All mechanical properties of powder metallurgy (P/M) materials, including sintered nickel steels, are direct functions of sintered density or its inverse, porosity. Mechanical properties, such as tensile strength, ductility, impact strength, and fatigue limit, are dependent on sintered density, alloy content, and heat treatment. Other properties, such as the modulus of elasticity and Poisson’s ratio, are solely functions of sintered density. Sintered nickel steel with less than 8% porosity can be case carburized. Case hardening improves the smooth and notched fatigue limit of high density sintered nickel steels.