Microstructure Of 316L Stainless Steel Research Articles

Effects of gas tungsten arc welding on the mechanical properties and microstructure of 316L stainless steel by powder bed fusion

Effects of gas tungsten arc welding on the mechanical properties and microstructure of 316L stainless steel by powder bed fusion

Influence of the Processing Parameters on the Microstructure and Mechanical Properties of 316L Stainless Steel Fabricated by Laser Powder Bed Fusion

Complex thermo-kinetic interactions during metal additive manufacturing reduce the homogeneity of the microstructure of the produced samples. Understanding the effect of processing parameters over the resulting mechanical properties is essential for adopting and popularizing this technology. The present work is focused on the effect of laser power, scanning speed, and hatch spacing on the relative density, microhardness, and microstructure of 316L stainless steel processed by laser powder bed fusion. Several characterization techniques were used to study the microstructure and mechanical properties: optical, electron microscopies, and spectrometry. A full-factorial design of experiments was employed for relative density and microhardness evaluation. The results derived from the experimental work were subjected to statistical analysis, including the use of analysis of variance (ANOVA) to determine both the main effects and the interaction between the processing parameters, as well as to observe the contribution of each factor on the mechanical properties. The results show that the scanning speed is the most statistically significant parameter influencing densification and microhardness. Ensuring the amount of volumetric energy density (125 J/mm3) used to melt the powder bed is paramount; maximum densification (99.7%) is achieved with high laser power and low scanning speed, while hatch spacing is not statistically significant.

Investigation of the Properties of 316L Stainless Steel after AM and Heat Treatment.

Additive manufacturing, including laser powder bed fusion, offers possibilities for the production of materials with properties comparable to conventional technologies. The main aim of this paper is to describe the specific microstructure of 316L stainless steel prepared using additive manufacturing. The as-built state and the material after heat treatment (solution annealing at 1050 °C and 60 min soaking time, followed by artificial aging at 700 °C and 3000 min soaking time) were analyzed. A static tensile test at ambient temperature, 77 K, and 8 K was performed to evaluate the mechanical properties. The characteristics of the specific microstructure were examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The stainless steel 316L prepared using laser powder bed fusion consisted of a hierarchical austenitic microstructure, with a grain size of 25 µm as-built up to 35 µm after heat treatment. The grains predominantly contained fine 300-700 nm subgrains with a cellular structure. It was concluded that after the selected heat treatment there was a significant reduction in dislocations. An increase in precipitates was observed after heat treatment, from the original amount of approximately 20 nm to 150 nm.

Additive manufacturing and characterization of a stainless steel and a nickel alloy

Abstract Recently, additive manufacturing is of interest, and there is a trend to study additively manufactured materials such as Inconel 718 and 316L stainless steel. Additive manufacturing brings the easiness of production of complex geometries, avoids expensive tools, helps achieve interesting microstructures and obtaining promising results for future applications. Since the additive procedure is sensitive to many fabrication variables thereby affecting the microstructure and mechanical properties. This motivation promotes investigating the additively manufactured microstructure of 316L stainless steel and Inconel 718. While 316L stainless steel was fabricated using an electron-based powder bed fusion manner, directed energy deposition was preferred for Inconel 718. Samples were examined utilizing optical and scanning electron microscopes. Results suggest processing of 316L stainless steel gives rise to the same porosity rate as Inconel 718. Bimodal equiaxed austenite grain morphology was observed whereas no dendrite presence was detected for 316L stainless steel. Additive manufacturing types do not cause a significant change in the level of porosity for Inconel 718 alloy. Unlike the case of stainless steel, additive manufacturing results in dendritic microstructure formation in Inconel 718 whereas powder bed fusion-type production triggers a better refinement compared to that of directed energy deposition.

