Effect of Ni addition on bainite microstructure of low-carbon special bar quality steels and its influence on CCT diagrams

This work presents the effect of Ni content on the bainite morphology and in the mechanical properties of low-carbon special bar quality steels. Four steel ingots with the same base chemical composition, having only Ni content as their main difference, were melted in a pilot scale vacuum induction melting furnace, hot forged into square bars followed by cooling in still air to room temperature. Samples for Charpy-V notch tests of every steel were tempered to equalize their hardness level. Through dilatometry tests, it was found that Ni slows down the transformation kinetics of ferrite and pearlite phases, and, acts in favor of bainite and martensite formation. In low-carbon bainitic steels, Ni also favors the formation of lower bainite at the expanse of upper bainite. The steels with lower Ni contents, 0.03 % and 0.70 %, have their microstructures mostly composed of upper bainite, with clusters of alloy carbides precipitated in the boundaries of the bainitic ferrite packages. The clusters of hard carbides act as stress raisers, facilitating the propagation of cracks, and, reducing the toughness of the steels. The other two steels studied with higher Ni, 1.40 % and 2.75 %, showed superior hardenability and predominant microstructures of lower bainite and martensite, respectively. The steels composed of lower bainite and martensite have higher toughness than the steels with upper bainite due to their more refined microstructure and homogenous dispersion of the alloy carbides in the steel matrix. It was also observed in the bainitic steels that tempering does not influence the location where the alloy carbides precipitated during bainite transformation. After tempering, these carbides remain at the bainitic ferrite boundaries or dispersed in the bainitic ferrite packages depending on the type of bainite formed