7.2.2 LP rotors
For low temperature rotors, the main requirements are relatively high tensile strength combined with high toughness.
The 3.5Ni Cr Mo V monobloc rotor forging is currently used as it avoids the complication of shrink fits in the case of built-up rotors. The manufacturing facilities and forging technology required for the manufacture of monobloc rotor forgings of very large size and weight are fully able to meet the designers requirements. No limitations are identified for the forgings of 2-pole 900 MW units currently under consideration.
The use of vacuum degassed ingots, single and double upsetting procedures and the use of larger presses capable of such forging operations, have all contributed to a marked improvement in quality of large LP rotor forgings. Rotor forging for 660 MW designs of 100 tonne forged weight, produced from ingots twice this size, are provided for LP rotors.
Vacuum degassing has eradicated the former problems of hydrogen embrittlement cracking and gives
good fracture toughness. Ultrasonic inspection techniques ensure freedom from any internal discontinuities greater than 5 mm effective diameter. The parallel application of visual and magnetic inspection of the axial bore with ultrasonic examination was the accepted approach to non-destructive examination of the rotor forging whose simple geometry, in the rough machined state, permits accurate location and diagnosis of any internal defects producing ultrasonic indications. Ultrasonic techniques allow the material in the vicinity of the bore hole, which is subjected to the highest tangential stresses during overspeed, to be examined critically. This provides reassurance of integrity in circumstances where the complex geometry of the machined rotor prohibits the use of external examination.
The purpose of the bore was to permit internal inspection, but it also removed some of the central part of the forging which is where defects or segregation in the original ingot might be located and provided the opportunity to obtain fracture appearance transition temperature measurements.
Inspection techniques have now developed to the extent where external ultrasonics can replace the former bore inspection. This is very significant as the LP rotor is subjected to high centrifugal stresses, the last-stage disc being the most heavily stressed part of the turbine. The centrifugal load of the large rotating blades sets up a tensile stress in the rim of the disc, and this stress increases with decreasing radius, its maximum value being at the hub. The larger the bore of the hub, the larger the maximum stress. If the bore is small, the hoop stresses are lessened.
Ni Cr Mo V steel containing up to 3.5% Ni has been used for both shafts and discs for built-up LP rotors and is the first choice to achieve the tensile properties necessary for large monobloc rotors. Welding techniques are established for the design of fabricated LP rotors employing a series of solid discs. Alloy steels for this application are subject to the conflicting demands for tensile strength and weldability.
Since LP turbine rotors may run at temperatures not much in excess of ambient, the provision of safeguards against the danger of brittle fracture is important. The use of materials for discs, shafts and monobloc rotors of the lowest possible fracture appearance transition temperature (FATT) and subject to stringent non-destructive testing and fracture mechanics assessment (see Section 7.3 of this chapter) provides the necessary framework for the safe operation of LP turbines.
3.5% Ni Cr Mo V steel has a low value of FATT. In the water-quenched and tempered condition, and with careful control of composition, high tensile properties can be matched with a FATT well below ambient temperature and a consequential high fracture toughness.