Transmission electron microscopy characterization of a Nb microalloyed steel for carburizing at high temperatures

The carbide particles in AISI 5115 steel modified with added 0.038 wt.% Nb were characterized by means of transmission electron microscopy after simulation of carburizing heat treatments at high temperatures. The advantage of a high-temperature carburizing treatment over the standard 930 degrees C heat treatment is the significant reduction in the processing time. However, higher carburizing temperature leads to general and abnormal grain growth, which must be avoided by grain-pinning precipitates. The carburizing heat treatment simulations were performed at 950, 1000, and 1050 degrees C for 1 and 2 h in samples with two prior microstructural conditions: hot rolled and spheroidized. Characterization of the precipitates regarding composition, morphology, size, and distribution was performed using carbon extraction replicas and thin foil specimens. The results confirm the existence of complex Nb carbides. Particle size distribution curves, as log-normal functions, are shown to be extremely important in the interpretation of grain boundary pinning, rather than the mean particle size. The spheroidized samples showed a susceptibility to abnormal grain coarsening.