Causes of pockmarks on screw surface of injection molding machine

There were pockmarks on the surface of 38CrMoAlA steel screw of an injection molding machine.The causes of pockmarks were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination and fracture analysis. The results show that the microstructure of the nitrided layer on the screw surface was poor, and there were vein nitrides and grain boundary network nitrides. The nitrides were extremely brittle and distributed in a network along the grain boundary, which destroyed the continuity of the metal matrix and led to the increase of surface brittleness. When the screw was polished after nitriding, due to the polishing pressure, the surface grain boundary network nitride cracked, causing the surface to fall off, resulting in pockmarks on the screw surface.The reason for the poor microstructure of nitrided layer was related to the poor microstructure of screw before nitriding.