Study On Microstructure And High Temperature Mechanical Properties Of Insulated Tubing Steel

With the economic developing rapidly,the demand for oil is growing,and as the unceasing exploitation of oil energy,heavy oil becomes an important part of the oil industry.On account of the feature of heavy oil which is big viscosity,large flow resistance,the exploitation of oil is very difficult,so we need to change the traditional production technology to improve the efficiency of collection.The fracture accident for oil tubing failure happens repeatedly,then it puts forward higher and higher performance for oil extraction equipment,improving the corrosion resistance,mechanical strength of tubing and prolonging the service life of the tubing become the primary task in the field of oil industry.The most research in oil pipe is about improving corrosion resistance performance to CO₂ and H₂S,optimizing the production efficiency of oil pipe,perfecting the heat insulation effect of heat insulated pipe to reducing thermal loss,or the improvement of the mechanical properties at atmospheric temperature.It is urgently needed to study on the high temperature mechanical properties,for example,the influence of aging on the microstructure and mechanical properties at serving temperature,some related work is finished in this article.Through instantaneous tensile experiment at high temperature,long-term aging test at 350℃/400℃and high temperature creep experimental research,this paper get the following conclusions:（1）The microstructure of insulated tubing steel N80-1,N80 Q and P110 is observed and analyzed,and the mechanical properties of the steel is studied under the environment of high temperature.The microstructure of N80-1tubing steel is ferrite+pearlite,and hardness is 234.8 HV;P110 steel is tempered sorbite,retaining the martensite position;The microstructure of N80-Q is also tempered sorbite.Compared to room temperature,the reduction of the strength of N80-1 is less than the others at350℃.At 400℃,the tensile strength of N80-1 changes less than the other materials.P110 reduces most obviously,with the tensile strength decreasing by 172 MPa,and the yield strength reducing by 176 MPa.The microstructure of N80-1 is more stable at high temperature,so strength reduces smaller.（2）Aging experiment at 350℃and 400℃for 1000 h,2000 h,5000 h for N80-Q and P110.With the extension of aging time,carbide particles growing up gradually.The density of carbon in P110 increases after 1000h,and the recrystallization occurs,generating the fine grain,then yield strength R_t0.6 increases significantly;The dislocation in martensite lath of N80-Q reverses,and most of the small angle grain boundary has disappeared,tensile strength R_m and yield strength R_t0.5 decreases;After2000h,the density of carbide in steel P110 decreases slightly,and the great mass of the martensite lath are smashed,retaining the original martensite position,and the strength decreases rarely;N80-Q begins to form the recrystallized grain,some tiny grain occurs,but not grows up,causing a fine-grain strengthen;The martensite morphology almost disappears after 5000h,and the equiaxed grain turns up,then strength decreases obviously with carbide growing.（3）The high temperature creep experiment for P110 and L80-1 is carried out in five groups of different temperature/stress,this paper obtains the steady-state creep rate by calculating the experimental data,the research gets the apparent activation energy Q at 350⁴50℃,the stress index of steady-state creep rate at 350⁴50 MPa and constant A through further calculation,then the formula of the steady-state creep rate is gained.