Abstract
The drive to increase the efficiency of fossil fired power generation to reduce CO2 emissions and to conserve energy resources has led Tata Steel to design new 10%Cr martensitic compositions for use at temperatures of 620°C and above with creep properties superior to steel 92 (9%Cr, 0·5%Mo, and 2%W), which is the best currently available steel of this type. In the new alloys, the chromium content was set at 10% to ensure the required oxidation resistance. The long term creep performance of steels with this level of chromium has been limited by the precipitation of Z phase nitrogen rich particles. These form at the expense of vanadium rich MN precipitates that are vital for long term creep strength. Compositions have been designed with the aim of suppressing or delaying Z phase formation. The principles underlying the new steel chemistries are discussed, and a detailed explanation is given of the role of heat treatment in optimising creep rupture strength at temperatures in excess of 600°C. The new steel compositions are based on published work supplemented by thermodynamic calculations to optimise the heat treatment cycle, to produce a fully martensitic microstructure and to maximise the stable nanoprecipitates that are essential to maximise creep rupture life.
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