Transformation and tempering of low-temperature bainite

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This thesis concerns a special class of novel bainitic steels, which can transform to bainite at around 200oC. The microstructure has been labelled `super-bainite', a mixture of fine bainite plates (20-40 nm thickness) in a matrix of austenite which is highly enriched in carbon. These high strength steels have found application as armour plate, and there has been further research to utilise them for aerospace applications.

The introductory chapter of the thesis contains a literature survey, starting from general background describing the solid-state transformations, and then developing with particular reference to earlier work on bainite, carbide-free bainitic steels, and on this new class of low-temperature bainitic steels.

Bainite transforms by a displacive mechanism, and chapter 2 characterises the surface relief due to transformation using a high-resolution surface technique. The measured shear component is larger than expected from past experience. This large shear component is consistent with the slender aspect of the ferrite plates. It may be that the small size of the features makes it much more difficult to measure the shear component as compared to transformation at higher temperatures.

Since the strengthening mechanism is a result of transformation, these steels offer a unique opportunity to achieve high strengths in large section sizes. In chapter 3 results of transformation of large samples are reported and experimental results are compared against calculated continuous cooling curves. It is demonstrated that uniform transformation of bulk sections is possible, and methods are presented that can be used to estimate limiting section sizes from the calculated time-temperature-transformation curves. It was found that in the alloy studied, the isothermal transformation kinetics were faster than calculated. It is proposed that this may be due to the formation of pro-eutectoid cementite.

The microstructure due to transformation at 150 and 200oC was characterised by X-ray diffraction, hardness testing, and thin-foil electron microscopy as presented in chapter 4. The bainite plates size was determined to be 39±1 nm after transformation at 200oC but unexpectedly increased upon transformation at 150oC. As previously reported, significant carb on supersaturation occurred in the austenite by transformation at 200oC as limited by free-energy change to super-saturated bainite, however no enrichment could be measured in the austenite transformed at 150oC. This indicates that there is a optimum temperature for achieving both fine plate size, and stabilisation of retained austenite. The microstructure as a result of transformation at 200oC was characterised by atom probe tomography (chapter 5). These direct measurements of carbon content are in agreement with the X-ray diffraction data reported in chapter 4. It can also be confirmed that substitutional element partitioning does not occur from the bainitic ferrite during transformation. The results are fully consistent with the displacive nature of the transformation.

The final two chapters of results deal with tempering of low-temperature bainite. The loss of hardness is consistent with the major strengthening mechanism being due to the plate size. X-ray measurements show that carbon in ferrite slowly reduces with increasing temperature, along with the recovery of heterogeneous strains. In contrast, hardness drops more rapidly after some critical temperature is reached. Carbides were identified by both X-ray diffraction of extracted residues, and using atom probe tomography, to be cementite. Tempering for 30 min at 400 and 500oC resulted in a fine carbide size around 10 nm, which can also contribute to strengthening, as well as being ideally positioned to prevent coarsening of the ferrite. Austenite decomposition takes some time, and a small amount can be retained even at 500oC for 15 min, or at 450oC for 1 h.

The final chapter deals with general conclusions and proposes future directions of research based on the results. Further work is needed to fully characterise the tempering behaviour of these steels, particularly at lower tempering temperatures which may be relevant in some applications.

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