The hardenability of steel and the type of heat treatment determine the hardness variations from the surface to the center. Since the strength of steel is directly related to the hardness, values of ultimate strength, yield point, and fatigue limit can be estimated both for the surface and points below the surface from measuredhardness values. With the strength properties determined, the load at which yielding will begin can be estimated. S-N curves can also be constructed for failure originating at the surface and below the surface. Applications of the method to through hardened, induction hardened, and case hardened machine components are demonstrated.