Cleavage fracture of A 533 B pressure vessel steel in martensitic condition

Abstract

Measurements of microscopic cleavage fracture stress σ_F* and fracture toughness K _1C at low temperatures have been made on A 533 B, in an autotempered martensitic condition, to assess the effect of austenitizing temperature on fracture in a system where the fracture mode is transgranular quasicleavage for all austenitizing temperatures. A wide range of prior austenite grain sizes (and hence packet sizes) has been studied, and detailed microstructural information has been obtained in an attempt to gain a clear understanding of the mechanisms of cleavage fracture in this steel. The microscopic cleavage fracture stress is found to be independent of austenitizing temperature, and in the temperature range from −100 to −160°C it is also independent of temperature. At −196°C, the value of σ_F* falls slightly. The temperature independence of σ_F* strongly suggests a tensile stress controlled fracture criterion. The coarse grained condition exhibits higher strength, slightly higher fracture toughness, and a lower transition temperature than the fine grained condition. Cleavage fracture is not controlled by the prior austenite grain size, packet size, or lath width. The lath width is essentially independent of prior austenite grain size. The distributions of auto tempered carbides show no significant variation with prior austenite grain size. Unless local stress elevation occurs, it is not probable that cracking of the very fine carbides will control the fracture process. The observed mechanical test results appear to be consistent with an inclusion controlled fracture process.