steel made with laser powder bed fusion (LPBF). The corrosion performance of LPBF stainless steel varies between studies and build parameters, thus motivating the search for postprocessing methods that enable wetted surface applications. Compressive surface stress has been demonstrated to reduce corrosion rate in additively manufactured metal and LSP is known to impart compressive residual stress into metal targets. Wettability also affects corrosion behavior and LSP induces hydrophobicity. LSP is therefore a promising tool for improving corrosion behavior of LPBF stainless steel. This paper examines the electrochemical properties of LPBF stainless steel before and after LSP with electrochemical impedance spectroscopy and potentiokinetic measurements. Contact angle, surface free energy, and surface finish are studied with dynamic contact angle measurements and profilometry. X-ray diffraction and energy-dispersive X-ray spectroscopy measures residual stress and surface chemistry. The top surface perpendicular to the build direction (XY) and the wall surface parallel with the build direction (XZ) are studied for all measurements due to the large differences in roughness and mechanical properties between these surfaces. LSP increases pitting potential for both XY and XZ surfaces and causes an increase to the surface electrochemical impedance. LSP also increases the contact angle of liquids on both surfaces. These changes to electrochemistry and wettability are attributed in part to surface morphology and surface chemistry alterations as well as the inducement of compressive residual stress.

Effects of pressurized water reactor environment and cyclic loading parameters on the low cycle fatigue behavior of 304L stainless steel

Austenitic stainless steels used in light water reactor coolant environments are susceptible to environmentally assisted fatigue due to non-monotonic loading conditions, primarily associated with load-follow, thermal transients, or intermittent plant shutdowns and start-ups. This study investigates the effects of a high-temperature pressurized water reactor (PWR) water environment and cyclic loading parameters on the low cycle fatigue behavior of austenitic 304L stainless steel. Prolonged exposure to a PWR environment and cyclic loading conditions such as a lower strain rate or a higher fraction of slow strain rate enhances the initiation and accelerates the crack growth rate of fatigue cracks, resulting in decreased fatigue life. The deformation-induced α'-martensite is observed in proximity to fatigue crack tips primarily in specimens tested in simulated PWR primary water, while cellular dislocation structures are more frequently observed near crack tips in specimens tested in high-temperature air. The deformation-induced martensitic transformation from γ-austenite to α'-martensite, occurring via the precursor ε-martensite phase, contributes to the accelerated fatigue crack growth rate in a PWR environment with hydrogen.

Effect of heat treatment and electroless Ni-P coating on mechanical property and corrosion behaviour of 316L stainless steel fabricated by laser powder bed fusion

ABSTRACT With the prosperity of laser powder bed fusion (LPBF) technology, post-processing tends to be increasingly attractive in further improving the performance of the LPBFed parts. In this study, post-heat treatment and electroless plating were performed on the LPBFed 316L stainless steel to investigate the evolution of surface topography, mechanical property and corrosion behaviour. The results show that the as-built part features melt tracks and fine cellular substructures, which presents higher deformation resistance than the cast and recrystallized counterparts. The uniform amorphous Ni-P coating can significantly increase microhardness of as-built 316L stainless steel from ∼300 to ∼800 HV. In addition, the as-built parts with/without coatings have better corrosion resistance than the corresponding cast ones, which can be further improved through 450°C annealing. Moreover, the coatings can also prevent the localised corrosion effectively. Therefore, this study provides available post-processing methods for further improve the performance of LPBFed 316L stainless steel.

Understanding Melt Pool Behavior of 316L Stainless Steel in Laser Powder Bed Fusion Additive Manufacturing.

In the laser powder bed fusion additive manufacturing process, the quality of fabrications is intricately tied to the laser-matter interaction, specifically the formation of the melt pool. This study experimentally examined the intricacies of melt pool characteristics and surface topography across diverse laser powers and speeds via single-track laser scanning on a bare plate and powder bed for 316L stainless steel. The results reveal that the presence of a powder layer amplifies melt pool instability and worsens irregularities due to increased laser absorption and the introduction of uneven mass from the powder. To provide a comprehensive understanding of melt pool dynamics, a high-fidelity computational model encompassing fluid dynamics, heat transfer, vaporization, and solidification was developed. It was validated against the measured melt pool dimensions and morphology, effectively predicting conduction and keyholing modes with irregular surface features. Particularly, the model explained the forming mechanisms of a defective morphology, termed swell-undercut, at high power and speed conditions, detailing the roles of recoil pressure and liquid refilling. As an application, multiple-track simulations replicate the surface features on cubic samples under two distinct process conditions, showcasing the potential of the laser-matter interaction model for process optimization.

