Nitriding problems on rolled rings made of 42CrMo4

Even though the existing nitriding processes allow precise parameters control, some components could not achieve sufficient hardening characteristics. The aim of this work is the determination of the main cause of the incorrect nitrided case formation on some large gears made of UNI EN 42CrMo4 steel, realized from rolled rings. Metallographic analyses were conducted on nitrided specimens made of different heats. Moreover different oil-quenching and tempering were performed on partial finished components (supplied in normalized conditions). Several maintenance time and temperature conditions were tested to understand the effects of grain-size and carbides precipitation on the nitrogen diffusion. Both segregation bands and excessive carbide precipitations near the surface lead to low surface hardness and insufficient effective case depth.