Metallurgical Analysis of Flaws in Carbon-Molybdenum and Nickel-Molybdenum-Vanadium Turbine Rotor Forgings

Abstract

The origins of metallurgical flaws in the vicinity of a 2.7Ni-0.5Mo-0.1V low-pressure (LP) rotor bore and on the bore surface of a 0.25C-0.5Mo high-pressure (HP) rotor, found after approximately 360 000 h of operation of a 50-MW tandem-compound, double-flow steam turbine, have been determined. LP rotor ultrasonic indications, the largest being equivalent to a 3.3-mm (0.13-in.)-diameter flat-bottom hole, were determined to be the result of void formation during ingot solidification shrinkage. Magnetic particle indications, maximum length of approximately 64 mm (2.5 in.) axially, on the HP rotor bore surface are a consequence of the intersection of a near-planar array of manganese-silicate inclusions by the bore hole. Evidence of service-induced transgranular growth of the HP-rotor flaw, predicted by the fracture mechanics analysis, is shown together with the actual-and-predicted flaw size and flaw location correlation for the LP and the HP rotors, respectively. The results of the metallurgical analysis, performed to characterize the nature of the LP and HP rotor flaws, and the low- and high-temperature tests, performed to determine whether the approximately 360 000 h of exposure of the 0.25C-0.5Mo HP rotor alloy to 722 K (449°C) adversely affected stress rupture strength or fracture toughness, are presented.