Effect of Pulsed/Continuous Double-Beam Hybrid Laser Cladding on Microstructure of 316L Stainless Steel

Effect of Pulsed/Continuous Double-Beam Hybrid Laser Cladding on Microstructure of 316L Stainless Steel

IMPROVING THE MECHANICAL CHARACTERISTICS OF 316L STAINLESS STEEL PRODUCED BY SELECTIVE LASER MELTING AND STUDYING THE EFFECT OF POROSITY ON STRENGTH

The results of experiments on the study of the physical and mechanical properties and microstructure of 316L stainless steel produced by additive technology selective laser melting are obtained. The dependences of the physical and mechanical properties (density, tensile strength, yield strength, elongation to failure, Young's modulus, nano hardness) of 316L steel on the main parameters of the selective laser melting (laser power, scanning speed) are obtained. 316L steel produced by selective laser melting has high mechanical characteristics: tensile strength – 710 MPa; yield strength – 610 MPa; elongation to failure – 48%; relative density – 99.5%, which is comparable to the values obtained for 316L steel produced by traditional methods. These characteristics were obtained using the optimal parameters of selective laser melting: laser power 100 W, scanning speed 200 mm/s, layer thickness 50 ?m, the distance between the scanning tracks is 80 ?m. It is shown that the use of the “volumetric energy density” parameter is usefully for carrying out a primary assessment of technological modes and solving the problem of optimizing the parameters of selective laser melting to obtain a material with high physical and mechanical properties. The nonlinear dependence of the mechanical properties of 316L steel on the main technological parameters can be explained by the influence of porosity arising at non-optimal modes. It is shown that selective laser melting allows controlling the porosity of 316L steel by varying the parameters of the technological mode. Thus selective laser melting makes it possible to produce both a material with a high density close to theoretical values and a material with controlled porosity.

Dependency of recrystallization kinetics on the solidification microstructure of 316L stainless steel processed by laser powder bed fusion (LPBF)

In this study, the recrystallization kinetics of two 316 L stainless steels produced by Laser Powder-Bed Fusion (LPBF) were compared. The as-built microstructures of the studied materials differed by their grain size, their grain boundary character distributions, and their populations of nanoprecipitates. Strong differences in their recrystallization kinetics were observed; they were attributed to a delayed nucleation in the steel exhibiting the finer grains, the lower density of low-angle boundaries (LAB), and the higher volume fraction of precipitates. This work shows that a high density of LABs stemming from the solidification stage in the LPBF process promotes recrystallization by locally increasing the stored energy close to grain boundaries, and hence the formation of recrystallization nuclei. The design of low-LAB microstructures could thus lead to higher stability at high temperatures. • Recrystallization kinetics are heavily influenced by the as-built microstructure. • Kinetics are not affected by a second-phase particles volume fraction increase. • Low angle boundaries promotes the formation of recrystallization nuclei.

Filament extrusion-based additive manufacturing of 316L stainless steel: Effects of sintering conditions on the microstructure and mechanical properties

Filament extrusion-based additive manufacturing of metals offers an alternative to the widespread beam-based counterparts. The microstructure obtained from extrusion-based techniques differs greatly from the ones obtained by beam-based additive manufacturing, as a sintering process is used, in contrast to the rapid solidification of a melt pool. In this study, the microstructure of 316L stainless steel fabricated by filament extrusion is investigated as a function of debinding and sintering conditions. High-speed nanoindentation correlated with energy-dispersive X-ray mapping is employed for microstructural characterization. High sintering temperatures of 1350 °C, an atmosphere of pure H2, and a cooling rate of 60 K/m are found to result in the optimal microstructure. High densities are obtained due to accelerated densification, enabled by the introduction of diffusion paths due to δ-ferrite formation. At the same time, hard phases like oxides or σ-precipitates with detrimental effects on the mechanical properties can be avoided. It is shown that the porosity can be quantified by analysis of hardness and modulus data from nanoindentation mapping. The values obtained are in good agreement with optical and Archimedes immersion method measurements. Tensile tests of 3D-printed and sintered specimens show excellent ductility and strength in comparison to literature. We demonstrate that 3D printing of 316L filaments and sintering with the optimized conditions results in material properties comparable to bulk values.

