7.2.1 HP and IP rotors
High temperature rotors are required to have a combination of creep strength, rupture strength and ductility. This is conferred by rotors manufactured from forged Chromium Molybdenum Vanadium steel (Cr Mo V), a ferritic material which provides the best possible creep properties.
Creep is a high temperature phenomenon and traditional design methods involve the selection of materials which provide the required creep strength and creep rupture ductility during the required design life.
Typically, high temperature turbine design is based on a creep strain of 0.2% during 105 h of operation. Typical curves of time to produce 0.2% strain and rupture are shown on Fig 1.110 for Cr Mo V rotor steel, and show the powerful effect of stress and temperature in controlling creep strain.
While the best possible creep properties are required on both HP and IP rotor forgings, the HP problem is simplified by modest dimensions of HP rotors. The IP rotor poses a much more serious problem, especially in turbines employing a reheat cycle. Full steam temperature is encountered on discs at the IP inlet stages, which have larger diameters and blades than equivalent stages on the HP rotor exposed to steam at the same temperature. Both at the rim and the bore, the stresses are such that steam cooling may be necessary, even with steels having the best creep properties.
The reduced temperatures encountered at the last two stages of the IP turbine are offset by increased bore stress due to the longer blade length, which demands an adequately high proof strength.
The requirement for good creep strength and the necessary proof strength in conjunction with a rotor forging of heavy section is achieved on current rotors by the use of ICr IMo 0.25V steel alloy.