Optimum hardness for maximum fatigue strength of steel

Observations relating to an optimum hardness for maximum fatigue strength of steels are reviewed. Data are presented to illustrate the influence of hardness on the mechanical behavior of SAE 1045 steel. Suggested causes for decreased strength and ductility at high hardness are discussed, and the idea is advanced that a transition from shear to tensile governed failure takes place as hardness increases. Resistance to shear stress is envisioned as increasing with hardness while the resistance to tensile stress decreases causing a sharp drop in ductility and strength. This view of the fatigue resistance of hard steels suggests ways to improve performance of hard steels permitting the full strength of potential to be realized