Improvement of creep performance of creep strength enhanced ferritic (CSEF) steel weldments through non-standard heat treatments

Significant improvement of creep-rupture life was observed in Grade 91 (modified 9Cr-1Mo) steel weldments when a non-standard heat-treatment was applied prior to welding. A lower temperature pre-weld tempering (LTT) than the typical heat-treatment resulted in a complete dissolution of M₂₃C₆ carbides in the fine-grained heat affected zone (FGHAZ) during welding, which allowed re-precipitation of the M₂₃C₆ as strengthening carbides after post-weld heat treatment. However, the LTT also raised the ductile-brittle transition temperature of the base metal above room temperature. A thermo-mechanical treatment (TMT) has been proposed in the present study as a way of balancing the need for improved creep properties in the weld region and an acceptable level of room temperature ductility in the base metal. Aus-forging and subsequent aus-aging promotes MX formation prior to martensitic transformation which effectively increases the creep resistance, even in the FGHAZ. The application of the standard tempering after the TMT process improves the room temperature ductility without losing the advantage of improved strength. Preliminary results indicate a successful improvement of the cross-weld creep properties of the TMT sample similar to the LTT sample.