Failure analysis of a burner ring made of 20Cr32Ni1Nb alloy in gas turbine combustion chamber

• Reason of reduced mechanical properties was enlargement of intra-dendritic carbides.
• Enlarged Nb rich precipitates were sites to generate voids in front of the crack path.
• Reduction of mechanical properties and providing void sites was the failure reason.

The failure analysis of an insert ring of a burner in a 156 MW gas turbine made of 20Cr32Ni1Nb alloy is presented. Normally, the useful service life of these parts under ideal conditions is about 30,000 h. However, more likely fracture and failure could be observed in 5000-h inspections, which resulted in the early and premature replacement of these parts.Microstructural changes of the alloy were carefully examined after utilization by means of optical microscopy and scanning electron microscopy (SEM). An increase of Nb element in carbides and dissolution of secondary interdendritic carbides after 5000 h of operation were observed. The studies indicate that the most damaging factors are the adverse working conditions and thermal stresses.