An experimental and modelling study of brittle cleavage crack propagation in transformable ferritic steel

Abstract

Flaws that can be present within pressure vessels, pipework and other engineering structures are assessed using the principles of engineering fracture mechanics. It is necessary to support such an approach with an understanding of the underlying fracture mechanisms. Moreover, many of these components are fabricated using transformable steels. In the present paper, the authors describe the fracture of an A508 type steel, heat treated to produce a tempered bainitic microstructure, and subsequently impact tested at −196°C. In particular, focused ion beam microscopy has been used to produce high resolution fractography, combined with information relating to the underlying microstructure and crystallography. The results of cleavage crack propagation across prior austenite grain, lath packet and lath boundaries are described and then correlated with predictions from a three-dimensional geometric model of brittle cleavage fracture in polycrystalline steel. This model includes a consideration of a lath substructure developed within the grains and is based upon a Kurdjumov–Sachs orientation relationship with the parent austenite grain.