- A power generation company experienced cracking in a number of stainless steel injector nozzles in a gas turbine generator. Where possible the parts were weld repaired and returned to service. Two nozzles which were too extensively cracked to be repaired were forwarded to TWI for investigation.
- To determine the primary cause of failure.
- From an understanding of the failure mechanism, make recommendations on how to improve the design and so increase the life of the part.
- Visual examination and photography of samples as received.
- Liquid penetrant inspection to establish the extent of cracking.
- Detailed examination of the fracture faces by binocular and scanning electron microscope.
- Metallographic sectioning to reveal the microstructure and crack path.
- Chemical analysis of the parent steels and weld metal.
- Confirmation of the failure mode of the part.
- Confirmation that the material quality and method of manufacture was acceptable.
- Improved weld design and the inclusion of a post weld heat treatment resulted in increased reliability and life.
The parts failed by fatigue from the weld toes attaching the branch tubes to the main nozzle tube. With no evidence of manufacturing flaws or environmental effects, it was concluded that the failures were a consequence of cyclic stresses that were either unforeseen at the design stage or significantly higher than envisaged. The design of the welds was improved so lowering the stresses at the weld toe.