This thesis deals with the changes in chemical composition and microstructure that occur when a joint between two different steels is heat treated for prolonged periods of time at a temperature below that at which austenite forms. Such joints are common place in power plant where components with different purpose are joined by welding. The aim of the work was to characterise the changes that occur at the weld junction and to quantitatively model them such that the method has predictive power. Metallographic analysis on dissimilar steel welds and bonds, heat-treated over a range of times and temperatures showed the ease of decarburised and carburised zone formation and their associated microstructures. In the low-alloy steel, rapid carbide dissolution close the dissimilar interface leaves room for grain growth that forms a decarburised zone. In the associated carburised zone, perception at and carbide coarsening occurred on grain boundaries and intra-granularly due to the associated increase in carbon concentration. In vanadium-containing low-alloy steels, stable vanadium-alloy carbides were found to complicate carbon partitioning generating wider, shallower decarburised zones compared to the smaller, almost carbide-free ones in vanadium-free steels. A model, developed for estimating the decarburised zone width in ferritic transition joints considers the carbon concentration as a function of distance from the dissimilar interface resulting from chemical potential gradients in the joint. Written in FORTRAN 77, the model uses the Crank-Nicholson implicit finite difference solution of Fick's second law, and was found to accurately predict decarburised zone widths based on measurement taken from welds and bonds from a given series of tempering tests. The existence of substantial carbon chemical potential gradients in the transition joint causes carbides to dissolve in the low-alloy steel during heat-treatment, releasing carbon to diffuse across the dissimilar interface to form a corresponding carburised zone. Naturally, the temperature and time of the heat treatment influences carbon diffusion, but the particular alloy combination in the transition joint also affects the amount of carbon partitioning.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598798 |
| Date | January 2002 |
| Creators | Elder, D. L. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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