Bainite is one of the common microstructures in special steel, which is mainly formed by a series of isothermal transformation of annealed ferrite and austenite. Its structure is more complex and diverse, and its growth behavior is still not clear. In order to systematically understand the characteristics of bainite growth and its relationship with key process parameters, a large number of research works have been conducted in the past few decades.
Over the decades, researchers have investigated the size effects on bainite grain growth through different approaches, such as electrical resistance measurements, optical microscopy and scanning electron microscopy. It was found that bainitic grain growth shows an inverse cubic power relationship with temperature, which can be expressed as: δ=K·T-3, where K was a constant coefficient related to the steel composition and the analytical techniques. The growth of bainite was also found to be dependent on the amount of dissolved carbon in the austenite, with a higher carbon concentration resulting in smaller bainite grains. It has also been observed that the shape of bainitic grains is largely influenced by the rate of cooling and the effective stresses in the material.
In addition to the size effects, the morphology of bainitic grains also changes with temperature. Baust et al.s experimental results have shown that there is a significant change in the morphology at higher temperatures, which can be roughly divided into three stages. At the lowest temperature, coarse and elongated laths were formed, while finer laths and interlath regions were observed at the highest temperature. This trend of morphology change is also strongly correlated with the nucleation and growth kinetics of bainite. The nucleation rate of bainite is strongly enhanced at high temperatures, which results in smaller and more numerous laths. On the other hand, the growth rate at high temperatures is much higher than at low temperatures, leading to shorter individual laths and larger interlath regions.
In sum, the growth of bainite grains is largely governed by the temperature, cooling rate and balance between the decomposition of the austenite and the formation of bainite. The amount of dissolved carbon in austenite also has a significant influence on grain size, while the morphology of bainitic grains changes with temperature, that is, coarser and more elongated laths at lower temperatures and finer laths and interlath regions at higher temperatures. All of these effects can be used as an effective tool to control the properties of special steels, such as hardness and strength.
