| In this dissertation,the objects of research are bainite/ferrite dual-phase X80 high-deformability pipeline steel and welded joints.The effect of dual-phase microstructutre on fatigue properties of X80 high-deformability pipeline steel has been explored.The reaserch is mainly focused on the effect of dual-phase microstructutre on fatigue crack propagation behaviour and the effect of welded joint on crack initiation and propagation.In addition,fatigue crack propagation behaviours in each subregion of heat affect zone(HAZ)are analyzed by welding thermal simulation method.Compairing crack propagation rate curve of actual welded joint with simulated microstructutre,evaluation methods of crack propagation life are discussed.Fatigue life test and crack propagation rate test were carried out using bainite/ferrite dual-phase X80 high-deformability pipeline steel.Fatigue life tests show that crack initiations locate on the surface of specimens,and propagation inward untill the occurrence of fracture.As the maximum stress is actual maximum service stress(400 MPa),the fatigue life of specimens is about 4.2×105 N.Fatigue crack propagation rate tests show that there is a turning point in da/dN-(35)K curve,which is related to the changes of crack path and fracture mode during fatigue crack propagation.Compairing with Paris equation,αβ model is more convenient and efficient to evaluate the life and design standards of safety for dual-phase X80 high-deformability pipeline steel.Pipeline steel will inevitably experience welding process in the manufacture and installation.Therefore,fatigue life test and crack propagation rate test were carried out using full-thickness specimens with welded joint of dual-phase X80 high-deformability pipeline steel.Fatigue life tests show that fatigue life of welded joint specimens decrease with the increase of maximum stress,and are less than fatigue life of base metal at different maximum stresses.When the maximum stress declines to the actual maximum operating stress(400 MPa),the fatigue life of welded joint specimens is about 1×105 N.The fatigue crack initiation of the full-thickness X80 welded joint specimens occurs at the outside weld toe because of stress concentration resulting from the inclusions and weld toe profile.The fatigue crack grows inward until the ultimate fracture forms in HAZ.The stable propagation distance of fatigue crack decreases with increased maximum stress,and the direction of ultimate fracture tends to ocuur at the low hardness zone.For welded joint specimens,fatigue crack propagation mainly occurs in HAZ.Therefore,thermal simulation test and fatigue test are carried out in different peak temperatures.Fatigue life tests show the similar results at the peak temperature of 1050 ℃and 850 ℃.However,when the peak temperature is 1350 ℃,fatigue life is lower than the former two.Fatigue crack propagation rate tests show that when the(35)K is lower than 27 MPa?m1/2,the value of da/dN reaches the highest at the peak temperature of 1350 ℃;when(35)K is higher than 27 MPa?m1/2,the value of da/dN reaches the highest at the peak temperature of 850 till the end of the crack stage which is related t℃ o the effect of the resistance of organization structure on crack propagation resistance.Comparing the da/dN-(35)K of the actual welded joint specimens with simulated specimens,evaluation methods of crack propagation life are discussed.Moreover,the evaluation of fatigue life in X80 high-deformability pipeline steel and its weleded joint are discussed according to stress-life analysis method.The results show that the S-N curve of X80 high-deformability pipeline steel and its weleded joint are fitted with Basquin equation,which can get the coefficient of c,with values 0.15 and 0.20,respectively.There are some discrepancies between the da/dN-(35)K of the actual welded joint specimens and simulated specimens.Therefore,it is suggested that the da/dN-(35)K of the actual welded joint should be used to evaluate fatigue crack propagation life. |
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