The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low-carbon steel

Abstract

The complex (B+C) diffusion layers have been formed on chromium- and nickel-based low-carbon steels. Gas boriding applied to these steels that have been previously carburized enables the production of wear-resistant borocarburized layers. After combined surface hardening with boron and carbon in the microstructure two zones have been observed: iron borides (FeB+Fe2B) and carburized zones. The iron borides in borocarburized layer show the tendencies towards lose of the needle-like nature. The depth of iron borides zone depend on pre-carburizing parameters. Although borocarburizing causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. An increase of distance from the surface is accompanied by a gradual decrease of carbon content and microhardness in carburized zone. Probably, the brittleness of borocarburized layer is lower. In the consequence, the frictional wear resistance of borocarburized layers is essentially higher than that obtained in case of only borided layers. There is every indication that the carbon content beneath iron borides has an important effect on wear resistance, too. The highest wear resistance of borocarburized layer has been observed in case of about 1.2% C below iron borides.