Fatigue Behavior of Nickel Based Super Alloy 718 in a Hot Corrosive Environment
14 May 2014-Materials Performance and Characterization (ASTM International)-Vol. 3, Iss: 2, pp 68-83
TL;DR: In this article, the high cycle fatigue (HCF) and fatigue crack growth (at 550°C) behavior of a nickel-based super alloy 718 in hot corrosive environment (Na2SO4+NaCl salt coating), typical of marine engine environment, is presented.
Abstract: Nickel based super-alloys when exposed to a combination of high temperature and low melting point fused corrosion products, result in early fatigue failure compared to their response in high temperature benign environment. The high cycle fatigue (HCF) (at 550 and 625°C) as well as fatigue crack growth (at 550°C) behavior of a nickel based super alloy 718 in hot corrosive environment (Na2SO4+NaCl salt coating), typical of marine engine environment, is presented in this paper. At least an order of magnitude decrease in fatigue life is noticed when the temperature is changed from 550 to 625°C at stress levels below 450 MPa. The statistical analysis of scatter in fatigue lives at different stress levels is performed and a Weibull–Inverse power law model is fitted to the stress–fatigue life data of alloy 718 in hot corrosive environment. Fracture surface examination of hot corrosion fatigue failures showed higher crack growth rates compared to uncoated high temperature HCF fracture surfaces. Fatigue crack growth rate at 550°C in a hot corrosive environment increases by an order of magnitude at 0.5 Hz in the Paris region compared to crack growth kinetics at 2 Hz in lab air environment at the same temperature. The fracture surface shows a mix of transgranular and intergranular mode of crack in the propagation region at 0.5 Hz. A fatigue failure diagram is proposed combining the thresholds of maximum stress level from endurance tests and maximum stress intensity factor from fatigue crack growth tests to demarcate the regions of propagating and non-propagating cracks during hot corrosion fatigue. The complexities in incorporating the effect of loading frequency in fatigue failure diagram are highlighted.
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TL;DR: In this paper, high cycle fatigue behavior of the superalloy IN718 was studied in the air at 600°C in fully reversed stress controlled mode (R = −1), in the as heat treated condition as well as pre hot corroded in the salt mixture of 75Na2SO4+15NaCl+10V2O5 (wt.%) at 600 °C for 100
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References
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TL;DR: In this paper, an explanation for non propagating fatigue cracks is presented based on the criterion that once the value of a particular strain intensity factor reduces to the threshold value for the material the crack should stop.
865 citations
TL;DR: In this paper, the macroscopic and microscopic characteristics as well as the proposed mechanisms of Type I (high-temperature) and Type II (low temperature) hot corrosion are reviewed.
517 citations
TL;DR: In this article, the basicity-dependent solubilities for NiO, Co3O4, Al2O3, Fe oxides, SiO2, and Cr 2O3 in fused Na2SO4 at 1200 K were investigated.
Abstract: Metals and alloys may experience accelerated oxidation when their surfaces are coated by a thin film of fused salt in an oxidizing gas. This mode of attack is called hot corrosion, and the most usual or dominant salt involved is sodium sulfate because of its high thermodynamic stability. The corrosive oxyanion fused salts exhibit an acid-base chemistry and are usually ionically conducting electrolytes, so that the corrosion attack must exhibit an electrochemical mechanism with certain characteristics analogous to aqueous atmospheric corrosion. Hot corrosion may involve fluxing of the protective oxides as either acidic or basic solutes in the fused salt. The thermodynamic phase stability can be described by high-temperature Pourbaix-type diagrams, and these can be used to interpret the basicity-dependent solubilities. Measured solubilities for NiO, Co3O4, Al2O3, Fe oxides, SiO2, and Cr2O3 in fused Na2SO4 at 1200 K exhibit remarkable agreement with the expected behavior and permit the calculation o...
140 citations
TL;DR: The performance of gas turbines has been improved by the development of alloys with progressively increasing high-temperature capabilities as mentioned in this paper, while both strength and corrosion resistance are importan-...
Abstract: The performance of gas turbines has been improved by the development of alloys with progressively increasing high-temperature capabilities. While both strength and corrosion resistance are importan...
137 citations
TL;DR: In this article, high temperature Pourbaix-type diagrams are used to interpret the basicity-dependent solubilities for NiO, Co3O4, Al2O3, iron oxides, Cr 2O3 and SiO2 in fused Na2SO4 at 1200 K.
123 citations