A numerical-experimental study of AISI 316L borided steels under cyclic contact loading

AISI 316L borided steels under cyclic contact loading were evaluated. Boriding was carried out by two powder-pack processes: the continuous and interrupted processes. Boriding processes were developed at 900 °C for 1 h (continuous process) and 4 h (interrupted process). A monolayer mainly formed by Fe₂B was obtained by the interrupted boriding with a thickness of 6.09 ± 0.31 μm and a surface hardness of 22.3 ± 1.4 GPa. In contrast, a FeB-Fe₂B bilayer was formed by the continuous boriding with a total thickness of 13.08 ± 0.39 μm and a surface hardness of 26.3 ± 2.1 GPa. Adhesion tests were developed, in which the monolayer showed better adhesion than that the bilayer. Cyclic contact tests were carried out by loading a ball on the borided steel surface. Circular cracks because of the applied critical load (monotonic load) were observed on the borided steel surface. Subcritical loads were applied to the borided steels, where cohesive damage was caused in the bilayer, whereas no damage was observed in the monolayer at low subcritical loads. In contrast, circular cracks were observed in both treatment conditions at high subcritical loads. In addition, the high percentage of the FeB phase caused a more severe damage upon the bilayer system. Additionally, the thinner thickness of the boride layer, as well as the presence of compressive residual stresses (−0.75 GPa) in the monolayer system, caused a decrease on the maximum principal stress (2.4 GPa at the highest applied load) caused by contact loading compared to bilayer system (5.2 GPa at the highest applied load). Therefore, the interrupted boriding (monolayer system) showed a better mechanical resistance under cyclic contact loads than the continuous boriding (bilayer system).