Nitriding problems on rolled rings made of 42CrMo4

Abstract

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.