A model for the development of microstructure in low-alloy steel (Fe-Mn-Si-C) weld deposits

H. K. D. H. Bhadeshia, L.-E. Svensson and B. Gretoft

Abstract

Experimentally observed microstructural variations in a series of low-alloy steel weld deposits containing different carbon concentrations (produced using experimental electrodes) are discussed in terms of a phenomenological model based on phase transformation theory. The model requires an input of austenite grain size, chemical composition and the cooling curve of the fusion zone; this allows the calculation of isothermal transformation diagrams and quantities necessary to define the start and finish temperatures of various reactions. Allotriomorphic ferrite growth is assumed to occur by a paraequilibrium transformation mechanism; its formation is found to determine the development of both Widmanstatten and acicular ferrite. It seems possible to rationalise the microstructural variations in terms of phase transformation theory, and although the presence of inclusions for the heterogeneous intragranular nucleation of acicular ferrite seems necessary, the inclusions, when present in a concentration beyond a limiting value, do not seem to control the overall development of microstructure.

This research is designed to predict the microstructural evolution of low-alloy steel weld deposits. By utilising phase transformation theory, the authors analyze how specific inputs-such as chemical composition, cooling rates, and austenite grain size-determine the final volume fractions of various ferritic phases. The study highlights that allotriomorphic ferrite is the primary phase to emerge during cooling, effectively limiting the space available for subsequent reactions. Furthermore, the model demonstrates that the development of Widmanstätten and acicular ferrite is heavily influenced by impingement effects and the presence of non-metallic inclusions. Ultimately, this theoretical framework aims to reduce the need for expensive empirical testing by allowing for the mathematical rationalisation of weld properties based on their thermal history.

Acta Metallurgica, Vol. 33, 1985, pp. 1271-1283.

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