Evaluation of the Mechanical Properties of 9Ni-4Co Steel Forgings

Material characterization data and design allowables were determined for two grades of the 9Ni-4Co steel group: 9Ni-4Co-0.30C quenched and tempered to 220-240 ksi tensile ultimate strength, and 9Ni-4Co-0.45C quenched to a bainitic structure with 260 to 280 ksi tensile ultimate strength. The effects of chemistry and processing variations on mechanical properties were evaluated by testing three heats of 9Ni-4Co-0.30C and four heats of 9Ni-4Co-0.45C. Both alloys were vacuum-arc-remelted with carbon deoxidation. Tension, compression, shear, and bearing allowables were developed per MIL-HDBK-5 guidelines in three grain directions at temperatures from -110F to 500F. The 9Ni-4Co-0.30C and 9Ni-4Co-0.45C were found suitable for aircraft structural forgings with minimum tensile ultimate strengths of 220 ksi and 260 ksi respectively. Material characterization data showed that both alloys have good stress corrosion resistance, competitive derived mechanical properties and fatigue properties, and satisfactory metallurgical stability. Grain direction did not significantly influence their mechanical properties. The 9Ni-4Co-0.30C showed superior plane-strain fracture toughness, whereas the 9Ni-4Co-0.45C compared favorably with other steels in the same strength range. Both showed excellent resistance when tested for delayed fracture in salt water. On the basis of the data obtained in this program, a proposed Military Material Specification was drafted for both alloys. (Author).