• PDF / 1,070,553 Bytes
• 8 Pages / 612 x 792 pts (letter) Page_size
• 50 Downloads / 197 Views

DOWNLOAD

REPORT

---

ON

THE austempering transformation in cast irons, which leads to end products with the best mechanical properties, is described as a three-stage process.[1–4] In stage I, parent austenite transforms into acicular ferrite and high-carbon austenite (␥p → ␣Fe ⫹ ␥hc), forming a microstructure called ausferrite. Carbon atoms are rejected from the growing ferrite plates, causing the enrichment of the surrounding austenite. The driving force for this transformation arises from the carbon-concentration gradient set up in the austenite as result of local equilibrium at the ferrite/austenite interface. Nucleation and growth processes govern the transformation kinetics.[5,6] The solid-solid nucleation theory[7] postulates that the driving force for nucleation is proportional to the volume free-energy change and the volume strain energy, while the interfacial free energy of the cluster acts as a barrier to this process. In terms of the temperature dependence of the nucleation process, from expressions reported by Doherty,[8] it could be deduced that, as the undercooling increases, the volume free-energy change increases, promoting a higher nucleation rate. Several authors have treated the growth of a ferrite plate in an austenite matrix from a theoretical point of view,[9,10] proposing that the rate of growth is controlled by the diffusion of carbon atoms through the austenite away from the tip of the advancing particle. The study of the thermal dependence of the austempering kinetics in cast irons can contribute to the determination of the times needed to reach the optimum mechanical properties. In the case of the compacted graphite (CG) cast iron, its high thermal-fatigue resistance makes the material optimum to be used in operational conditions of thermal cycling. This feature could be complemented by the improvement K.F. LANERI, Teaching Assistant, and J. DESIMONI and R.C. MERCADER, Professors, Departamento de Fisica, Facultad de Ciencias Exactas, UNLP, La Plata University, 1900 La Plata, Argentina, are also Fellow and Researchers, respectively, at CONICET, Argentina. R.W. GREGORUTTI, Teaching Assistant, and J.L. SARUTTI, Professor, Laboratorio de Entrenamiento Multidisciplinario para la Investigacion Technologica, La Plata University, are also Research Assistants, CICBA, Argentina. Manuscript submitted January 11, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A

of the mechanical properties achieved through the austempering heat treatment. A previous study[11] determined that the evolution of the fraction of ␥hc during the stage I transformation of CG cast irons has a sigmoidal behavior and the kinetics of transformations through the k and n parameters of the Johnson–Mehl’s equation was quantified.[5,6] The results indicated that the transformation proceeds through a localized nucleation and a phase transformation controlled by an interface reaction.[6] After the nucleation sites get saturated, the advance of the transformation is further controlled by a diffusion process.[12] However, the dependence of the parameters on