Effect of the plasma nitriding on the mechanical properties of the 18Ni300 steel obtained by selective laser melting

Abstract

Selective laser melting is an additive manufacturing technique used to build complex parts at reduced cost and time with the same mechanical properties obtained by conventional methods, where post-heat treatments such as solution and thermal aging are commonly used to improve the hardness and wear resistance. In this paper, the effect of the plasma nitriding time on the mechanical properties of the 18Ni300 steel obtained by selective laser melting is investigated. Samples with 30 mm diameter and 10 mm thickness were built by selective laser melting with 99.94 % relative density and nitrided by 1, 2 and 5 h with a reactive gas mixture composed by nitrogen and argon. Samples were evaluated by X-ray diffraction, rugosimeter, pin-on-disc sliding wear test, Vickers microhardness test, field emission scanning electron microscopy, energy dispersive spectroscopy and optical microscopy. Results show that the non-nitriding sample is composed of martensite and retained austenite, and the surfaces of the nitride samples are mainly composed of nitrogen-based phases, such as γ′-Fe4N, ε-Fe3N and expanded martensite. Microscopy techniques show the presence of the white layer and diffusion zone for the nitrited samples, and characteristic microstructures for samples obtained by selective laser melting. The tribological investigations, after plasma nitriding, show the increase of the surface hardness and decrease of the wear rate for values similar to those obtained by traditional methods. The hardness profiles of the nitrided samples, which were investigated with 180 μm depth, present higher values when compared with the non-treated steel. Among the samples studied, 2 h treatment presented the best mechanical properties, ensuring the combination of the SLM and plasma nitriding as an alternative technique for production complex aged parts.