C crystal structure. It is this crystalline structure which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material. Mild steel carbon steel with up to about 0. Since bainite shown as ledeburite on the diagram at the bottom of this page and pearlite each have ferrite as a component, any iron-carbon alloy will contain some amount of ferrite if it is allowed to reach equilibrium at room temperature.
The exact amount of ferrite will depend on the cooling processes the iron-carbon alloy undergoes as it cools from liquid state. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process. The term is not limited to steels, but can be applied to any constituent formed by a shear process which does not involve atom diffusion or composition change.
The martensite transformation normally occurs in a temperature range that can be defined precisely for a given steel. The transformation begins at a martensite start temperature M s , and continues during further cooling until the martensite finish temperature M f is reached.
Many formulae have been proposed to predict the martensite start temperature. Most are based on the composition of the steel, and a selection are listed in the following table:. More recently, M s models have been developed through the use of neural networks, trained on experimental data and using further data to validate and test the model, a reasonable approximation of M s can be identified.
Such models are available on the web [2] and can be used with compositional information. Neural networks based on the relationship between the chemical composition, transformation temperature and kinetics during continuous cooling enable calculation of a CCT diagram for the steel. These also take into account the influence of alloying elements on the phase transformation curves, as well as the resulting hardness. It is also possible to predict quantitatively the microstructure of the steel e.
Models combining the kinetics of martensitic transformation with mechanics, in view of microstructural development are also applicable. Finite element analysis enables evaluation of the local stress and strain fields as well as monitoring the kinetics of martensitic transformation and development of the understanding on critical parameters such as effect of austenite grain size on the resulting martensitic microstructure. In-situ experimental studies based on synchrotron radiation can also result in valuable data to support computer models, as real-time study of such diffusionless phase transformations will be crucial to broaden the understanding of microstructural development and related structure-property relationships.
Bainite is formed at cooling rates slower than that for martensite formation and faster than that for ferrite and pearlite formation. There are two forms of bainite, known as upper and lower bainite. There are several proposed formation mechanisms, based on the carbon content and transformation temperature of the steel, resulting in slightly different morphologies.
Low carbon steels exhibit fine bainitic laths, nucleated by a shear mechanism at the austenite grain boundaries. Carbon solubility in bainitic ferrite is much lower than in austenite, so carbon is rejected into the austenite surrounding the bainitic ferrite laths.
As the carbon content increases, the cementite filaments become more continuous, and at high carbon contents, the bainitic ferrite laths are finer with the cementite stringers more numerous and more continuous. The structure can appear more like pearlite, and is termed 'feathery' bainite. The transformation nucleates, like upper bainite, by partial shear. However, the overall mechanism of lower bainite formation is independent of carbon content in the main. The appearance of lower bainite strongly resembles that of martensite, but lower bainite is formed by a mixture of shear and diffusional processes rather than just shear.
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