HP-40Nb乙烯裂解炉管开裂失效分析

Failure Analysis of Cracking in HP-40Nb Ethylene Cracking Furnace Tubes

HP-40Nb(Cr25Ni35Nb系合金)乙烯裂解炉管的设计服役时间为43800 h,服役21900 h后发生鼓胀开裂失效现象。本工作利用显微硬度计、金相显微镜、扫描电子显微镜以及EDS等技术对乙烯裂解炉管的失效原因进行分析,并与供货态铸管样品进行对比分析。供货态乙烯裂解炉管管壁的内壁等轴晶区与外壁侧柱状晶分别占47%和53%,服役21900 h后发生鼓胀开裂处的渗碳层厚度达炉管壁厚的近61%,内壁等轴晶区已完全渗碳。炉管开裂处的内外壁均存在大量蠕变空洞以及裂纹,空洞主要分布在一次碳化物附近。开裂断口上存在大量的球形析出物、带状焦炭,断裂类型为脆性断裂。炉管内壁碳沉积在渗碳过程中是不可避免的,渗碳过程中的碳的积累使炉管内壁局部区域产生结焦,造成炉管提前失效的主要原因是未及时清焦和柱状晶区占比偏少。

The design service time of the HP-40Nb ethylene furnace tube is about 43800 h.But it had been failure for some tubes in service after21900 h.The microstructure of pre-failure ethylene cracking furnace tubes,compared with that of as-cast tube,was examined by microhardness tester,the optical microscope(OM),scanning electron microscope(SEM)and EDS technique.Wall thickness of equiaxed grains and columnar grains are 47%and 53%of as-cast ethylene cracking furnace tubes,respectively.The proportion of wall thickness for the carburizing inner layer accumulated up to 61%,and the carburizing layer had completely penetrated the equiaxed grains zone.There are a lot of creep voids and cracks at the failed ethylene cracking furnace tube,and the voids are mainly distributed in the primary carbides.Spherical precipitates and coke are found on the fracture morphology.It showed the brittle intergranular fracture mechanism.The carbon deposition on the inner wall of the tube due to accumulation during carburization had been unavoidable.The main mechanism leading to failure before the service life for HP-40Nb ethylene cracking furnace tubes is not decoking in time and the thickness of the outer columnar grain zone of as-cast furnace tube accounted for a less proportion of the whole thickness.