Crystalographic analysis of acicular ferrite and bainite microstructure

Acicular ferrite (AF) is known to nucleate at intragranular inclusions. The high toughness of this austenite (A) decomposition product is attributed to the fine grained interlocking arrangement of fine plates growing by displacive mechanism. AF is formed in the same temperature range and by the same transformation type as upper bainite (B). On the contrary to AF, ferrite particles in B are nucleated at A-grain boundaries and form packets in which the plates have parallel arrangement [1]. In medium- and low-carbon steels, the formation of AF is usually associated with a favourable combination of strength and toughness values. The achieved AF microstructure toughness values are related to the increased density of high-angle boundaries (interfaces) characterized for this microstructure. This is beneficial microstructural arrangement because these interfaces act as obstacles to cleavage crack propagation, forcing the cleavage crack to change the microscopic plain of propagation in order to accommodate the new local crystallography. Low-angle boundaries forming B-packets are not effective obstacles and consequently have not influence on the achievement of improved toughness level in steel. This arrangement forms crystallographic packets defined by Gourgues et al. [2] as a continuous set of ferrite plates with crystallographic misorientation lower than a certain level of angle in contradiction to the AF secondary plates nucleation realized at surface of primary AF plates. The fan shaped formation of primary AF plates represents a further favourable parameter contributing to the increase of cleavage fracture resistance of this microstructure [3]. The object of present contribution is to compare the effect of AF and B microstructure parameter in relation to the obtained mechanical properties, especially toughness values in low-carbon steels. This comparison will be based on the misorientation of crystallographic parameters observed in above mentioned microstructure types.