Failure Analysis of Weld Repaired Turbine Rotor

The first stage of an IP turbine rotor failed after approximately eight years of service. The rotor had been weld repaired using a 360 degree all weld metal buildup. The rotor material was specified as ASTM A-470 Class 8, 1Cr-1Mo-1/4V forging material, and the weld metal was Modified 9Cr-1Mo known as Grade 91 in the wrought product forms. The weld repair procedure used a controlled weld bead deposition process to refine the grain size of the HAZ. A failure analysis was performed including metallurgical examination and stress analysis. For the failure analysis, the primary features of interest were the fracture appearance, cracking/failure location, and failure mechanism. The fracture appearance was “lack of fusion” where the fracture surface nominally conforms to the weld bead shape. The primary cracking and failure was located at the fusion interface of the weld repair. The failure mechanism was creep rupture. Based on the thermal and stress analysis, the probable root cause of failure was improper location of the weld fusion line during the design stage. Both the operating stress and temperature were higher than design. The operating temperature was higher than the design temperature because of the lower cooling of the rotor experienced in service than predicted. Thus, the HAZ was located at a radial position having higher than allowable temperatures and stresses for the creep weak HAZ.