The top tube of a horizontal superheater bank in the reheat furnace of a steam generator ruptured after seven years in service. The rupture was found to have occurred in the ferritic steel tubing (2.25Cr-1Mo steel (ASME SA-213, grade T-22)) near the joint where it was welded to austenitic stainless steel tubing (type 321 stainless steel (ASME SA-213, grade TP321H)). The surface temperature of the tube was found to be higher than operating temperature in use earlier. The ferritic steel portion of the tube was found to be longitudinally split and heavily corroded in the region of the rupture....