Cooling rate effects on acoustic emission-microstructure relationships in ferritic steels

Abstract

The study reported here systematically investigates the nature of the relationship between defect dynamics and acoustic emission in specially prepared low alloy steels containing 3.25 wt% Ni, 1 wt% Mn with carbon contents between 0.06 to 0.49 wt% as the rate of cooling from the austenitic state is varied. During plastic flow, the strongest acoustic emissions are generated from slowly cooled microstructures with a ⩾10 μm ferrite dimension, a low initial dislocation density and very widely spaced precipitates. This emission is believed to originate from the propagation of high velocity (>100 ms−1) dislocation groups in the ferrite phase. It is consistent with an emission model in which the product of the dislocation glide distance and velocity (which are both controlled by microstructure) determines the amplitude of the acoustic emission. In air cooled samples containing retained austenite, additional emission is seen and suggests that stress-induced martensitic transformations are a second emission source. During subcritical microfracture, intergranular and alternating shear modes of microcracking occur in high strength conditions and generate strong signals. Both mechanisms involve the rapid propagation of cracks over distances of 10–100 μm and the resulting emission is consistent with the model predictions. The ductile dimple mode of fracture is found to generate no detectable signals regardless of dimple spacing and fracture stress, which is consistent with the view that such fracture occurs under essentially quasistatic conditions with little or no mechanical instability.