Efeito da temperatura, tempo e taxa de resfriamento na formação do eutetóide divorciado em soldas de trilhos de aço perlítico

Abstract

The hardness reduction that occurs at the end of the Heat Affected Zone (HAZ) of welded joints of railway is generated through of the Divorced Eutectoid formation (DE). This transformation happens in the weld during the cooling of the austenite when there is presence of undissolved carbides, which, in turn, act as preexisting cores for the formation of this microstructure, forming a combination of spheroidized cementite + ferrite. In order to understand this microstructural transformation, samples of premium and superpremium rails were tested by dilatometry, with a full factorial Design Of Experiments (DOE) that evaluated the effects of variables temperature, holding time and cooling rate; and their interactions. The samples after thermal cycling were characterized by microhardness test and scanning electron microscopy analysis. The results showed that the highest DE fractions were obtained at higher temperatures (750 °C) and with higher equivalent carbon. However, by increasing the cooling rate, from 1.5 to 9 °C/s, resulted in a reduction of the DE fraction of 49% for the premium rail at 750 °C and 75% for the superpremium rail at 740 °C. Regarding hardness, the behavior was inversely proportional, where the increase in the cooling rate provided an increase in the hardness of the rails (about 21%), in addition, the samples simulated at 740 and 750 °C and cooled to 1.5° C/s showed hardness similar to that was found in the region of lower hardness of rails welded by Flash Butt Weld (FBW). The factors that had the greatest effect on DE formation, in increasing order, were temperature, equivalent carbon and cooling rate.