Proposition of an Empirical Model for Determination of Critical Temperatures During Continuous Cooling in Heat Affected Zones of IF Steels Welded by the TIG Process

Abstract

IF steels are widely used in the automotive industry. In the course of the welding applications, they undergo the transformation of austenite (γ) → ferrite (α) phase that occurs by nucleation and grain growth and, under isothermal conditions, is described by the JMAK equation. However, post-weld phase transformations occur under continuous cooling conditions. Thus, the present work characterized the transformation kinetics γ → α and proposed an empirical model to predict critical temperatures during continuous cooling in heat affected zones of three types of IF steels. Samples of the steels were submitted to dilatometric tests with different rates of continuous cooling. The critical phase transformation temperatures (γ → α) were determined and an empirical equation was obtained which correlated the critical temperature with the cooling rate, which enabled the predictability of phase transformation kinetics. Mechanized TIG process was applied to IF steels with thermal cycling monitoring and, in regions of the HAZ with austenitizing conditions and cooling rates similar to those used in the dilatometric tests, it was verified that the obtained empirical equation is a viable alternative to predict the critical temperatures and the microstructure during the continuous cooling in the HAZ of these steels.

Key-words:
Austenite decomposition; Kinetic models; Predictability; Welding of IF steel