HY80 steel is a low-carbon steel known for embodying high strength and toughness properties. This steel is used in submarine applications. Temper embrittlement, which is the reduction of fracture toughness, occurs in steels when subject to aging and drastic temperature fluctuations. These changes occur in submarines over time while in underwater environments. During temper embrittlement, impurity atoms and carbides migrate to grain boundaries, which make the steel more susceptible to fracture. A non-destructive testing (NDT) method is desirable to assess the temper embrittlement damage in HY80. Magnetic Barkhausen Noise (MBN) is of interest as being a potential NDT method for analyzing HY80. Focusing on microstructural characterization and its effect on MBN could have implications for establishing an MBN based method to detect varied stages of temper embrittlement in HY80 steel. In this research, samples of HY80 were prepared and heat treated for 16–336 hours to mimic various degrees of temper embrittlement. Microstructural changes with heat treatment were characterized and connected to the MBN produced at each holding time. Methods consisted of performing scanning electron microscopy (SEM) and using an MBN measurement system. It was observed that as holding time increases, grain size increases and carbide density within the grains decreases. These carbides, which act as pinning sites, make it more difficult for domain walls to move, consequently affecting MBN energy.