Abstract: The author discusses the elements responsible for the development of inner surface cracks at the rear section of the shell hot-pierced from the 25-22Mo2N steel bloom and put forwards measures to improve the process so as to prevent the said defects. The investigation shows that the crack is produced due to the co-effect by various stresses when piercing, local grain-boundary weakness and plasticity reduction are caused in the metal owing to over temperature-rise thereof and has nothing to do with metallurgical quality.