Initial stage of isothermal decomposition of austenite to ferrite and graphite in spheroidal graphite cast iron

摘要

The isothermal decomposition of austenite to ferrite and graphite in a spheroidal graphite cast iron was investigated by dilatometry metallography, and image analysis. The results gave quantitative, although indirect, information on the kineticsof carbon transfer from austenite to graphite, as well as on the kinetics of the austenite to ferrite and graphite transformation. The relationship between the kinetics and the graphite nodule count was highlighted. The incubation period before ferritegrowth was found to be associated with a global contraction of the material, in spite of the related increase in the graphite volume fraction. Calculations showed good agreement with experimental results, except for the rate of carbon transfer which wasfound to be much slower experimentally than predicted by calculations based on the volume diffusion of carbon. Possible reasons for this discrepancy are discussed.