Abstract
In a wide variety of applications, metals and alloys that can maintain their desirable mechanical properties in a high temperature environment are essential. Research has been ongoing in this area for many years and there is constant industrial demand for alloys that provide high levels of oxidation resistance in a range of environments.A review of the current literature in this area was carried out focusing on high temperature isothermal and cyclic oxidation of stainless steels and some selected chromia and alumina forming superalloys. The current practices used in high temperature oxidation testing were closely considered along with post exposure characterization techniques used to evaluate the type and structure of the oxide layers formed on the alloy samples. This study of the literature highlighted particular areas where it was felt that more work was needed to clarify alloy oxidation behaviour, in particular was the lack of application specific representative testing. It was found that there was a lot of research carried out in varying environments where isothermal high temperature oxidation was concerned, however, when cyclic oxidation at high temperature was investigated it seemed that there was significantly less variation in the testing environments covered by the literature. This was especially surprising when specific applications of the alloys were mentioned by researchers, yet the testing that they carried out was not in an environment that would closely represent the working environment of that application, therefore prompting the questioning of the usefulness of the results to that industrial process.
To address the key areas of interest in the literature a comparison would be made between representative tests and tests carried out in a more simplistic static air environment. It was hoped that the results from this would highlight the importance of testing alloys for a specific application in a way that matches as closely as possible the conditions that the alloy would experience in service.
It was decided that samples should be exposed to a number of 8h cycles in static air at 950°C, and to air at 950°C with a flow velocity of approx. 3.5m/s. In order to carry out this work a test rig was designed that would house a minimum of 20 samples at one time inside a chamber furnace with controllable temperature and controllable flow velocity across the samples surfaces. Using the test rig a relatively short term data set was produced indicating significant differences in alloy performance in both environments. From this data set, using the COSP, predictions for long term cyclic oxidation behaviour could be made for alloys which showed promising oxidation resistance during the short term testing. These predictions of long term behaviour in a flowing air environment using the COSP have not been carried out elsewhere in the literature and provide substantial novelty and usefulness to this Thesis. The alloys chosen for this study were austenitic stainless steel grades 304, 316 and 310 along with the Ni-based superalloy Inconel 625 and the Fe-based superalloy Kanthal APMT. 304 and 316 stainless steel are the most commonly used stainless steels in elevated temperature environments, while the other three were chosen because of their known ability to provide significant resistance to high temperature oxidation along with resistance to creep.
Prior to this an isothermal investigation of selected stainless steels was carried out across a range of high temperatures, close to the maximum suggested limits of the alloys, over a range of time periods. This was carried out to investigate the limits of the stainless steel alloys and to provide information on the breakdown of the alloys protective qualities. Three different alloys were tested and compared across a range of temperatures. The findings from this investigation were used to show that certain alloys may not be acceptable for long term high temperature operation and that further study of more advanced alloys was required.
| Date of Award | Dec 2019 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Savko Malinov (Supervisor) & Roy Douglas (Supervisor) |