Abstract The kinetics of phase transformations in isothermal processes is well described by the classic JMAK model. However, it is known that most industrial facilities employ continuous cooling processes and, for this condition, the JMAK model is adapted but, sometimes, without success. Considering the importance to predict critical temperatures, the present work proposes an alternative empirical functional equation to describe the kinetics of steel phase transformation under continuous cooling, being useful when the JMAK use is not successful. In this study, the continuous cooling austenite to ferrite transformation was experimentally characterized by dilatometry for an IF-Ti-stabilized steel. The proposed equation was compared with the classic adapted JMAK model regarding to evaluate its efficacy to fit the dilatometric experimental data. Using the constants obtained by the both model fittings as input parameters, a computational simulation was performed to determine the CCT diagram of the IF-Ti steel. The proposed functional equation was efficient to predict the critical temperatures, the kinetics of austenite decomposition and the CCT diagram of the studied steel. The results predicted by the proposed model greatly met the experimental data measured by dilatometry.

Resumo Aços IF são amplamente empregados na indústria automobilística. No decorrer das aplicações de soldagem, eles passam pela transformação de fase austenita (γ) → ferrita (α) que ocorre por nucleação e crescimento de grão e, em condições isotérmicas, é descrita pela equação JMAK. Todavia, as transformações de fases pós-soldagem acontecem a partir de resfriamento contínuo. Assim, o presente trabalho caracterizou a cinética de transformação γ → α e propôs um modelo empírico para prever as temperaturas críticas durante o resfriamento contínuo em zonas termicamente afetadas de três tipos de aços IF. Amostras dos aços foram submetidas a ensaios dilatométricos com diferentes taxas de resfriamento contínuo. Determinaram-se as temperaturas críticas de transformação de fase (γ→α) e obteve-se uma equação empírica que correlacionou a temperatura crítica com a taxa de resfriamento e que viabilizou a previsibilidade da cinética de transformação de fases. Processo de soldagem TIG mecanizado foi aplicado nos aços IF com monitoramento de ciclos térmicos e, em regiões da ZTA com condições de austenitização e taxa de resfriamento similares às utilizadas nos ensaios dilatométricos, verificou-se que a equação empírica obtida é uma alternativa viável para prever as temperaturas críticas e a microestrutura durante o resfriamento contínuo na ZTA destes materiais.

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.