A Discussion of the Effect of Cobalt on the Ductile-to-Brittle Transition Temperature of Ultra-High Strength Steels

The literature on the effects of alloying elements on the ductile-to-brittle transition temperature of ferritic-pearlitic steels and martensitic steels will be reviewed with an emphasis on cobalt.   The literature strongly suggests that cobalt additions to these steels results in an increase in the ductile-to-brittle-transition temperature while nickel additions result in a decrease in the ductile-to-brittle transition temperature.  The toughness of several ultra-high strength  steels which contain significant amounts of cobalt but either no nickel or low amounts of nickel will be discussed.  All of these steels are characterized by very low toughness at room temperature.  While several steels noted for their high toughness such as the maraging steels and the secondary hardening steel HY180 and its higher strength modifications contain large amounts of cobalt all of these steels also contain large amounts of nickel which seems to reduce the detrimental effect of cobalt.  Approaches, other than the addition of nickel, to lowering the ductile-to-brittle transition temperature of ultra-high strength steels containing cobalt  will be discussed.