C38+N2非调质钢曲轴法兰开裂原因分析

Cause for Cracking of Crankshaft Flange of C38+N2 Non-quenched-and-tempered Steel

某厂的C38+N2非调质钢曲轴,在装机测试压力时法兰部位开裂。通过化学成分分析、金相检验、断口检验、能谱分析及硫印试验等研究了曲轴开裂的原因。分析结果如下:①材料化学成分和法兰部位淬硬层深度均符合要求;②断口上部为准解理断裂,下部为“朽木状”撕裂;③在锻造分模面有硫化物聚集,甚至有长达1mm的以MnS为主的超长硫化物。硫化物聚集割裂基体材料是曲轴开裂的主要原因,曲轴锻后晶粒粗大,法兰的螺孔部位厚度较薄,是其开裂的次要原因。

In a factory,crankshaft made of C38+N2 non-quenched and-tempered steel presented crack in its flange part when installing and testing the pressure.The reason why the crankshaft cracked was investigated withthe aid of chemical composition analysis,metallographic examination,fracture test,EDS,and sulphur print test.The results were as follows:①The chemical composition and the depth of hardened case in the flange all conformed with the requirements;②The fracture was quasi-cleavage fracturing in upper part and"rotten wood"tear in lower part;③It was discovered that sulfides aggregated in forging parting surface,there being even sulfides as long as 1 mm in which MnS was dominant.The crank shaft cracking stemmed from the aggregated sulfide cleaving the matrix to a large extent and from both coarse grain after forging and thinner screw hole of flange in a certain extent.