Kazuyuki Sakuraya

BN type inclusions formed in high Cr ferritic heat resistant steel

Abstract Scanning electron microscopy observations of fractured steel rod samples were made to clarify the formation and dissolution behaviour of boron nitrides in high Cr ferritic heat resistant steels during heat treatment at high temperature. A large number of coarse size BN type inclusions of 2 to 5 μm were distributed at the bottom of the dimples. They did not dissolve during annealing at 1150°C. However, they had a tendency to begin dissolving and reducing its particle size with time at 1200°C and dissolved completely in a short time at 1250°C. From the chemical analyses of boron and nitrogen in many high Cr heat resistant steels and SEM observations of their fractured samples, critical boron and nitrogen concentrations for the formation of coarse size BN type inclusions of 0·001%B and 0·015%N were found, and the solubility limit of BN was represented as log [%B]=−2·45log [%N]−6·81 at 1150°C.