Research On Microstructure And Fatigue Performance Of Welded Pipe’s Welded Joint For Three Electeric Resistance Electeric Resistance Welds

As an important way to transport oil and gas, the construction of pipe depends on the research and development of pipeline steel. At present, X52, X65 and L415 Electrical Resistance Welds are commonly used for oil and gas transmission. ERW welded pipe is short for Electrical Resistance Welds. Fatigue fracture may cause catastrophic accidents, so the research of Electrical Resistance Weld fatigue property is necessary.Metallographic microscope, scanning electron microscope(SEM), electron back-scattered diffraction(EBSD), microhardness test and tensile test are employed to study microstructure and mechanic properties of three kinds of experiment steels’ welding joint. The experimental results showed that the microstructure of these three kinds of steels are both ferrite and pearlite, the grain size of X65 is finest and appears banded structure. Three kinds of experiment steels’ hardness reach maximum in the welded seams, hardness decrease with the distance from the welding line is increasing, the main reason is pearlite content decline in microstructure. Compared with X52, L415, X65 showed the highest tensile strength and yield strength, the lowest yield ratio.Welded joint fatigue test are carried out on a MTS Landmark 370 type servo hydraulic fatigue testing machine to research fatigue behavior of three kinds of steels and get the fatigue S-N curve of three kinds of steels, the fatigue ultimate strength and loop hysteresis return line. Results show that X52 has the highest ultimate strength, L415, Secondly, X65, the lowest, three kinds of steels all performance cyclic softening. Metallographic microscope, scanning electron microscope(SEM) are employed to analyze fatigue fracture position, fatigue fracture morphology and fatigue deformation and fracture behavior, The experimental results showed that the fatigue fracture position of three kinds of steel are all occurred in the welding heat affected zone, which is related to the microstructure deterioration. The fatigue fracture morphology of three kinds of steels performance mixed type of ductile and brittle fracture, fatigue sources generally form on the surface, fatigue extension area has obvious fatigue striations and secondary cracks, transient fracture zone performances toughening characteristics. X52、L415 has many secondary cracks, in contrast, X65 secondary crack almost negligible. Secondary crack extension is mainly along the ferrite, the type of crack propagation is trans-granular fracture, when faced with a hard phase such as perlite the crack propagation direction will change.