The working environment of the brake drum is abominable. Under the impact thermal stress caused by the frequent brakes, each brake is equivalent to a thermal stress, because the heat dissipation of the drum is not good and the temperature rises. If the stability of the pearlite is not good, the decomposition will occur, the volume will expand, the phase change stress and the phase change will be caused. If the superposition peak of stress and thermal stress exceeds the thermal fatigue stress, thermal fatigue cracking will occur. Similarly, if the environment is worse or the heat condition is worse, the temperature rises above the eutectoid transition temperature and forms austenite, and the martensite, bainite or Soxhlet will be formed under the condition of strong cold, such as water cooling or strong wind cold. The hardening phase is the reason why the cracked brake drum is cracked while it forms a hard spot on the surface.