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Abstract
For commercial production it is necessary for any steel or thermomechanical treatment to be cheap and practicable. It is particularly important for the kinetics of transformation to be such that the alloys can be manufactured on a large scale using continuous cooling transformation. The work reported in the thesis therefore establishes the mechanism and kinetics of transformation, and then continues to apply the concepts to the successful development of rail steels.
The thesis begins with a survey of the literature about solid-state transformations in steels, together with a detailed assessment of bainite. This is followed by a description of the experimental techniques used. Chapter 3 contains measurements of the isothermal transformation kinetics of a variety of silicon-rich steels, and the data are demonstrated to be reasonably consistent with a mechanism in which bainitic ferrite grows without diffusion. This conclusion is reinforced with the results presented in Chapter 4, which for the first time identify quantitatively, the factors responsible for the thickness of austenite films found in bainitic microstructures. Such films have in the past been shown to be of the utmost importance in controlling toughness.
Some new information of fundamental importance was revealed during the course of routine metallographic investigations (Chapter 5). Firstly, it was found that when the driving force for transformation is large, it is possible for separately nucleated platelets of bainite to coalesce into coarse plates. Chapter 6 contains observations which prove that the bainitic ferrite/austenite interface behaves like that of martensite, in that it bows between obstacles. In addition, there is clear evidence that the shear associated with bainite growth can be accommodated by mechanical twinning when differently oriented plates collide.
Chapter 7 establishes for the first time that the number of carbide variants that form in lower bainite can be influenced by the application of an external stress.