Detailed transmission and analytical electron microscope studies have been carried out on a series of high strength martensitic 12CrMoV and 12CrMoVNb steels which exhibited sigmoidal creep rupture behaviour wehn tested at temperatures of 500 deg C and above out to durations beyond 100,000 hours. The results indicate that this behaviour is associated with marked softening of the materials, due to microstructural degradation effects which mainly take the form of dissolution of the fine strengthening M_2X and MX phases in conjunction with coarsening of interlath/subgrain and prior sustentie grain boundary M_(23)C_6 precipitates. These processes result in the creep rupture strength of the materials changing during creep eposure from being initially controlled by precipitation strengthening to being denendent in the long term on solid solution hardening, thus accounting for the observed sigmoidal inflexion in their high temperature creep rupture characteristics.
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