Fatigue damage assessment from cyclic spherical contact of borided and nitrided H13 steel

In this study, boride and nitride layers formed on H13 steel were assessed by cyclic spherical contacts. Boriding was conducted at 800 °C for 1 and 5 h, and nitriding at 580 °C for 1 h. The hardness and strain hardening exponents n of the layers were estimated by spherical indentations (5 µm radius). The standing contact fatigue (SCF) performance was investigated by cyclic spherical contacts (Al₂O₃, 3 mm diameter), first applying monotonic load tests from 300 to 1000 N to establish the critical loads considering the onset of certain cracks for each layer; later, between 10² and 10⁵ cycles applying different subcritical loads. The results revealed strain hardening exponents of n_B1h = 0.226, n_B5h = 0.230 and n_N1h = 0.241. In SCF, a critical load of 600 N in both layers was found. In cyclic tests, the thicker boride layer presented larger cracking and moderate spallations, attributed to its mechanical properties; the thinner layer exhibited a better SCF resistance. Regarding nitriding, well-defined circumferential cracks without spallations were observed.