Effect of Micro-Phases on the Machinability of Titanium, Niobium Added Case-Hardening Steel

High-temperature case-hardening steels using Ti or Nb have been developed for the purpose of minimizing the carburizing time. Case-hardening steels require high machinability as well as grain growth resistance. In this paper, the effect of micro-phases on the machinability of case-hardening steels containing Ti or/and Nb is studied. Following results are obtained.1) Ti and Nb form carbo-nitrides such as Ti(C, N) and Nb(C, N) in the case of high-temperature case-hardening steels. In Ti added steels, MnS decreases and carbo-sulfides increase as the nitrogen content decreases.2) Carbo-nitrides and carbo-sulfides that exist in Ti or/and Nb added steel do not influence machinability(cutting resistance). Only MnS influences the machinability of Ti or/and Nb added steels. MnS decreases the coefficient of friction and improves lubricity between tools and chips.3) The machinability performance of Ti added case-hardening steels decreases as a result of losing the effect of lowering the friction since the sulfur compound may be changed from MnS into Ti4C2S2.