Steels such as P91 and P92 (9Cr martensitic steels) with higher creep strength were initially developed for use in the newer ultra supercritical power plants at temperatures above about 1050°F (570°C). However, soon these steels became popular materials as replacement components for conventional power plant and for HRSGs (Heat Recovery Steam Generators) operating at relatively lower temperatures. The main attraction was in terms of their lower weight per unit volume thus reducing construction, transportation and welding costs. Furthermore, due to privatization and restructuring of the electricity industry many of the existing power plants are now required to operate in cycling mode and this requires the use of materials with high resistance to thermal fatigue. Here high strength steels offer an additional benefit in that the lower section thickness reduces the level of through wall temperature gradients in thick section components. Because of this envisaged additional benefit a number of operators/owners of the existing plant have been replacing some of the ageing components in their plant with those made from these higher strength steels. For the HRSGs, there is a requirement to produce more compact units and here the higher strength steels are used to make smaller size components. This paper discusses these issues and the perceived benefits with the actual plant and more recent R&D experience. It further refers to the potential future techniques for damage detection and life estimation.
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