Enhanced surface hardening of AISI D2 steel by atomic attrition during ion nitriding

• AISI D2 steel was nitrided by using atomic attrition in a two step nitriding process.
• Surface hardness after two step nitriding increased up to about 1500 HV.
• Atomic attrition resulted in inhomogeneous recrystallization at the surface region.
• It also led to higher nitrogen concentration and more chromium nitrides at the surface.
• Those factors induced enhanced surface hardness of AISI D2 steel.

An experimental study was conducted in order to investigate the surface hardness enhancement of AISI D2 steel, by ion nitriding through atomic attrition. Following a conventional ion nitriding process of 120 min under an atmosphere of H2 and N2, an atomic attrition process was applied to the surface of AISI D2 steel under a gas mixture of Ar and N2 for 360 min. A conventional ion nitriding of 480 min was also performed for the purposes of comparison. The results obtained demonstrate that the surface hardness of AISI D2 steel undergoing ion nitriding and atomic attrition was about 1500 HV; which is the highest surface hardness among all the specimens tested, and almost 250 HV higher than that of a conventionally ion nitrided specimen. After nitriding by atomic attrition, those grains located along grain boundaries and in the vicinity of precipitates in the surface region were recrystallized, which is believed to be caused by the stress field formed between grains and different phases. In addition, the specimen that was ion-nitrided by atomic attrition exhibited a higher nitrogen concentration and a consequently greater quantity of chromium nitrides at the surface region, when compared to a single nitrided specimen. This could be attributed to the sputtering characteristics of argon in the plasma nitriding. The refined grains, higher nitrogen concentration, and greater amount of chromium nitrides all represent significant factors in the enhanced surface hardening of AISI D2 steel.