This thesis deals with the mechanism of the bainite transformation in steels, with emphasis on a special class of carbide-free bainitic steels. Their microstructure is designed to consist of a mixture of bainitic ferrite, retained austenite and martensite. The steels are destined for applications in the railway industry, and generally wherever resistance to mechanical wear is an important selection criterion.

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 transforma-tion 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 fer- rite/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. It reinforces a published hypothesis that the frequently observed single variant of carbide in lower bainite is a consequence of stress-affected precipitation.

Chapters 8 and 9 deal with the wear properties of the bainitic alloys and include extensive data on microstructures generated by continuous cooling transformation. There is also an outcome from difficulties encountered during the manufacture of thick section, that the embrittlement caused by phosphorus can be overcome by the addition of a controlled amount of molybdenum to the steels.

The thesis finishes with a brief chapter summarizing the key conclusions and proposing some ideas for future work.