Experimental analysis of bending fatigue strength of case hardened steel specimens for gear manufacturing

ISO 6336 Standard allows designers to use fatigue data obtained from plain and notched specimens to design gears. This work presents the mechanical properties obtained from specimens of case-hardened steels commonly used to manufacture gears for high power transmissions. Static tests showed the beneficial effects of case-hardening treatment, that leads to an increase of ultimate tensile strength by a factor of three. Both axial and four-point plane bending fatigue tests under R=0.1 stress ratio are presented. Fatigue results show that the high-cycle fatigue knee is located approximately at 2E5 cycles for all specimens’ geometries. This value is much lower than standardized curves reported in ISO 6336 Standard. Notch sensitivity was experimentally evaluated for a notch radius of 0.8 mm. Attention was paid to material microstructure and hardness. Fracture surfaces were observed and compared among different load type and magnitude.