With the desire for higher operating temperatures and pressures to improve the thermal efficiency of new power generating plant there have been significant changes in the materials used. For operation up to 620°C, a new range of ferritic steels with 9-13%Cr has been developed. With proper control of composition and heat treatment these materials, including Grades 91 and 92,exhibit predominantly martensitic microstructures and a good balance between strength and ductility. However, fabrication processes such as welding and bending, normally combined with extreme operating conditions have resulted in in-service damage. Examples of factors leading to accelerated creep, creep fatigue and oxidation damage are described.

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