Abstract
The 35Cr45Ni steel with the high service temperature of about 1150°C was wildly used in ethylene pyrolysis device. However, the structure of 35Cr45Ni steel will change greatly after it was in service for a long time at high temperature. Therefore, in this paper, focusing on the 35Cr45Ni furnace tubes with different service time (0 and 6 years, i.e. original sample and failed sample), the microscopic morphology and elemental composition analysis were carried out to determine its failure behaviour. The results showed that the fracture morphology of the failed tube was mainly characterized by intergranular cracking, and there were carbides on the local surface of the crack, which was brittle cracking at low temperature. The precipitated phase in original 35Cr45Ni steel was mainly austenitic, M7C3 carbide and MC carbides. In a long-time service at 1000°C, the transformation of M7C3→M23C5 and MC(NbC)→Nb3Ni2Si occurred, and dispersed secondary M23C6 carbide was precipitated in the grain. Additionally, The NbC→Nb3Ni2Si transformation promoted the M7C3 to M23C5 process and the carbide precipitation as well as coarsening.
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