Pearlite transformation during continuous cooling and its relation to isothermal transformation.

Abstract

Synopsis An attempt was made to predict the cooling transformation over the entire range of pearlite reaction , from the isothermal transformation kinetic data. A kinetic equation of pearlite transformation during cooling was derived by applying an additivity rule to the transformed fractions. A good agreement was observed between the measured and calculated cooling transformation curves although the pearlite reaction would not precisely satisfy any one of the additive conditions. A possible deviation of a cooling transformation kinetics from that estimated with the additivity rule was considered based on the nucleation and growth process. It was found that the transformed fractions of pearlite appeared to be additive firstly because the change in the ratio of the nucleation to the growth rate as a function of supercooling was small, and secondly because the cooling transformation occurred either nearly isokinetically when the cooling rate was slow or almost in the condition of site saturation when the cooling rate was fast. II. Additivity Rule and Cooling Transformation Kinetics 1. Additivity Rule The additivity rule was advanced by Scheill) and later, independently, by Steinberg,2) to predict the start of transformation under non-isothermal conditions. Scheil' proposed that the time spent at a particular temperature, ti, divided by the incubation time at that temperature, zi, might be considered to present the fraction of the total nucleation time required. When the sum of such fractions (called the " fractional nucleation time ") reaches unity , transformation begins to occur, i.e., I. Introduction In the practical heat treatment of steels, the decomposition of austenite usually occurs during cooling rather than in isothermal condition. However, experimental and theoretical aspects of the decomposition are most conveniently studied isothermally. So it is desirable to relate the transformation behaviour of a steel during continuous cooling with its iso-thermal transformation data. There have been several attempts to predict the cooling transformation from an experimentally determined set of isothermal transformation curves.l-12) Scheil,l) and later, Steinburg,2) presented an additivity rule for calculating the temperature at which transformation begins during continuous cooling. An additive reaction implies that the total time to reach a specified stage of transformation is obtained by adding the fractions of the time to reach this stage isothermal-ly until the sum reaches unity. This additivity rule has provided a basis for many of the subsequent works on the subject.3-12) However, most of the previous studies have been limited to the beginning of transformation , and not many work13-15) appear to have been carried out about the continuous cooling transformation over the entire range of reactions. The present study was undertaken to predict an entire range of cooling transformation to pearlite from an isothermal transformation kinetic data. The progress of pearlite transformation during continuous cooling was measured and compared with that predicted by assuming that the transformed fractions are additive.