Cause Analysis and Prevention on Cracking of End Face Pinhole of Needle Valve Body

Cracking occurred in the pinhole at the end face of a needle valve body made of 18CrNi8 steel during reliability test for about 10 h. The cracking causes of pinhole were analyzed by macroscopic examination, chemical composition analysis, scanning electron microscopy analysis, metallographic examination and hardness test. The results show that the cracking mode of needle valve body pinhole was brittle fracture. The surface carbon potential of pinhole was so high that reticulated carbide formed due to improper heat treatment process, which destroyed the continuity of matrix, the pinhole part became high hardening zone and the grain boundary was weakened. Under the action of internal stress and external axial stress, crack initiated and propagated and eventually led to the pinhole cracking. Finally, the preventions were put forward.