The technical evaluation core of pressure piping components after exposure to fire was performed by considering various factors affecting the pipelines subjected to fire damage, including the determination of material deterioration degree, and the fitness-for-service assessment of the piping equipment. Based on the analysis results and testing data, the safety status and performance loss of the pressure piping after exposure to fire could be obtained. On this view, the influences of fire test and heat exposure as the fire-condition simulation on the grade X70 pipeline steel (API Spec.5L), which was widely used for piping equipment, was carefully investigated. According to the division of fire zone with different factors of temperature and fire suffering preservation, the material micro-structure deterioration and performance degradation were analyzed in detail, after that the grade X70 pipeline steel was heat-treated in muffle furnace to simulate the fire exposure under different conditions of the temperature, heat holding period and cooling mode. In the present foundational research, the mechanical tensile tests, Charpy impact tests, hardness measurements and the metallographic examinations were then conducted in detail. Thus, the data accumulation of performance degradation was believed to benefit to the fitness-for-service assessment of piping components of the grade X70 pipeline steel. Moreover, the material thermal degradation mechanism and the fitness-for-service assessment process of the grade X70 pipeline steel after exposure to fire was further discussed.

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