Effects of Multiple Post Weld Heat Treatments on Microstructure and Precipitate of Fine Grained Heat Affected Zone of P91 Weld

To understand the potential impact of multiple post weld heat treatments (PWHTs) on service life of P91 weld, the multiple PWHTs are designed and carried out to explore their influences on microstructure and precipitate of fine grained heat affected zone (FGHAZ) of the P91 weld. The results show that the multiple PWHTs cause the recovery of martensite matrix (specifically, the elimination of lath-like structure and the decrease of dislocation's density) and the evolution of M23C6 carbide (specifically, the coarsening of M23C6 carbide and the change of its distribution). The recovery of martensite matrix and the coarsening of M23C6 carbide are undesirable in terms of the creep resistance of the FGHAZ. Besides, the change trend of hardness is consistent with that of M23C6 carbide density, indicating that the dispersion hardening introduced by the M23C6 carbide plays a dominant role in hardening effect.