TRUE SHORT-TERM CREEP AND LONG-TERM AGEING OF THE 9% CR MARTENSITIC STEEL WITH DIFFERENT KINDS OF STRENGTHENING PHASES

Abstract

The behaviour of the 9% Cr martensitic steels with different kinds of strengthening phases during long-term ageing and short-term creep has been investigated. In the 0.02 wt.% low carbon steel alloyed with tantalum, the main strengthening phase was TaX carbonitrides with an average size of 11 nm and a volume fraction of 0.19%. In the steel with a standard carbon content of 0.1 wt.%, alloyed with niobium and vanadium, the main strengthening phase was M23C6 carbides with an average size of 90 nm and a volume fraction of about 2%, NbX carbonitrides with an average size of 40 nm and VX with an average size of 20 nm with a total volume fraction of 0.25%. During longterm ageing, the transverse size of martensite laths increased by 23% and 5% for 0.02%C-Ta and 0.1C-VNb steels, respectively, that is caused by different resistance to thermal coarsening of the strengthening phases when held at 650 °C, as well as in additional hardening of the 0.1C-VNb steel by Laves phase particles. The behaviour of the steels during creep is also determined by the stability of the strengthening phases to coarsening during deformation. Thus, 0.02%C-Ta steel hardened with TaX demonstrates an increase in time to rupture by 2.4 times and a decrease in the minimum creep rate by 2 orders of magnitude compared to 0.1C-VNb steel hardened with carbides and the Laves phase under the high applied stress. However, as the applied stress decreases, the difference between the steels in time to rupture and the minimum creep rate is disappeared.