Microstructure evolution due to carburising of a 25Cr35NiNb+MA ethylene pyrolysis furnace tube was investigated after service for approximately 4 years. The microstructure of 25Cr35NiNb+MA alloy was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy (TEM). The results revealed that M23C6 carbides of the 25Cr35NiNb+MA alloy had coarsened and transformed to M7C3 carbides with a heavily faulted structure during carburising. The content of M7C3 carbide increased closer to the inner wall. A large number of dislocations had been observed surrounding the carbides and some formed dislocation walls. The carbide transformation mechanism and the effect of the dislocation walls are discussed.

Carburization phenomena of the ethylene pyrolysis furnace tube made of 25Cr35NiNb alloy were investigated after service for approximate 30,000 h in a petrochemical plant. Three different zones, namely, oxidation zone, carburization zone, and aging product zone, were observed in the cross section of furnace tube by microscopic analysis. The oxidation zone near the inner surface has a thickness of about 700 mu m and possesses the characterization of many loose pores and voids. The inner surface has a continuous thin Cr2O3 layer and the grain boundaries near the inner wall are mainly composed of mixed oxides of Cr2O3 and SiO2. In addition, segregation of impurities S and P was detected at grain boundaries. The dark gray SiO2 distributes in the front of the oxidation zone. Carbides distribute at grain boundaries in the carburization zone with lots of voids and micro cracks. The width of grain boundary is broadened. According to the experimental results, the initiation of cracks may be promoted by carbides at grain boundaries. The uneven distribution of the Cr element was found in the carburization zone, and direct experimental evidence of Cr element diffusion was detected. In the aging product zone, the carbides evolution was observed. The chromium carbides are composed of Cr7C3 and Cr23C6. The microstructure and composition evolutions of the furnace tube and their effects on service life of ethylene pyrolysis furnace tube were also discussed.

本研究面向石油化工、煤化工、航空航天、电力、核能等国家重要领域，围绕承压设备高温、腐蚀等极端环境，开展了多种气氛环境高温蠕变疲劳性能测试仪的研发。研究工作对提升我国超高温疲劳试验设备的自主研发能力，引领和带动行业的技术进步，具有重要意义。研制出了三台套由结构独特的机电伺服加载机构、超高温与多种气氛环境装置组成的蠕变疲劳性能测试仪，达到了发达国家同期水平，该测试仪的研制为相关理论模型的试验验证、超高温环境下蠕变疲劳性能测试提供了有效的技术手段。开发出了一种全数字式控制器及相应软件，构成了一套嵌入式系统，实现了标准化、精确化、小型化，保证了系统的通用性、可靠性和稳定性，可定时自动校准力、变形、位移测量系统，保证了测量系统具有较强的抗干扰能力和长期稳定性。开发了一套超高温环境下的变形测量机构，填补了国内空白，达到甚至超过了发达国家同期水平，该装置的研制成功为超高温环境下的变形测量提供了有效的手段和测试方法。The research and development of the measuring instrument for high temperature creep fatigue properties under various atmosphere environment has been conducted,concerning the extreme environment of pressure equipments such as high temperature,corrosion,etc.in the petrochemical,coal chemical,aerospace,electric power,nuclear power industries,etc.The research work has great significance for promoting the ability of research and development of ultra high temperature fatigue test equipments in our country,leading and driving the technical progress of high temperature creep fatigue instrument industry.Three sets of creep fatigue testing equipment including the structure of the unique mechanical and electrical servo load mechanism,ultra high temperature and atmosphere environment has been developed for the first time,filling the domestic gap in this field and meeting the technical level of developed countries over the same period.The development of the testing equipment provides effective technical means to the experimental verification of theoretical model and the testing of creep fatigue performance under ultra high temperature environment.A fully digital controller and corresponding software have been developed to constitute a set of embedded system.The standardized,accurate and miniaturized system ensures versatility,reliability and stability.It can be timed automatically to calibrate force,deformation and displacement,with strong anti-interference ability and long-term stability.A set of deformation measurement facility for ultra-high temperature has been developed for the first time,filling the domestic gap in this field and meeting or exceeding the technical level of developed countries over the same period.The successful development of this device provides an effective measuring and testing method for the deformation under ultra-high temperature environments.