MICROSTRUCTURAL ENGINEERING APPLIED TO THE CONTROLLED COOLING OF STEEL WIRE ROD .2. MICROSTRUCTURAL EVOLUTION AND MECHANICAL-PROPERTIES CORRELATIONS

In the second part of this paper, the microstructural evolution and mechanical properties of plain-carbon steel rods which have been subjected to known cooling conditions are described. Specifically, the isothermal phase transformation kinetics for the decomposition of austenite into ferrite and pearlite have been determined with a diametral dilatometer and characterized in terms of empirical coefficients in the Avrami equation. The continuous cooling transformation (CCT) start time, fraction ferrite, ferrite grain diameter, and pearlite interlamellar spacing have been quantified and correlated with steel composition and cooling rate. Tensile tests have been conducted to obtain yield strength (YS) and ultimate tensile strength (UTS), which, with literature data, have been related to the microstructure and composition of the steels. These correlations, which apply to both hypoeutectoid and eutectoid steels, have been incorporated in a mathematical model of the Stelmor process, to be described in Part III of this article.[44]