Coarse Size BN Type Inclusions formed in Boron Bearing High Cr Ferritic Heat Resistant Steel Free

This study investigates the behavior of boron nitride (BN) inclusions in high-chromium ferritic heat-resistant steels like P92 and P122. Boron is added to improve creep resistance, but its role is not fully understood. Here, the formation and dissolution of BN inclusions during high-temperature heat treatment were examined. Microscopic analysis revealed coarse BN inclusions (2-5 μm) alongside smaller alumina inclusions. Annealing experiments showed that these BN inclusions only dissolved at temperatures exceeding 1200°C, suggesting they form during casting or forging processes below 1150°C. Chemical analysis identified a critical boron and nitrogen concentration threshold (below 0.001% B and 0.015% N) for BN inclusion formation.