Seal welding of the joint between the tubes and the rear flue sheet after they had been rolled caused a gap to open on the water side of the joint. That combined with excessive free oxygen in the water supply caused accelerated corrosion of the tubes, ultimately resulting in tubes failing just ahead of the flue sheet.
Recommended corrective measures include lightly re-rolling the tubes after seal welding to close the gaps, and implementing better water quality control. There was no evidence of excessive rolling.
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