On the kinetics of transgranular particle embrittlement during simulated carburizing in steel containing grain-refining additions of aluminum and niobium plus aluminum

摘要

The kinetics of toughness degradation resulting from transgranular particle embrittlement are evaluated as a function of composition and processing history for simulated carburizing operations in air-melt steel containing grain-refining additions of aluminum and aluminum plus niobium. The kinetics of particle embrittlement are inherently linked to the ripening of A1N precipitates after extended austenitization in steel containing carbon contents representative of both the case and core of a carburized component. Embrittlement in steel containing A1N occurs with an activation energy similar to the value for aluminum diffusion in austenite, although an A1N volume fraction effect on the embrittlement kinetics is manifested as decreases in activation energy with decreases in the [A1]/[N] ratio of steel. In contrast, the presence of niobium substantially retards the kinetics of particle embrittlement in steel containing 120-200 ppm N. Observations of A1N precipitates coated with Nb(C,N) indicate that the decreases in embrittlement kinetics are related to a reduction in the potential for A1N ripening during austenitization.