A detailed fitness-for-purpose assessment of turbine valve spindles

This paper describes a fitness-for-purpose assessment of a number of turbine valve spindles fabricated from a forged stainless steel type A286, following failure incident in service. Detailed microstructural characterisation and material analysis were carried out to aid the assessment of the valve-components using a range of experimental techniques including metallography, secondary ion mass spectrometry, scanning Auger microscopy, focused ion beam, SEM/EDAX and analytical electron transmission microscopy. In addition, Charpy properties, fracture path type and environmental bend crack growth details were investigated.It was found that the steel at the spindle head location was in a grain boundary degraded condition that resulted from an improper forging schedule. In addition, environmental bend test revealed that the degraded steel microstructure was highly susceptible to intergranular stress corrosion cracking and, as such, it was not recommended that the reserve valve spindles enter service.