【作者】 李存杰；

【导师】 于志伟；

【作者基本信息】 大连海事大学 ， 载运工具运用工程， 2010， 硕士

【摘要】 两件大型渗碳齿轮在磨齿及静置的过程中发生开裂,现场检查发现两件齿轮的裂纹均出现在齿轮辐板圆孔。本论文以两件开裂的大型渗碳齿轮为研究对象,利用扫描电子显微镜(SEM)、光学电子显微镜(OPM)详细观察研究了齿轮的宏观及微观组织特征；利用发射光谱仪(OES)、X射线能谱仪(EDX)研究了齿轮材料的基体成分及微区成分。通过对开裂齿轮的宏观断口及微观断口的观察研究确定了两件齿轮的开裂机制及失效原因。论文主要结论如下：(1)这两件齿轮的材质为20CrMnTi钢,正常区基体化学成分符合技术要求,齿轮表面进行了渗碳处理。(2)这两件齿轮基体普遍存在冶金缺陷-宏观点状偏析,偏析等级为此类缺陷中最严重的4级,尤其在裂纹源区出现了较大尺寸的偏析区(24x47mm和27×12mm),在宏观点状偏析区的Si、Mn、P、S、Cr等元素含量均高于正常区基体。(3)点状偏析区与正常区域的微观组织存在明显差异：辐板表层偏析区由淬火马氏体和残余奥氏体组成,正常区由回火马氏体组成；辐板心部偏析区组织为贝氏体和沿晶断续铁素体,正常区基体组织为细片状珠光体和沿晶粗网状铁素体。(4)两件齿轮断口呈沿晶断裂形貌,其沿晶小刻面上存在韧性撕裂或“鸡爪”痕迹,表现出氢致开裂的断口特征,两件齿轮的失效形式均属于氢致沿晶开裂。(5)辐板心部存在的点状偏析区为氢脆开裂提供了组织条件：偏析区存在过量的P、S元素、出现了对氢脆更为敏感的贝氏体组织,在齿轮辐板裂纹源区存在的较大尺寸的偏析缺陷成为吸氢的“陷阱”,随着偏析区氢含量的增加,界面强度降低,最终导致富氢区开裂。氢可能是在材料熔炼或铸造的过程中引入的。更多还原

【Abstract】 Two large carburized gears cracked when grinding teeth and on a statically placed state. Inspection in site show that for the two gears the circumferential cracking occurr-ed on the spoke plate between the two holes of gear. The macrostructure and micro-structure of gear material were analyzed by SEM and OPM in detail. Chemical compo-sition of core and micro-composition were examined by OES and EDX. Macro and micro fractures were investigated by visual and SEM to determine the cracking mechanism and the failure causes of the two gears. The main results can be concluded as follows:(1) Severe macro point-like segregation is present in the gear material. Content of Si, Mn, Cr, especially P, S in the segregation zones is much higher than in the non-segregation zones. The excessive alloying elements in the segregation zone result in the microstructure difference between the segregation and non-segregation zones.(2) The cracks initiated from the larger size segregation zones in the core of spoke plate of gear and propagated toward the two hole surfaces on the spoke plate and the upper and low ends of gear. Failure in intergranular mode and the intergranular facets associated with ductile tearing was found. Evidences indicate that the failure of the gea-rs is due to hydrogen embrittlement. The possible source for hydrogen was introduced during melting and casting operating.(3) The large segregation zones in the core of spoke plate of gear supply the favor-able metallurgical condition for hydrogen-induced fracture:presence of excessive impurities of S, P and microstructure of bainite susceptible to hydrogen embrittlement, which became the trap of hydrogen. As the hydrogen content in the segregation zone increased, the strength of interfaces decreased to lead to cracking of the hydrogen-enriched zone.更多还原