Influence of heat treatment on formation behavior of boron nitride inclusions in P122 heat resistant steel

To clarify the behavior of the formation of boron nitrides in P122 heat resistant steel containing 0.003 mass% B and 0.06 mass% N, the influence of heat treatments, remelting and hot working were investigated by SEM observations on boron nitrides at the fractured surfaces of the steel samples and by the EDS analysis. Boron nitrides start to precipitate at temperatures between 1 150 and 1200 degrees C during the cooling process after hot forging or rolling. They agglomerate to a very large size of about 20 to 30 mu m at a very slow cooling rate of 100 degrees C/h. However, they only grow to 1 to 3 mu m at a medium slow cooling rate and never precipitate at a very fast cooling rate such as in water quenching. The precipitation behavior of boron nitrides has also been found to be affected by the cooling rate after normalizing but not by the microstructure of the steel resulting from casting or forging.