N-Infiltration of Surfaces of Pre-Carburized 20CrMnTi Steel in Three Nitriding Techniques

The pre-carburized 20 CrMnTi steel surfaces were nitrided in resistive-, induction-and ion-heating in vacuum. The influence of the pre-caburization and itriding methods on the microstructures, phase-structures and tribological properties of 20 CrMnTi steel was investigated with conventional surface and mechanical probes. The results show that the pre-caburization combined with ion heating nitriding had a major impact. For example, the carburized/nitrided surface comprised the Fe₄N, N-containing ferrite and tempered sorbate in top-down order; the 30 μm thick, compact N-infiltration layer significantly improved the hardness, wear-resistance and brittleness. Formation of soft residual austenite, distributed in martensitic matrix greatly reduced the brittleness of the nitrided layer; and when the martensitic matrix, formed in carburization and quenching, changed into tempered sorbate in ion-heating nitriding, the vacancies, left by release of carbide from α-Fe phase, were was favorable for diffusion/adsorption of N₂ and considerably shortened the N-infiltration time.