Effect of long-term service on the precipitates in P92 steel

The precipitates in P92 steel after long-term service in an ultra-supercritical unit were investigated by field-emission scanning electron microscopy and transmission electron microscopy and were found to mainly consist of M₂₃C₆ carbides, Laves phase, and MX carbonitrides. No Z-phase was observed. M₂₃C₆ carbides and Laves phase were found not only on prior austenite grain boundaries, martensite lath boundaries, and subgrain boundaries but also in lath interiors, where two types of MX carbonitrides—Nb-rich and V-rich particles—were also observed but the “winged” complexes were hardly found. Each kind of precipitate within the martensite laths exhibited multifarious morphologies, suggesting that a morphological change of precipitates occurred during long-term service. The M₂₃C₆ carbides and Laves phase coarsened substantially, and the latter grew faster than the former. However, MX carbonitrides exhibited a relatively low coarsening rate. The effect of the evolution of the precipitate phases on the creep rupture strength of P92 steel was discussed.