Carbide coarsening has been investigated in a 12 percent CrMoV steel. Studies in the gauge length (creep exposed) and in the head (stress free ageing) for both interrupted and ruptured creep specimens tested at an initial stress of 80 MPa at various temperatures and at 600°C at different initial stresses have been performed. The measured quantities include hardness, carbide sizes in the tempered martensite, at the former austenite grain boundaries and at the δ-ferrite boundaries as well as mean free distance between carbides, total number of carbides, and carbide size distribution. The contributions to the increase of strain rate in the tertiary creep are discussed in terms of plastic deformation, cavitation accumulation, and microstructural degradation. By using a strain and strain rate relation, expressions to quantitatively account for each contribution to increase of strain rate are given. The increase of strain rate, thereby rupture, is mainly caused by the microstructural degradation due to carbide coarsening and by the cavitation accumulation.

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