Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancem
- PDF / 2,132,909 Bytes
- 13 Pages / 612 x 792 pts (letter) Page_size
- 37 Downloads / 190 Views
I.
INTRODUCTION
THE isothermal heat treatments applied to microalloyed steels produce very different types of structures, depending on the temperature and time of treatment.[1] Contingent upon the transformation temperature, ferritic-pearlitic structures originated by diffusion-controlled processes, bainitic or martensitic structures originated by a displacive mechanism, and all the variants within each type of microstructure can be formed. Other factors, such as composition and austenitization temperature and time, which determine the austenite grain size, also influence the resulting microstructure.[2,3] Acicular ferrite formation is a mechanism competitive with bainite formation, as generally recognized.[4] In the latter case, clusters of bainitic plates grow, emanating from austenite grain surfaces, whereas acicular ferrite plates nucleate intragranularly. The acicular ferrite formation, promoted by the presence of nonmetallic inclusions, is well documented.[4,5,6] These second-phase particles, acting as point sites from which intragranular nucleation is developed, promote the formation of acicular ferrite. Another determining variable is the austenite grain size. The reduction of the austenite grain boundary area per unit volume[3,7] diminishes the bainite nucleation sites, a condition obtained by using a high austenitization temperature. The process of nucleation in the inclusions, together with sympathetic nucleation, leads to the fine-grained interlocking microstructure of acicular ferrite.[4,7–11] As a particular case in bainitic structures,[1] acicular ferrite is a microstructure generally associated with welding,[12–15] giving an optimal combination of mechanical properties as compared to bainitic structures. This is why, even if the mechanisms
I. MADARIAGA, Ph.D. Student, J.L. ROMERO, Researcher, and I. GUTIE´RREZ, Senior Researcher, are with the Department of Materials, CEIT, 20009 San Sebastian, Basque Country, Spain. Manuscript submitted April 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
associated with its nucleation are not fully understood, it is a desired phase in welds. Recent work[16–20] has shown that acicular ferrite structures obtained in medium-carbon microalloyed steels by thermomechanical treatments lead, as in the case of lowcarbon steels for welding, to a good combination of mechanical properties. In the present article, the influence of the time and the isothermal treatment temperature on the microstructure of a microalloyed steel have been investigated with a special regard to the composition and the austenitization treatment suitable for the formation of acicular structures. Such study has included both the amount and distribution of the different components produced during the transformation of austenite to acicular ferrite, the fine structure being resolved with transmission electron microscopy (TEM) using thin foils and carbon extraction replicas. Special emphasis has been placed on investigating the presence of inclusions responsible for the nucleation of ferrite units
Data Loading...
