Environmental conditions can affect the microstructural properties of a material, which ultimately determines a component's structural integrity. The steels used in submarine applications are susceptible to embrittlement when they are heated or slowly cooled through the embrittling temperature range of 370 to 600 °C. Evaluation of the state of temper embrittlement in HY-80 steel (submarine steel) can contribute to risk assessments that provide assurance that in-service components will not undergo failure. The present work evaluated the response of Magnetic Barkhausen Noise (MBN) to changes in temper embrittlement in HY-80 cast steel. Three steel samples were subjected to a constant temperature (525 °C) at different holding times, to produce different amounts of embrittlement in each sample. The MBN measurement system used a flux controlled waveform, which facilitates reproducibility of the measurements and permits extraction of variations in permeability between samples. MBN signal response was observed to decrease as a function of holding time, which was attributed to migration of impurity elements that act as pinning sites for domain walls, to prior austenitic grain boundaries. In addition, microstructural characterization was performed on the samples using optical microscope.