AbstractThe addition of Co to 18%Ni maraging steels produces an increase in hardness due to a synergistic effect. A similar effect is reported in this paper for maraging steels based on Fe–5Mn–6Ni. However, Co is thought to have an effect on the ductile-brittle transition temperature (DBTT) of mar aging steels. The addition of 9%Co to the base alloy plus 2·5%Mo increases the DBTT to 120°C in unaged material. Similar to reports by other workers on Fe–Co alloys, it is considered that this increase in DBTT is due to Co restricting cross-slip. In contrast, Mo is thought to lower the DBTT by increasing cross-slip. The reason for 2·5%Mo changing the mode of brittle fracture from intergranular to transgranular cleavage in the base alloy is discussed. Mention is also made of the effect of Ni on the DBTT in lath martensite structures. In classical 18%Ni maraging steel, the DBTT of the Fe–18Ni base is probably so low that the addition of Co does not seriously embrittle the steel.MST/1828