The upper bainite is a form of transformation from austenite to bainite that occurs at relatively high temperatures (~400-600°C) and involves a diffusion-controlled process rather than the nucleation process that characterizes most phase transformations. Upper bainite is usually formed between 400-550°C depending on alloy composition and transformation rate.
Upper bainite is a distinctive microstructure usually found in martensitic steels, in which the austenite is transformed into ferrite and cementite at relatively high temperatures due to relatively rapid cooling. It is bainitic microstructure that is typically characterized by relatively large blocky ferrite grains surrounded by a network of cementite particles. The cementite particles are often smaller than those found in lower bainite, and the ferrite grains are generally larger and have fewer particles.
The combination of the bainitic ferrite grains and the cementite particles creates a microstructure that is harder and tougher than both lower bainite and pearlite, but this higher hardness/toughness combination sometimes results in decreased ductility and impact toughness. The hardness and strength of the structure depend on the cooling rate. If the cooling rate is high enough, the alloy will not transform completely and the remaining austenite will form martensite, making the structure even harder and stronger.
Upper bainite microstructure is typically formed when steels are rapidly cooled, typically resulting in a greater proportion of bainite and a smaller proportion of martensite. The microstructure consists of large plate-like or blocky ferrite grains surrounded by a network of cementite particles. The ferrite grains are typically much larger than pearlite and lower bainite ferrite grains, and they often have low dislocation densities with low numbers of intragranular cementite particles. The cementite particles are often considerably smaller than those found in the lower bainite microstructure, and the network of cementite particles generally has a less distinct boundary than that of the lower bainite.
This distinctive microstructure imparts a number of beneficial properties to steels, including higher tensile strength and yield strength, improved wear resistance, and better fatigue and impact resistance. The microstructure also imparts improved hot-workability, which can be beneficial for certain manufacturing applications. Furthermore, upper bainite does not require significant post-treatment machining or grinding, making it attractive for certain applications. The combination of these beneficial properties makes upper bainite an attractive microstructure for a variety of applications, including components for automobiles, construction equipment, and tools.
