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针对乙烯裂解炉管在服役过程中的结焦失效现象,采用XRD、扫描电镜和电子探针等手段对服役2.5年Cr35-Ni45型炉管的结焦机制、组织特征及力学性能进行了研究。结果表明,炉管的焦体由催化结焦形成的丝状焦和热裂解结焦形成的球状焦油滴组成,焦体对氧化膜破坏较大,导致了严重的渗碳。结焦后炉管内壁外侧的Cr2O3氧化层基本消失,只剩下了少量沿晶界分布的SiO2,内壁附近的碳化物贫化区消失,在其亚表层出现了一个很宽的渗碳层。结焦炉管中有3种碳化物,分别是内壁最外侧少量富铬的M3C2,渗碳区富铬的M7C3,心部基体的M23C6。此外,结焦引起的碳化物粗化及组织不均匀,导致炉管的力学性能下降。造成结焦炉管组织性能变化的主要原因是该合金在高温服役时不同碳、铬浓度下的析出相稳定性不同。
Aiming at the coking failure of ethylene pyrolysis tube during service, the coking mechanism, microstructure and mechanical properties of the aged Cr35-Ni45 furnace with service life of 2.5 years have been studied by means of XRD, SEM and electronic probe. The results show that the coke body of the furnace tube consists of filamentous coke formed by catalytic coke formation and spherical tar droplets formed by pyrolysis coking. The coke body greatly damages the oxide film, resulting in serious carburization. After coking, the Cr2O3 oxide layer on the outside of the inner wall of the tube disappeared, leaving only a small amount of SiO2 distributed along the grain boundary. The carbide depletion zone near the inner wall disappeared, and a very wide carburized layer appeared on the subsurface. There are three kinds of carbide in the coke tube, which are the outermost inner wall of a small amount of chromium-rich M3C2, carburizing chromium-rich M7C3, the heart of the matrix M23C6. In addition, carbide coarsening caused by coarsening and uneven organization, resulting in the mechanical properties of the furnace tube decreased. The main reason for the change of the microstructure and performance of the coke tube is the different precipitation stability of the alloy under different carbon and chromium concentrations when it is used at high temperature.