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- Moverare, Johan, 1973-, et al.
(författare)
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Environmental aspects on LCF-life of Ni-base superalloys during long term operation
- 2012
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Rapport (refereegranskat)abstract
- The applicability of fine grain polycrystalline nickel base superalloys in gas or steam turbine applications is often limited by their susceptibility to fast intergranular cracking during fatigue in combination with extended hold times at high temperatures and high tensile stresses. This effect is further enhanced in corrosive environments even at moderate temperatures such as 400-600°C. In this study the negative effect of SO2 + water vapour on the low cycle fatigue resistance of three different nickel based superalloys (Nimonic 901, Inconel 718 and 718plus) has been studied at 450°C and 550°C. A negative effect was found on both the crack initiation and crack propagation process. The effect increases with increasing temperature and is likely to be influenced by both the chemical composition and the grain size of the material. The negative effect of water vapour + SO2 is manifested by a decreased resistance to cyclic plastic deformation and a transition from transcrystalline to intercrystalline fracture behaviour. In Nimonic 901 this negative effect increases with the degree of plastic deformation. For lower mechanical strain amplitudes where the number of cycles and the total exposure time is increased the environmental impact is reduced. Similar trends can also be noticed for the other alloys. The surface corrosion in air and in SO2 + water vapour are found to be rather similar in this study and it is therefore concluded that the surface scale can remain adherent and protective if the strains on the oxide scale are low. However, for LCF tests with higher strain ranges, the oxide scale will rupture preferably at the grain boundaries and intergranular microcracks will initiate which promotes inward diffusion of embritteling elements such as oxygen and sulphur. Inconel 718 seems to be more sensitive to SO2 + water vapour than Nimonic 901 when tested at 450°C. The overall resistance to LCF is however still higher in Inconel 718 for the test conditions investigated in this study, this is due to the exceptionally good resistance to low cycle fatigue that this alloy has from the start (as seen for the tests in air).
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