THE EFFECT OF ALLOYING ELEMENTS ON THE TEMPERATURE RANGE OF PEARLITE TO AUSTENITE TRANSFORMATION IN LOW ALLOY HYPOEUTECTOID STEELS

The formation of austenite above A1 temperature plays very important role in the heat treatment of hypoeutectoid steels, especially automotive DP steels. It is widely accepted that the formation of austenite during intercritical annealing takes place in three stages: 1 - very rapid pearlite to austenite transformation, 2 - slower growth of austenite into ferrite, 3 - slow final equilibrium of ferrite and austenite. According to the literature data, important factors that influence the phase transformation kinetics are the cementite morphology, the grain size and the heating rate. In this work, experimental results of dilatometric examinations of low alloy normalized hypoeutectoid steels show that during heating at the same rate, temperature range of pearlite to austenite transformation strongly depends on the amount of alloying elements and whether they are ferrite or austenite stabilizers. ThermoCalc and DICTRA computational tools were also used to calculate the eutectoid regions of the equilibrium phase diagrams for some low alloy steels.