Borocarburizing of decarburized gears made from 21NiCrMo2 (AISI 8620) steel

Abstract

In the present study, gears made of 21NiCrMo2 (AISI 8620) steel were subjected to heat treatments including carburizing, decarburizing, boriding and hardening. Carburizing, decarburizing and boriding were performed in a gaseous atmosphere, a salt bath consisting of 60% NaCl and 40% NaCO3, and a liquid medium containing borax and silicon carbide, respectively. Borocarburazing was carried out in two steps: carburizing and then boriding. Some specimens were treated to obtain different carbon concentrations and to observe the effect of carbon content on the boriding process. This process, consisting of carburizing followed by decarburizing and finally boriding, is named borodecarburizing. The microstructures and phase compositions of the diffusion layers were examined by means of X-ray diffractometry, scanning electron microscopy and optical microscopy. The microhardness profiles of these layers were studied by a Vickers indenter. The hardness value obtained by the borodecarburizing process is 10% higher than a borocarburized specimen’s hardness. One-phase iron boride zone (Fe2B) was observed in the layers. This phase is preferred due to its mechanical properties.