Microstructure and Wear Behaviour of 38CrMoAl Steel Laser-Processed with the Addition of MoS2 or WS2
R. Zhou, Z.P. Tong, G.F. Sun, Z.H. Ni and W. Zhang
Wear resistant composite layers were fabricated on 38CrMoAl steel by laser processing with NiCr/Cr3C2-CeO2 powders with addition of MoS2 or WS2. The composite layers were heat treated at 150 and 300°C (MoS2), and 300 and 600°C (WS2). Microstructure, phases, microhardness, wear behaviour at room temperature and crack behaviour under loading of the laser processed and heat treated layers were investigated. Results indicate that undissolved MoS2 or WS2 were observed in all the layers. Heat treatment changed the microstructure and phases of all the laser processed layers. When the original majority phase of the laser processed layer was martensite, volume fraction of retained austenite increased after heat treatment; while when the original majority phase was austenite, volume fraction of retained austenite decreased after heat treatment. Full width at half maximum (FWHM) of martensite peaks in the laser processed layer with addition of 20 wt.% MoS2 decreased with heat treatment temperatures, indicating reduced defects in the heat treated layers. Laser processed layers with addition of 10 wt.% MoS2, 10 wt.% or 20 wt.% WS2 without heat treatment showed the best wear resistance while that with addition of 20 wt.% MoS2 showed the best wear resistance after heat treatment at 300°C. The wear mechanism of all the layers is abrasive, adhesive and oxidative wear, accompanied by the formation and removal of oxide film and sulphide containing tribofilms. The oxide film and sulphide lubricating phases formed on the worn surface during sliding wear play an important role in reducing friction and wear loss. Un-dissolved MoS2/WS2 and crack behaviour in case of loading is closely related to the microstructure of the pressed matrix.
Keywords: Fibre laser, steal, coatings, heat treatment, oxidation, inorganic, composites, metallic matrix, microstructure, corrosion, wear, microhardness