Effect of additional boron amount on surface roughness after lathe turning in h-BN dispersed type 304 stainless steels

The authors have reported that the machinability (lathe turning, drilling and sawing) of SUS304 (Type 304) austenitic stainless steel is improved by precipitating hexagonal boron nitride (h-BN). In this study, we investigated the surface roughness after lathe turning in the h-BN precipitated SUS304 steels, where the nitrogen content was fixed at 0.2% and the boron content was changed as 0, 0.03, 0.05, and 0.1% (mass%). Surface roughness increased as the boron content increased, especially at a lower cutting speed of 22 m/min. However, SUS304 steel with boron content of up to 0.05 mass% had similar or smaller surface roughness compared with a commercial SUS304 steel, and the SUS304 steel containing 0.1 mass% boron had smaller surface roughness compared with a SUS303 (Type 303) sulfur-added free-cutting steel. Precipitation of h-BN increased the size of the build-up edge and increased the surface roughness. On the other hand, solute nitrogen increased the Vickers hardness, leading to decreased separation size of the build-up edge and decreased surfaced roughness.