Tempering of a Hard Mixture of Bainitic Ferrite and Austenite
C. Garcia-Mateo, M. Peet, F. G. Caballero and H. K. D. H. Bhadeshia
Materials Science and Technology, Vol. 20, 2004, 814-818.
Abstract
Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV, tensile strength about 2.3 GPa and a toughness of some 30 MPa m1/2. Because these properties rely on the fine scale of the microstructure, a study has been carried out on the tempering resistance of the steel over the temperature range 350-750°C. It is found that significant softening occurs only after the plates of ferrite begin to coarsen. The coarsening process is hindered by the intense precipitation of carbides resulting from the decomposition of the carbon-enriched retained austenite. The carbides themselves lead to some precipitation strengthening during the early stages of tempering. The ferrite is found to contain excess carbon, beyond its solubility limit, and X-ray analysis indicates that the carbon is associated with heterogeneous strains in the microstructure. It does not readily precipitate until the onset of substantial recovery during annealing.
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