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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Phase Changes in the Surface Layer of Stainless Steel Annealed at a Temperature of 550 °C

To investigate the effects of different scanning speeds on the mechanical properties of selective laser melting 316L stainless steel (SLM 316L SS), process specimens with scanning speeds at 350 mm/s, 650 mm/s, 950 mm/s, and 1250 mm/s were prepared using the SLM technique in this paper. The quasistatic and dynamic compressive mechanical properties of the four specimens were tested by electrohydraulic servo experimental machine and Hopkinson compression bar experimental device, and the mechanical differences of the specimens were analyzed by microscopic observation. Finally, the modified Johnson-Cook (J-C) model was adopted to describe the dynamic mechanical properties of SLM 316L SS. Results showed that the four process steel specimens exhibited typical viscoplastic characteristics and significant strain rate strengthening effects in the mechanical property tests. Moreover, the scanning speed significantly affects the internal defects and melt pool characteristics of the SLM 316L specimens, and the yield strength decreases significantly with the disappearance of the melt pool characteristics. Finally, the modified J-C model can better describe the mechanical behavior of SLM 316L SS material more accurately. This study can provide theoretical references for improving the fabrication process of SLM 316L SS to broaden the practical application of this material. 

Mechanical Properties and Constitutive Model of Selective Laser Melting 316L Stainless Steel at Different Scanning Speeds

Post-Process Treatments for Additive-Manufactured Metallic Structures: A Comprehensive Review

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. 

/pdf/mossbauer-study-of-thermal-behavior-of-cl20es-and-cl50ws-3cfv20rp.pdf

Mössbauer study of thermal behavior of CL20ES and CL50WS steel powders used in selective laser melting

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.

/pdf/influence-of-heat-treatment-on-thermo-mechanical-fatigue-and-2hukvr9p.pdf

Influence of heat treatment on thermo-mechanical
fatigue and high temperature creep properties
for 316l steel

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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Effects of Heat Treatment on the Microstructure and Mechanical Properties of Selective Laser Melting 316L Stainless Steel


 A 316L stainless steel material is widely used in the design of impact-resistant structures. Using selective laser melting (SLM) technology to form 316L stainless steel to study its mechanical behavior under dynamic loading is vital to improve the service performance of this product. This study investigates the dynamic compression mechanical properties and the constitutive models of 316L stainless steel specimens formed at different scanning speeds. The quasi-static and dynamic compression mechanical properties of SLM316L stainless steel specimens formed at four scanning speeds were tested using an electro-hydraulic servo experimental system and split Hopkinson pressure bar experimental apparatus. Microstructure observation was used to analyze the differences in the mechanical properties of specimens with different forming parameters. Finally, the modified Johnson-Cook (J-C) constitutive model was established and compared with the experimental data to illustrate the applicability of the modified model in describing the dynamic mechanical properties of SLM316l stainless steel. In dynamic compression mechanics experiments, the results show that SLM316L stainless steel specimens exhibit typical viscoplastic characteristics and significant strain rate strengthening effects. Furthermore, the scanning speed significantly affects the stacking characteristics of SLM-formed specimens, and the yield strength in axial compression decreases with the loss of stacking characteristics. Finally, the modified J-C model can accurately describe the mechanical properties of SLM316l stainless steel. This study can provide a theoretical model and methodological support for the design and development of SLM316l stainless steel.

/pdf/dynamic-mechanical-properties-and-constitutive-model-of-ppbx0lyws1.pdf

Yongsheng Ge

Papers

Effects of Heat Treatment on the Microstructure and Mechanical Properties of Selective Laser Melting 316L Stainless Steel

Dynamic Mechanical Properties and Constitutive Model of Selective Laser Melting 316L Stainless Steel at Different Scanning Speeds