Critical cooling temperatures and phase transformation kinetics in structural steels determined by mean flow stress and dilatometry

Abstract

Austenite to ferrite transformation temperatures during cooling (Ar ₃ , Ar ₁ ) have been determined using two different techniques. The first technique consisted of simulation of a rolling sequence by torsion, following the method of Jonas et al., representing mean flow stress (MFS) versus the inverse of the temperature for each pass. This method also makes it possible to estimate the influence of the residual stress of austenite on the transformation temperatures. The second approach has been the classic dilatometry technique, and comparison of the results has led to an evaluation of the possible errors that can occcur when apparent values, related with a notable change in the slope of the dilatometric curve or in the MFS straight line, are taken as Ar ₃ and Ar ₁ , respectively. The kinetics of austenite transformation during cooling have been determined and an Avrami expression established from the data. The present study has considered six steels with different Ti and N contents to assess the influence of chemical composition on the transformation temperatures.