Effects of rescanning parameters on densification and microstructural refinement of 316L stainless steel fabricated by laser powder bed fusion

• The effect of laser power and scan speed on microstructure of 316L SS is explored. • Cell size of single scan can guide microstructural expectation of multilayer scan. • Rescanning with a 2nd parameter achieved 70.2 % cell size refinement. • Laser rescanning increased the relative density by 0.21−0.42%. • Spatially heterogeneous microstructure was achieved by applying rescanning. A challenge with microstructural control and refinement in laser powder bed fusion (LPBF) is maintaining high density when choosing parameters for desired microstructures. Rescanning during LPBF has been reported to improve densification and decrease surface roughness for many different alloys. However, little has been reported regarding the effects of locally rescanning with varying processing parameters on sub-grain cell size refinement for 316L stainless steel (SS). This study presents a novel solution to enable high densification with microstructural control in 316L SS by using a set of initial scanning parameters to achieve densification and a different set of rescanning parameters to refine the microstructure. Results showed that rescanning resulted in heterogeneous microstructure with coarse cell size of 0.84 μm and locally refined cell size of 0.35 μm, while maintaining a high level of densification (99.96 %), therefore enabling potential variations in component strength and hardness. The spatial distribution of local microstructure refinement was dictated by the melt pool dimensions of initial scanning and rescanning relative to the powder layer thickness. To better understand the link between LPBF process parameters and microstructure, the Wilson-Rosenthal equation was used to predict cooling rate ( G × R ) and correlate with sub-grain cell size. Such variation in properties may be useful for applications requiring parts with hardened surfaces, or localized strengthening at stress concentrations and sites of expected failure.

Anisotropic cellular structure and texture microstructure of 316L stainless steel fabricated by selective laser melting via rotation scanning strategy

In this work, the effects of solidification and remelting under chessboard 67° rotation scanning strategy on the microstructure and properties of selective laser melted (SLMed) 316L stainless steel (316L SS) were studied, including dendrite microstructure, element distribution, grain orientation, dislocation structure, microhardness and corrosion resistance. We put forward a complete foundation relationship among texture, cellular structure and mechanical property influenced by anisotropy. Main results are summarized as follows: The 〈101〉 texture formation is the result of dendrite epitaxial growth. The cellular dislocation structure highly overlapped with dendrite significantly changed in remelting zone. Hence the high ratio of 〈101〉 texture signifies the anisotropy distribution of epitaxially growth dendrite which released the internal stress. The dislocation structure has a contribution to strength similar work hardening strengthening. Meanwhile, the 316L SS obtains Orowan strengthening via the nano oxide pinning effect in dislocation wall. Thus, the dislocation density and oxide distance obvious changed in different directions of cellular dislocation structure, resulting in the cellular dendrite regions performed higher microhardness than the columnar dendrite regions. The drastic change of dendrite increased the crack sensitivity of SLMed 316L SS, and the junction of different epitaxially growth direction dendrites formed high energy grain boundary.

Multi-scale characterisation of microstructure and texture of 316L stainless steel manufactured by laser powder bed fusion

316L stainless steel produced by additive manufacturing (AM) offers superior mechanical and corrosion properties compared to parts of the same steel processed by conventional methods. However, thorough understanding of the detailed microstructural evolution during various additive manufacturing techniques is lacking when compared to conventionally produced counterparts. As such, the present work contributes to filling this gap in knowledge by using advanced microscopy techniques to analyse the microstructure of 316L stainless steel builds produced by laser powder bed fusion. The results show that changes in processing parameters lead to variations in grain structures, texture, and hardness. For instance, the dominant texture and grain structure is heavily influenced by changes to the laser scanning rotation between individual layers, while increasing laser power reduces hardness. The current study also provides additional insights into how grains in AM 316L steel accommodate strain. Due to the complex strain fields in austenite grains, they are usually split into complex-shaped regions separated by dislocation boundaries with characters similar to deformation- and micro-band boundaries in conventionally deformed austenite. Through electron probe micro-analysis over a large area, the current study reveals the tendency of Mo and Si concentrations to fluctuate around the cell and melt pool boundaries.

