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.
- To determine the primary cause of failure.
- To determine if the part was manufactured to the required standard.
- 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.
- Confirmation of the failure mode of the part.
- Improved environmental control to avoid further stress corrosion cracking.
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.
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