Study On Mechanical Properties Of Ultra-high Strength Drill Pipe Steel At Different Loading Rates

In this paper,the tensile properties and fracture toughness of high-strength drill pipe steel G105,S135 and ultra-high-strength drill pipe steel V150,U165 were mainly studied at different loading rates by means of static hydraulic universal testing machine.At the same time,the microstructure and fracture morphology of the drill pipe steel was studied by SEM and TEM,and the comprehensive analysis of the mechanical fracture mechanism and micro structure of the four drill pipe steels provide a basis for designing the mechanical properties of the drill pipe steelThe microstructures of the four high-strength drill pipe steels are tempered sorbite structure,and the grain size of the ultra-high strength drill pipe steels U165 and V150 is very fine.The carbide particles in the ultra-high-strength steel grade U165 and V150 are mainly(Cr,Fe)23C6,which is mainly dispersed in the original austenite grain boundaries,but is not continuous,so it is not much harm s to the drill pipe performanceThe hardnesses of the four steel grades are sorted from large to small,U165>V150>S135>G105,which is mainly determined by the grain size of the material structure.For the four types of steel,as the steel grade increases,the rate of increase in the tensile strength of the material is lower than that in the yield strength.The yield point strain and yield strength show a positive correlation,and the tensile strength point strain and tensile strength show a negative correlation The total elongation of G105,S135,V150 drill pipe steel generally showed a downward trend with the increase of steel grade,but the elongation of U165 slightly increased,and the static toughness was the highest,which indicates that U165 steel Excellent comprehensive tensile properties.As the loading rate increases,the yield strength of the four steels increases monotonously,but the increase rate will gradually slow down,especially when the loading rate exceeds 18 mm/min,the increase rate decreases sharply.At the same time,the test also shows that the loading rate has the same rule for the tensile strength of the material.As the loading rate increases,the elongation of the four types of drill pipe steels tends to decrease.As the strain rate increases,the yield strength and tensile strength of the material will increaseAs the loading rate increases,the strains of the tensile strength points of the four types of drill pipe steel all show a downward trend.In contrast,the higher the steel grade,the less the strain of the tensile strength point corresponding to the loading rate.For the four types of steel,the strain corresponding to the tensile strength has a significant reduction.In contrast,the higher the strength,the smaller the impact,which is the opposite of the strength change trend of the material.For the four types of steel,as the loading rate increases,the static toughness first decreases significantly,then slowly decreases.Through the single specimen method and the three-point bending specimen,the fracture toughness of four different grades of drill pipe can be obtained;as the loading rates of the four types of drill pipe steels increases,the fracture toughness tends to decrease.Compared with U165,G105,S135 and V150 have a smaller decrease in fracture toughness J(0.2bl(7.5))under low loading rate.As the loading rate increases,the reduction speed of the fracture toughness J(0.2bl(7.5))of all four steels is slowed down.