Microstructure And Properties Of Thick-walled X80 Hot Induction Seamless Bend And Optimization Of Heat Treatment Process

As an important component of long-distance pipeline,the bend bears complex working conditions during service,which requires it to have excellent comprehensive mechanical properties.The thick-walled X80 hot induction seamless bend has the significant advantage of the pipe body without welds,which can avoid the serious deterioration of the microstructure and properties of pipe body and weld of the longitudinal welded pipe prepared by the traditional controlled rolling and controlled cooling process during the hot bending and subsequent heat treatment process,and is gradually applied to the long-distance oil and gas pipeline project.Due to the large wall thickness of thick-walled bend,the nonuniform distribution of microstructure and properties of wall thickness section occurs after hot bending.Based on this,this paper takes the thick-walled X80 hot induction seamless bend as the research object.OM,SEM,TEM,XRD and EBSD were used to characterize its microstructure and tensile test,low temperatur eoscilloscope impact test and fracture toughness test were carried out.The relationship between the microstructure and mechanical properties of the thick section of the seamless bend after hot bending was studied and analyzed.By optimizing the heat treatment process of the seamless bend,the homogenization of the microstructure and mechanical properties of the thick section of the seamless bend was realized,and then the seamless bend suitable for service in extreme environments was prepared.Therefore,this study has important scientific value and engineering significance.In this paper,the influence of microstructure of thick wall section of seamless bend after hot bending on its mechanical properties is studied,especially the fracture properties.The microstructures of the outer surface,the wall thickness center and the inner surface of the seamless bend are mainly composed of lath bainite,granular bainite and polygonal ferrite,respectively.The difference of induction heating temperature and cooling rate during hot bending is the main reason for the nonuniform microstructure of the wall thickness section.The yield strength of seamless bend decreases from outer surface to inner surface.The low proportion of high angle grain boundaries in the outer surface is the reason for the poor low temperature impact toughness.The high angle grain boundaries in the center of the wall thickness and the inner surface are larger,showing better strength and toughness.The wall thickness center material shows good resistance to crack propagation ability and low temperature fracture toughness,with high resistance curve and critical fracture toughness and low ductile brittle transition reference temperature T₀,followed by the inner surface and outer surface.The higher the crack tip constraint level is,the larger the constraint factor A₂is,and the lower the fracture toughness is.The deep crack tip subjected to bending load has a higher constraint and significantly reduces the fracture toughness.In this paper,the effects of quenching and tempering process parameters on the microstructure and mechanical properties of seamless bend were studied.The bainite transformation kinetics was analyzed by simulating the quenching process with thermal expansion test.The results show that the bainite transformation kinetics curve has a typical S-type.With the increase of austenitizing temperature,the phase transformation turns to the low temperature region and the transformation rate increases.The strength increases and the impact toughness decreases with the increase of quenching temperature.As the tempering temperature increases to 650℃,the strength gradually increases and the impact toughness also increases.After quenching at 950℃and tempering at 650℃,the structure and properties of the thick section of the seamless bend wall are evenly distributed and have good strength and toughness.A quenching and tempering treatment process for preparing seamless bend pipes with good comprehensive mechanical properties is explored,which improves the service safety under conventional conditions.In this paper,the effects of the“quenching+intercritical quenching+tempering”process on the microstructure and mechanical properties of seamless bends were studied,and the kinetics of reversed austenite phase transformation was analyzed.The results show that a dual phase microstructure composed of intercritical quenched bainite and intercritical ferrite is formed by intercritical quenching process.The transformation kinetics of reversed austenite based on JMAK model shows that the isothermal transformation process of intercritical quenching is controlled by the diffusion mechanism.The reversed austenite nucleates in the non-isothermal stage and grows rapidly and coalesces in the subsequent holding stage.As the inercritical quenching temperature increases,the strength increases,and the impact toughness decreases.With the increase of tempering temperature,the strength decreases and the impact toughness increases.The inetrcritical ferrite can effectively participate in the synergistic deformation and significantly improve the plasticity.Subjected to quenching at 950℃+intercritical quenching at 800℃+tempering at 625℃,seamless bends exhibit excellent comprehensive mechanical properties,at the expense of reducing partial strength to achieve significantly increase in plastic deformation ability,work hardening ability,and low temperature toughness.Therefore,an optimized heat treatment process is proposed to prepare seamless bend with high strength,high toughness and excellent plasticity suitable for service in extreme environments.