铸铁汽蚀破坏机制研讨

本文通过扫描电镜和振动式汽蚀机对铸铁的汽蚀过程进行了研究。从水力学汽蚀冲击力开始,系统地讨论了汽泡渍灭时产生的球形应力波、射流冲击力的大小,和冲击力作用于金属边壁后如何使其内部的全位错半位错滑移并形成裂纹核,认为力学冲击是铸铁汽蚀的主要原因,冲击力使材料内产生剪应力并使表面石墨脱落。根据实验结果具体分析了铸铁中石墨的存在对裂纹成核倾向的影响,说明了石墨的存在一方面降低了铸铁的微区强度,另一方面应力集中和切口效应使裂纹容易在其周围萌生和扩展的原由,指出石墨是铸铁汽蚀源。根据裂纹核在铸铁汽蚀面的位置指出了三种起裂方式并用实验进行了验证,三种起裂纹方式为①汽蚀面上的石墨尖端和晶界处,②垂直石墨尖端,③ 距表面层△b厚(20～100μm)区内的硬质点和石墨与基体界面处。指出裂纹沿着或向着石墨片和硬质点的方向扩展。

An Investigation into the Mechanism of Cavitation Errosion of Cast Iron

Using SEM and vibration test machine, the cavitation errosion of cast iron is studied in this paper. Starting from the cavitation impact pressure defined by hydraulics,the nodular stress waves formed affer bubble collapse,the strength of impact pressure,and thes formation of crack nuclei due to the dislocation and semi-dislocation slip caused by the impact onto the metal are systematically discussed. The mechanical impact is considered to by main cause of cavitation errosion of cast iron. Impact pressure forms shear stress in the material which makes the graphite on the surface drop. According to the results of test,the influence of existence of graphite in cast iron on the formation of crack nuclei is analysed specificallg. It shows that the existence of the graphite will reduce the microzone strength of cast iron on the one hand and,on the other hand, it makes the cracks sprout and propagate easily around itself due to stress concentration and notch effect. It is pointed out again that the graphite is the origin of cavitation errosiox of cast iron. According to the position of crack nuclei on the cavitation surface,three ways of crack formation is advanced and checked by the experimet. They are. (1) crack nuclei on the graphite tops and crystal interfaces. (2) crack nuclei on vertical graphite tops inside the cast iron. (3) crack nucled on the hard particles and the interfaces between the graphite and base metal. It is poinled out that the crack propagates along or toward the graphites and hard particles.