Fatigue fracture of surface-modified layers in 18CrNiMo7-6 carburized steel

•The fatigue performance and fracture of the gradient carburizing layer was analyzed.•The fracture mode changed from brittle fracture to ductile–brittle mixed fracture as the depth of the modified layer increased.•The fatigue fracture varies with stress amplitude in the same modified layer depth.•The fatigue fracture and tensile fracture of the modified layer with different depth were compared.