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某机械厂使用100 mm规格的45钢圆棒加工一种挖掘机油缸活塞,在加工过程中发生矫直断裂。对45钢圆棒和断裂样品进行了化学成分、低倍酸洗、断口、金相组织等项目分析。结果表明:用于加工活塞杆的45钢圆棒力学性能、化学成分符合标准要求,钢中夹杂物含量少,组织是正常的铁素体+珠光体。活塞杆矫直断裂为脆性断裂,断裂源位于工件表面,断口微观形貌为冰糖状脆性断口。断裂件经酸洗后发现纵向表面存在较多沿周向分布的裂纹及较深、较粗的车加工刀痕,金相分析结果表明裂纹为中频淬火过程中形成的沿晶裂纹,裂纹区域存在回火马氏体硬脆相组织。活塞杆发生矫直断裂的原因是工件矫直前已存在中频淬火形成的淬火沿晶裂纹,在矫直应力的作用下,裂纹沿基体扩展直至发生断裂。通过改进中频淬火和回火工艺,提高工件表面加工光洁度可消除表面淬火裂纹,后续产品未再发生矫直断裂。
A machine shop machined an excavator cylinder piston using a 100-gauge 45-gauge steel bar that straightened and fractured during processing. The chemical composition, low pickling, fracture, microstructure analysis of 45 steel round bar and fracture samples were analyzed. The results show that the mechanical properties of 45 steel round rod used for processing piston rod meet the standard requirements. The content of inclusions in steel is small and the organization is normal ferrite + pearlite. Straightening rod fracture brittle fracture, fracture source is located on the workpiece surface, the fracture morphology of the rock candy-like brittle fracture. After the pickling was cracked, there were more circumferential cracks and deeper and coarser cutting edges on the longitudinal surface. The results of the metallographic analysis showed that the crack was the intergranular crack and the cracked area formed during the quenching Tempered martensite hard and brittle phase organization. Straightening rod piston rod fracture reason is the existence of quenching before the workpiece straightening hardening quench along the crystal crack, the role of straightening stress, the crack along the substrate until the crack occurs. By improving the IF quenching and tempering process to improve the surface finish of the workpiece surface quenching cracks can be eliminated, the follow-up products no longer occur straightening fracture.