Abstract
Austenitic stainless steels produced by laser powder bed fusion (L-PBF) are quite interesting materials owing to their specific microstructure consisting of dendrite walls built of dislocations pinned by many nano-oxides that involves significant strengthening without loss of ductility. In this work, different plasma treatments were performed to harden the surface of 316 L steel manufactured by L-PBF. The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), light microscopy (LM) and micro-hardness tests. The experimental results show that all the plasma treatments enhance the hardness of the surface because a C-enriched layer of austenite (S-phase) forms with a thickness up to 25 μm. The plasma gas mixture, consisting of 2.5% (CH4) + 97.5% (H2), resulted in being the most effective and produced a surface hardness (547 ± 27 HV) more than double with respect to that of the untreated material. The treatment temperature was 475 °C, which represents a good compromise between the necessity to avoid the precipitation of M23C6 carbides and the compatibility of treatment time with the industrial practice. Moreover, it has been observed that a 2 μm-thick over-layer of amorphous C forms on the sample surface. The hardness of such over-layer, which depends on the specific treatment and is related to the degree of topological disorder, is generally greater than that of S-phase. The work demonstrates that plasma carburizing is quite effective in hardening the surface of 316 L steel manufactured by L-PBF and further improves its mechanical properties, which are basically superior to those of the same material prepared by conventional processes.
Highlights
IntroductionIn spite of recent improvements achieved by additive manufacturing [1], low hardness and wear resistance still represent an intrinsic limit to the different applications
In the sample top view (Figure 4a), the laser tracks rotated of 90◦ during building process are clearly visible, while the melting pools can be observed in the cross-section (Figure 4b)
All the plasma treatments enhance the hardness of the 316 L steel, owing to the formation of a C-enriched layer of austenite (S-phase) with thickness ranging from 20 to 25 μm
Summary
In spite of recent improvements achieved by additive manufacturing [1], low hardness and wear resistance still represent an intrinsic limit to the different applications For these materials, conventional thermo-chemical surface treatments, such as carburizing, nitriding and carbo-nitriding, are not helpful. Since these treatments are usually performed at temperatures higher than 550 ◦ C, they induce the precipitation of Cr carbides with consequent Cr depletion of the surrounding matrix, the improvement of hardness takes place with detrimental effects on corrosion resistance.
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