BN type inclusions formed in high Cr ferritic heat resistant steel

In order to make clear the formation and re-dissolution behavior of boron nitrides in high Cr ferritic heat resistant steels during heat treatment at high temperature, SEM observations of the fractured steel rod samples were carried out. A large number of coarse size BN type inclusions grown to 2 to 5 mum were distributed at bottom of dimples. During annealing at 1150 degreesC, coarse size FIN type inclusions were not dissolved, but at 1200 degreesC coarse size BN type inclusions had a tendency to dissolve reducing its particle size with time and all BN type inclusions were re-dissolved completely in a short time at 1250 degreesC. From the chemical analysis of boron and nitrogen in several high Cr heat resistant steels and SEM observations of its fractured samples, it was found that the critical boron and nitrogen concentration for formation of coarse size BN type inclusions are 0.001% B and 0.015% N, and the equation related [%B] and [%N] is represented as log[%B] = -2.45 log[%N]-6.81.