Engineering Concepts: Segregation and Banding in Steel
Most metals are classified as being either cast or wrought. This classification is somewhat misleading since even the wrought product is initially cast and therefore subject to chemical inhomogenieties during solidification. Dendritic microstructures are common in almost all engineering alloys; a dendritic microstructure for a cast aluminum bronze is shown in Fig. 1.
It should be noted that the alloy can be segregated to either the dendrite core or to the interdendritic regions depending upon the liquidus and solidus lines of the phase diagram (Fig. 2). The first liquid to solidify (center of the dendrite) will be alloy rich if the alloy addition raises the melting temperature, whereas the alloy will be segregated to the interdendritic regions if the alloy addition lowers the melting temperature.
In wrought materials, the dendritic microstructure is partially broken down by plastic deformation, which causes the dendritic arms to be reoriented parallel to the rolling direction. This creates the banded microstructures of ferrite and pearlite that are observed in plane carbon steels resulting from the segregation of manganese during solidification. A greater rolling reduction produces a finer spacing between the bands. The segregation of manganese also produces a partitioning of the carbon because manganese is an austenite stabilizer. Thus, as the steel is cooled through the critical temperatures to form ferrite and pearlite, it is the manganese rich regions that form the last austenite to transform. Perhaps a better way to look at this phenomenon is that the manganese-lean region has a higher critical temperature for the formation of ferrite.
Segregation of alloy can produce tremendous headaches with regard to controlling hardenability in steel. At intermediate quenches, the difference in hardenability between alloy-rich and -lean regions is manifested as banded microstructures of martensite and bainite as shown in Fig. 3 for an AISI 4150 alloy steel. In carburized and hardened microstructures, the alloy-rich regions have greater amounts of retained austenite because the segregated alloy reduces both the martensite start and finish temperatures.
Microradiography can be used to detect segregation in steel. This technique relies on the elemental dependence of x-ray absorption. For example, the absorption of cobalt Ka radiation is nearly an order of magnitude greater for manganese than it is for iron. To reveal segregation, a thin slice of polished metal (approx. 0.04 mm, or 0.0015 in. thick) is placed directly on a fine-grained photographic paper. As the specimen is irradiated, the manganese-lean regions absorb fewer x-rays and result in a higher intensity being recorded on the photographic paper. The film is then examined at magnifications as high as 200X to reveal the scale and magnitude of the segregation.
Segregation and banding in steel has always existed, but in the past it was economically feasible to homogenize the steel in soaking pits. Currently, ingot-soaking pits are no longer practical as most steels are continuously cast, and soaking pits would not be considered simply because of the added cost of equipment, space, time and energy.