Rainbow Test of Laser Reconditioned Frame 6 Buckets

The procurement and reconditioning of turbine buckets and vanes drive the maintenance cost for gas turbines. The use of advanced repair methods like laser welding can help to reduce these costs. The main advantage of the laser welding process is that high-grade filler metals can be used with the possibility to extend current repair-limits. Before the laser welding process can be applied for industrial repairs the method has to be extensively tested for attained mechanical properties and for the performance during exposure in an actual turbine. This paper discusses the first rainbow test as performed on a frame 6 gas turbine of the Bergum power plant in the Netherlands. For a set of first stage buckets with 12139 operating hours (OH) 6 buckets were reconditioned using conventional TIG repair and 6 buckets using the laser-powder welding process. In both cases In 625 was used as the filler metal. The remaining buckets were not reconditioned. Monitoring of the performance of the reconditioned buckets is done using pyrometry and by boroscopic inspections. The reconditioned buckets are about 17°C cooler than the others. Moreover, it is found that the buckets repaired using laser welding have lower tip temperatures compared to the buckets reconditioned using conventional TIG welding. The overall lower temperature is partly caused by the difference in colour between the reconditioned and non-reconditioned buckets, which affects the emissivity. This difference was found still present during a boroscopic inspection 1776 OH after the overhaul. The other possible factors affecting the temperature difference are identified but are not yet fully understood.