The properties of the nickel based superalloy Inconel 718 (IN 718) are insufficient for its use in the higher temperature stages at the rear of the gas turbine jet engine compressor. Considerable effort is being made to adapt the microstructure and improve the properties by thermomechanical processing, to enable the cheaper IN 718 alloy to replace the current alloy, Waspaloy, in the fabrications of these rear-end compressor disks. The initial casting of the alloy leads to an unsuitably coarse microstructure. The alloy is therefore forged in order to optimise the microstructure and hence mechanical properties. Isothermal compression tests were carried out on the alloy in the as-cast condition to investigate both dynamic and static microstructural events between 900°C and 1121°C. Strain rates ranged from 0.001s<SUP>-1</SUP> to 10.0s<SUP>-1</SUP> with strains from 0.1 to 0.6. Microstructural investigations revealed that complete dynamic recrystallisation was not evident in any of the samples, however partial recrystallisation occurred in samples forged at the higher temperatures and strain rates. The amount of dynamic recrystallation was found to increase primarily with temperature but also with strain rate. Complete static recrystallation was found to have occurred in tests conducted with a post-forge hold time at temperature. The degree of this recrystallisation and the resulting grain size were found to depend primarily on temperature and strain and less so on strain rate. Stress levels varied systematically with temperature and strain rates. Higher stress levels resulted from lower testing temperatures and higher strain rates. Currently property data of material in the as-cast condition is not widely available. Constitutive analysis was performed on the Stress-strain data for subsequent implementation in the forging simulation program "BILLIE". These results would then be used to provide simulations of the industrial Inco Alloys forging process.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:636261 |
| Date | January 2000 |
| Creators | Clark, A. C. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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