| The stainless steels are used widely in petroleum and chemical industry because of its excellent mechanical properties and hydrogen embitterment resistant abilities. The damage and fracture of stainless steel serviced in hydrogen environment with high temperature and pressure are very important for the safety production of petroleum and chemical industries. In the present paper, damage and fracture of the furnace tube used to heating hydrogen circularly, which had served for 106 hours, were mainly studied. The microstructure degradation of Incoloy800H furnace tube was analyzed by use of metallurgical examination, and the change of mechanical properties at room temperature and high temperature was investigated by mechanical tests. The reasons of resulting in damage of the furnace tube serviced in the hydrogen environment with high temperature and pressure were discussed. The fracture properties of the furnace tube are studied by measuring the high temperature fracture toughness of Incoloy800H alloy. According to obtained mechanical properties and fracture toughness of material , the high failure assessment diagram of the Incoloy800H furnace tube serviced for 106 hr at 6800C was constructed, which was applied in practical engineering case. The major research works and conclusions of this paper are as follows: (1) The change of the microstructure and damage distribution of the serviced furnace tube along axial and radial direction were analyzed by metallurgical examination. Little damage occurs in a certain extent for the Incoloy800H furnace tube serviced for 106 hr in the hydrogen environment with high temperature and pressure. The microstructure feature of the hydrogen damage was not observed, which implies that the Incoloy800H alloy is not susceptive of hydrogen damage. (2) Mechanical property tests of Incoloy800H furnace tube were carried out at room temperature and high temperature. The research results show that the mechanical properties of serviced Incoloy800H tube have be somewhat degraded. The decrease in plasticity is obvious at elevated temperature. Especially for weldment of the furnace tube, its impact toughness is relatively small in the room temperature, and the plasticity obviously falls down,which implies that the weldment become brittle in some extent and will be one of the weak links. Compared that new material, the mechanical properties of the serviced furnace tube decrease. However, the serviced Incoloy800H furnace tube still retain higher strength and toughness and has some safety stock. (3) Based on the data of creep test and long term term, the remaining life of the Incoloy800H furnace tube is predicted by Larson Miller life extrapolation methods and is about 106 hours at operating condition. By comparing and analyzing the degradation extent of the mechanical properties of the serviced furnace tube, such conclusions are obtained that the temperature and pressure are the main factors of causing mechanical property degradation of the Incoloy800H furnace tube and the effect of hydrogen on its mechanical properties is low. (4) By performing high temperature fracture toughness test, the fracture toughness of the serviced Incoloy800H alloy was measured at 680℃. The test results indicate that the fracture toughness is lower at higher temperature zone of the furnace tube. In general, the serviced Incoloy800H furnace tube still has the excellent ability of resisting the crack growth. The influencing factors of measurement of elevated temperature fracture toughness were also discussed in this paper. (5) Based on the mechanical property data of Incoloy 800H obtained from test, high temperature failure assessment diagram of the Incoloy800H alloy serviced for 106 hr was constructed. It was successfully applied to the elevated temperature fracture safety assessment for the Incoloy800H furnace tube with defects, and the safety assessment result indicates the furnace tube with some defects is safe at assessment condition.
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