Pack-boriding of low alloy steel: microstructure evolution and migration behaviour of alloying elements

Low alloy steel was pack-borided at different processing temperatures (at 850, 950, and 1050°C) and times (2, 4, and 6 h). The microstructural characterisation of boronized steel showed the presence of three zones, namely boronized region containing finer grains and columnar geometry of (Fe, M)2B (where M = Cr, Mn, Mo, and Ni), transition zone, and non-boronized core. The concentrations of the alloying elements in (Fe, M)2B were increased from the surface to the core of the specimen. The pattern of slope variation of boron concentration–depth profile (obtained using GDOES) was linked with the boride morphology and process temperature. Pack-boriding of steel led to the development of systematic trend in slope variation of overall concentration–depth profiles of the alloying elements. The composition and morphology of boride affected the trend of slope variation for the boride-forming alloying elements. However, for Al and Si, the trend of slope variation was connected to the boride morphology and the composition of the matrix. Chemistry of the matrix was strongly dependent on the migration kinetics of the alloying elements during the boride growth. The migration kinetics of Cr, Mn, Mo, and C were found almost equivalent to the rate of boride growth. However, Ni, Al, and Si were migrated at a slower rate. Si showed the lowest migration kinetics among the alloying elements. The concentrations of the alloying elements having higher migration kinetics remained constant in the matrix during the boride growth.