Transfer from M3B2 boride to BN nitride in 9Cr3W3CoB martensitic heat-resistant steel

Abstract

Abstract Boron is generally added into heat-resistant steel to improve the creep strength. The existence of M3B2 borides and BN nitrides should be avoided due to the consumption of effective B. Before eliminating the harmful borides, it is important to elucidate their thermodynamic behavior during aging. Therefore, the formation of BN nitride in a 9Cr3W3CoB steel during high temperature aging was investigated. It was found that majority of B was presented as M3B2 borides in the as-treated steel. Due to the low content of N and the existence of M3B2 borides, only a few BN nitrides were distributed along PAGBs. Metastable M3B2 borides would gradually dissolve during isothermal exposure and then lead to the precipitation of BN nitride, which has been verified by employing thermo-mechanical treatment (TMT) to eliminate M3B2 borides. It was revealed under scanning electron microscopy (SEM) that BN could not precipitate during high temperature aging in the TMT sample without M3B2 borides. The nucleation and growth of BN nitride was thermodynamic processes affected by both aging time and temperature. Its number density and size increased with increasing aging temperature. During aging at 800\xa0°C, BN nitride nucleated quickly and the shape evolved from subsphaeroidal to irregular even with sharp corners. Unfortunately, the BN particles with sharp corners were detrimental to toughness.