Effect of prior creep cavitation on brittle fracture in heat affected zone of ferritic steel weldments

Abstract

Many components used by a range of industries, such as electrical power generation, operate at temperatures where creep damage is accumulated during the service life. In this process the materials undergo a range of microstructural changes including accumulation of cavities on grain boundaries oriented predominantly normal to the direction of the maximum principal stress. A theoretical model has been developed to consider the influence of such prior creep cavitational damage on subsequent brittle fracture of ferritic steels. The model allows the modes of transgranular cleavage and intergranular fracture to be quantified. It shows that if there is less than ∼20% of cavitated boundaries they have little effect on the nature of the fracture process or the fracture strength. Material encompassing the heat affected zone has been removed from a region of a manual metal arc weldment in a CrMoV steel containing creep cavities arising from stress relief heat treatment. Fracture toughness tests using reconstituted Charpy geometry specimens were undertaken at a temperature of 170 K to assess the effect of this damage on the lower shelf fracture toughness. The results are discussed with respect to the predictions of the theoretical model.