Improvement of Creep Rupture Ductility in Ni Base Superalloy Nimonic 80A and Its Material Properties.

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Superalloys of Refractaloy 26 (R-26) and Nimonic 80A are used as high temperature bolt materials for steam turbines. R-26 has both high creep rupture strength and high creep rupture ductility, and is therefore highly reliable as a high temperature bolt material. However, because it is an austenitic steel having a large thermal expansion coefficient, whereas the material for the turbine casing in ultra high temperature turbine plants is a high Cr cast steel of the martensite system, the resulting differential thermal expansion leads to insufficient fastening force for the turbine casing. Nimonic 80A, an austenitic steel like R-26, does not show such a large thermal expansion coefficient, and the problem as seen in R-26 can thus be eliminated in terms of bolt fastening force for the turbine casing. However, because the creep rupture ductility of Nimonic 80A is low, its reliability in terms of high temperature strength can be insufficient. <br>In the present study, therefore, improvement of creep rupture ductility in Ni base superalloy Nimonic 80A was undertaken through improvement of chemical composition and heat treatment method. The investigation results confirmed that creep rupture ductility can be improved by reducing the Ti content and providing 3-step aging treatment. Furthermore, based on these findings, an actual size bolt material sample was produced to evaluate its material properties, and this material demonstrated good material properties as seen in basic test results.

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