Total Service
The process will lend itself to any component that can be rotated. Typical applications would include repairs and up-grades to all types of power plant. Examples being: Gas Turbines, Steam Turbines, Boiler feed pumps, Cooling Water pumps, Emergency stop valves, Contactor heads. Reasons for the original failure should be investigated and discussed prior to any recommendations for the recovery process are made.
Our work includes:
Our case study is based on the repair of a 4.5 ton high criticality circulating rotor. Our study will take you through all of the aspects of the repair, from initial conception to completion.
Initial investigations and consultation with the owner of the rotor revealed that the subject rotor was deemed to be unfit for service in its damaged condition. With replacement costs in excess of £250K and delivery times exceeding 18 months from the OEM. We were asked to investigate the possibility of recovering the rotor using spiral weld technology.
Initially we were asked to prepare a method plan and weld procedures for submission to the clients metalurgist / welding engineer for consideration prior to conducting initial full-scale weld tests.
Following acceptance of our procedures. A similar piece of material in specification and size as the subject rotor shaft was pre-machined for spiral welding tests of one, two and three layer overlays. Upon completion this test piece was subjected to MPI, Ultra sonic, hardness values assessments. Following this, the test piece was cut into several segments and numerous sections were taken along the deposited overlay. In some areas, notably the two to three lay deposit there was evidence of carbon dilution near the fusion boundary. This had been tempered by the post weld heat treatment (PWHT). Macro hardness tests were also conducted along with mechanical tests. The test piece was a complete success with no signs of cracking of the interface, and the weld substrate interface was shown to be superior to that of the substrate. Where martensite had been produced in the dilution layer, the PWHT tempered the material, removing any risk of stress corrosion cracking.
Having now established that the spiral weld recovery process was a complete success. The next stage was to carry out the repair of the actual rotor. This entailed setting the rotor into a lathe and pre machining the worn areas.
Upon completion of pre machining, the rotor was prepared for welding. The rotor was pre heated to 225 Degrees C and due to the size of the rotor it was crucial that this temperature was maintained during the complete welding cycle. To do this the rotor was wrapped with a ceramic thermal heating jacket, with minimal access allowed for the welding torch itself. On completion of the weld the temperature was raised to 300 degrees C and the rotor tilted into a vertical position for the preparation of the post weld heat treatment.
The rotor temperature was then increased at 50 degrees C per hour to a maximum temperature of 620 degrees C + or - 10 degrees and soaked at this temperature for 7 hours (1 hour per inch diameter). Following this, the rotor was then slow cooled at 50 degrees C per hour to 250 degrees C and all thermal insulation then removed and allowed to cool to ambient. The rotor was then reset into the lathe, checked for concentricity and proof machined to + 0.020" to establish weld integrity by full NDT as per the original test samples.
On completion of successful testing the rotor was then finished machined to the original manufacturers drawing. All heat treatments were fully certified.
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