X80 pipeline steel is reinforced by high strength and toughness deformation. X80 steel is also ultra-fine grain and high purity steel. Therefore, steel has special requirements for welding process, mainly how to prevent grain coarsening, local softening and embrittlement in heat affected zone, how to achieve the purity and grain refinement of weld metal, how to select welding methods and improve welding process. Grain coarsening is inevitable in HAZ, and grain growth weakens the properties of welded joints. In order to control the grain growth in HAZ of pipeline steel under high heat input, small welding heat input or high energy beam welding are usually used to reduce the coarse grained zone in order to avoid affecting its applicability. Welded joints. However, cold cracks may occur at the weld under small line energy. Therefore, it is necessary to preheat the weld before welding and to study the effect of preheating on the cold crack sensitivity of X80 pipeline steel by adjusting the preheating temperature. At the same time, preheating can also improve weldability and reduce stress. In this case, the preheating temperature of pipeline steel during welding was studied.


At different preheating temperatures, the metallographic structure thickness and outer diameter of welded joints of X80 pipeline steel pipe are 7.9 mm and 610 mm respectively. With the increase of the appearance of granular bainite, the bainite lath is segmented, which makes the bainite lath have the same orientation thin and thin. At the same time, the boundary of the slab can be the same as the grain boundary. When the crack propagates to the boundary of the slab, the crack will occur, which effectively prevents the crack propagation in the process of low temperature fracture. The crack rate of section of X80 pipeline steel is about 30% at room temperature and 50 C. With the increase of the preheating temperature, the crack rate of the section decreases to 3.55% when the preheating temperature reaches 100 and only 0.85% when the preheating temperature rises to 150. C.  This shows that the HAZ of weld metal has excellent crack resistance when it is preheated at 100 C under the rigorous conditions of inclined Y groove weld crack test. With the increase of preheating temperature, the crack rate of test joints of small iron rock specimens decreases obviously; when the preheating temperature reaches 100 C, the crack rate of section is far less than 20%, and the crack sensitivity decreases greatly. The occurrence of cold cracks is caused by the combination of hardening tendency, hydrogen content and constraints. Pipeline steel has the tendency of low carbon content, easy hardening and reducing cold cracking. However, with the increase of strength level, the plate thickness increases, and there is still a certain tendency of cold cracking. In 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.  In the case of large on-line energy, grain growth will seriously affect the performance of welded joints.


Hydrogen is one of the main factors causing cold cracks in welding process. It takes a certain time for hydrogen to diffuse and accumulate, which results in a certain delay of cold cracks. Under preheating conditions, the hydrogen diffusion rate in welded joints is significantly accelerated, and the hydrogen concentration in welded metals decreases rapidly. The cooling rate of welded joints decreases under preheating condition. The higher the preheating temperature, the slower the cooling, and the longer the time of hydrogen diffusion and escape. A large amount of diffused hydrogen will escape through accelerated diffusion. The lower the hydrogen content in the heat affected zone is, this is one of them. The reason for the increase of preheating temperature leads to a significant decrease in fracture rate.


Summary:
1. X8 0 pipeline steel has low carbon content and low hardening tendency. Normally, heat treatment and other insulation measures are not required after welding.
2. When X80 pipeline steel is tested in the small iron test, the metallographic photos of cracks near the fusion zone show that the cracks are transgranular, which is a typical cold crack in the weld.
3. When X80 pipeline steel is tested with small iron, the crack rate decreases significantly with the increase of preheating temperature.
4. When X80 pipeline steel is tested with small iron, the microstructure of melting zone is granular bainite + lath bainite + ferrite. With the increase of preheating temperature, the linear energy decreases and the Fusi decreases.