Correlation of ferrite formation to creep properties of Cr-Mo steel welds: Henry Granjon prize 2013 winner category B: materials behaviour and weldability

A factor of four decreases from 16.0 × 10⁻⁴ to 4.1 × 10⁻⁴ %/h in steady state creep rate was observed in the fine-grained heat-affected zone (FGHAZ) of a Cr-Mo steel weld, by reducing the pre-weld tempering temperature from 760 to 650 °C. The current study used electron backscatter diffraction and synchrotron x-ray diffraction techniques to characterize the microstructure in the FGHAZ of the two tempering temperature conditions. The results showed carbide-free ferrite that formed in the FGHAZ of weldments that were pre-weld tempered at 760 °C, contributed to void formation resulting in lower creep strength. It is proposed that the formation of ferrite in the FGHAZ is due to the incomplete dissolution of Cr₂₃C₆ carbide at heating process during welding, which results in Cr enrichment adjacent to the undissolved Cr₂₃C₆ carbide. Dictra simulation confirmed ferrite formation at the carbide/austenite boundaries during Cr₂₃C₆ carbide dissolution.