Abstract
Martensitic steel P91 with higher creep strength was first introduced as thick section components in power plants some 18 years ago. However, more recently a number of failures have been experienced in both thick and thin section components and this has given rise to re-appraisal of this steel. Thick section components are generally known to have failed due to Type IV cracking. Furthermore, due to the restructuring of the electricity industry worldwide many of the existing steam plant are now required to operate in cycling mode and this requires the use of materials with high resistance to thermal fatigue. Here high strength P91 is assumed to offer an additional benefit in that the reduced section thickness increases pipework flexibility and reduces the level of through wall temperature gradients in thick section components. Because of this envisaged benefit a number of operators/owners of the existing plant, especially in the UK, have been substituting these new higher strength steels for the older materials, especially when a plant is moved from base load to cyclic operation. There has also been a perceived advantage of higher steam side oxidation resistance of superheater tubes made from high Cr steels. For the Heat Recovery Steam Generators (HRSGs) used in Combined Cycle Gas Turbines (CCGTs) there is a requirement to produce compact size units and thus high strength steels are used to make smaller size components. This paper discusses these issues and compares the envisaged benefits with the actual plant experience and more recent R&D findings. In view of these incidents of cracking and failures it is important to develop life assessment tools for components made from P91 steel. ETD has been working on this through a ‘multi-client project' and this aspect will be discussed in this paper.
