The isothermal decomposition of austenite in the bainite region was investigated by means of a magnetic balance. The progress of the transformation was followed at intervals of fifteen to thirty seconds. The data thus obtained showed that the austenite to bainite transformation occurs in at least two steps. A graphical solution for the activation energy proved that the rate of transformation of austenite to bainite is governed by the rate of diffusion of carbon through ferrite. A general theory has been advanced to account for the various structures, and the mode of formation of bainite from undercooled austenite. This theory has been formulated on the basis of the results of this investigation and examined in the light of the findings of previous investigators. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The isothermal decomposition of austenite in the bainite region has been examined. The progress of the transformation in several hypoeutectoid and eutectoid steels was investigated metallographically from a qualitative point of view. A survey was made of the information available on the initiation, course, and end product of the transformation. The factors entering into the transformation were examined separately, their temperature-dependence and interactions investigated. A theory of the decomposition of austenite was proposed, and this theory examined in the light of the phenomena associated with the isothermal and anisothermal progress of the transformation. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

Typescript (photocopy) / 111 leaves, [20] leaves of plates : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--Dept. of Chemical Engineering, University of Adelaide, 1982

Based on Thesis (PH. D.)--University of Michigan, 1942. / Bibliography: p. 16-17.

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Prova complementar para doutoramento, na Faculdade de Engenharia da Universidade do Porto

This dissertation is a part of a collaborative work between the University of Texas, Austin-Texas, the University of Oulu, Oulu-Finland, and Outokumpu Stainless Oy, Tornio-Finland, , to develop commercial austenitic stainless steels with high strength and ductility. The idea behind this work involves cold-rolling a commercial metastable austenitic stainless steel -- AISI 301LN stainless steel to produce strain-induced martensite, followed by an annealing treatment to generate nano/sub-micron grained austenite. AISI 301LN stainless steel sheets are cold-rolled to 63% reduction and subsequently annealed at 600°C, 700°C, 800°C, 900°C and 1000°C for 1, 10 and 100 seconds. The samples are analyzed by X-Ray diffraction, SQUID, transmission electron microscopy, and tensile testing to fundamentally understand the microstructural evolution, the mechanism for the martensite [implies] austenite reversion, the formation of nano/sub-micron austenite grains, and the relationship between the microstructure and the strength obtained in this stainless steel. The results show that cold-rolled AISI 301LN stainless steel consist of dislocation-cell martensite, heavily deformed lath-martensite and austenite shear bands. Subsequent annealing at 600°C for short durations of 1 and 10 seconds leads to negligible martensite to austenite reversion. These 600°C samples exhibit a similar microstructure to the coldrolled sample. However, for samples annealed at 600°C for 100 seconds and those annealed at higher temperatures (700°C, 800°C, 900°C and 1000°C) exhibit equiaxed austenitic grains of sizes 0.2[mu]m-10[mu]m and secondary phase precipitates. The microstructural analysis also reveals that the martensite [implies] austenite reversion occurs via a diffusion-type reversion mechanism. In this regard, a generalized form of Avrami's equation is used to model the kinetics of martensite [implies] austenite phase reversion. The results from the model agree reasonably well with the experiments. Furthermore, the activation energy for grain growth in nano/sub-micron grained AISI 301LN stainless steel is found to be ~ 205kJ/mol, which is comparable to values observed in coarse grained commercial stainless steels (AISI 304, 316). However, the driving force for grain growth in nano/sub-micron grained AISI 301LN stainless steel is considerably higher when compared to other stainless steels. Finally, the average grain sizes in AISI 301LN stainless steels are related to the mechanical properties obtained, through the Hall-Petch relationship. / text

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Observations on some steels used in high performance gears are presented in this thesis. The object was to understand how microstructure and residual stress influenced mechanical properties, particularly fatigue strength. The investigations were carried out using fatigue testing, metallographic techniques, shot peening, hardness testing and Xray diffraction to determine residual stress and the amount of retained austenite. The work is divided into two main parts. In the first part, the opportunity was taken of investigating a batch of case-carburised gears manufactured from 17CrNiMo6 steel that contained an abnormally high level of retained austenite. . The benefits or otherwise of retained austenite is a matter of some controversy in the literature and this was an opportunity of testing the effect of high retained austenite in gears. In the second part, the properties of a recently formulated through hardening steel was investigated. The steel is inexpensive and capable of being fully hardened by air-cooling. It is believed that use of the material could reduce the cost of gear manufacture by eliminating the carburisation process, oil quenching and the subsequent need to correct distortions that arise from quenching. Gears were manufactured using l7CrNiMo6 steel. After carburization, one batch was found to have an abnormally high level of retained austenite (40 to 60%). Normally, gears with this level of retained austenite would be rejected. Nevertheless, gears from this batch were found to have surface and bending fatigue strengths that were not very different from those with much lower retained austenite contents. Despite the fact that the material is relatively soft, it was found that a very thin surface layer had hardened during back-to-back gear testing, probably by strain-induced transformation of the retained austenite. It was deduced that this thin layer of hardened material is sufficient to maintain a good level of surface contact fatigue strength. The second part of the work includes a basic study of the through-hardening steel. The study includes heat treatment and hardness, estimation of fracture toughness and the development of residual stress during heat treatment. Specialised standing contact fatigue (SCF) tests were also done using this material. The SCF test consists of cyclic loading of a hard ball in contact with the surface of the specimen, which is meant to simulate asperity contact in surface contact fatigue. Radial cracks or ring cracks or both are nucleated just outside the indentation circle in these tests, depending on the load and the development of plasticity. The stresses near the indentation were modelled using Finite Element analysis and were found to be consistent with the experimental results. Finally, the effect of residual compressive stress induced by shot peeing was studied using the SCF test. It was found that shot peening suppressed the formation of radial cracks.