Some Effects of Microstructure on the Fracture of Steel

Abstract

<p> The fracture behaviour of a medium strength bainitic steel (SAE 4340 in the 11 as transformed and in the "warm rolled" condition) . and four carbon-manganese structural steels (in the hot rolled ferritepearlite condition) was investigated. The purpose was to isolate those features of the microstructure which exert control over the fracture properties. </p> <p> The detailed nature of the microstructure of the steels was studied with transmission and scanning electron microscopy, qualitative x-ray analysis and quantitative metallography. An attempt was made to correlate the fracture behaviour with the microstructure through models which relate to the structure properties to the unnotched tensile properties. </p> <p> In the case of the bainitic steels it was found that the carbide morphology, dislocation substructure and prior austentite grain size have the major influence on fracture properties. In contrast, the fracture properties of the structural steels were controlled by the volume fraction of inclusions and to some extent by the shape of the inclusions. </p>