TWI Bulletin, March - April 2009

Case Study: Examination of failure in fir tree connection in disc from steam turbine 101-JT

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 numerousstart-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 and was asked to determine the primary cause of failure and if the part was manufactured to the required standard.

Visual examination and photography of the samples were carried out followed by detailed examination of the fracture faces by binocular and scanning electron microscope. Metallographic sectioning to reveal the microstructure andcrack path of the material was undertaken plus chemical analysis and Charpy impact testing in order to determine if the material had suffered temper embrittlement.

It was discovered that 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 which was most probably theresult of a caustic solution concentrating in the turbine during use. The failure was not due to a design fault or any non-conformance issue.