Importance

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Abstract

A system for repairing worn, distorted, cracked, or degraded portions of high temperature rotors such as those used in high-pressure and reheat steam turbines is disclosed. The repairs are applicable to low alloy steels generally described in ASTM Specification A-470 classes 3, 7, and 8. Explicit controls on the welding process, the welding consumables, and the placement of the weld fusion line are disclosed. For the welding process, a novel staging of the 'relative heat input' for applying the initial cold wire gas tungsten arc weld (GTAW) buttering layer is disclosed. Significantly, the optimum weldment properties are achieved in the cold wire GTAW by utilizing a lower heat input for the crucial second layer relative to the first layer. Faster deposition or weld build-up is achieved over the buttering layer by applying the balance of welding through utilization of the hot-wire GTAW process. Hot-wire weld integrity is assured by control of a helium-argon cover gas mixture, application of a trailing gas shroud, weld-head oscillation, and control of the wire insertion point into the molten puddle. For the weld deposit, a specially modified 9Cr-1Mo filler metal based on the 'Grade 91' alloy developed by the Oak Ridge National Laboratory is selected. Additional stringent controls are placed on the chemical composition of the weld wire. Finally, judicious placement of the weld fusion line to insure long service is achieved by a detailed finite-element stress analysis. Near the fusion line, the stresses are limited to values below the minimum stress-rupture strength of the base metal as described by a correlation using the Larson-Miller time-temperature parameter.

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First Claim