Failure Analysis of Ozark, Arkansas, Power Plant Socket-Head Cap Screws
CONSTRUCTION ENGINEERING RESEARCH LAB (ARMY) CHAMPAIGN IL
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This research analyzed the socket-head cap screws which failed in the turbines of the Ozark Power Plant, Ozark, AR. The cause of failure was found to be fatigue in improperly heat-treated cap screws. The reduced fatigue strength of the cap screws was further aggravated by the presence of a sharp fillet where the shaft and head joined and by the nonuniform preloads imposed on the cap screws during installation. Several procedures are recommended to improve the service life of the turbine cap screws 1 Insure that all cap screws or studs are heat-treated in the following manner a Provide full normalizing heat treatment after forging at 1600 F 870 C. b Austenitize for 2 to 3 hr at 1550 F 843 C, and quickly oil quench. c Temper at 800 F 423 C to a hardness of 38 to 42 Rcapproximately 3 12 hr. 2 Use rolled rather than machined threads for both cap screws and studs. The threads should be rolled on after heat treatment. 3 Keep cap screw and stud hardness below 44 RC to prevent stress corrosion cracking. Machine cap screw and studnut seats to insure good alignment and perpendicularity this will eliminate bending stresses.
- Electric Power Production and Distribution
- Structural Engineering and Building Technology