Determination of Martensite Start Temperature for Engineering Steels Part II. Correlation between Critical Driving Force and Ms Temperature

Abstract

Various physical models are discussed based on different correlations established between the Ms temperature, the critical driving force, and the steel 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 driving force was calculated by thermodynamic software and the Ms temperature was predicted by a validated artificial neural network model. Two basic physical models are discussed: the Ms-dependent model and the chemistry-dependent model. In the Ms-dependent model, the critical chemical driving force is linearly related to the Ms temperature; the effect of the steel chemistry 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 account. The chemistry-dependent model is based on the hypothesis that the critical driving force can be fully represented in terms of the steel chemistry. The critical driving force has been estimated using either linear, exponential, Pythagorean or mixed superposition laws. Comparisons of the critical driving force predicted by these addition methods with the thermodynamic result indicate that an exponential addition method, with the optimum exponent index value of 2.07 (approximately square) gives the best predictive result. The quality of the linear relation between the critical driving force and the Ms temperature is improved slightly if the critical driving force is corrected for the elastic strain energy, estimated by assuming that the elastic moduli, lattice constants, and molar volumes of ferrite and austenite are both temperature- and chemistry-dependent, is removed from the critical driving force. Analysis indicates that the simple Ms-dependent model will be improved after considering the extra minor effect of steel chemistry. In contrast, the error of the chemistry-dependent model can be hardly related to the Ms temperature. Based on the above analysis, an accurate mixed Ms-dependent plus chemistry-influence model, in which the elastic strain energy is considered, is developed, and which yields for the chemical driving force −ΔG_r^*=3247−4.8446 · Ms (°C), with a standard error of only 40.7 J/mol.