Effect Of Pre-compression Deformation On Fracture Toughness Of X80 Pipeline Steel

In recent years,with the rapid development of economy,the demand of natural resources such as natural gas,oil and so on continues to grow.Large diameter,high density and high pressure transportation mode promote the development of pipeline steel to high steel grade.In the actual service process of pipeline steel,the surface subsidence,extrusion,bending and other factors affect its internal plastic deformation.In this paper,X80 pipeline steel is taken as the research object.Through the pre-compression deformation,tensile test after pre deformation and three-point bending test,the actual service situation of X80 pipeline steel is simulated to explore the influence of pre-compression deformation on its mechanical properties and fracture properties.Firstly,the pipeline steel was pre-compressed at 0.5%,1.0%,and 1.5%,and then the samples with four kinds of deformation were tensile tested to get the change rule of yield and tensile strength;the single sample flexibility method was used to carry out three point bending test to study the change rule of fracture toughness under different pre-deformation;then the change of microstructure micro-hardness near the crack tip was measured The change rule between the size of plastic deformation zone and the amount of pre-deformation at the crack tip was analyzed.The microstructure and crack growth of X80 pipeline steel were analyzed by OM and SEM.The tensile test results show that the yield strength of X80 pipeline steel decreases obviously with the increase of pre-compression deformation,the deformation increases from0%to 1.5%,and the corresponding yield strength decreases from 574 MPa to 503 MPa.There is no obvious change in tensile strength,so the yield ratio decreases from 0.92 to 0.83.In addition,with the increase of pre compression deformation,Bauschen effect becomes more and more significant,which is shown by the downward movement of stress-strain curve.The elongation of the material decreases after fracture,which indicates that the pre compression deformation reduces the plasticity of the material to a certain extent.The fracture toughness test results show that when the pre-compression deformation of X80 pipeline steel increases from 0 to 1.5%,the corresponding fracture toughness crack tip opening displacement value decreases from 0.75 mm to 0.27 mm,and the critical J integral value from 780 k J/m~2 It dropped to 280 k J/m~2.As the amount of deformation increases,the area of the plastic zone at the crack tip gradually decreases.The structure and crack propagation mode of X80 pipeline steel obviously change with the increase of pre-compression deformation,which provides a theoretical basis for the safety evaluation of pipeline steel performance.