Kinetics of Delta-Ferrite to Austenite Phase Transformation in a Two-Phase Fe-Al-C Alloy TIHE ZHOU, HATEM S. ZUROB, ELACHMI ESSADIQI, and BENOIT VOYZELLE The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer in a Fe-Al-C alloy. The results showed that the austenite phase nucleated along the delta grain boundaries. The transformed austenite morphology changed from cellular to Widmansta¨tten pattern when the holding temperature decreased from 1398 K to 1123 K (1125 °C to 850 °C). Full partitioning of the substitutional alloying elements was observed and the spacing of the austenite plates was controlled by the diffusing distance of the substitutional elements. Interestingly, growth of the austenite front was controlled by the long-range diffusion of carbon from the center of the delta grains to the growing front. Deformation of the delta phase enhanced the nucleation of austenite at existing grain boundaries and newly formed subgrain boundaries. DOI: 10.1007/s11661-011-0747-3 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION BECAUSE of their good combination of strength and toughness, microalloyed (MA) steels are used in a wide range of applications including oil and natural gas transmission pipelines, offshore equipment, automotive components, and machinery. [1–4] In order to achieve good toughness (low ductile-to-brittle transition tem- perature), weldability, formability, and corrosion resis- tance, the carbon content of most MA steels is maintained at ~0.03 to 0.08 wt pct. [5,6] When the carbon content is lower than 0.08 wt pct, MA steels would solidify as delta-ferrite. Fast diffusion in the delta phase leads to rapid homogenization of the as-cast micro- structure. Upon further cooling, the delta-ferrite trans- forms to austenite, and at yet lower temperatures, austenite transforms to alpha-ferrite. While the theory of solidification of MA steels [7–10] and the kinetics of austenite to alpha-ferrite phase transformation [11–14] were extensively studied and developed, comparatively little information is available concerning the kinetics of the delta-ferrite to austenite transformation and its effect on the grain size. The reason for this is that the final alpha-ferrite grain size is largely controlled by the parent austenite grain size. [11,12,15] A smaller austenite grain size will lead to refinement of the alpha ferrite grain size. In modern MA steel production processes such as thin slab casting direct rolling (TSCDR), the opportunities for control of the microstructure through thermomechanical are limited by the small number of deformation passes available. Previous study shows that the delta-ferrite and austenite grain size (before entering the homogeni- zation furnace) is nonuniform with large grains being present inside the slab center. [16] It is often observed that during TSCDR, thermomechanical processing can re- duce the average austenite grain size, but cannot eliminate the nonuniformity; a small number of large austenite grains persist in the microstructure. [17,18] Therefore, understanding and controlling the delta- ferrite to austenite phase transformation becomes increasingly important to obtain fine and uniform austenite grains and, subsequently, final alpha-ferrite grain size. The main difficulty in studying this transfor- mation (d fi c) is that any untransformed delta-ferrite would transform on quenching, making it impossible to observe the progress of the transformation in quenched specimens. A second obstacle is that the d fi c transformation temperature is very high, which leads to frequent problems of oxidation and decarburization. In order to overcome these obstacles, this article used Fe-1.5 pct Al model alloy. In this alloy, the delta phase can be retained down to room temperature (RT) while the austenite transforms to martensite, which makes it possible to follow the transformation in quenched specimens. The possibility of increasing austenite nucle- ation sites using deformation of the delta-ferrite prior to the phase transformation is also investigated. II. EXPERIMENTAL PROCESS In order to observe the delta-ferrite to austenite phase transformation by standard optical metallography tech- nology, 1.5 pct Al was added to a low carbon alloy in order to stabilize the delta phase on quenching, as shown in the Fe-1.5 pct Al phase diagram in Figure 1. The addition of Al is based on the fact that Al is the most efficient delta-ferrite stabilizer. The composition of TIHE ZHOU, Postdoctoral Fellow, and HATEM S. ZUROB, Assistant Professor, are with the Steel Research Centre, McMaster University, Hamilton, ON L8S 4L7, Canada. Contact e-mail: zurobh@mcmaster.ca ELACHMI ESSADIQI and BENOIT VOYZELLE, Research Scientists, are with the CANMET Materials Research Laboratory, Ottawa, ON K1A 0G1, Canada. Printed by permission of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2011. Manuscript submitted January 24, 2011. Article published online June 3, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, NOVEMBER 2011—3349