某注塑机38CrMoAlA钢螺杆表面存在麻坑,采用宏观观察、化学成分分析、金相检验、断口分析等方法对麻坑的产生原因进行分析。结果表明:螺杆表面氮化层显微组织不良,存在脉状氮化物及晶界网状氮化物,该氮化物脆性极大,且沿着晶界呈网状分布,破坏了金属基体的连续性,导致表层脆性增加;在对螺杆进行氮化后抛光时,受抛光压力的作用,表层晶界网状氮化物发生开裂,使得表面产生脱落现象,导致螺杆表面出现麻坑;造成氮化层显微组织不良的原因与螺杆在氮化前存在不良显微组织有关。

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.