Effects of Boron and Nitrogen on Graphitization and Hardenability in 0.53%C Steels

Abstract

The effects of B and N on the graphitization and the hardenability in 0.53 mass% C steels were investigated in order to utilize both effects of BN and free B on the properties of machine structural steels. The number of BN particles after normalizing heat treatment is increased by the increase in the amount of B or the decrease in N. In the steel that has lower content of N, BN can be resolved and finely precipitates during normalizing. BN provides the preferred site for graphite nucleation, so the distribution of graphite particles is very similar to that of BN prior to graphitization. The growth rate of graphite particle is controlled by diffusion of carbon in ferrite matrix, and the time required for completion of the graphitization is the function of the radius of graphite particles after the end of graphitization, which depended upon the number of graphite particles. The hardenability of the steels can be estimated by the amount of sol. B at heating temperature prior to quenching. Sol. B of 6 ppm or more is required to obtain full martensitic microstructure. The preferable graphite distribution and the enough hardenability of the steels can be simultaneously achieved by optimizing the heating process and the amount of boron and nitrogen.