Structure Integrity Assessment Of A Welding Repaired Free Turbine

K4648alloy, which has the highest chromium count in all of thenickel-based cast superalloys, is used for integrally cast turbine guide deviceand other mechanical parts due to its moderate strength as well as excellenthigh temperature oxidation resistance and corrosion resistance. The vanes of afree turbine guide device are made of K4648nickel-based alloy. Duringservice, cracks appear near the roots of the vanes. After TIG welding repair,cracks still exist in welding joint. It is necessary to evaluate the safety of thewelded turbine guide device and predict its service life. Thus, structureintegrity assessment of repaired turbine guide device is implemented.First of all, based on analysis of the working condition and failure statusof the turbine guide device, the thermodynamic processes of solid vane andhollow vane are simulated by means of finite element analysis (FEA), and thecorresponding temperature field and stress field are obtained. The resultsshow that the vanes have suffered high amplitude “compressive-tensile”alternate stress composed of thermal stress and residual stress, and the dangerzones with the highest stresses are the joints between the margins of the vanesand the inner ring or outer ring as well as the edge of the chamber of hollowvane. Residual stress test verifies that FEA can accurately predict residualstress distribution in the turbine guide device.The turbine guide device’s defects include mainly deformation andcracks. Three penetrated cracks on the vanes are observed and analyzed withscanning electron microscope, and features of fatigue crack, such as origins offatigue cracks, radial edges, fatigue striations and river pattern are detected.So the cracks can be confirmed as fatigue cracks that initiate and propagateunder fatigue load during service. Safety assessment and life prediction forthe vanes should be based on the theory of propagation of fatigue crack.In order to obtain the material properties required for safety assessmentand life prediction, the fracture toughness and fatigue crack propagation rate of TIG welded joints of K4648alloy (including base metal, weld metal andheat affected zone) are tested and studied based on national standard.On the basis of the results above, corresponding calculation procedureand method are worked out based on British standard BS7910, and thecalculation program is composed. The damage tolerance and allowed initialcrack length are calculated for repaired turbine guide vanes.This research can be guidelines for establishing quality controlspecification of welding repaired free turbine guide device.