The growth of single Fe2B phase on low carbon steel via phase homogenization in electrochemical boriding (PHEB)

Abstract

In this study, we introduce a new electrochemical boriding method that results in the formation of a single-phase Fe2B layer on low carbon steel substrates. Although FeB phase is much harder and more common than Fe2B in all types of boriding operations, it has very poor fracture toughness; hence, it can fracture or delaminate easily from the surface under high normal or tangential loading. We call the new method “phase homogenization in electrochemical boriding” (PHEB), in which carbon steel samples undergo electrochemical boriding for about 15min at 950°C in a molten electrolyte consisting of 90% borax and 10% sodium carbonate, then after the electrical power to the electrodes is stopped, the samples are left in the bath for an additional 45min without any polarization. The typical current density during the electrochemical boriding is about 200mA/cm2. The total original thickness of the resultant boride layer after 15min boriding was about 60μm (consisting of 20μm FeB layer and 40μm Fe2B layer); however, during the additional phase homogenization period of 45min, the thickness of the boride layer increased to 75μm and consisted of only Fe2B phase, as confirmed by glancing-angle x-ray diffraction and scanning electron microscopy in backscattering mode. The microscopic characterization of the boride layers revealed a dense, homogeneous, thick boride layer with microhardness of about 16GPa. The fracture behavior and adhesion of the boride layer were evaluated by the Daimler-Benz Rockwell C test and found to be excellent, i.e., consistent with an HF1 rating.