锻造MR曲轴缺陷分析及成因探讨

Defect analysis and mechanism discussion on forged automobile MR crankshaft

在由某中碳微合金钢(C0.40%-Mn0.68%-S0.022%-V0.049%)棒材经感应加热锻造制成的汽车MR曲轴法兰盘一侧端头上发现裂纹和折叠缺陷,分别处于锻造飞边及与之垂直的平面上。利用金相显微镜、扫描电镜结合电子探针等手段对该曲轴缺陷进行了分析。结果表明,裂纹附近存在深约0.2 mm的脱碳层,深度与正常部位相当,裂纹呈沿晶断裂特征,周边存在魏氏组织;(Mn,Fe)S夹杂呈晶间及晶内魏氏析出形貌;折叠缺陷微观形貌与裂纹相近。分析缺陷产生原因,认为裂纹缺陷是由于感应加热过程造成了材料局部过热形成表面横向裂纹,在锻造外力作用下扩展所致;折叠则是由于棒材温度的不均匀分布造成锻造时材料的流动速度不同,材料端头变形弯曲或凹陷,在终锻时金属汇流而成。

Crack and folding defects existing on one end of automobile MR crankshaft flange processed by induction heating and forging for continuous casting micro-alloyed steels with CO. 40% - Mn0. 68% - SO. 022% - V0. 049% were found at the forging flash and the vertical plane respectively, and the above defects were analyzed and discussed by metallographic microscope, scanning electron microscopy com- bined with electronic probe and other means. The results show that there are decarburization of 0. 2 mm near the crack with the same depth as the forged condition, the crack is characterized by intergranular fracture with widmannstatten structure around, the inclusions of （Mn, Fe） S present intergranular and intracrystalline widmannstatten precipitate morphology, and the microstructure of folding defect is similar to that of the crack. Furthermore, the causes of crack defects are the surface transverse cracks caused by the local overheating of material in the induction heating process and extended by external forging forces. However, the folding is formed by metal material merging into a stream in the final forging because of different flow rates of forging material caused by the uneven distribution of bar temperature making the material bending or sinking at the end.