Time-Temperature-Transformation Diagram within the Bainitic Temperature Range in a Medium Carbon Steel

Abstract

The time-temperature-transformation (TTT) diagram within the medium temperature range of medium carbon steel has been determined. A single type of C-curve is found within the bainite temperature range for the studied steel. Distinct reaction C-curves were not observed for both types of microstructure, upper bainite and lower bainite in the TTT diagram. Experimental results on the kinetics of the isothermal formation of bainite at different temperature have demonstrated that both type of microstructure, upper and lower bainite, possesses similar overall transformation kinetics. Some applications of phase transformation theory towards the formation of bainitic microstructures are discussed, with particular emphasis on the bainite start temperature, transition temperature from upper to lower bainite, martensite start temperature and the thickness of bainitic plates. Bainite can be regarded as a non-lamellar mixture of ferrite and carbides, but within this broad description, it is possible to identify two classical morphologies, traditionally named upper and lower bainite. Both upper and lower bainite consist of plates of ferrite, known as sub-units, separated by cementite. The plates of ferrite grow in aggregates called sheaves of bainite. Within each sheaf, the plates of ferrite are parallel and share a common crystallographic orientation. The essential difference between upper and lower bainite is with respect to the carbide precipitation. In upper bainite, the bainitic ferrite is free of precipitation; carbides grow from the regions of carbon enriched austenite, which are trapped between the sub-units of ferrite. By contrast lower bainitic ferrite contains a fine dispersion of plate-like carbides within the bainitic ferrite plates. Both upper and lower bainite are formed by the propaga- tion of displacive ferrite sub-units followed by the partition of the excess carbon into the residual austenite. Cementite can then precipitate from the enriched austenite between the ferrite plates. The growth of the bainitic sub-units is accompanied by a change in the shape of the transformed region, which can be described as an invariant-plane strain with a large shear component. The parent austenite cannot accommodate the large shape deformation elastically and relaxes by plastic deformation in the region adjacent to the bainite. This plastic deformation stifles the growth of bainite sub-units before they hit any obstacle, and the transformation proceeds with the formation of a new sub-unit. 1) The mechanism of transformation continues until the carbon concentration of the residual austenite reach the To curve, at which ferrite and austenite with identical chemical compo-