Building rate effect on microstructure and high temperature mechanical properties of Austenitic 316L stainless steel manufactured by laser directed energy deposition

Improving the build rate of additive manufacturing (AM) processes has been of great industrial interest to enhance productivity, reduce costs and increase efficiency. Increasing the build rate in AM is typically achieved by optimizing the process parameters. However, the impacts of these process parameters changes on the resulting microstructure and high temperature mechanical performance in AM is not yet understood. The present study highlights that increasing the build rate, achieved by enlarging the laser beam diameter and laser power while maintaining the energy density, influences the microstructure, tensile properties, and creep behaviour of 316L stainless steel processed using laser directed energy deposition (LDED). Results demonstrate that nearly fully dense LDED samples were achieved at various deposition conditions. Remarkably, LDED samples produced with an increase build rate exhibited superior tensile properties at high temperature and enhanced creep performance. Regarding anisotropy, a lower build rate with optimized process parameters promoted the formation of a finer and more anisotropic microstructure, resulting in significant anisotropy in creep behaviour. In contrast, a higher build rate yielded a uniform microstructure, leading to uniform creep behaviour across different orientations. This study highlights the potential of LDED for achieving superior and uniform high temperature mechanical performance through an increased build rate.

Effect of heat treatment on the microstructure and pitting corrosion behavior of 316L stainless steel fabricated by different additive manufacturing methods (L-PBF versus L-DED): Comparative investigation exploring the role of microstructural features on passivity

This work explores the relation between microstructural features and pitting corrosion resistance of additively manufactured 316 L stainless steel. Specimens fabricated using two different laser-based manufacturing processes (L-PBF and L-DED) were studied in as-built and heat-treated conditions following a comparative approach. Potentiodynamic polarization tests and corrosion morphology analysis served to evaluate their corrosion resistance and passive behavior. This work shows that, contrary to previous reports, the absence of MnS inclusions or presence of the microsegregation structure are not the main factors enhancing passivity of these materials. Instead, dislocation density was the only microstructural feature matching the trend in their passivity behavior.

Elastoplastic Behavior of 316L Stainless Steel (SS) and the Analysis of the Ratcheting Phenomenon Under Cyclic Loading

The present work is concerned with the evaluation of the nonlinear behaviour of 316L steel used in several fields, industry, automotive, aeronautics, medicine .... subjected to cyclic movements. This investigation allowed us to analyze the different responses of steel in low cycle fatigue, and to perceive the phenomenon of Ratcheting leading, to the damage of the structure by an excess of plastic strain, under loading cycles. The prediction of the behaviour of the steel studied, reveals that the Ratcheting increases according to the average stress when the stress amplitude is constant, and decreases according to the attenuation of the stress amplitude when the average stress is constant. The simulation campaign allowed us to present an assessment, of the conditions favoring the acceleration of the Ratcheting, includes several scenarios of periodic loadings, with strain and stress imposed under uniaxial tension/compression. The predictions of this model are evaluated by subjected to cyclic loading within the software ZébuLon.

Influence of Surface Finish on the Dry Sliding Wear and Electrochemical Corrosion Behaviour of 316L Stainless Steel

316L stainless steel is known for its high corrosion resistance and extensively being used for biomedical implants and for the fabrication of the parts in nuclear reactors, and fuel cells. In the current study, 316L stainless steel samples were grounded using different grades of surface grits (400, 1200, and mirror-finished) by mechanical polishing, and influence of surface finish on the wear and corrosion characteristics was studied. The dry sliding wear experiments were carried out on ball-on-plate setup against hardened steel ball and the corresponding friction curves were obtained and specific wear rates were measured. The wear mechanisms were identified by examining the worn-out surface using scanning electron microscope and Raman spectrometer. It was evident that the formation and removal of iron oxides is the dominant mechanism for material loss from the samples during wear tests. The higher wear resistance of the mirror-finished sample was attributed to the formation of stable chromium oxide layer on the wear track. Potentiodynamic polarization tests were performed on the samples and better corrosion resistance was observed for the mirror-finished sample.

Effect of microstructure evolution induced by LP on hydrogen permeation behavior of 316L stainless steel

In order to investigate the hydrogen permeation behavior of 316L stainless steel during the microstructural evolution induced by laser peening (LP), an electrochemical hydrogen charging system for initial hydrogen charging of LPed and non-LPed specimen was developed. Afterward, the microhardness, residual stress, and microstructures of the samples were determined and analyzed. Finally, electrochemical hydrogen permeation experiments were undertaken to verify LP's influence on hydrogen permeation parameters of 316L. The results showed that LP reduced the hydrogen-induced hardening rate of the alloy and additionally invoked high magnitude compressive residual stress on its surface. At the layer close to the face of the specimen, the grain refinement rate was as high as 56.18%, which was accompanied by the appearance of high-density dislocations. Compared with the non-LPed sample, the hydrogen permeation time increased significantly, and the saturation current density in steady state hydrogen permeation also decreased gradually.

