Cause Analysis on Early Cracking of S136 Steel Molds

Early cracking failure of S136 steel molds occurred during its use. The failure causes of the molds were analyzed by means of macro observation, chemical composition analysis, scanning electron microscopy, energy spectrum analysis and metallographic examination. The results show that the main reason for the cracking of large pieces mold was that the excessive pulse current in the electrical machining of the inner hole caused serious quenching and modification layer on the edge of the inner hole, which resulted in the surface tension stress cracking. At the same time, there were reticulated carbides in the matrix structure, which can reduce the strength and toughness of the material, increase the tendency of cracking in the inner hole of the mold and accelerate the process of crack propagation. The main reason for the cracking of small pieces mold was that the chamfer of the groove was too small and the stress concentration increased remarkably. Under the action of working load, stress concentration and cracking were formed.