Effect Of Temperature On Mechanical Properties Of Ultra-High Strength Drill Pipe Steel Grade

In this paper,the tensile properties,impact toughness and fracture toughness of ultra-high strength drill pipe steel U165 and V150,and high strength drill pipe steel S135 and G105 at different temperatures are systematically studied by means of static hydraulic universal testing machine,impact testing machine and low temperature box,at the same time,SEM and TEM were used to study the microstructure and the fracture morphology of drill pipe steels.The mechanical fracture mechanism and microstructure of these drill pipe steels are analyzed comprehensively,which provides basis for the design of mechanical properties of drill pipe steel.The main conclusions are as follows:The four kinds of drill pipe steels studied in this paper are tempered sorbite structure.With the decrease of drill pipe steel grade,the coarser the microstructure,the lower the grain size grade,and the smaller the strength and hardness.The carbides distributed along the grain boundary of ultra-high strength drill pipe steel are discontinuous particles,which play the role of strengthening the structure.With the decrease of temperature,the yield strength and tensile strength of the four drill pipe steels increase,the yield ratio decreases.With the increase of drill pipe steel grade,the tensile temperature sensitive coefficient qb and the yield strength temperature sensitive coefficient qs of the materials are obviously decreased,while their ratio is obviously increased.The yield strength and k(the ritio of tensile strength temperature sensitive coefficient to yield strength temperature sensitive coefficient)are fitted by Boltzmann function,and the correlation coefficient is 0.982.The ritio of tensile strength temperature sensitive coefficient to yield strength temperature sensitive coefficient can be used to characterize the effect of temperature on the mechanical properties of materials.The larger the value k is,the faster the tensile strength increases compared with the yield strength and the better comprehensive mechanical properties of materials at low temperature.In addition,the value k of these drill pipe steels studied in this paper is larger,so their comprehensive mechanical properties are better at low temperature.Under the impact load,the impact energy of these drill pipe steels all decrease with the decrease of temperature,and the impact energy of different drill pipe steels in different temperature ranges all decrease abruptly.Moreover,with the decrease of drill pipe steel grade,the ductile-brittle transition temperature of materials also decreases.There are three types of load-displacement curves of the oscillographic impact curves of drill pipe steel studied in this paper,which are respectively corresponding to ductile fracture,quasi-cleavage fracture and brittle fracture.With the decrease of temperature,the crack formation energy is first increased and then decreased,and the crack propagation energy shows a significant downward trend.The decline of crack propagation energy is generally larger than the change of crack propagation energy,and the sum of the two is the impact energy.The ratio of crack formation energy to total impact energy increases with the decrease of temperature,and the ratio of crack propagation energy to total impact energy decreases.The fracture toughness J0.2BL(7.5)of ultra-high strength drill pipe steel U165 decreases with the decrease of temperature.The fracture toughness J0.2BL(7.5)of V150 drill pipe steel increases first and then decreases with the decrease of temperature.However,the fracture toughness J0.2BL(7.5)of S135 and G105 is obviously increased.