Abstract: A waterwall tube rupture in a subcritical natural-circulation W-flame boiler that resulted in an unplanned unit outage was investigated through macroscopic examination, mechanical-property and metallographic examinations of tube specimens, and compositional analysis of deposits. The rupture exhibited a trumpet-shaped opening, and diametral swelling and microstructural transformation were observed in the rupture area and adjacent tube sections, indicating short-term overheating. Hard deposits at the steam-drum end consisted mainly of iron oxide. Inadequate preservation after chemical cleaning and passivation, combined with repeated start-ups and shutdowns and low-load operation, promoted the formation, deposition, and consolidation of corrosion products. Large deposits entering the waterwall circuit locally impeded flow and degraded cooling conditions, ultimately inducing a short-term overheating rupture. Preventive measures are proposed, including preservation after chemical cleaning and during standby periods, water-filling flushing and blowdown, abnormal wall-temperature monitoring, and reinspection and acceptance of foreign materials after maintenance.