PURPOSE: To sharply reduce an abnormal layer consisting of slack quenched structure as well as of intergranular oxides formed on the surface of a steel material and to manufacture a steel material having excellent fatigue strength, by subjecting the steel material surface to boronizing treatment ad further to carburizing treatment.
CONSTITUTION: A steel material to be treated is buried in a solid boronizing agent composed mainly of B4C, which is heated and held at 800°C for 1hr to undergo boronizing treatment. Then the above material is subjected to heating and holding (B) at 930°C for 17hr in a carburizing-gas atmosphere (A) containing 0.35% CO2, to successive heating and holding (C) at 860°C for 1hr, and then to dipping into oil of 100°C to undergo hardening (D) treatment. In this way, the abnormal layer consisting of slack quenched structure and intergranular oxides formed on the steel material surface is sharply reduced, so that steel material having sufficient fatigue strength can be obtained.
ASANO HIYOUE
MORI TAKASHI
