Effects of Heat Treatment on the Microstructure and Mechanical Properties of Selective Laser Melting 316L Stainless Steel
TL;DR: In this paper, the dynamic compressive mechanical properties of the deposited, annealed, and solution-treated specimens were tested at high strain rates by using a split Hopkinson pressure bar (SHPB) experimental apparatus.
Abstract: 316L stainless steel materials are widely used in impact-resistant structures. Heat treatments could affect the mechanical properties of 316L stainless steel parts formed by selective laser melting (SLM), which is vital for ensuring service safety. This study aimed to analyze the mechanical behavior of SLM 316L stainless steel under different heat treatment methods. Therefore, test specimens were prepared using the SLM technique and then annealed at 400°C for 1 h. The solution was treated at 1050°C for 20 min. The dynamic compressive mechanical properties of the deposited, annealed, and solution-treated specimens were tested at high strain rates by using a split Hopkinson pressure bar (SHPB) experimental apparatus. Moreover, the microstructures of the previously mentioned samples were analyzed by optical microscopy and scanning electron microscopy. The experimental results showed that the three-state samples exhibited strain rate sensitivity in the dynamic mechanical tests and that solution treatment could alter their mechanical properties significantly. In addition, the microstructure of the deposited specimens presented cylindrical cellular crystal features, which have a higher dislocation density. Hence, the yield strength of deposited specimens is higher than that of the solution-treated ones. After annealing, the microstructures of the samples did not change obviously, and their dynamic yield strength remained almost unchanged. After solution treatment, its cellular crystal disappeared and dislocation density dropped dramatically, resulting in a sharp decrease in yield strength. Finally, this research can provide a theoretical reference for broadening the practical application of SLM 316L material parts.
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TL;DR: In this paper , the surface morphology and elemental and phase composition of stainless steels were studied using conversion X-ray Mössbauer spectroscopy (CXMS), conversion electron microscopy (SEM), energy-dispersive Xray spectroscopic analysis (EDS), and XRD.
Abstract: Stainless steels have the advantage of forming a protective surface layer to prevent corrosion. This layer results from phase and structural changes on the steel surface. Stainless steel samples (1.4404, 316L), whose alloying elements include Cr, Ni, Mo, and Mn, were subjected to the study of the surface layer. Prism-shaped samples (25 × 25 × 3) mm3 were made from CL20ES stainless steel powder, using selective laser melting. After sandblasting with corundum powder and annealing at 550 °C for different periods of time (2, 4, 8, 16, 32, 64, 128 h), samples were studied by conversion X-ray Mössbauer spectroscopy (CXMS), conversion electron Mössbauer spectroscopy (CEMS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The main topics of the research were surface morphology and elemental and phase composition. The annealing of stainless steel samples resulted in a new surface layer comprising leaf-shaped crystals made of chromium oxide. The crystals grew, and their number increased as annealing time was extended. The amount of chromium increased in the surface layer at the expense of iron and nickel, and the longer the annealing time was set, the more chromium was observed in the surface layer. Iron compounds (BCC iron, mixed Fe–Cr oxide) were found in the surface layer, in addition to chromium oxide. BCC iron appeared only after annealing for at least 4 h, which is the initial time of austenitic–ferritic transformation. Mixed Fe–Cr oxide was observed in all annealed samples. All phase changes were observed in the surface layer at approximately 0.6 µm depth.
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TL;DR: In this article , phase and structural changes of steel powders CL20ES and CL50WS used in additive manufacturing were studied by Mössbauer spectroscopy and X-ray diffraction.
Abstract: Abstract The phase and structural changes of steel powders CL20ES and CL50WS used in additive manufacturing were studied by Mössbauer spectroscopy and X-ray diffraction. Investigated powders were annealed in the temperature range 500 °C–1100 °C in oxidizing and inert atmosphere. Annealing in an oxidizing atmosphere resulted in the formation of different iron oxides in both steel powders depending on the annealing temperature. In addition, a phase change of ferrite to austenite was identified when annealing CL50WS steel powder in an oxidizing atmosphere. This phase change was confirmed by annealing given CL50WS steel powder in an inert nitrogen atmosphere. The transformation of austenite to ferrite phase was observed only, when annealing CL20ES steel powder in an inert atmosphere at 1100 °C. Morphological changes on the surface of the spherical powders were observed by scanning electron microscopy.
TL;DR: In this paper , the authors developed the characteristics of high-temperature structure degradation processes under the synergistic effects of thermo-mechanical fatigue and high temperature creep for samples made of 316L steel in the delivery condition and after heat treatment.
Abstract: The main purpose of the work was to develop the characteristics
of high-temperature structure degradation processes under the
synergistic effects of thermo-mechanical fatigue and high temperature
creep for samples made of 316L steel in the delivery condition
and after heat treatment. The use of heat treatment consisting of
solution treatment at 1100°C for 45 minutes and water cooling improved
the mechanical properties of 316L steel. Moreover, in fatigue
tests, in every case, increasing the strain value in a single cycle leads
to a faster sample rupture. An additional aim of the research was
also to develop and verify a new innovative research methodology
concerning the combination of fatigue cycles with the creep process
at elevated temperature, the aim of which is to better reflect the behaviour of the material in real working conditions.
References
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TL;DR: In this paper, the high thermal gradients occurring during SLM lead to a very fine microstructure with submicron-sized cells, which can be modified to a weak cube texture along the building and scanning directions when a rotation of 90° of the scanning vectors within or between the layers is applied.
1,431 citations
TL;DR: The potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications is demonstrated, with austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibiting a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels.
Abstract: Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.
1,385 citations
TL;DR: In this article, the authors show that the pre-existing dislocation network, which maintains its configuration during the entire plastic deformation, is an ideal modulator that is able to slow down but not entirely block the dislocation motion.
557 citations
"Effects of Heat Treatment on the Mi..." refers background in this paper
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11 Feb 2015-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a fine columnar sub-grain structure of size 0.5μm was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10-100μm.
Abstract: Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6±0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 μm was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10–100 μm. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.
526 citations
TL;DR: In this paper, the effect of the MPBs on microscopic slipping, macroscopic ductility and fracture mechanism of the SLM parts made along different directions, which were analyzed and evaluated using slip theory and experiments.
327 citations
"Effects of Heat Treatment on the Mi..." refers methods in this paper
...Afterward, the data is transformed into a stressstrain curve through data processing [21, 22]:...
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