The role of microstructure in toughness behaviour of microalloyed steels

The micromechanisms involved in cleavage propagation in the brittle and ductile-brittle behaviour of microalloyed steels are analysed. The cleavage process is divided into three different steps which must take place dynamically in order to succeed: nucleation of a microcrack in a microstructural feature, propagation of the microcrack across the particle-matrix boundary and finally, propagation across the matrix surmounting high angle boundaries. The microstructural parameters involved in the three steps are considered, together with their effective surface energy values. Depending on testing temperature, the microstructural feature controlling cleavage fracture changes. In relation to the ductile-brittle transition, the main parameter is the volume fraction of coarse microstructural units surrounded by high angle boundaries.