A combination of thermodynamic and kinetic theory describing the atomic mechanism of displacive transformation has been used to develop steels which transform to bainite at temperatures as low as 125°C. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2500 MPa together with an ability to flow plastically before fracture. The toughness is in excess of 30-40 MPa m1/2. The low bainite-start temperature is a consequence of the high carbon concentration and to a lesser extent, solutes such as manganese, chromium which in the present context increase the stability of austenite relative to ferrite. The alloys also contain sufficient silicon to suppress the precipitation of cementite from austenite during bainite formation.
The microstructure is generated at temperatures which are so low that the diffusion of iron is inconceivable during the course of the transformation to bainite. As a result, slender plates of ferrite, just 20-40 nm thick are generated, giving rise to those extraordinary properties. However, it may take several days in order to achieve the required degree of transformation at low temperatures. In some commercial scenarios it may be useful to accelerate transformation without losing the ability to utilise low temperatures. Certain elements such as cobalt and aluminium have been found to increase the free energy change when austenite transforms and hence to accelerate its decomposition.
In this work we report metallographic details of the very fine bainitic microstructure associated with the incredibly low transformation temperature. X-ray diffraction analysis of the retained austenite has shown that it is enriched in carbon to a concentration close to To boundary, as expected from the incomplete reaction phenomenon. The X-ray analysis also revealed higher ferrite carbon concentrations than the expected paraequilibrium solubility levels. Atom probe microanalysis results on the carbon concentration bainitic ferrite confirmed the X-ray data and revealed that a substantial quantity of carbon was trapped at dislocation in the vicinity of the ferrite/austenite interface.
Proceedings of Super-High Strength Steels, 2--4 November 2005, Rome, Italy, Associazione Italian di Metallurgica, pp. 1--8.
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