Nondestructive evaluation of steels using acoustic and magnetic barkhausen signals—I. Effect of carbide precipitation and hardness

Abstract

The effect of microstructures on acoustic and magnetic Barkhausen signals has been investigated in a quenched and tempered steel and spheroidized steels with various carbon contents. Many different features between the acoustic and magnetic Barkhausen signals were observed. A major peak of the acoustic Barkhausen signal was induced when a magnetic field was increased from zero to the saturation state. A minor peak of the acoustic signal and a single peak of the magnetic signal appeared during the decreasing field. It was found in all the steels that the peak value of the acoustic Barkhausen signal shows a linear dependence on the sweep rate of a magnetic field while that of the magnetic Barkhausen signal does a nonlinear one. The increasing tempering temperature which gives rise to a decrease in hardness and an increase in carbide size and spacing caused that acoustic and magnetic Barkhausen peak voltages to increase precipitously and gradually, respectively. In the spheroidized steels, the acoustic peak voltage monotonically decreased with increasing carbon content from 0.17 to 0.96 wt% and the magnetic peak voltage was greatest when the carbon content was 0.46 wt%. The source mechanisms of the two types of the Barkhausen signals which are affected by the nucleation and growth processes of domains in a magnetic field are discussed to explain the effect of the change in carbide precipitate morphology and hardness.