Transmission electron microscopy was used to investigate the particle size distribution, morphology, composition, and crystallography of microalloy (Ti, Nb) precipitates in 4130 steels between 900 to 1250 deg C. Considering the Ti fraction, the size and the morphology, two types of precipitates were identified: cuboidal coarse TiNbMo carbonitrides, rich in TiN and a fine dispersion between 2 to 25 nm of TiNbMo carbides. The progressive formation of higher soluble phases, such as NbC and MoC, was observed on the pre-existent, TiN and TiC precipitates. In the studied conditions, nitrides were found to be insoluble and quite resistant to coarsening. On the contrary, carbides not only began dissolution in the range of 960 - 1000 deg C according to the microalloy content in the steel, but also produced abnormal grain growth.
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