The effect of preexisting VC carbides on the bainite transformation in medium-carbon high-alloy steel

Bainite transformation in steels is influenced by various factors. In the present work, bainite transformation in medium carbon high alloyed steel was investigated focusing on the influence of preexisting VC carbides on the morphology and transformation kinetics of the subsequently formed bainite. Hot-work die steels were held at 950 °C for various times to precipitate VC carbides, then rapidly cooled from 950 to 350 °C and held at this temperature for the bainite transformation. It is found that the bainite transformation was obviously accelerated by the preexisting VC carbides precipitated at the austenite region. The precipitation of carbides leads to a decrease in carbon concentration in the matrix, which decreases the effective activation energy and increases the highest temperature for the nucleation of bainite. Besides, bainite was observed to grow beside the VC carbides. It suggests that the VC carbides in the matrix act as nucleation sites for the bainite transformation. In the specimens, the bainite transformation is accelerated, and a higher fraction of bainite is formed when there are carbides in the matrix.