Mechanism of Formation and Characteristics of Microstructure of Novel Nanobainitic Steel

The effect of high-temperature annealing pretreatment above Ac_cm on the evolution of bainitic austempering microstructure in Fe – 0.95C – 1.37Si – 1.04Cr – 0.44Mn (wt.%) steel is investigated by differential thermal analysis, x-ray diffraction and high-resolution transmission electron microscopy. It is found that a novel nanobainite with a needle-like morphology forms in the experimental steel under conventional bainitic austempering after annealing pretreatment at 950°C. A thin film crystal of the novel nanobainite with a thickness as small as 50 nm has a body-centered cubic lattice, in which the opposite stacking fault is the main crystal defect. The interface of the novel nanobainite crystal is composed of both coherent and non-coherent areas. The steel containing the novel nanobainite structure has a higher hardness than that with conventional bainite.