The Expanded Lever Rule on the Phase Transformation Rate Curve for Phase Volume Fraction Calculation in Thermal Simulations

The expanded lever rule is more efficient for calculating phase volume fraction when two phases are partly overlapped. It can make up the deficiency of metallography analysis when phases are similar in morphology. In this study, the expanded lever rule on the phase transformation rate curve is introduced, the approximate symmetry principle in applying the expanded lever rule is modified, and the resulting measurement uncertainty is discussed. Results show that (i) the calculated volume fraction of pearlite by the expanded lever rule in an overlapped two-phase process of a high strength low alloy (HSLA) weld metal is in accordance with the result by metallography analysis, and (ii) the expanded lever rule has an advantage over metallography analysis as it can calculate upper bainite, lower bainite, and lath-like martensite volume fractions in a three-phase process with two overlaps of a HSLA steel. The expanded lever rule is applicable to overlapped two-phase processes of any material theoretically since each phase process can be described by Johnson–Mehl–Avrami–Kolmogorov equation.