Cleavage fracture model for granular bainite in simulated coarse-grained heat-affected zones of high-strength low-alloyed steels

A model of cleavage fracture in granular bainite was developed. It involves two-dimensional elastic interaction between a main crack and multiple microcracks induced by the existence of martensite-austenite (M-A) particles in front of the main crack. A factor of fracture toughness reduction due to the presence of M-A particles (f(MA)) was introduced to describe the effect of M-A particles on the stress intensity factor at the tip of the main crack. The values of f(MA) can be expressed as the inverse of the amplification coefficient of the stress intensity factor as a result from the interference between microcracks and a main crack in brittle materials. An equation was derived which describes the relationship between fracture toughness and microstructural variables, including the influence of the effective grain size as well as the average width and interspacing of M-A particles. Moreover, simple linear regression equations were used to check the validation of the present model for predicting cleavage fracture toughness in simulated coarse-grained heat-affected zones of quenched and tempered high-strength low-alloyed steels.