Effect of nickel and cobalt on the properties of medium-carbon high-strength steel

1. To obtain a martensitic structure with quenching in oil (cooling rate 20–30 deg/sec) for steel containing 0.3% C and 4% Co the nickel content should be at least 9%. 2. With increasing nickel concentrations from 5 to 9% in the thermally hardened steel containing ∼0.3% C and ∼4% Co the standard mechanical properties change little (σb, σ0.2, δ5, θ), and there is no change in the work of crack propagationap. The principal effect of nickel in these steels, as in other structural steels, is an increase of resistance to brittle fracture, manifest in a lower ductile-brittle temperature. 3. The addition of cobalt to the steel with ∼0.3% C and 9% Ni increases the strength with no substantial reduction of the ductility or toughness, and with a substantial drop of the ductile-brittle temperature. This effect is evident with a change of the cobalt content from 0 to 4%; a larger amount of cobalt has a negative effect on the ductility.