TEMPERING EFFECTS ON ACOUSTIC-EMISSION MICROSTRUCTURAL RELATIONSHIPS IN FERRITIC STEELS

A systematic study of the effect of tempering on acoustic emission-mic rostructure relations has revealed that intermediate tempering treatme nts of three Fe-3.25 wt % Ni alloys with 0.06, 0.17 and 0.49% carbon l ead to a pronounced acoustic activity during subsequent ambient-temper ature tensile testing. The maximum emission intensity occurs from samp les tempered near 250-degrees-C for 100 min, and increases with carbon content. Mechanical property measurements reveal the emission maximum to be correlated with strengthening, the maximum strengthening (betwe en 250 and 300-degrees-C) coinciding with the maximum emission. The ob servations can be accounted for by a model which involves the high-spe ed cooperative motion of groups of dislocations over distances corresp onding to the lath packet dimension. The mechanism that induces cooper ative dislocation motion is suspected to be a precipitate shearing pro cess, a process that has not been significantly considered for quenche d and tempered ferritic steels before. A second, much weaker source of emission has been identified in material subjected to prolonged tempe ring at 625-degrees-C. The mechanism responsible for this emission is believed to be the rapid multiplication, and high-speed propagation, o f groups of dislocations between widely distributed cementite particle s. No evidence has been found to support the view that carbide fractur e in quenched and tempered steels is a direct source of acoustic emiss ion.