Maraging steel grade18Ni300 produced by powder bed fusion (PBF) in its as built condition was plasma nitrided at three different temperatures. The aim of the work was to investigate the impact of the ...nitriding temperature on the microstructural changes as well as on the surface properties such as hardness, wear and corrosion resistance. The microstructural features in the bulk as well as in the nitride layer were investigated using electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-Ray diffraction (XRD) analysis. The bulk microstructure consists of martensite with a small amount of retained austenite, the amount of which increases with a higher nitriding temperature. The nitriding process also causes the formation of precipitates and can therefore also act as an aging treatment. A specific lamellar structure occurs on the surface during the nitriding process, which in the majority of cases consists of the Fe4N phase. The retained austenite also transforms during nitriding to the nitride phase Fe4N. It was found that nitriding at higher temperatures leads to the formation of cracks in the nitride layer. The crack formation is related to nano and micro segregation during the LPBF. These segregations lead to austenite formation, which also takes place along the grain boundaries and transforms during nitriding to Fe4N. Higher nitriding temperatures lead to a thicker nitride compound layer and to better wear resistance. The impact of the cracks on the static mechanical properties is negligible. However, the corrosion resistance is governed by the formation of cracks at higher nitriding temperatures.
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•A higher nitriding temperature increases the amount of austenite and causes cracks.•During nitriding, the austenite on the surface transforms to Fe4N and TiN.•Cracks are related to segregations and transformations of the austenite to nitrides.•A higher wear resistance is obtained at higher nitriding temperatures.•A corrosion enhancement is seen at lower nitriding temperatures.
As a surface-hardening technique, plasma nitriding is a common procedure for improving the properties of conventional Ni-based alloys. The diffusion of nitrogen hardens a layer on the surface of the ...alloy, leading to better wear resistance and a higher coefficient of friction, as well as a higher surface hardness. This study reports the effect of plasma nitriding on additive-manufactured (AM) Inconel 625 (IN625) compared to its conventional manufactured and nitrided counterparts. The samples produced with the laser powder-bed fusion (LPBF) process were subsequently plasma nitrided in the as-built condition, stress-relief annealed at 870 °C and solution treated at 1050 °C. The plasma nitridings were carried out at 430 °C and 500 °C for 15 h. The growth kinetics of the nitride layer of the AM samples depends on the prior heat treatments and is faster in the as-built state due to the specific cellular structure. The lower nitriding temperature leads to the formation of expanded austenite in the nitride layer, while at the higher nitriding temperature, the expanded austenite decomposes and CrN precipitation occurs. The XRD and SEM analyses confirmed the presence of two layers: the surface layer and the diffusion layer beneath. The lower nitriding temperature caused the formation of expanded austenite or a combination of expanded austenite and CrN. The higher nitriding temperature led to the decomposition of the expanded austenite and to the formation/precipitation of CrN. The higher nitriding temperature also decreased the corrosion resistance slightly due to the increased number of precipitated Cr-nitrides. On the other hand, the wear resistance was significantly improved after plasma nitriding and was much less influenced by the nitriding temperature.
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•Nitriding of IN625 alloy causes combination of austenite/expanded austenite and diffusion layer.•Growth kinetics depends on production route, heat treatment, nitriding temperature.•At lower nitriding temperature expanded austenite forms in the nitride layer.•Expanded austenite decomposes at higher nitriding temperature and CrN forms.•Better corrosion performance is obtained at lower nitriding temperature.
Fe-Mn alloys were produced for bioresorbable applications using the laser powder bed fusion (LPBF) process with varying process parameters. The feedstock alloy powder for LPBF was derived from ...conventional cast/forged bars using plasma ultrasonic atomization. Additionally, a conventionally produced Fe-Mn alloy with the same composition was investigated to compare material properties. The influence of the processing route and LPBF process parameters on microstructure evolution, particularly the formation of Σ boundaries, was examined and correlated with the observed corrosion rate obtained from potentiodynamic curves in Hank's solution. The concentration of released Fe and Mn ions after immersion tests in lactic acid was also evaluated. The initial corrosion mechanism of the LPBF alloy was elucidated through X-ray photoelectron spectroscopy (XPS). Furthermore, in vitro tests were conducted using MG-63 human osteoblast-like osteosarcoma cells to assess the biocompatibility response. The present study established a correlation between microstructure and corrosion rate, while the biocompatibility tests affirmed the suitability of additively manufactured Fe-Mn alloys for bioresorbable applications.
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