Evaluation of Platform Weld Repairs on F-Class, Stage 1 Buckets

Abstract

Abstract The higher turbine inlet temperatures coupled with dry low emission combustors on the widely used F-class gas turbines produces high heat loadings on the stage 1, hot section components, particularly focused on the platform section of rotating buckets/blades. This paper provides a brief design and durability history overview of the platform areas of buckets. High heat loadings combined with cyclic operation, and variation in casting supplier quality, resulted in various levels of extensive cracking and high scrap rates based on prior conservative repair limits. Currently, the consensus amongst repair shops is that platform cracking extending beyond a limited area near the edge is irreparable, and the bucket/blade should be scrapped. As repair technology is ever changing and evolving, what once was a limit may now be excessively conservative. To reduce scrap frequency and increase component repair yields, newer weld filler materials and alternative welding processes were tested and evaluated. Metallurgical evaluation of various types of weld filler metals as applied to the platforms of F-Class, first stage buckets cast from DS GTD111 material were undertaken. The buckets were equally processed up to but excluding weld filler and weld process type. The bucket platform welds were then simultaneously evaluated via optical microscopy. Crack free weld repairs, conducted on engine run platforms, given the appropriate heat treatments, pre- and post-welding, can be achieved with solid solutioned strengthened Inconel 625 filler, and low volume fraction gamma prime strengthened Nimonic 263 filler using conventional GTAW. Crack free weldments or minor cracking (cracks of a small number and length) can also be achieved using Laser Cladding and/or elevated temperature GTAW with IN-738 filler metal. Surprisingly the newer weld filler metal Haynes 282 and older/traditional Haynes 230, showed evidence of hot-cracking and/or micro-fissuring (strain age cracking. A large number and length of cracks was observed when using Waspaloy as the weld filler metal. Tensile and stress rupture testing of various types of welds as applied to the platform areas were also undertaken to down-select the best filler metal. Samples were removed from the platform area of engine run buckets. Some samples were then used to obtain a baseline set of parameters of the base material. Other samples were used to test weldments of various filler metals and weld processes against those baseline values obtained. Testing of elevated temperature GTAW weldments and laser cladding with IN-738 filler metal as well as conventional GTAW with Haynes 282 filler metal produced satisfactory tensile strength and ductility properties. Elevated temperature welds using IN-738 filler were able to achieve between 76–79% stress rupture life of the base metal, while laser cladding using the same filler only yielded a 60–64% value of the base material stress rupture life. The GTAW Haynes 282 samples yielded approximately between 57–64% of the base material stress rupture life. Based on the test results, the recommended procedure for GTD111DS blade platform weld repair is to use IN-738 weld filler with the elevated temperature GTAW process.