Interrupted boriding of medium-carbon steels

The results of an extensive study on the microstructure, microhardness, corrosion, and tensile properties of continuously borided and interrupted borided specimens of medium-carbon steel are compared. Carbon repartitioning away from the surface is one of the principal modes to accommodate the high strains introduced on boron diffusion into the case. However, this is a kinetically constrained process and is more predominant on interrupted boriding. The effect of such a carbon redistribution is to result in microstructural modifications including (1) blunting of boride needle tips, (2) precipitation of nearly spherical and fine borocarbides, and (3) enhanced carbon segregation at the boride needle/steel matrix interface on interrupted boriding. The mechanisms aiding the change in the morphology of the boride needles are discussed. The improvements in the mechanical and corrosion properties of the interrupted borided specimens over continuously borided specimens are described.