Abstract:
Modified 9Cr–1Mo steel, with minor additions of V and Nb in conventional 9Cr–1Mo steel, is widely used for steam generators of nuclear reactors. It is also a candidate material for future sodium cooled fast breeder reactors. It possesses higher thermal conductivity, lower thermal expansion coefficient, higher tensile and creep strength, better weldability, higher microstructural stability and resistance to stress corrosion cracking in water–steam systems in comparison to those of low Cr–Mo steel, conventional 9Cr–1Mo steel and austenitic stainless steels. During service, the components of steam generators are often subjected to repeated thermal stresses as a result of temperature gradients arising from heating and cooling during start–up and shutdown operations. These transient thermal stresses induce cyclic strains and cause low cycle fatigue (LCF) damage to material. LCF at elevated temperatures is known to be influenced by time dependent processes like dynamic strain ageing (DSA), creep, oxidation and microstructural degradation. DSA has been found deleterious to tensile ductility, LCF life and creep resistance of structural components.