Tribocorrosion behavior of 316L stainless steel sliding against different ceramics in 0.5 M H2SO4 solution

The influence of ceramic ball wear on the tribocorrosion behavior of 316L stainless steel is little studied. This study investigated the tribocorrosion behavior of 316L stainless steel sliding against three different ceramic balls in 0.5 M H2SO4 solution. The results showed that the three ceramic balls exhibited quite different wear behaviors and the tribocorrosion behavior of 316L stainless steel was significantly influenced by the wear of the ceramic counterpart. Surface analysis indicated Si-water reactions on the Si-containing balls at high shear stress and high flash temperature. This study provides clear evidence that the selection of the counterpart materials and their wear behaviors need to be paid more attention to in tribocorrosion experimental investigations of stainless steels.

Effect of scanning speed on fatigue behavior of 316L stainless steel fabricated by laser powder bed fusion

The effect of the scanning speed on the fatigue behavior of Laser Powder Bed Fusion (LPBF)-fabricated 316L steel is investigated in this paper. To this end, fatigue limits of specimen manufactured by different scanning speeds are determined by the self-heating approach. EBSD experiments and relative density measurements are carried out to characterize the microstructure and porosity. To analyse the influence of scanning speed on the microstructure (grain morphology, texture, dislocation density and stored energy) and fatigue property, a dislocation-density, crystal plasticity and stored energy-based fatigue model is developed. The inverse optimization method is combined with EBSD experiments and uniaxial tension experiments to identify the model parameters. The experimental results show a critical scanning speed, below which the fatigue limit stays almost unchanged and decreases drastically while the scanning speed is increased beyond. Furthermore, the simulation results show that the predicted fatigue limits correspond well to the experimental fatigue ones. From experimental and numerical results, it is deduced that the critical stored energy density and maximum temperature variation are functions of the porosity and can be used to differentiate the types of fatigue: microstructure-dominated or defect-dominated. This article provides new insights which can be further used in the optimization of fatigue behavior of LPBF 316L steel with respect to the scanning speed.

Effects of Residual Stress Induced by Laser Shock Peening on Fatigue Crack Propagation Behavior in Directed Energy Deposition Stainless Steel

To investigate the effect of laser shock peening (LSP) on the fatigue crack propagation behavior of 316L stainless steel fabricated by directed energy deposition (DED), three‐dimensional finite element models of DED and compact tensile specimens before and after LSP are developed. The residual stress fields induced by DED and LSP are simulated, as well as the effects of different residual stresses on the stress intensity factor and effective stress ratio based on the contour integral method are also analyzed. The microstructure of the LSP region is observed by scanning electron microscope. When the crack length increases from 12 to 22.5 mm, the average effective stress ratio of the DED specimen is 0.133, and the average effective stress ratio of the DED specimen after LSP decreases to 0.110, which decreases by 17.3%. The fatigue lives of the DED specimen before and after LSP are 62.7% and 105.2% of that of the hot‐rolled plate. After LSP treatment, the fatigue life of the DED specimen is increased by about 1.68 times. The fracture morphology in the transient fracture zone changes from ductile and brittle mixed fracture to ductile fracture.

Fatigue behavior of 316L stainless steel fabricated via Material Extrusion Additive Manufacturing

Material Extrusion Additive Manufacturing (MEAM) is a novel technique that widens the field of additive manufacturing applications in producing metal parts. There is limited knowledge about the process-induced properties of the metal parts obtained with such a process, especially when dealing with cyclic loading. The present work aims to assess the fatigue life of 316L components fabricated via MEAM, investigating the fatigue failure mechanisms. The resulting S-N curve shows a lower fatigue limit compared to wrought 316L. Internal defects are detrimental to the fatigue life since crack initiation sites are observed from the pores resulting from the deposition strategy of the material during the printing phase.

