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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。
The acetylene vacuum carburizing method was used to accelerate the carburizing treatment of heat-resistant steel HP40Nb, which was not used for 5 years and was used for 5 years. The carburizing behavior of the inner wall of carburizing furnace before and after carburizing was analyzed by SEM and quantitative electron probe System analysis. The results show that the uncoated HP40Nb tube wall is mainly composed of chain carbides mainly composed of M7C3 and NbC before carburizing. After vacuum carburizing, carbide morphology in the intensive carburized region changed greatly with the increase of depth. Carbide species gradually changed from M3C2 and M7C3 to M23C6 with increasing depth. Carburizing caused by increased surface hardness, increased brittleness. The service life HP40Nb tube wall cross-section is divided into surface oxide layer, sub-surface lean carbide region and the internal carburizing zone three regions, the surface oxide layer can be divided into Cr2O3 and Si O2 two regions. The surface oxide has a great influence on the carburization resistance. Cr2O3 has a certain anti-carburization capacity, but after a long time under high-carbon activity Cr2O3 will gradually unstable carbonation. The intergranular oxidation zone is mainly dominated by Si O2. Si O2 is relatively stable under high carbon activity and difficult to carbonize, resulting in a decrease in carburization rate.