Abstract
Pitting corrosion is a critical issue for steam turbine operators since localised surface degradation causes stress concentration which may lead to fatigue failure. Dual certified 403/410 martensitic 12% Cr steel – which is a standard material for steam turbine blades in the low pressure part – was tested using ultrasonic fatigue testing technique. Experiments were performed up to the very high cycle fatigue regime on both smooth and pre-pitted specimens. For the latter, corrosion pits of defined size comparable to those found in failed turbine blades were generated artificially. Test environments were air at 90 ◦C and aerated 6 ppm Cl− solution at 90 ◦C (for details see [1]). In this work, the results of extensive fatigue tests [1, 2] are evaluated using two different approaches. Fatigue assessment was performed using the √ area parameter model developed byMurakami and Endo [3] and the small-crack model by El Haddad et al. [4]. The prediction equation for the first model is expressed as w = a(Hv + 120) (√ area )1/6 · ( 1− R 2 ) (1)
Highlights
Pitting corrosion is a critical issue for steam turbine operators since localised surface degradation causes stress concentration which may lead to fatigue failure
For pre-pitted specimens in air, the prediction error is within ca. ±20% (for pit sizes of 50 m (+14% at R = 0.05 and −13% at R = 0.5), 100 m
No acceptable results were found for 6 ppm Cl− solution where the prediction error is as high as −53%
Summary
Pitting corrosion is a critical issue for steam turbine operators since localised surface degradation causes stress concentration which may lead to fatigue failure. Experiments were performed up to the very high cycle fatigue regime on both smooth and pre-pitted specimens. For the latter, corrosion pits of defined size comparable to those found in failed turbine blades were generated artificially. [3] and the small-crack model by El Haddad et al [4]. The prediction equation for the first model is expressed as w. Where 0 is the fatigue limit for smooth specimens, l is the crack/defect length and l0 is the fictitious crack length l0 = 1 ·.
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