IGSCC Failure of Refractalloy 26 Turbine Bolts

Refractalloy 26 turbine bolt failures were discovered during refurbishment activities. Failure analysis was performed to determine the failure mechanism and to develop a comprehensive/plausible root cause of failure scenario. The failed bolts are used to attach the nozzle block to the nozzle chamber and were installed during the 1993/1994 overhaul of the turbine. The unit was retired in 1997 and had been in operational service approximately 40% of the time following the 1993/1994 overhaul. Fifty of the 53 broken bolts were located in the lower or bottom half of the turbine and a white deposit was found covering the nozzle block and two rows of the stationary blades adjacent to the nozzle block for the lower cylinder. Based on the fractographic and metallographic examinations, the damage features are: (1) numerous, branched, intergranular cracks that initiate at the stress concentrated regions of the transition radius from the head to uniform section of the bolt, and at the thread root, and (2) continuous, surface connected cracks that are filled with corrosion product. The failure mechanism was identified as intergranular stress corrosion cracking (IGSCC). The failures require contaminant ingress of typical boiler feedwater chemicals and carryover. Based on SEM and x-ray diffraction results, the probable contaminants are Na, P, S, and Cl and the failures result from carryover of either NaOH or di-sodium phosphate into the steam. Based on a review of operating history, two root cause failure scenarios were developed: (1) single acid contamination even followed by significant caustic corrective action, and (2) multiple upset events, termed “decreasing temperature excursions”, that typically occur during an increase in steam flow. Both scenarios require low steam flow (such as that found at low loads) in order to cause agglomeration of the contaminants at the bottom of the turbine. The actual events that caused the IGSCC damage are not definitively known. Nevertheless, corrective actions that eliminate carryover are expected to be effective in preventing any future SCC damage to the turbine.