EFFECTS OF NIOBIUM ADDITIONS ON THE STRUCTURE, DEPTH, AND AUSTENITE GRAIN-SIZE OF THE CASE OF CARBURIZED 0.07-PERCENT-C STEELS

Carbon (0.07%) steel samples containing about 0.04% Nb singly and in c ombination with nitrogen were carburized in a natural Titas gas atmosp here at a temperature of 1223 K (950 degrees C) and a pressure of abou t 0.10 MPa for 1/2 to 4 h, followed by slow cooling in the furnace. Th eir microstructures were studied by optical microscopy. The austenite grain size of the case and the case depths were determined on baseline samples of low-carbon steels and also on niobium and (Nb + N) microal loyed steel samples. It was found that, when compared to the baseline steel, niobium alone or in combination with nitrogen decreased the thi ckness of cementite network near the surface of the carburized case of the steels. However, niobium in combination with nitrogen was more ef fective than niobium in reducing the thickness of cementite network. N iobium with or without nitrogen inhibited the formation of Widmanstatt en cementite plates at grain boundaries and within the grains near the surface in the hypereutectoid zone of the case. It was also revealed that, when compared to the baseline steel, niobium decreased the case depth of the carburized steels, but that niobium with nitrogen is more effective than niobium alone in reducing the case depth. Niobium as n iobium carbide (NbC) and niobium in the presence of nitrogen as niobiu m carbonitride, [Nb(C,N)] particles refined the austenite grain size o f the carburized case, but Nb(C,N) was more effective than NbC in inhi biting austenite grain growth.