Microstructure and properties of composite (B + C) diffusion layers on low-carbon steel

Combined surface hardening with boron and carbon was used for low-carbon 5120 steel. The microstructure, carbon profiles and chosen properties of borided layers produced on the carburized 5120 steel have been examined. These composite (B + C) layers are termed borocarburized layers. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB + Fe2B) and a carburized layer. It has been found the depth (100–125 μm) and microhardness (1500–1900 HV) of iron borides zone. The carbon content (0.83–1.46 wt pct) and microhardness (950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. An increase of distance from the surface is accompanied by a decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of composite layers (B + C) on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be greater in comparison with that for only borided or only carburized layers.