The principle of additivity and the proeutectoid ferrite transformation
Creators: name: Kamat, Rajeev Gurunath
Publisher: name: University of British Columbia
Year: 2011

description: This study critically examines the additivity of the proeutectoid ferrite transformation. The study has been carried out in two parts with the first part involving experimental verification of the additivity of the proeutectoid ferrite transformation and the second part a theoretical assessment of the additive nature of the ferrite transformation with the aid of mathematical models. Austenite-to-proeutectoid ferrite transformation kinetics were measured under a number of isothermal, stepped-isothermal and continuous-cooling conditions for three plain-carbon hypo-eutectoid steel grades (AISI 1010,1020 and 1040) using a dilatometer and a Gleeble 1500 thermomechanical simulator. Isothermal transformation kinetics were characterized using the Avrami equation. The stepped-isothermal transformation tests were designed to experimentally assess the additive nature of the austenite to proeutectoid ferrite transformation by measuring transformation kinetics partially at one temperature and after a rapid temperature change to another temperature. Results on the 1010 and 1040 steels showed that the proeutectoid ferrite transformation with polygonal morphology is additive under changing temperature in that the ferrite transformation kinetics at the second temperature are quite similar to the isothermal kinetics at that temperature. Stepped-isothermal transformation measurements were made on the 1020 steel with the resulting ferrite morphology either remaining Widmanstatten at both temperatures or changing from allotriomorph to predominantly Widmanstatten at the two temperatures. In both cases the results showed additive behavior. However, the stepped-isothermal test in which the proeutectoid ferrite was transformed to an equilibrium amount and equilibrated at the first temperature and then rapidly changed to the second temperature was not additive. Characterizing the isothermal formation of proeutectoid ferrite in the three steels using the Avrami equation resulted in a reasonably constant value of n and the b parameter increasing with increasing transformation temperature. Early site saturation was evident in a number of test specimens. In the second part of the study, two mathematical models with planar and spherical interface geometries were developed to theoretically assess the additivity of the proeutectoid ferrite transformation. A finite-difference numerical technique was employed to describe the austenite to ferrite diffusion controlled moving-interface problem for a system having finite boundaries. A test of additivity of the proeutectoid ferrite transformation was made by predicting the ferrite growth kinetics and the associated carbon gradients under stepped-isothermal conditions. The predictions were consistent with the observed experimental additivity of the proeutectoid ferrite transformation in the 1010 steel. The spherical model predicted isothermal ferrite growth kinetics compared more favorably with the experimentally measured kinetics of the 1010 steel than the planar model. An unusual phase was detected in the 1020 steel. A number of tests were performed to measure the isothermal transformation kinetics of the new phase. Scanning electron microscopy (SEM) and scanning-transmission electron microscopy (STEM) were used to further investigate the structural details in this new phase. The results indicated that the new phase is a type of bainite but having some of the characteristics attributable to massive transformation products.