Assessment of premature failure in a first stage gas turbine nozzle

Premature failure of a first stage gas turbine nozzle has been the subject of a precise study and the present investigation comprehensively describes it. It was determined that failure occurred by thermal fatigue mechanism during start-up/shutdown of gas turbine. The cracks initiated from the surface of trailing edge section in which intergranular oxidation caused by high temperature hot corrosion (HTHC) was significant. In addition, the presence of continuous film of carbides at the grain boundaries was a direct result of transformation of primary carbides M6C type to secondary carbides M23C6 type as a result of high temperature operation of the nozzle. Therefore, the nozzle crack initiation and propagation was facilitated by grain boundaries brittleness caused by formation of continuous film of carbides M23C6 at the grain boundaries. Furthermore, to experimentally confirm the degradation of the nozzle, mechanical test data was taken into consideration. It was found that the tensile properties of the alloy have decreased. In the similar test conditions, the hardness of the exposed alloy increased considerably compared to the as-cast sample, which could be attributed to the microstructural degradation, especially formation of M23C6 carbides in the large quantity in the matrix alloy.

► HTHC (high temperature hot corrosion) could result initiation sites for cracking in the nozzle.
► Maximum intergranular oxidation occurred in the trailing edge section.
► Phase transformation of primary carbides M6C type to secondary carbides M23C6 type caused to the formation of continuous film of carbides at the grain boundaries of the base material.