Steam turbine rotors

1. A steam turbine rotor comprising:

at least one first region including a plurality of forged discs formed from a first material having a first yield strength sufficient to prevent a vulnerability to stress corrosion cracking in a wet steam environment of the rotor at steam temperatures of less than 300° C. and configured to receive an earlier stage of moving blades and a second region including a plurality of forged discs formed from the first material having a second yield strength and configured to receive a last stage of moving blades, wherein the forged discs in the at least one first region are heat treated differently than the forged discs in the second region such that the second yield strength is greater than the first yield strength, and

wherein the forged and heat treated discs in the at least one first region and the second region are welded together in axial series.

2. The steam turbine rotor as recited in claim 1 , further comprising at least one intermediate region disposed between the second region and the at least one first region and having an intermediate yield strength, the intermediate region configured to receive an intermediate stage of moving blades, wherein the intermediate yield strength is less than the second yield strength and greater than the first yield strength.

3. The steam turbine rotor as recited in claim 2 , wherein the second region, the first region and the intermediate region are composed of a plurality of forged discs welded together in axial series, each of the plurality of forged discs corresponding to one of the second, first, and intermediate regions.

4. The steam turbine rotor as recited in claim 3 , wherein each of the plurality of forged discs is composed of the first material, and wherein the forged discs in the at least one intermediate region of the steam turbine rotor are heat treated differently than the forged discs in the at least one first region and the second region.

5. A method of manufacturing a steam turbine rotor having at least one first region configured to receive an earlier stage of moving blades and a second region configured to receive a last stage of moving blades, the method comprising:

heat treating a plurality of forged discs formed from a first material in the second region so as to achieve a second yield strength;

heat treating a plurality of forged discs formed from the first material in the at least one first region so as to achieve a first yield strength sufficient to prevent a vulnerability to stress corrosion cracking in a wet steam environment of the rotor at steam temperatures of less than 300° C., wherein the second yield strength is greater than the first yield strength; and

welding the forged discs together in axial series to produce the steam turbine rotor having the regions of different yield strength.

6. The method as recited in claim 5 , wherein the rotor includes at least one intermediate region disposed between the second region and the at least one first region, the at least one intermediate region being configured to receive at least one intermediate stage of moving blades, and wherein the method further comprises heat treating an intermediate material in the intermediate region so as to achieve an intermediate yield strength between the first and second yield strengths.

7. The method as recited in claim 6 , wherein the intermediate material and the first material are the same.

8. The method as recited in claim 5 , further comprising determining an optimum yield strength of each region of the steam turbine rotor by reference to threshold curves based on expected ranges of stress and operating temperature for each region of the steam turbine rotor.

9. A method of manufacturing a steam turbine rotor, the method comprising:

selecting the same material for all discs in the rotor, the material having a predetermined range of yield strengths in the wet steam environment of the finished rotor;

forging each disc to a desired shape;

heat treating the forged discs to achieve different yield strengths in accordance with a region of the rotor to be constituted by each disc such that at least the first region is not vulnerable to stress corrosion cracking in the wet steam environment of the rotor al steam temperatures of less than 300° C.; and

welding the discs together in axial series to produce the steam turbine rotor having the regions of different yield strength.

10. A method of manufacturing a steam turbine rotor, the method comprising:

selecting at least first and second different disc materials, the first material having a lower yield strength than the second material in the wet steam environment of the finished rotor;

forging said first disc material into the disc corresponding to the second region of the rotor;

forging said second disc material into at least one disc corresponding to the at least one first region of the rotor;

heat treating the forged discs separately to produce the regions of different yield strength such that at least the first region is not vulnerable to stress corrosion cracking in the wet steam environment of the rotor at steam temperatures of less than 300° C.; and

welding the discs together in axial series to produce the steam turbine rotor.