Microstructural Modifications Due to Tungsten and Tantalum in 9Cr Reduced Activation Ferritic Martensitic Steels on Creep Exposure

Replacement of Mo and Nb by W and Ta in modified 9Cr-1Mo class of steels has been significantly useful for achieving reduced activity and improved mechanical properties. The addition of W and Ta strongly influences microstructure which comprises of the substructural changes and precipitation kinetics under both thermal and stress exposure. This study focusses on the effect of W and Ta on microstructural changes on thermal and creep exposure at 823K in 9Cr-W-Ta-0.2V- 0.1C steels, with W and Ta concentrations varying from 1 to 2% and 0.06 to 0.14% respectively. Increase in W from 1 to 1.4 or 2%, was beneficial to retain lath structure and retard the coarsening of M23C6 during long term thermal aging. Detailed microstructural investigation of the steesl after creep deformation showed extensive subgrain formation and coarsening of M23C6 carbides than just thermal exposure. Influence of W on subgrain size was more pronounced than Ta. Increase in W was found to be useful to control the coarsening rate of M23C6 carbides, even under stress, while MX precipitate size showed no significant change.