The low-pressure spindle of a 165,000-kw cross-compound steam turbine burst during a routine overspeed-trip test. An intensive investigation of the design, metallurgy, and manufacture of the spindle and of the operational history of the turbine was completed within four months of the date of the accident. The nickel-molybdenum-vanadium material currently being specified by the turbine industry for large turbine spindles and generator rotors is known to be susceptible to brittle failure in the presence of notches in heavy sections. It was determined that the initiating cause of the Ridgeland accident was flakes or thermal cracks developed in the shaft during heat-treatment. These flaws provided the notches necessary to trigger a catastrophic burst of the shaft. The incident and subsequent investigation focus attention on the metallurgy, production, and inspection of large turbine-spindle forgings.

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