On the Cobalt

Abstract

Recently, novel martensitic creep resistant steels strengthened by slowly coarsening Laves phase or stable M23C6 precipitates have been identified both computationally and experimentally. The coarsening kinetics of these precipitates, traditionally considered to be very detrimental in creep steels, can be suppressed to a degree which makes them attractive strengthening factors by alloying such steels to high Cobalt levels. As high Co levels are undesirable for various reasons, in the present work, the characteristics of Laves phase and M23C6, in particular the volume fraction, coarsening rate, and precipitation strengthening factor, in newly designed alloys are computationally compared with those of existing Co-containing creep steels. The binary analyses of Co–M balance show that the Co–W levels are highly coupled for creep steels strengthened by Laves phase deposits and W can be used to partially replace Co to yield the same precipitation strengthening. For the M23C6 strengthened alloys, irrespective of the Cr level, a high Co concentration is predicted to be necessary for a high creep resistance.