A steam turbine wheel with fractures at the locking bucket location was received for fracture and microstructure examination, chemistry, and mechanical property testing.
The wheel chemistry and mechanical properties meet the requirements of the customer supplied specification. Examination of the fracture surfaces on the wheel indicated a primarily intergranular fracture surface that was somewhat compromised by corrosion. An intergranular fracture surface of the wheel material is indicative of either an embrittlement mechanism and/or corrosion. Metallographic examination confirmed that the wheel experienced intergranular stress corrosion cracking (SCC).
The chemistry and hardness of the locking pin are consistent with the tool steel as specified on the drawing. The locking pin fracture surfaces exhibited some coarse transgranular texture in undamaged areas. Only a very small area of ductile dimples were observed. The fracture surfaces appeared quite complex indicating a possible corrosion mechanism was present. Fractography and microstructure examination confirmed that the pin experienced transgranular SCC.
Sodium was confirmed to be present in the corrosion products. The observed corrosion mechanisms of the wheel and locking pin are consistent with caustic SCC caused by exposure to NaOH in the steam. The corrosion damage weakened both the locking pin and the wheel. It could not be determined if the final fracture of one component (wheel or locking pin) preceded the other.
IMAGE CAPTIONS (LEFT TO RIGHT)
- Secondary electron image of pin cross section near the fracture surface. Note complex crack pattern indicative of caustic stress corrosion cracking.
- Secondary electron image of Charpy impact fracture surface. Ductile dimples are present. Brittle fracture in service conditions points to embrittlement or corrosion mechanisms.
- Etched section of wheel fracture surface. Branched intergranular cracks are visible. Tempered martensite is present in the base structure (2% nital etch).
- River patterns (arrows) are indicative of a brittle fast fracture mechanism of the pin.