Abstract: Microstructure and properties of the HR3C steel after service for 50 000 h were studied by means of room temperature and high temperature mechanical properties test, high temperature impact test, stress rupture test, metallographic structure, scanning electron microscopy (SEM) and energy dispersive spectrum (EDS) analysis. The results show that after service for 50 000 h, the HR3C steel has obvious brittleness, the plasticity decreases greatly, while the room temperature strength and hardness are satisfied with the standard specification. Compared to room temperature, the toughness and plasticity of the embrittled HR3C steel at 665 ℃ are significantly resumed, and the high temperature strength also meets the standard requirements. A large number of second phases precipitate in the crystal or grain boundary, bulk M₂₃C₆ phase accumulated at the austenite grain boundary is the main cause of embrittlement, while the NbCrN phase in the crystal produces dispersion strengthening effect. The creep rupture strength of the HR3C steel at 665 ℃ is only 10% lower than the standard recommended value, and taking final-stage superheater tube as an example, the residual life of the steel is estimated to be still more than 100 000 h.

Key words: HR3C steel, high-temperature service, embrittlement, bulk M₂₃C₆ phase, NbCrN phase, 100 000 h