Spheroidal inclusion effects on acoustic emission-microstructural relations in ferritic steels

Abstract

A comparison of the acoustic emission accompanying the deformation and fracture of high- and low-sulphur steels subjected to various heat treatments has revealed that an additional component of acoustic emission exists in steels with high manganese sulphide inclusion content. The inclusion component of the emission has been correlated with matrix properties and shows some dependence upon yield strength. Results are consistent with both interfacial decohesion and microplasticity in the ensuing region of matrix-stress intensification acting as the source of the extra acoustic emission signals. The results also shed light upon the ways in which inclusion and carbide distributions interact to influence the ambient-temperature fracture mode. In particular, there is a transition from an alternating shear to a cup and cone fracture mode as the carbide size increases. The quasi-static ductile-dimple mechanism of the cup and cone fractures does not generate detectable signals. The unstable alternating shear fracture is found to be capable of generating detectable signals, an observation that has important consequences for attempts to utilize acoustic emission for the detection of crack growth.