Cleavage fracture in bainitic and martensitic microstructures

Abstract

This paper addresses the mechanisms of cleavage fracture in the pressure-vessel steel A533B. Microstructures of single bainite, single auto-tempered martensite and a mixture of bainite-plus-martensite have been investigated. Upper-bainite microstructures exhibit a higher propensity for brittle cleavage fracture than do those of auto-tempered martensites. The K 1 c values of mixed microstructures are determined by the statistical distribution of the two phases and the range of the values is bounded by limits set by those for the single-phase microstructures. The results are explained in terms of the RKR model, which involves a local cleavage stress σ F * and a distance ahead of the macrocrack tip, X, as two critical parameters. It is found that the carbides or carbide “colonies” act as critical microcrack nuclei, and hence play a key role in determining the fracture toughness, although packet boundaries in bainite may give rise to “pop-in” arrests in displacement-controlled tests.