Abstract: Cracking failure occurred in a nut and conical washer assembly of M16 in the field assembly process. The causes of washer cracking were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis, metallographic examination and hardness test. The results show that the tempering temperature of the conical washer was less than 350℃ during the heat treatment process, which was the first type of tempering brittleness temperature zone (200~350℃), resulting in the tempering brittleness of the washer. The micro-cracks on the surface of the gasket propagated along the grain boundary, and the brittleness of the gasket was eliminated after holding at 380℃ for 2 h, oil cooling and tempering.