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.

A?os IF s?o amplamente empregados na ind?stria automobil?stica. No decorrer das aplica??es de soldagem, eles passam pela transforma??o de fase austenita (?) ? ferrita (?) que ocorre por nuclea??o e crescimento de gr?o e, em condi??es isot?rmicas, ? descrita pela equa??o JMAK. Todavia, as transforma??es de fases p?s-soldagem acontecem a partir de resfriamento cont?nuo. Assim, o presente trabalho caracterizou a cin?tica de transforma??o ? ? ? e prop?s um modelo emp?rico para prever as temperaturas cr?ticas durante o resfriamento cont?nuo em zonas termicamente afetadas de tr?s tipos de a?os IF. Amostras dos a?os foram submetidas a ensaios dilatom?tricos com diferentes taxas de resfriamento cont?nuo. Determinaram-se as temperaturas cr?ticas de transforma??o de fase (???) e obteve-se uma equa??o emp?rica que correlacionou a temperatura cr?tica com a taxa de resfriamento e que viabilizou a previsibilidade da cin?tica de transforma??o de fases. Processo de soldagem TIG mecanizado foi aplicado nos a?os IF com monitoramento de ciclos t?rmicos e, em regi?es da ZTA com condi??es de austenitiza??o e taxa de resfriamento similares ?s utilizadas nos ensaios dilatom?tricos, verificou-se que a equa??o emp?rica obtida ? uma alternativa vi?vel para prever as temperaturas cr?ticas e a microestrutura durante o resfriamento cont?nuo na ZTA destes materiais.IF steels are widely used in the automotive industry. In the course of the welding applications, they undergo the transformation of austenite (?) ? ferrite (?) 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 ? ? ? 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 (? ? ?) 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