Variação da redução de área na estricção com o tempo, em condições isotérmicas, em um aço médio-alto teor de carbono

Abstract

Due to their high resistance to stress relaxation, high carbon steels alloyed w ith Cr and Si have increasing application in the manufacture of quench and tempered components for the automobile industry. It has been observed that the reduction in area of wire rod stock produced w ith this class of steel at the João Monlevade plant of Belgo Siderurgiaincreased w ith time. For the identification of mechanism associated w ith this change, specimens stored at -18°C after cooling in the Stelmor® w ere heat treated in different conditions of time and temperature, aiming to determine the kinetics of the process responsible for the ductility variation w ith time of the steel studied. The equation expressing the variation of reduction in area w ith time suggests that the mechanism responsible by the process is hydrogen dessorption. The apparent activation energy obtained, 63,9kJ/mol, indicates that this mechanism is controlled bydiffusion of hydrogen in ferrite, temporarily trapped in micropores localized in matrix/non-metalic inclusion interfaces. The effects of the strain rate and test temperature on the ductility of the steel studiedwere also evaluated, as an auxiliary method to identify the mechanism responsible for the variation of reduction in area w ith time. The results obtained for the samples stored at -18ºC are compatible w ith the effect of hydrogen in the ductility of steels, whose presence causes a decrease on the fracture strain w ith the creasing strain rate The fracture surfaces of specimens stored for 40 days at 25ºC and those of the samples tested after cooling in the Stelmor® w ere analyzed to identify their macro andmicroscopic aspects. The surfaces of samples stored at room temperature show ed dimples, characterizing ductile fracture. At the fracture surfaces of specimens tested after cooling, cleavage pattern w as identified, indicating embrittlement.