SPHEROIDAL INCLUSION EFFECTS ON ACOUSTIC-EMISSION MICROSTRUCTURAL RELATIONS IN FERRITIC STEELS

A comparison of the acoustic emission accompanying the deformation and fracture of high- and low-sulphur steels subjected to various heat tr eatments has revealed that an additional component of acoustic emissio n exists in steels with high manganese sulphide inclusion content. The inclusion component of the emission has been correlated with matrix p roperties and shows some dependence upon yield strength. Results are c onsistent with both interfacial decohesion and microplasticity in the ensuing region of matrix-stress intensification acting as the source o f the extra acoustic emission signals. The results also shed light upo n the ways in which inclusion and carbide distributions interact to in fluence the ambient-temperature fracture mode. In particular, there is a transition from an alternating shear to a cup and cone fracture mod e as the carbide size increases. The quasi-static ductile-dimple mecha nism of the cup and cone fractures does not generate detectable signal s. The unstable alternating shear fracture is found to be capable of g enerating detectable signals, an observation that has important conseq uences for attempts to utilize acoustic emission for the detection of crack growth.