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No description available.

no abstract provided by author / Master of Science

Call number: LD2668 .T4 1956 C65 / Master of Science

No description available.

M.S. / Materials Science and Engineering / The most fundamental step in the development of a predictive model for microstructure and residual stress distribution in steels is the accurate representation of the transient temperature field. Three constituents of a database of thermophysical properties, namely the thermal conductivity, volumetric specific heat capacity and convective heat transfer coefficient, were isolated and their effects quantified on the accuracy of temperature field predictions using finite element analysis (FEA). The most critical parameter in the heat transfer process was ultimately identified to be the temperature dependent convective heat transfer coefficient. It was determined using an inverse heat transfer method, which was successfully applied to accurately establish the thermal boundary conditions for an arbitrary 3D steel geometry. The temperature dependency of the volumetric specific heat capacity in the transformation range of temperatures has to be known a priori, for which a reliable model describing alloy dependent reaction kinetics has to be developed first. Thermal conductivity and its dependency on temperature have secondary effects on the accuracy of FEA predictions. The impact of the outcome of this study lies in its relevance to the heat treatment industry.

published_or_final_version / Industrial Engineering / Doctoral / Doctor of Philosophy

Bibliography: pages 105-113. / This thesis deals with the effect of heat treatment on the localized corrosion resistance of the low carbon, chromium containing steels, designated 825, 102A and 122 which recently have been developed. The potentiodynamic polarization technique has been employed to determine the corrosion characteristics of the three steels. The results do not accurately reflect the effect heat treatment has on corrosion rates but scanning electron microscopy of corroded surfaces does allow a characterization. Both tempering temperature and time at temperature have a significant influence on the corrosion behaviour of chromium steels because the type, size and morphology of carbide precipitates are determined by the temperature and time of temperi ng. Localized pitting corrosion predominates for specimens tempered at temperatures below 450°C. Intergranular corrosion together with general corrosion occur after tempering at temperatures. in excess of 450°C. The resul ts of hardness tests show that secondary hardeni ng occurs after tempering between 450°C and 600°C. Secondary hardening suggests the presence of chromium carbides which deplete the surrounding matrix of chromium leaving it susceptible to active general corrosion (within the grains) and intergranular corrosion (at grain boundaries). A model showing the effect that 12% chromium, in comparison to 8% chromium, has on the corrosion resistance, is proposed. The significance of these results with regard to the application of the steels is discussed.

The effect of a specific type of pre-exposure heat treatment on the susceptibility of AlSI type 304 stainless steel to stress corrosion cracking was studied in terms of time for crack nucleation and rate of crack propagation. U-bend specimens were exposed to 42 weight percent magnesium chloride aqueous solution after pre-exposure heat treatments at 140°C and 154°C. The straight-line relationship between maximum crack depth and the logarithm of exposure time expressed by the empirical equation log t = log t_o + D/M was obtained. The stress corrosion constants derived from the empirical equation indicate that this type of pre-exposure heat treatment promotes crack nucleation because of the formation of less protective surface films, and retards the rate of crack propagation because of effects on internal structural changes within the alloy. / Master of Science

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Masters of Science in Engineering. University of the Witwatersrand, Johannesburg, 2016 / With the current tough economic times and depressed commodity prices, focus is being placed on achieving higher power densities in gears without increased cost or reduced achievable life and reliability. An investigation into the use of different carburisation methods and processes to minimise post heat treatment distortion, with the aim to reduce and even eliminate the requirement for post heat treatment grinding and grinding stock allowances is presented. The investigation included the processing of test pieces, as per the recommendations of AGMA 2004-B89: Gear Materials, Heat Treatment and Processing Manual. Four alloy steels, namely AISI 3310, AISI 8620, AISI 9310 and 17CrNiMo6, were selected for this research, based on availability, cost and hardenability. The carburising cycles were derived for both gas and vacuum carburising for a 0.10% and 0.20% carbon steel respectively. Two quenching options were applied, oil quenching and intensive quenching. The test pieces were 3-dimensionally measured to determine distortion through the changes in diameter and ovality. The microstructures of the case and core were analysed, as well as effective case depth. This research found that current methods, atmospheric carburising and oil quenching, and steel alloy combinations are inadequate to produce low distortion carburised internal gears, while the use of vacuum carburising and intensive quenching as a process combination can achieve such gears. It was also found that the use of AISI 3310, performed the best for both current and proposed process combinations. It is recommended that future work be conducted, including a full-scaled manufacturing trial with more detailed process adjustments to ensure the quality and repeatability of the final carburised gear. / MT2017