Influence of process duration on structure and chemistry of borided low carbon steel

In this study, we employed an ultra-fast boriding technique to grow hard boride layers on low carbon steel substrates using an induction furnace at 900 °C. The technique utilizes an electrochemical cell in which it is possible to achieve very thick (i.e., about 90 μm thick) boride layers in about 30 min. The effects of process duration on boride layer thickness, composition, and structural morphology were investigated using microscopic and X-ray diffraction (XRD) methods. We also developed an empirical equation for the growth rate of boride layers. XRD results revealed two principal boride phases: FeB and Fe2B thickness of which was very dependent on the process duration. For example, Fe2B phase was more dominant during shorter boriding times (i.e., up to 15 min.) but FeB became much more pronounced at much longer durations. The growth rate of total boride layer was nearly linear up to 30 min of treatment. However during much longer process duration, the growth rate assumed a somewhat parabolic character that could be expressed as d = 1.4904 (t)0.5 + 11.712), where d (in μm) is the growth rate, t (in s) is duration. The mechanical characterization of the borided surfaces in plane and in cross-sections has confirmed hardness values as high 19 GPa at or near the borided surface (where FeB phase is present). However, the hardness gradually decreased to 14 to 16 GPa levels in the region where Fe2B phase was found.