Nitrogen alloyed 9-12% chromium steels for high temperature applications

Worldwide advanced steam and gas turbines are currently being designed to operate at increased temperature and/or pressure in order to improve the thermodynamic efficiency. To meet the demand of the advanced conditions new ferritic steels with improved creep strength and improved high temperature strength must be developed. 9-12% chromium steels with high nitrogen and reduced carbon contents turn out to be very promising candidate materials. Fundamental knowledge about the nitriding process of 9-12% chromium steels are presented in the first part of this work. Alloys with nitrogen contents up to 0.8% were produced in pressure-ESR-facilities or in a pressure-induction-furnace. The mechanical properties, especially the ratio of strength to toughness, may be distinctly influenced by the addition of nitrogen to 9-12% chromium alloys with reduced carbon contents. It has been shown that for special properties of nitrogen alloyed steels the way of heat treatments is very important. The toughness of these steels is significantly influenced by the solution temperature. At low solution temperatures the nitrogen-steels show higher toughness values than the conventional carbon-steel X20CrMoV121, at high solution temperatures the toughness of both type of steel is almost the same. At high temperatures nitrogen-steels offer significant advantages. At operating temperatures of 600oC for components in steam turbines high creep rupture properties are desired. At solution treatments around 1150oC and annealing treatments at 750oC the nitrogen alloyed 9-12% chromium steels prove high creep rupture strength and significantly exceeded those of the carbon-steels. (author) 54 figs., 13 tabs., 116 refs