COMPARATIVE STUDY BETWEEN DIRECT AND INTERRUPTED BORIDING OF C35 STEEL IN A SOLID MEDIUM

ABSTRACT: This study investigates the influence of boriding mode-direct and interrupted- on the microstructure, microhardness, and corrosion properties of C35 steel treated in a solid medium. Boriding treatments were conducted at temperature of 900 °C under continuous (direct) and cyclic (interrupted) conditions. The resulting boride layers were systematically characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers microhardness testing, and corrosion rate established by weight loss measurment. Microstructural analysis revealed that direct boriding leads to the formation of a typical bilayer structure composed of an outer FeB phase and an inner Fe₂B layer with a pronounced saw-tooth morphology. In contrast, interrupted boriding, promoting the formation of a dense, uniform, and predominantly Fe₂B monophase coating. Microhardness measurements showed that while direct boriding achieves higher surface hardness values (up to ~2200 HV) due to FeB formation. Conversely, interrupted boriding yields slightly lower hardness (about 1800 HV) but significantly improved corrosion resistance. This improvement is attributed to reduced residual stresses and enhanced microstructural homogeneity.