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 10Cr martensitic compositions for use at temperatures of 620°C and above with creep properties superior to steel 92 (9Cr, 0·5Mo, and 2W), 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.
A near-eutectoid lower-carbon vanadium-alloyed steel has been under consideration for bearing steel components where improved toughness, and therefore better micro-defect tolerance, is required. Due to the steel’s relatively lower concentration of carbon—that is, compared with a typical approximately 1 wt.% C through-hardened bearing steel, the hardenability was found to be higher. The steel design and heat treatment concepts have been investigated experimentally using, for example, dilatometry and transmission electron microscopy characterizing small-scale lab melts. The novel steel has been designed with a particular focus on continuous casting as a suitable process.
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