旋压车载CNG气瓶封头底部裂纹形成原因分析

Causes Analysis of Bottom Crack Formation of CNG Vehicle Gas Cylinder Dome by Spinning

采用热旋压工艺将30Cr Mo无缝钢管成形为气瓶后,在气瓶封头底部熔合处出现裂纹,严重影响了气瓶质量。本文通过金相、扫描电镜、能谱分析等方法,对气瓶裂纹部位进行了金相组织观察及夹杂物分析。结果发现,裂纹周边出现大量脱碳铁素体,强度较低,且铁素体晶粒粗大,组织中伴有混晶,形变时受晶界影响的区域相对缩小,大尺寸晶粒间易造成应力集中,导致裂纹产生。裂纹周围还存在大量氧化夹杂物,破坏了母体金属的连续性,降低了材料塑性,如果这时局部塑变过大,超过微区塑变极限,就会形成微裂纹,并进一步扩展为宏观裂纹。

After the 30CrMo seamless steel tube was formed by hot spinning process, cracks appear at the bottom of the gas cylinder dome, which seriously affects the quality of the gas cylinder. For the crack part of gas cylinder, the metallographic microstructure was observed and the inclusion was analyzed by the metalloscope, SEM and EDS. Results find that a large number of decarburization ferrite appears in crack surrounding and the strength is lower, and ferrite grain is coarse, there is mixed crystal in microstructure, deformation effected by grain boundary area is relatively narrow. Large size grains are easy to cause stress concentration, which leads to crack formation. There are still a lot of oxide inclusions around the crack, the continuity of the base metal is damaged, and material plasticity is reduced. If the local plastic becomes too large, and exceeds plastic deformation limit of the micro-zone, micro-crack will be formed. After further extending it becomes macroscopic crack.