HP40Nb型乙烯裂解管的高温真空渗碳行为

High-temperature vacuum carburization behavior of HP40Nb steel ethylene cracking tubes

采用乙炔真空渗碳方式对未服役和服役5年的乙烯裂解炉管耐热钢HP40Nb进行了加速渗碳处理,并利用SEM和定量电子探针对渗碳前后炉管内壁的渗碳行为进行了系统分析。结果表明,未服役HP40Nb炉管内壁在渗碳前分布在晶界上的主要是以M7C3和Nb C为主的链状一次碳化物。真空渗碳之后,强烈渗碳区域的碳化物形貌随着深度增加发生较大的变化,碳化物种类随深度增加也逐渐发生由M3C2、M7C3到M23C6的转变。渗碳造成表面硬度升高,脆性增加。服役态HP40Nb炉管的内壁横截面分为表面的氧化层、亚表层的贫碳化物区和内部的渗碳区3个区域,表面氧化层区又可以分为Cr2O3和Si O2两个区域。表面氧化层对于抗渗碳性能具有较大的影响。Cr2O3具有一定的抗渗碳能力,但在高碳活度下较长时间后Cr2O3会逐渐不稳定发生碳化。晶间氧化区主要以Si O2为主,Si O2在高碳活度下比较稳定,难以碳化,因而造成渗碳速率下降。

HP40Nb steel tubes in different service states were carburized by low-pressure vacuum carburizing（ LPVC） at 1100 ℃. The carburization behavior in the inner wall was studied by means of SEM,EPMA,hardness test and EDS analysis. The results show that,the chained carbides of M7C3 and Nb C are distributed at grain boundaries before carburization for the original HP40 Nb steel tube. After carburization,the carbides morphology changes with the increasing depth in the carburized layer,while the carbide types change from M3C2,M7C3 to M23C6. Carburization causes the increase of surface hardness and material brittleness. For the serviced HP40 Nb tubes,three regions of the surface oxide scales,sub-surface carbide free zone and inner carburization zone are observed. The oxide scale region consists of Cr2O3 and Si O2 layers,which significantly influence the anti-carburizing performance. Cr2O3 shows the definite carburization resistance,but it would be carbonized gradually in the atmosphere of high carbon activity. The intergranual oxide is mainly Si O2,which keeps stable at high carbon activity and leads to decrease of carburization rate.