I was going through some literature on AUSC steam turbine (operating temperature 700 deg C). In many of the papers i have seen that 400 MPa yield strength at operating temperature was used as material selection criteria for turbine rotor. At the same time some people are using Alloy 617 for same application, for which creep rupture is quite high (around 137MPa at 700 deg C) at operating temperature but yield strength is around 200 MPa.
Can someone please explain how higher yield strength at operating temperature (creep regime) is advantageous also if for two material all properties are same (including ductility and toughness also) and there is difference only in their yield strength, against which failure mode this higher YS will be advantageous. What is the basis of 400 MPa YS at operating temperature?
As far as turbine blades go,
As far as turbine blades go, I've seen creep rupture strength used as a smaller consideration compared to the operating temperature yield strength, because the blades can be viewed as replaceable parts until the next scheduled outage. Because creep is obviously a time dependent parameter, as long as the blades are replaced within 100kHr/200kHr rating for the creep rupture before the outage, I think yield strength can be be a more important consideration.
From my experience, the creep rupture strength data has very large scatter. Should you use the minimum value, average value for design? I think most designers use a combination. FWIU the data are from accelerated high temp. tests and fitted to curves. I think creep and relaxation material data from these tests should not be boiled down to a single number, but presented as a statistical range.
The rotor can be interesting in terms of materials depending on the OEM construction type. As you might know, in the wetter stages <140C LP's, higher yield strength is avoided in the rotors because it has been shown to be susceptible to stress corrosion cracking.