Microstructural Characterization of P91 Steel in the Virgin, Service Exposed and Post‐Service Re‐Normalized Conditions

P91 steel has been widely used in power generation industry. Generally, P91 creep resisting steel is produced by normalizing and tempering. The normalizing temperature is in the range of 1040–1080°C and tempering temperature is in the range of 750–780°C. The microstructure after tempering is tempered martensite with precipitates of carbides, M23C6, and vanadium/niobium rich carbo‐nitride of the type MX (M = V or Nb and X = C or N). The presence of carbide precipitates improves creep rupture strength due to precipitation hardening. The carbide coarsening and microstructure degradation during service will result in deterioration in creep strength. It is important to understand the microstructural evolution of P91 steel during long‐term operation. In this work, the steels in the virgin condition (normalized and tempered), service exposed (9 years at 600°C) and post service exposed re‐normalized and tempered in an attempt to restore the original microstructure were characterized. A range of microscopy techniques, predominantly TEM, were applied to understand the effect of these thermal histories on the microstructure of the materials.