Influence of long-term thermal ageing (>10,000h) on low stress creep deformation of P92 heat-resistant steels

Microstructure evolution of P92 heat-resistant steel during long-term service has high influence on the creep property. In this paper, P92 heat-resistant steel samples were aged at 898 K for 10,000 h, 15,000 h and 25,000 h. The effect of thermal ageing on microstructure was analysed through optical microscopy and scanning electron microscopy. The creep behaviour at specific shear stresses (14.7 MPa, 20 MPa, 25.6 MPa, 41 MPa) was investigated through helicoid-spring creep testing. The relationship between microstructure evolution and creep properties was discussed. The results demonstrated that the basic morphology of P92 heat-resistant steel samples following long-term thermal ageing was martensite strip structure, while the precipitated phases were MX, Laves and M23C6. The coarsening of M23C6 carbide was slower than Laves phase during thermal ageing of 15,000 25,000 h. Precipitation strengthening for the grain interior was corresponding to the Orowan mechanism. In the regions of high shear stresses (14.7 MPa, 20 MPa), the Laves phases played an important role in dislocation motion inhibition at grain boundaries, while in the regions of low shear stresses (25.6 MPa, 41 MPa), the M23C6 phases played a significant role in dislocation motion inhibition at grain boundaries. Moreover, the main precipitate which affected the creep of P92 heat-resistant steels under the lower shear stresses of 14.7 MPa and 20 MPa was Laves phase and the higher shear stresses of 25.6 MPa and 41 MPa was mainly M23C6 phase.