EFFECT OF COBALT ON IMPACT TOUGHNESS OF STEELS

The 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 tho ught to have an effect on the ductile-brittle transition temperature ( DBTT) of maraging steels. The addition of 9% Co to the base alloy plus 2.5% Mo increases the DBTT to 120-degrees-C in unaged material. Simil ar 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 a lso 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 p robably so low that the addition of Co does not seriously embrittle th e steel.