Changes in Solidification Mode, and the Measurement of Cooling Rates following Solidification during Arc Welding

The solidification process in a weld pool has been shown to have a considerable influence upon the properties of the resultant weld. It influences the development of microstructure (Edvardsson et al., 1976), and determines the distribution of alloying elements, and hence the homogeneity of the weld. Previous work on the cooling behaviour of welds (Garland and Kirkwood, 1974; Svensson et al., 1986) has tended to concentrate on the measurement and prediction of cooling rates for the temperature range 800-500°C, because these are important in determining the mechanism of austenite decomposition in the weld. This work, however, is concerned with cooling conditions and solidification processes at, and near, the melting temperature. The microsegregation that occurs during solidification is closely related to the size and morphology of the solidifying phase, the crystal structure of which in turn is dependent upon the cooling rate. For example, it is known that with fast cooling rates, low carbon steels may solidify directly as austenite, when 8-ferrite would be expected for cooling conditions closer to equilibrium (Edvardsson et al., 1976), since substitutional atom diffusion is slower in austenite, solidification with non-equilibrium austenite as the primary phase would profoundly influence the homogeneity of the weld deposit.