Study On The Welding Property Of X80Steel Pipes

In recent years, it is difficult to transport the oil and gas found in the remote areas to the center cities, therefore the transportation of the oil and gas pipeline steel should have a good ability of strong toughness, large deformation, excellent welding performance, etc. The pipeline steel with strain-based design is usually characterized by high yield strength and tensile strength, low yield stress to tensile strength ratio, initial work hardening rate, etc. Firstly the dynamic CCT curve was drawn by the thermal expansion test, the metallographic test and the hardness test. Then welding thermal simulation experiment was carried out to study the performance characteristics of X80pipeline steel with strain-based design. Finally, the performance of pipeline steel pipe and welded joint of the X80pipeline steel with acicular ferrite and the X80pipeline steel with strain-based design were studied by scanning electron microscopy (SEM) test, the charpy impact test, the vickers hardness test and the tensile test and other methods. The main contents are given as follows:The dynamic CCT curve showed that the ferrite decreases, the grain refine, bainite and hardness increase, when the cooling speed increased. When the cooling speed was greater than3℃/s, the main microstructure was bainite. The two phase of hard and soft combination could be obtained at the cooling rate of2℃/s, and the two phase was ferritic and bainite, which met the requirement of pipeline steel with strain-based design.With the increase of welding heat input, the toughness of the material increased first and then decreased. When the welding heat input was20KJ/cm, the coarse grain heat-affected zone area showed good toughness. When the welding heat input is35KJ/cm, with the increase of peak temperature, the toughness of material reduced, the hardness and grain size increased. When the condition of welding heat input was20KJ/cm, material still possessed good toughness and hardness and the performance was not deteriorated. When the condition of the heat input was35KJ/cm and the metal experienced secondary thermal cycle, the toughness of material increased but the hardness decreased at the temperature of800℃; the toughness and hardness of the material were lower at the temperature of900℃. When the condition of the heat input was20KJ/cm and the metal experienced secondary thermal cycle, impact toughness and hardness of the material reduced, but impact toughness and hardness are higher than the condition of the heat input35KJ/cm.The microstructure of X80pipeline steel with strain-based design was mainly ferrite plus granular bainite, and the proportion of granular bainite was about20.6%. The toughness and hardness of welded joint of the X80pipeline steel with strain-based design were lower than base, but these two kinds of performance still met requirement of standard. The microstructure of the weld contained intragranular nucleated acicular which was beneficial to the toughness and hardness of the weld. The grain of coarse grain in HAZ near two phase region(heating temperature was about900℃) refined significantly. The heating temperature of base near HAZ was lower than Ac1, and some fine carbide precipitated easily. But the bainite reverted and recrystallized at this temperature, so the hardness of this region reduced. The main types inclusion of coarse grain heat-affected zone was class D inclusion. FeO inclusions were not good combination with matrix, which were harmful to the steel. AlO3·CaO·MnO inclusion was small and good combination with matrix, which were innocuity to the steel.The welded joint of the X80pipeline steel with acicular ferrite did not appear obvious softening phenomenon. The change of the hardness of welded joint were similar between the X80pipeline steel with acicular ferrite and the X80pipeline steel with strain-based design. The hardness of near the base of HAZ reduced due to M/A island spheroidizing. Inside weld hardness was higher outside weld and reached250HV because of grain refining and precipitation strengthening. The main inclusions of coarse grain heat-affected zone were class D inclusions. Calcium aluminate compound phase inclusions combined with matrix were better. But there was the gap between oxide inclusion and matrix.The hardness of the welded joint of the X80pipeline steel with acicular ferrite was higher than X80pipeline steel with strain-based design, but the hardness of the base metal was the seem with each other. The hardness of coarse grain zone was different, because the M/A of X80pipeline steel with acicular ferrite was more scattered and distribution.