Thermodynamics of the Acicular Ferrite Transformation in Alloy-Steel Weld Deposits

J. R. Yang and H. K. D. H. Bhadeshia


The mechanism by which acicular ferrite grows is not understood and this hinders the development of phase transformation models used in the prediction of microstructure. In this work, we report experiments on the thermodynamics of the formation of acicular ferrite from austenite. It is found that on cooling below the Bs temperature, the degree of transformation to acicular ferrite increases, from zero at Bs, to an extent dependant on the undercooling below the Bs temperature. However, at any given isothermal transformation temperature, the transformation ceases prematurely when the carbon concentration of the austenite reaches the level where diffusionless decomposition becomes thermodynamically impossible. The transformation thus exhibits the "incomplete reaction phenomenon" typical of the bainite reaction. Acicular ferrite is found to be similar to bainite, its growth being diffusionless, with carbon redistribution occurring after the initial formation of the ferrite plate. Acicular ferrite differs morphologically from bainite because it nucleates intragranularly on inclusions and because of hard impingement betweenplates nucleated on adjacent inclusions.

Advances in Welding Technology and Science,
ASM, Metals Park, Ohio, U. S. A., 1987, pp. 187-191.

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