The metallurgical mechanism responsible for the initiation and propagation of circumferentially oriented, fireside cracks on waterwall boiler tubes was investigated. The materials studied were Cr-Mo low alloy steels in the as-received and post-service (exhibited cracking) condition. It was determined that cracks were initiated by a corrosion/fatigue interactive mechanism at the fireside tube surface. The corrosion mechanism is sulfidation while the material fatigue degradation occurs due to thermal stress cycles. Crack propagation occurs by an environmentally assisted thermal fatigue crack propagation mechanism.

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