Microstructural evolution in seven 2.25Cr–1Mo superheater outlet headers resulting from service exposure

Abstract

Seven 2.25Cr–1 Mo superheater outlet headers with service exposures ranging from as fabricated through to 190 kh (21.7 years) have been examined using analytical TEM. Parent plate, heat affected zone, and weld regions have been characterised to understand the influence of service exposure on the microstructure and the carbide make-up. Post-weld stress relief heat treatment in a virgin header produced a starting microstructure devoid of M₃C, and consisting of M₇C₃ and M₂₃C₆ in the bainite and M₂C in the ferrite. Service induced aging produced coarsening of M₂C and the development of denuded zones along prior austenite grain boundaries. The carbide compositions in the bainite shifted towards thermodynamically stable phases, rich in Mo and Cr. One header exhibited anomalous behaviour; parent plate compositional effects may have contributed to this, along with fabrication and/or operational differences. Most of the individual carbide phases studied were found to undergo compositional changes with parabolic kinetics. These microstructural and compositional changes may find application in the assessment of effective time at temperature.