There has long existed a controversy regarding the formation of the bainite phase in steels. Nonetheless, following the clear identification of bainite in 1930, several characteristic features associated with the bainite transformation became well established. These features include: the classification into upper and lower bainite; the existence of a bainite start temperature; and the incomplete reaction phenomenon. Accordingly, two competing theories have been developed to explain the bainite transformation. However, no overriding consensus has been reached as to which is correct. The first theory invokes a diffusion controlled transformation, describing the growth of bainite via the propagation of ledges – a series of steps on the transformation interface. The second interpretation favours a displacive, diffusionless transformation. Bainite growth occurs via the autocatalytic nucleation and growth of successive subunits or platelets. Over time, a wealth of techniques has been implemented in order to increase the understanding of the mechanism by which bainite forms. From the early approaches that involved thermodynamic and kinetic considerations; through detailed work on the crystallography of the transformation; to studies involving advanced characterisation techniques that focused on the distribution of atoms, etc. Some of the theories have been progressively adapted to new evidence and new concepts as they emerge. For example, the concepts of the T0 curve and of paraequilibrium transformation. This work presents a chronological summary of the two theories of the bainite transformation, charting their progression and noting the variety of evidence both in support of and in contradiction to each viewpoint. The bainite controversy is still debated by steel metallurgists today, although it has mostly been reduced to the question of whether or not bainitic ferrite initially forms with a supersaturation of carbon. This review gathers and evaluates some of the accumulated work in the hope that this question may finally be answered.

The diffusion of hydrogen in austenite is slower than in ferrite. Experiments have been conducted to study the behaviour of hydrogen in a nanostructured steel sample consisting of a mixture of thin plates of bainitic ferrite and intervening films of retained austenite, with the latter phase present in a quantity larger than the percolation threshold, i.e. it has threedimensional connectivity. The structure was then heat treated to control the fraction of austenite, and hence to study the role of hydrogen when the austenite decomposes below the value required to sustain percolation. The experiments have involved both thermal desorption analysis and permeation, and when combined with theoretical analysis, indicate a significant influence of percolating austenite in hindering the passage of hydrogen into the steel during hydrogen charging, and its permeation through the composite nanostructure. The effect is not as large as might be expected from a simple comparison of independent data on the diffusivities of hydrogen in the two lattices, because the effective diffusivity in ferrite is found to be much smaller than in the defect-free ferrite, owing to trapping effects.

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A nanostructured steel has been produced using a clean steel-making technique. The mechanical properties have been comprehensively characterised. The maximum strength of the material recorded was 2.2 GPa at yield, with an ultimate tensile strength of 2.5 GPa, accompanied by a Charpy impact energy of 5 J, achieved by heat treatment to refine the prior austenite grain size from 145 to 20 µm. This increased strength by 40% and the Charpy V-notch energy more than doubled. In terms of resistance of the hardness to tempering, the behaviour observed was similar to previous alloys. Despite reducing the hardness and strength, tempering was observed to reduce the plane-strain fracture toughness.

Adiabatic shear bands represent intense plastic deformation that is localised because the rate at which the heat generated by deformation is greater than that at which it is dissipated. The structure of such bands generated by ballistic testing is examined in order to reveal the governing mechanisms. We attempt to distinguish in particular whether local reaustenitisation occurs, or if the microstructural change are a reflection simply of intense deformation.