Welding cold crack test of X80 pipeline steel

X80 pipeline steel is made of high strength and toughness by deformation strengthening, and X80 steel is also ultra-fine grain, high purity steel. Therefore, the steel puts special requirements on the welding process, mainly in how to prevent grain coarsening, local softening and embrittlement in the heat affected zone, how to achieve the purity of the weld metal and grain refinement, how to choose Welding methods and improved welding processes. Grain coarsening in the heat affected zone is an unavoidable phenomenon, and the grain growth weakens the performance of the welded joint. In order to control the grain growth of the heat affected zone of the pipeline steel at high heat input, the small welding heat input or high energy beam welding method is often used to narrow the coarse crystal zone so as not to affect the serviceability of the welded joint. However, cold cracks are likely to occur at the weld seam under small line energy. Therefore, it is necessary to preheat the weld before welding, and to study the influence of preheating on the cold crack sensitivity of X80 pipeline steel by adjusting the preheating temperature. Simultaneous preheating also improves weldability and reduces stress. In this case, the preheating temperature during the welding of pipeline steel is studied.

The metallographic structure of the welded joint of the X80 pipeline steel pipe with a thickness of 7.9 mm and an outer diameter of 610 mm is different at different preheating temperatures. With the increase in the appearance of granular bainite, the bainite laths are divided to make the bainite laths having the same orientation thin and thin. At the same time, the boundary of the slats can act like a grain boundary, and the crack will occur when the crack propagates to the slab boundary, thereby effectively blocking the crack propagation during the low temperature fracture process. Under the preheating conditions of X80 pipeline steel at room temperature and 50°C, the crack rate of the section is about 30%. With the preheating temperature of the test plate rising, the crack rate of the section is reduced to 3.55% when the preheating temperature reaches 100°C, and the crack rate of the section is only 0.85% when the preheating temperature is raised to 150°C. This indicates that under the severe conditions of the oblique Y groove weld crack test, the weld metal heat affected zone has excellent crack resistance at 100°C preheating. With the increase of preheating temperature, the crack rate of the test joint of Xiaotieyan test piece is significantly reduced; when the preheating temperature reaches 100°C, the crack rate of the section is much less than 20%, and the crack sensitivity is greatly reduced. The occurrence of cold cracks is caused by the combination of hardening tendency, hydrogen content and restraint. Pipeline steel has a low carbon content, a tendency to harden and a tendency to reduce cold cracking. However, as the strength level increases, the plate thickness increases, and there is still a certain tendency of cold cracking. In the field welding, due to the use of high-hydrogen welding materials such as cellulose electrode and self-shielded flux-cored wire, the line energy is small and the cooling speed is fast. Under the condition of large line energy, the grain growth will seriously affect the performance of the welded joint.

Hydrogen is one of the main factors in the generation of cold cracks in welding, and it takes a certain time for hydrogen to diffuse and aggregate, which leads to a certain delay in the generation of cold cracks. Under preheating conditions, the rate of hydrogen diffusion in the welded joint is significantly accelerated, and the hydrogen concentration in the weld metal is rapidly reduced. In the preheating case, the cooling speed of the welded joint is slowed down. The higher the preheating temperature is, the slower the cooling is, and the longer the diffusion hydrogen escapes, the large amount of diffused hydrogen will escape through accelerated diffusion, and the diffused hydrogen content in the heat affected zone The lower, this is one of the reasons why the preheating temperature is raised to cause a significant decrease in the crack rate.

Summary:
1. X80 pipeline steel has low carbon content and low tendency to harden. Generally, heat treatment and other heat preservation measures are not required after welding.
2. When the X80 pipeline steel was tested in a small iron test, the photo of the cracked metallographic phase near the fusion zone indicates that the crack is transgranular, which is a typical cold crack of the weld.
3. When the X80 pipeline steel is tested in small iron, the crack rate is significantly reduced as the preheating temperature increases.
4. When the X80 pipeline steel is tested in small iron, the microstructure of the fusion zone is granular bainite + lath bainite + ferrite. With the increase of preheating temperature, the linear energy decreases, and the fusion zone is granular. The body structure is increased, the bainite slats are divided to refine the grains, and the toughness of the weld is enhanced to reduce the cold crack sensitivity of the welded joint.