Nitriding problems on rolled rings made of 42CrMo4

Even though the existing nitriding processes allow precise parameters control, some components could notachieve sufficient hardening characteristics. The aim of this work is the determination of the main cause ofthe incorrect nitrided case formation on some large gears made of UNI EN 42CrMo4 steel, realized from rolledrings. Metallographic analyses were conducted on nitrided specimens made of different heats. Moreoverdifferent oil-quenching and tempering were performed on partial finished components (supplied in normalizedconditions). Several maintenance time and temperature conditions were tested to understand the effects ofgrain-size and carbides precipitation on the nitrogen diffusion. Both segregation bands and excessive carbideprecipitations near the surface lead to low surface hardness and insufficient effective case depth