Microstructure and properties of flash butt welding joints on corrosion resistant steel for marine flexible pipe

In order to investigate the instability of welded steel joints for high-strength Marine flexible pipe armor, taking the hydrogen damage resistant corrosion resistant steel 600 MPa grade as the test object, and the effects of different flash allowance and upset allowance on the microstructure and mechanical properties of the joints were studied. The results show that with the increase of the upset allowance, the polygonal ferrite content of the weld and coarse grain zone in the heat-affected zone increases, and the microstructure gradually coarsens, and the joints strength increases firstly then decreases. When the flash allowance exceeds 5 mm, the joint strength deteriorates sharply. With the increase of flash allowance, the weld and heat-affected zone coarse-grained zone the bainite ferrite gradually coarser, and the joint strength gradually increases and then decreases sharply. When the flash allowance is 8 mm, the joint strength deteriorates seriously. Both of the parameters increase the elongation gradually, but the influence of the upset allowance on the elongation is greater. When the flash allowance and upset allowance are 7 mm and 4 mm, the mechanical properties of the welded samples are the best. The width of the ferrite lath in the heat affected zone is about 0.59 μm, the tensile strength is 688 MPa, the yield strength is 586 MPa, the elongation is 15%, and the tensile fracture form is ductile fracture. The fine and dispersedly distributed granular bainite can be formed by the matching of the flash allowance and the upset allowance parameters, which can effectively strengthen the steel matrix.