Abstract: Microstructure and mechanical properties of different areas of the Super304H superheater steel tube of an ultra-supercritical unit after 40 000 h service were analyzed by means of optical microscope, scanning electron microscope and transmission electron microscope. The results show that the Super304H steel tube has abnormally grown austenite grains in the outer wall after 40 000 h service. TEM test shows that the coarsening of M₂₃C₆ particles in the outer wall is obvious, and continuous M₂₃C₆ particles precipitate on grain boundary, while the growth of MX phase and Cu rich phase is not obvious. The abnormally grown grains cause the room temperature tensile strength and impact property of coarse-grained zone of outer wall of the Super304H steel tube in service to decrease by 21.9% and 50%, respectively, compared with the supply state. The fracture characteristics of the impact specimen in the coarse-grained zone are obvious, and its brittleness is significantly increased, which poses a threat to the service safety of the Super304H steel tube. Supervision should be strengthened to eliminate the potential safety hazards such as tube explosion caused by the abnormal growth of austenite grains on the outer wall.

Key words: ultra-supercritical unit, Super304H steel tube, coarse-grained zone, microstructural evolution, mechanical properties