Revealing relationships between heterogeneous microstructure and strengthening mechanism of austenitic stainless steels fabricated by directed energy deposition (DED)

The heterogeneous trans-scale structures play important roles in achieving high strength and toughness in austenitic stainless steels fabricated by directed energy deposition (DED). In this study, systematic annealing investigations were demonstrated the evolution of trans-scale heterogeneous structures such as dislocations, chemical cells, nano-oxides, grains and recrystallization behavior. Their effects on yield strength were quantified. The experimentally observed yield strength of as-received and annealed samples agrees well with the estimated yield strength using standard Taylor hardening formulae. The dislocation intensity accounted for 200 MPa and became the main strength contributor, among which GND type of dislocation accounted for more than 60% of the total dislocation. Despite the development of DED steel micro inclusions, the synergistic effect of deformation twinning and pre-existing GND-induced back stress results in high tensile plasticity. This research helps to better understand the interplay between DED processing, the microstructure of 316L stainless steel and the mechanical properties that are crucial to meeting new demands.

Effects of ion irradiation on microstructure of 316L stainless steel strengthened by disperse nano TiC through selective laser melting

The manufacturing of the new generation of radiation-resistant structural materials is an extremely interesting and challenging topic in the field of additive manufacturing research. Understanding the special microstructure characteristics and the influence on the radiation resistance of these additive manufactured materials is still superficial. In this study, high-quality bulk 316L stainless steels (SSs) strengthened by dispersed nano TiC were successfully prepared by selective laser melting (SLM). The results of transmission electron micrograph and small angle neutron scattering showed that TiC existed in the matrix of 316L SSs in the form of nanoparticles with average size less than 50 nm. TiC particles were distributed inside the subgrains and on the subgrain boundaries. Smaller helium bubbles were observed after the same flux of He2+ ion irradiation in the case of 316L SSs with 4% TiC compared with pure SLM 316L SSs. In comparison with the case on the grain boundaries and intragranular, the helium bubbles at TiC/316L interfaces have the smallest size and the largest density. The results Nanoindentation results showed that 4% TiC doping had a remarkable inhibiting effect on irradiation-induced hardening at a low dose. This condition is because numerous interfaces of TiC/316L acted as sink/trap sites for the irradiation-induced defects.

Effects of process parameters, debinding and sintering on the microstructure of 316L stainless steel produced by binder jetting

Binder Jetting is becoming increasingly important in the scenario of metal Additive Manufacturing processes due to the absence of rapid melting and solidification steps that can induce defects in most sensitive alloys and lead to unwanted reactions. However, this technology requires to consider the effects of different factors such as powder packing, wettability with the binder and sinterability to define suitable process parameters and to achieve desirable material properties. Although several research works focus on individual aspects of Binder Jetting and sintering of parts, a comprehensive understanding of material evolution under different processing conditions has not been achieved yet.The present research explores the effects of different process and thermal parameters on the porosity and mechanical properties of binder jetted 316L samples. The effect of layer thickness and binder saturation, considering the powder bed features, and debinding and sintering atmospheres, referring to the post-processing stages, were investigated at the green and sintered stages via microstructural and compositional analysis and mechanical characterization. Thermal treatments simulations were employed to determine the microstructural evolution during sintering. The 316L steel produced in this work by Binder Jetting exhibited a fine equiaxed microstructure, tensile strength values comparable to those of cast products and superior ductility compared to other additive techniques.

Mechanical properties and microstructure of 316L stainless steel produced by hybrid manufacturing

Hybrid manufacturing is a combination of additive (deposition) and subtractive (machining) manufacturing in a single machine tool. Such a system can be used for near net shape manufacturing and component repair using either similar or dissimilar materials. Integrated into a single system, transition between additive and subtractive manufacturing can occur immediately and be leveraged to generate large components by alternating between the processes. This investigation shows how the interleaved capabilities can reduce overall cycle time by up to 68 %, improve average relative elongation to failure by 71 %, and reduce the average relative porosity fraction by 83 % when compared to traditional additive manufactured components. Results from this investigation builds the foundation needed for hybrid manufacturing to be applicable towards the manufacture of large complex components such as nosecones and marine propulsors.

