Determination of martensite start temperature for engineering steels part II. Correlation between critical driving force and Ms temperature

Various physical models are discussed based on different correlations estab lished between the Ms temperature, the critical driving force, and the stee l chemistry. These correlations were derived for a group of Fe-(0.2-0.5)C-( 0.5-2.0)Mn-(0.5-2.0)Si-(0.5-2.0)Cr-(0.1-0.7)Mo test alloys, which serve as a good representative for most low alloy engineering steels. The chemical d riving force was calculated by thermodynamic software and the Ms temperatur e was predicted by a validated artificial neural network model. Two basic p hysical models are discussed: the Ms-dependent model and the chemistry-depe ndent model. In the Ms-dependent model, the critical chemical driving force is linearly related to the Ms temperature: the effect of the steel chemist ry is indirect. The standard error of the simple Ms-dependent model is 51.9 J/mol when the spontaneous Zener ordering of carbon atoms is taken into ac count. The chemistry-dependent model is based on the hypothesis that the cr itical driving force con be fully represented in terms of the steel chemist ry. The critical driving force has been estimated using either linear, expo nential, Pythagorean or mixed superposition laws. Comparisons of the critic al driving force predicted by these addition methods with the thermodynamic result indicate that an exponential addition method, with the optimum expo nent index value of 2.07 (approximately square) gives the best predictive r esult. The quality of the linear relation between the critical driving forc e and the Ms temperature is improved slightly if the critical driving force is corrected fur die elastic strain energy, estimated by assuming that the elastic moduli, lattice constants. and molar volumes of ferrite and austen ite are both temperature- and chemistry-dependent is removed from the criti cal driving force. Analysis indicates that the simple Ms-dependent model wi ll be improved after considering the extra minor effect of steel chemistry. In contrast, the error of the chemistry-dependent model can be hardly rela ted to the Ms temperature. Based on the above analysis, an accurate mixed M s-dependent plus chemistry-influence model, in which the elastic strain ene rgy is considered, is developed, and which yields for the chemical driving force -Delta G(r)* = 3247 - 4.8446 . Ms (degrees C), with a standard error of only 40.7 J/mol.