| P12 heat-resistant steel is a kind of pearlitic steel with good high temperature resistance,so it is widely used in chemical industry,energy industry and other fields.However,metal materials which have been in service for a long time at high temperatures are prone to creep and then to fracture.Now,the high temperature steam pipe of P12 heat-resistant steel in service of a petrochemical enterprise has occurred creep rupture failure before it was much lower than the design life.Revealing the high temperature creep fracture mechanism of P12 heat-resistant steel may contribute to its relevant design and evaluation and ensure the safe operation of the equipment.At present,the research on creep fracture of materials at home and abroad mainly focuses on the micro level and macro level.At the micro level,micro imaging technology is mainly used to explore the changes of material microstructure and composition.And at the macro level,the influence of creep on the mechanical properties of materials is mainly studied through various mechanical tests.In this paper,the creep fracture mechanism of P12 heat-resistant steel at high temperature was investigated from the micro level.Creep experiments were carried out at the condition of 630℃ and 70 MPa tensile stress for different times,and then creep samples from 30 hours to 360 hours were obtained.Then the samples were metallographically examined and analyzed by SEM to obtain the microstructure changes of P12 heat-resistant steel during high temperature creep.In addition,the computer image processing technology was used to quantitatively analyze the acquired image of microstructure evolution to explore the grain size,pearlite spheroidization,carbide precipitation and other situations.In order to further determined the effect of microstructure deterioration on creep fracture of P12 heat-resistant steel,a finite element model of microstructure of P12 heat-resistant steel was established based on Thiessen polygon algorithm.Then the effects of grain boundary weakening,pearlite spheroidization and carbide precipitation on stress distribution,tendency of microcrack formation and dynamic crack propagation in microstructure were studied.The results show that during the creep process,in addition to pearlite and ferrite,there were also a large number of carbides in the metallographic microstructure of P12heat-resistant steel,and the carbides precipitate continuously with the aggravation of creep,and their size was also increasing.And all test samples had no obvious change in grain size with the 9 level.The proportion of pearlite decreased with the creep process.In addition,the pearlite structure also had obvious spheroidization,and the number and size of cementite in pearlite also decreased.Furthermore,the weakening of grain boundary,the intensification of pearlite spheroidization and the deepening of carbide precipitation leaded to increased the stress and the tendency of microcrack formation of P12heat-resistant steel microstructure,and also leaded to accelerated crack propagation.The research results may have an important reference for the creep fracture analysis of P12 heat resistant steel. |
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