Deformation mechanisms of the subgranular cellular structures in selective laser melted 316L stainless steel

Selective Laser Melting (SLM) of certain alloys such as Stainless Steel yields a unique microstructure consisting of complex subgranular cell structures produced by ultra-fast cooling during solidification. Owing to the presence of these cell structures, SLM-fabricated 316L Stainless Steel displays a superior yield strength and hardness. Despite the significance of the distinctive cellular structures, their deformation characteristics and the mechanisms responsible for superior mechanical properties are not well-understood yet. In this paper, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The atomic configurations in the computational models of the cell structures are inspired by the SEM images of the microstructure of 316L Stainless Steel. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nano-twins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Ni concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. However, the flow stress is found to be the highest when both the cell boundary and cell interior are austenitic with higher Ni concentration in the cell interior. Moreover, governed by the initial dislocation density, the subgranular cell size has an inverse relationship with mechanical strength. Finally, the nano-twinned cells exhibit higher strength in comparison with twin-free cells as a result of the dislocation pinning at the twin boundaries. The investigation on the effect of twin spacing reveals a Hall-Petch type phenomenon in which smaller twin spacing strengthens the nano-twinned subgranular cells further.

Influence of cryorolling on properties of L-PBF 316l stainless steel tested at 298K and 77K

The goal of the present work is to evaluate mechanical properties and to analyse the microstructure of 316L stainless steel produced by Laser Powder Bed Fusion (L-PBF) follow by rolling with different thickness reduction under ambient and cryogenic conditions. The samples before rolling were heat treated. The static tensile test was realized at ambient and cryogenic (77K) conditions. The L-PBF powder metal production technology approved that is a key technology in the AM area, especially for metal powder materials. Mechanical properties tested at 298K and 77K shows that the application of various thermo-deformation rolling conditions increases of strength properties. Achieved mechanical properties are comparable to conventional bulk materials. The strength properties after the rolling under ambient and cryogenic conditions were significantly increased.

Microstructure and mechanical properties of stainless steel 316L obtained by Direct Metal Laser Deposition

The microstructure of 316L stainless steel obtained by layer-by-layer direct metal laser deposition is reviewed. Mechanical tests of the samples were performed in accordance with GOST 1497-84. Studies show that changes in power of laser radiation to grow parts lead to changes in their mechanical properties. The research shows dependencies between the strength characteristics of materials and the power of laser radiation. Causes of the forementioned changes are studied through the analysis of the microstructure. Nanosized inclusions of spherical shape were found in the process of studying the microstructure of materials. A study in the nature of the formation of these inclusions and their effect on the properties of the obtained material was performed.

Effect of heat treatment on microstructure and mechanical properties of 316L steel synthesized by selective laser melting

The influence of annealing at different temperatures (573, 873, 1273, 1373 and 1673 K) on the stability of phases, composition and microstructure of 316L stainless steel fabricated by SLM has been investigated and the changes induced by the heat treatment have been used to understand the corresponding variations of the mechanical properties of the specimens under tensile loading. Annealing has no effect on phase formation: a single-phase austenite is observed in all specimens investigated here. In addition, annealing does not change the random crystallographic orientation observed in the as-synthesized material. The complex cellular microstructure with fine subgrain structures characteristic of the as-SLM specimens is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K). The strength of the specimens decreases with increasing annealing temperature as a result of the microstructural coarsening. The excellent combination of strength and ductility exhibited by the as-synthesized material can be ascribed to the complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains.

Corrosion Behavior of 316L Stainless Steel Fabricated by Selective Laser Melting Under Different Scanning Speeds

This work discussed the effects of different scanning speeds (800, 1083, 1200 and 1400 mm/s) on the microstructure of 316L stainless steel manufactured by selective laser melting (SLM) and the related corrosion behavior. Results showed that there were more voids with faster scanning speeds, and there were oxide powder and non-melt silicon inside the defects. The SLM 316L exhibited a full gamma austenite phase filled with sub-grains, and the average grain size of SLM-1083 mm/s 316L was approximately 42 μm, three times larger than that of the quenched 316L. The pitting potentials for the SLM 316L were all approximately 300 mV higher than that of the quenched due to the modification of inclusions in SLM, but the corrosion rate for the SLM 316L was faster, which was attributed to the voids and unstable passive film. The number of pitting sites increased with the scanning speed, and the pits occurred preferentially at the voids in SLM 316L.