Certain impurity species trapped in the grain boundaries of HY80 steel during the post-fabrication heat treatment are known to cause temper embrittlement [1]. On the other hand, the local residual stress fields due to lattice mismatch at grain boundaries are the main pinning sites causing irreversible magnetic domain wall motion that creates magnetoacoustic emission (MAE). The peak amplitude of MAE depends on two major factors; the waveform of applied AC magnetic field and the strength of the 90° domain wall-defect interaction which creates an effective potential barrier against the domain wall motion. As the degree of embrittlement increases, therefore, the peak amplitude of MAE burst should increase since the higher concentration of impurities enhances the 90° wall-defect interaction at the grain boundaries.