Type and technique important
The weld repair used the shielded metal arc, or "stick" welding process. Electrodes ranging in diameter from 3/32 inch to 1/8 inch were used. Because the cracking within the original girth weld was likely related to the low-carbon electrode used during the header's fabrication, AWS E9018-B3 electrodes were used rather than AWS E-9018-B3L electrodes. The former type is at most 0.15% carbon, making it a good match for the original base material used during the header fabrication.
Welding technique also played a key role in the repair. In this case, Thielsch technicians used a specially developed low-stress buttering technique. The buttering process involves laying stringer beads with 3/32-inch-diameter weld electrodes up the sides of the weld cavity prior to filling in the center of the groove. By reducing the amount of heat input, the technique significantly reduces residual welding stresses.
Post-weld steps
After welding was completed, the header temperature was raised from the preheat temperature of 550F to 1,325F at a rate of 100 degrees/hr. The ligament repair weld was soaked at the stress relief temperature for four hours and then cooled slowly to 400F at a rate of 100 degrees/hr. The resistance heaters were secured after the header reached 400F. It then was left to cool naturally to 150F with the insulation left in place.
A 1/8-inch reinforcement cap was applied over the section of the header surrounding the two tube bore openings that were weld-repaired. The two tube bores then were machined in place, and new tube stub-to-header sockets were machined (Figure 6) to facilitate reinstallation of the tube stubs. Figure 7 is a sketch of the machined joints. Care was taken to properly align the new bore openings with the remaining tube stubs.
6. Precision bore. Two tube bores were precision-machined for the new tube-to-header connections. Courtesy: Thielsch Engineering Inc.
7. Plan ahead. A sketch of the tube-to-header joint used in a high-temperature header weld repair. Source: Thielsch Engineering Inc.
The final repair welds were subjected to NDE by qualified personnel. The completed repair restored the reheat outlet header to a level of integrity suitable for continued service with minimum unit downtime.
—Peter Kennefick is vice president of the Utility Engineering Services Division of Thielsch Engineering. He can be reached at 401-467-6454 or pkennefick@thielsch.com.
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