Creep resistant materials invariably undergo microstructural changes during long-term exposure in service at high temperatures that lead to degradation of their mechanical properties. This is reflected in softening of the materials together with reductions in their tensile and creep strengths. In certain circumstances embrittlement may also occur resulting in reduced toughness and increased notch sensitivity. The suitability of particular materials for high temperature applications is consequently limited by their resistance to degradation during service exposure. In this paper the results of quantitative microstructural studies on a series of creep resistant 12CrMoVNb steels which have been exposed for durations up to 100,000 h at temperatures in the range 748-873 K are reported. Changes in their microstructures and the composition of phases such as M23C6 are discussed as a means of assessing the remaining service life of components manufactured from these materials.
Modelling of Microstructural Evolution in Creep Resistant Materials, eds A. Strang and M. McLean, The Institute of Materials, London, 1999, pp. 129-150.