Modelling of Phase Transformation Kinetics by Correction of Dilatometry Results for a Ferritic Nb-microalloyed Steel

Abstract

Using the dilatometry technique, g→a transformation kinetics has been determined at different cooling rates in a steel with low carbon and low niobium contents (0.09 and 0.017 mass% respectively). First of all the real and the conventional transformation temperatures of the steel were determined. The real start temperature for proeutectoid ferrite formation (AЈ r3 ) corresponds to the point where the dilatometric curve starts to diverge from the straight during cooling. The conventional start and finish temperatures for proeutectoid ferrite formation (A r3 and A r1 ) are given by two points close to the minimum and the first maximum of the curve, respectively. The real start and finish eutectoid transformation temperatures -(AЈ r1 ) s and (AЈ r1 ) f -correspond to the second point of inflection and a point close to the second relative maximum of the curve, respectively. Carbon enrichment of the remaining austenite, as the transformation to ferrite advances, is corrected taking into account the dependence on the carbon content of the atomic volume of austenite. On the other hand, the dilatometric data have also been corrected with regard to the different expansion coefficients of austenite and ferrite. In this way it has been seen that the lever-rule method applied to the dilatometric curve is useful for determining transformation temperatures, but not for determining transformation kinetics, since the amount of proeutectoid ferrite calculated with this method was up to 10 % greater than the real amount measured with an image analyser. Finally a model based on Avrami's law has been developed for the real g→a transformation kinetics.

Method to Evaluate the Kinetics of Bainite Transformation in Low-Temperature Nanobainitic Steel Using Thermal Dilatation Curve Analysis

“…A great deal of research has been conducted to develop a model of lattice parameters by regression of chemical composition of alloy elements with the help of XRD (X-ray diffraction) [21]. There are other forms of lattice parameter equations except Equations (9) and (11) [1,5,24]:…”

“…Dilatometry is a powerful technique to study the phase transformation kinetics of steels [1][2][3][4][5][6][7][8].…”

Development of Technique for Identification of Phase Transformation Model Parameters on the Basis of Measurementof Dilatometric Effect-Direct Problem

“…20) In the present work, the dilatometric test is carried out at a constant cooling rate. In such case, the temperature T is directly proportional to the time t As has been shown by Gomez et al, 21) the dilatometric transformation curves obtained under constant cooling rate present two differentiated zones that corresponded to the austenite to proeutectoid ferrite and remaining austenite into pearlite transformations and can be described by an Avrami equation as a function of time. Taking this into consideration, the following equation has been used to model the austenite to ferrite transformation in this part of the work, assuming that the amount of pearlite is significantly smaller than the amount of ferrite for relatively low cooling rates:…”

Mathematical modeling of dilatometric behavior of 420A and 440C martensitic stainless steels used in surgical tools

“…The fraction of phase A is considered , where x is the distance between the extended line of field I (f 1 ) and the dilatometric curve, and y is the distance between the extended line of field III (f 3 ) and the dilatometric curve (Gomez and Medina, 2003).…”

“…Dilatometry is a technique applied to study phase transformations in solid state, allowing real-time monitoring of the transformation progress in terms of dimensional changes, according to the thermal cycling program (Caballero et al, 2001a;Gomez and Medina, 2003). Using this technique diagrams of the dilatometry temperature variation versus the length of the material are obtained under controlled heating and cooling.…”