Models to describe brittle and ductile fracture in ferritic steels

Abstract Theoretical models have been developed to describe the various fracture mechanisms that occur in polycrystalline α-iron and ferritic steels over a range of temperature which spans the ductile-to-brittle transition. At low temperatures, the models enable the proportions of transgranular cleavage and intergranular brittle failure to be explored for different ratios of the fracture energies for the two mechanisms. This allows, for example, the effect of embrittlement arising from grain-boundary segregation of minor alloying and impurity elements to be investigated. In addition the effect of texture on the predictions obtained has been considered and the results are presented. At higher temperatures, the models have been developed to accommodate ductile fracture. They have also been extended to consider the influence of prior creep cavitational damage at grain boundaries on both low- and high-temperature fracture processes and the corresponding fracture energies. The effect of this prior damage on subsequent ductile failure is explored. The predictions show that, if fewer than about 20% of the grain boundaries are fully cavitated, then there is little change in either the mechanism of the fracture process or the fracture strength of the material. These predictions are compared with measured upper-shelf fracture toughness values obtained for a Cr-Mo-V ferritic steel containing proportions of up to about 40% of prior creep damage.