AbstractsEngineering

Characterization of high temperature fatigue mechanisms in Haynes 282 nickel based superalloy

by Kyle A. Rozman




Institution: Oregon State University
Department: Materials Science
Degree: PhD
Year: 2014
Keywords: Haynes 282; Heat resistant alloys  – Effect of high temperatures on
Record ID: 2043982
Full text PDF: http://hdl.handle.net/1957/53470


Abstract

Electric power needs will only grow over the next decades as more humans ascend from poverty into the middle class. Currently, the majority of electric power is generated by burning fossil fuels. To help mitigate the undesirable effects of burning fossil fuels research is being done to increase the efficiency of power produced. In order to increase the efficiency of power production, the operating temperature of steam turbines must be increased, which presents a materials challenge. Haynes 282 is a nickel based superalloy which has been proposed as a potential rotor alloy for steam turbines operating at high temperature (760°C). Other authors have previously looked at creep, oxidation, low cycle fatigue and other properties of Haynes 282; however, lacking from the literature are studies on the fatigue crack growth mechanisms in Haynes 282. This project investigated Haynes 282 from a fatigue crack growth perspective with an aim to fill this literature gap and assess the utility of Haynes 282 as a steam turbine rotor alloy. This dissertation has specifically evaluated the fatigue crack growth rates of Haynes 282 as functions of temperature and frequency. The testing method utilized was ASTM E647, "Standard test method for measuring fatigue crack growth rates." Temperatures investigated were 550°C, 650°C and 750°C. The loading frequencies were both 25 Hz and 0.25 Hz. In general increasing the test temperature increased fatigue crack growth rate. Thermally activated cross-slip and dislocation annihilation were the primary mechanisms responsible for the increased fatigue crack growth rate. The noted activation energy for dislocation jog migration, which is related to dislocation annihilation, was about 12.5 kJ/mol. This value fit with the measured activation energy. The post-test microstructure showed greatly reduced dislocation density at the highest temperature. Fractography of the crack growth region showed transgranular crack growth at 550°C and 650°C with signs of isolated intergranular features at 750°C. The effect of frequency on the fatigue crack growth rates was minor at 550oC but much more significant at 650°C and 750°C. For the temperatures investigated the effect of decreasing loading frequency was to increase the fatigue crack growth rate. At high loading frequency, the isolated intergranular features were present only at stress intensities below about 11 MPa√m. At 0.25 Hz loading frequency, isolated intergranular features persisted into the high stress intensity range. While the isolated intergranular features are of some concern, the measured activation energy was well below published creep and/or oxidation activation energies. This means the crack path did not have enough energy to sustain an intergranular crack path. In summary, the work discussed in this dissertation investigated the fine details of fatigue crack growth of Haynes 282. This study has closed some of the existing gap in the literature regarding the fatigue crack growth rates of Haynes 282. Previous studies have shown no adverse…