This memorandum briefly discusses the physical metallurgy, heat treatment, mechanical properties, stress-corrosion properties, and fabrication of the 9Ni-4Co-XC steels. This family of steels was developed specifically to meet the requirements of the aircraft and aerospace industries for materials possessing high strength and high toughness. By varying the composition and heat treatment, the 9Ni-4Co-xC steels can develop strengths in the 200 to 300 ksi tensile-strength range and reportedly possess high toughness. The lower carbon grades have excellent weldability, developing strength in the 180 to 200 ksi range with high toughness without the need for preheat or postweld-heat treatment. (Author).