Some aspects of plant and research experience in the use of new high strength martensitic steel P91

Due to privatisation and ensuing competition coupled with environmental requirements there has been a quest for higher output ultra supercritical power plants operating at higher temperatures and pressures. Steels with better creep resistance are therefore required. Competition in the electricity industry has also meant more frequent operation of power plant in cyclic mode and the need to reduce damage to components due to ensuing thermal fatigue. Here high strength steels can offer an additional benefit in that the reduced section thickness increases pipework flexibility and reduces the level of through wall temperature gradient in thick section components. Because of this envisaged benefit a number of operators/owners of existing plant are substituting these new higher strength steels for the older materials, especially when a plant is moved from base load to cyclic operation. The use of new higher strength steels also has benefits for the manufacturers as the transportation, erection, welding and such other costs are decreased for reduced thickness components made from these steels. This equally applies to petrochemical and other high-temperature plant. 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 turbine (CCGT) plants there is a requirement to produce compact size units and thus high strength steels are used to make smaller size components.This paper discusses the above issues and compares some of the perceived benefits with the actual plant experience and new R&D findings. The paper is based on a comprehensive study carried out by European Technology Development of the worldwide plant experience and synthesis of the R&D findings [Shibli IA, Robertson DG, Review of the use of new high strength steels in conventional and HRSG boilers: R&D and plant experience, ETD Report—No: 1045-gsp-40, October 2006].