Porosity, roughness, and passive film morphology influence the corrosion behavior of 316L stainless steel manufactured by laser powder bed fusion

The development of additively-manufactured (AM) 316L stainless steel (SS) using laser powder bed fusion (LPBF) has enabled near net shape components from a corrosion-resistant structural material. In this article, we present a multiscale study on the effects of processing parameters on the corrosion behavior of as-printed surfaces of AM 316L SS formed via LPBF. Laser power and scan speed of the LPBF process were varied across the instrument range known to produce parts with >99 % density, and the macroscale corrosion trends were interpreted via microscale and nanoscale measurements of porosity, roughness, microstructure, and chemistry. Porosity and roughness data showed that porosity φ decreased as volumetric energy density Ev increased due to a shift in the pore formation mechanism and that roughness Sq was due to melt track morphology and partially fused powder features. Cross-sectional and plan-view maps of chemistry and work function ϕs revealed an amorphous Mn-silicate phase enriched with Cr and Al that varied in both thickness and density depending on Ev. Finally, the macroscale potentiodynamic polarization experiments under full immersion in quiescent 0.6 M NaCl showed significant differences in breakdown potential Eb and metastable pitting. In general, samples with smaller φ and Sq values and larger ϕs values and homogeneity in the Mn-silicate exhibited larger Eb. The porosity and roughness effects stemmed from an increase to the overall number of initiation sites for pitting, and the oxide phase contributed to passive film breakdown by acting as a crevice former or creating a galvanic couple with the SS.

Constructing passivation film on plain carbon steel similar to that on 316L stainless steel

A high corrosion-resistant passivation film with cerium-molybdenum-chromium-nickel ions has been synthesized on plain carbon steel rebar through a chemical transformation method which significantly lowered the pitting susceptibility of the rebar in the chloride-containing concrete environment. The results indicated that the prepared film has a re-passivation self-healing property in a concrete environment with a concentration of 0.1 M NaCl, which is similar to the behavior of 316L stainless steel. The charge transfer resistance increased markedly with the extension of immersion time, reaching 3.70 × 106 Ω cm2 in about 1500 h of immersion (about 10 times of the initial Rt value). The high corrosion resistance mechanism of the film was discussed here in depth.

Experimental and constitutive modelling studies of type 316L stainless steel based on internal stress under low cycle fatigue and creep-fatigue interaction

Low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of 316L stainless steel at 550 °C were investigated using experimental and modelling approaches. Results demonstrate that friction and back stress contribute to the initial hardening stage, whereas the former is primarily responsible for the subsequent softening stage. Besides, friction stress also accounts for the non-masing feature and varied softening rate. Increasing holding time induces a rise followed by a decline in friction stress, while the back stress decreases steadily to a saturated value. Moreover, a short tensile holding duration leads to planar structures and higher dislocation density. However, a longer tensile holding time promotes dislocation annihilation and subgrains growth, and elongates dislocation cells. Finally, an improved viscoplastic constitutive model was proposed based on the internal stress analysis. The kinematic hardening rule was improved by considering the cycle-dependent plastic modulus and stress relaxation, and the non-masing and variable softening rate were captured by modifying the isotropic hardening rule. In addition, holding time-related factors were also coupled into the model. The proposed model is robust for describing the cyclic features under both LCF and CFI.

Impact-sliding fretting tribocorrosion behavior of 316L stainless steel in solution with different halide concentrations

Impact-sliding caused by random vibrations between tubes and supports can affect the operation of heat exchangers. In addition, a corrosive environment can cause damage, accelerating the synergism of corrosion and wear. Therefore, the focus of this work was the impact-sliding fretting tribocorrosion behavior of 316L heat exchanger tubes at different halide concentrations. A device system incorporating the in situ electrochemical measurements of impact-sliding fretting corrosion wear was constructed, and experiments on 316L heat exchanger tubes in sodium chloride (NaCl) solution with different concentrations (0.0, 0.1, 0.5, 1.0, 3.5, and 5.0 wt%) were carried out. The synergism between wear and corrosion was also calculated and analyzed. The wear and damage mechanisms were elucidated by correlating the corrosion-wear synergism, morphologies, and material loss rates. The results indicated that the stable wear stage occurred at approximately 9–12 h, after which the corrosion current increased with the expansion of the wear area. As the halide concentration increased, the scale of damage on the wear scars gradually decreased, changing from being dominated by cracks, delaminations, and grooves to being dominated by scratches, microgrooves, and holes. There was an obvious positive synergism between wear and corrosion. The material loss was dominated by pure mechanical wear and wear enhanced by corrosion, but corrosion enhanced by wear contributed more than tangential sliding fretting corrosion. The total mass loss increased gradually in the range of 0.0–0.5 wt% and decreased in the range of 0.5–5.0 wt%. Large-scale damage enhanced by corrosivity and small-scale damage reduced by lubricity dominated the material loss at low and high concentrations, respectively.