Tempering effects on acoustic emission-microstructural relationships in ferritic steels

Abstract

A systematic study of the effect of tempering on acoustic emission-microstructure relations has revealed that intermediate tempering treatments of three Fe-3.25 wt % Ni alloys with 0.06, 0.17 and 0.49% carbon lead to a pronounced acoustic activity during subsequent ambient-temperature tensile testing. The maximum emission intensity occurs from samples tempered near 250 °C for 100 min, and increases with carbon content. Mechanical property measurements reveal the emission maximum to be correlated with strengthening, the maximum strengthening (between 250 and 300 °C) coinciding with the maximum emission. The observations can be accounted for by a model which involves the high-speed cooperative motion of groups of dislocations over distances corresponding to the lath packet dimension. The mechanism that induces cooperative dislocation motion is suspected to be a precipitate shearing process, a process that has not been significantly considered for quenched and tempered ferritic steels before. A second, much weaker source of emission has been identified in material subjected to prolonged tempering at 625 °C. The mechanism responsible for this emission is believed to be the rapid multiplication, and high-speed propagation, of groups of dislocations between widely distributed cementite particles. No evidence has been found to support the view that carbide fracture in quenched and tempered steels is a direct source of acoustic emission.