Upper bainite consists of lath-like ferrite with cementite (Fe3C carbide) on high angle lath boundaries, whereas, lower bainite is made of lath-like ferrite with Fe2.4C carbide inside the ferritic laths. The concentration of high-angle boundaries is more in upper bainite than that in lower bainite. This kind of boundary acts as an obstacle to cleavage crack propagation, forcing the cleavage crack to change the microscopic plane of propagation in order to accommodate the new local crystallography. This leads to higher energy absorption before failure. Lower bainite contains more of low angle boundaries which are not effective obstacles and, consequently, seem to have no influence on the toughness of steels.
Bottom line, upper bainite exhibits superior toughness than lower bainite. in carbon and alloy steels.
Upper bainite with lath-like microstructure will show higher impact toughness. Bainite packet sizes are also important.
Upper bainite tends to form in higher temperatures, and since the transformation temperature is affected by carbon content, it is also an essential parameter.
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