For plants with ultra critical steam conditions with live steam temperatures up to 720°C and pressure up to 300 bars optimized materials are required. These are a ferritic-bainitic material (T24) for applications up to 550°C, modified 9 Cr steels (E911 and P92), a new Co-alloyed martensitic 12 Cr steel for usage up to 630°C and Nickel based alloys (Alloy 617) for temperatures above 650°C. Experimental work has been done to create a reliable data base for design and inspection. Special emphasis was put on long term creep characteristics of base material with specific consideration of cross welds, microstructural investigations with regard to optimization of chemical composition and heat treatment and numerical modeling.

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