在低温下对QT400-18L铁素体球墨铸铁件进行Charpy缺口示波冲击实验, 测试温度对冲击过程中裂纹形核和裂纹扩展能力的影响, 采用OM观察分析不同温度下铁素体球墨铸铁裂纹萌生与扩展路径以及断口附近组织的演变规律. 结果表明, 韧脆转变温度以上, 冲击后断口附近大量石墨-基体界面发生开裂, 石墨界面开裂后产生的孔洞起到钝化裂纹和降低裂纹扩展速率的作用; 韧脆转变温度区间, 冲击试样表现为韧窝和解理混合断口形貌, 断裂模式和裂纹形核均与石墨球有关; 韧脆转变温度以下, 垂直交叉孪晶成核进而导致微裂纹扩展, 解理断裂主要是孪晶起裂, 这种形变孪晶引起的裂纹形核和扩展方式造成了裂纹形成功与裂纹扩展功的剧烈下降.

Due to its excellent ductility and moderate strength, QT400-18L ferrite ductile iron has been widely used in producing core components of wind power equipment such as the hub of a wind turbine. Most of the researches have focused on the exploration of mechanical properties at low temperature, but none of them give the explanation on microcosmic mechanism of ductile iron during low temperature impact and the mechanism of crack nucleation and propagation of ferrite ductile iron during impact fracture has not been analyzed. In this work, the impact toughness of QT400-18L ferrite ductile iron was measured by V-notch Charpy impact test at different temperatures, the influence of low temperature impact toughness and the fracture behavior of ferrite ductile iron were discussed. The results show that the cleavage fracture resistance of QT400-18L ferrite ductile iron is reduced with the decrease of impact temperatures. Above ductile-brittle transition temperature (DBTT), most of the total fracture energies are expended during the crack propagation process. Below DBTT, both crack initiation energy and crack propagation energy decrease obviously. By using in situ fracture metallographic observation method, crack initiation and propagation of QT400-18L ferrite ductile iron under different temperatures were analyzed. Above DBTT, graphite nodules play the role of crack blunting and reducing crack propagation rate; in DBTT range, the fracture morphology shows mixed fracture with cleavage and dimples, which are related to graphite nodules; below DBTT, deformation twins lead to the nucleation of microcrack and result in cleavage fracture, the deformation twinning could possibly play a significant role in the ductile to brittle transition of QT400-18L ferrite ductile iron.