The influence of alloying element additions on the boriding of steels

Abstract

The influence of the alloying elements carbon (0%–2%), nickel (0%–14%) and chromium (0%–26%) on the boriding characteristics of steels was systematically studied using a new boriding technique on laboratory-cast binary and ternary steels and on industrial steels. The concentration profiles, phases (structure and texture) and microstructures were determined by glow discharge optical spectroscopy, X-ray analysis and classical metallographic techniques. In high concentrations, all three elements tend to favour the formation of relatively flat boride layers, even when these layers are highly textured, in contrast with the single-phase acicular Fe 2B layer obtained on low carbon steels. The concentration profiles developed during boriding revealed different behaviours: carbon segregates towards the matrix, nickel segregates towards the surface, whereas chromium is scarcely affected. The segregation of nickel to the surface at high boron activities severely hinders the successful boriding of highly alloyed steels, e.g. austenitic stainless steels. It is shown, however, that low boron activity gaseous phase boriding can produce good quality homogeneous Fe 2B layers with reduced nickel segregation.