Research On Hot Cracks And Cold Bending Performance For Welded Joint Of X70 Pipeline Steels

High pressure, large diameter and high strength pipes are inevitable trend for the oil and natural gas transporting pipeline. X70 pipeline steels, microalloyed steel and low carbon produced by TMCP process, possess high strength and good toughness. Before welding the microstructures are fine acicular ferrite, but through the thermal cycling effect of SAW, the microstructures in WM comprise of proeutectoid ferrite and acicular ferrite, and those in CGHAZ comprise of coarsely granule bainite. After welding the materials deterioration in performance, sometimes hot cracks will appear, and cracks will take place in the cold bending test. In this thesis, the mechanics of appearing the hot cracks and cold bending cracks for X70 pipeline steel will be studied, and some improvement measurements will be proposed.Through analyzing the metallic phase and constitute of specimens shown hot cracks, it can be concluded the cracks are all crystal cracks. It is the main reasons for them that the segregation of Sulfur and phosphorus elements in the substrate, welding profiled coefficient less than 1 and too much energy input.Through the cold bending cracking specimens' metallic phase and fracture ESEM analysis, it can be concluded that the cracks start with CGHAZ and finish with normalized zone. In order to research the relationship between the cold bending cracks and CGHAZ, thermal simulation on X70 CGHAZ is performed making use of Gleeble-2000 thermal simulation machine, only changing t8/5.By comparing the microstructures of CGHAZ in real weld with the simulated specimens of spiral welding when t8/5 is 40s, they are approximately anastomotic. It indicates that the thermal simulation can reflect the real CGHAZ microstructures and properties. It can be shown by analyzing the simulated specimens that austenite crystal coarsens, the CGHAZ microstructures are coarse grain bainite. Making use of lepera reagent revealing the M-A constituents, its pattern, size and distribution's effect on toughness of CGHAZ are analyzed. With the t8/5 increasing, the M-A constituents alongγ grain boundary become more and coarser, the M-A constituents in ferrite strip transform strip into block, the average size grows bigger, and the bulk possessed increases, and its average size exceeds Griffith crack critical size, it can act as the cracking resources and damage the toughness of CGHAZ, and it's the dominant factor which has effect on the toughness. The experimental results of hardness, tension and -20℃impact test show that the hardness of CGHAZ decreases gradually, the b and s also decrease, the elongation percentage have no obvious change, and the impact toughness decrease, with the t8/5 increasing. In addition, the results of prestrain test show that strain embrittlement produce, the impact energy reduce. Finally, the results of microstructures and properties show that the CGHAZ toughness decreases with the t8/5 increasing, resulting in the cold bending performance decreasing.