Background
During the commissioning of a steam turbine in a chemical fertiliser factory in Iraq, failure occurred in the third stage disc of a forged rotor in a 'fir tree' blade fixing feature. The steam turbine, which had undergone numerous start-ups and shutdowns during its commissioning period, had failed after only the equivalent of four months continuous service. TWI was provided with a single portion of the disc rim and blade from the same location.
Objectives
- To determine the primary cause of failure.
- To determine if the part was manufactured to the required standard.
Approach
- Visual examination and photography of samples as received
- Detailed examination of the fracture faces by binocular and scanning electron microscope
- Metallographic sectioning to reveal the microstructure and crack path
- Chemical analysis of disc and blade material
- Charpy impact testing in order to determine if the material has suffered temper embrittlement
Conclusion
The turbine failed due to cracking in the outer rim of the disc in the fir-tree blade-fixing feature. Failure occurred due to stress corrosion cracking initiating at the surface. The stress corrosion cracking was most probably the result of a caustic solution concentrating in the turbine during use.
The failure was not due to a design fault or any non-conformance issue.
Recommendation
Improve environmental control to avoid further stress corrosion cracking.
