Thermal cracking refers to the cracks generated at high temperature, also known as high temperature cracks or crystal cracks, which usually occur inside the weld, and sometimes may also appear in the heat affected zone. The manifestations are: longitudinal cracks, transverse cracks, root cracks, crater cracks And heat-affected zone cracks. The reason is that there is segregation in the welding pool during the crystallization process. The low-melting eutectic and impurities exist in the form of liquid interlayer during the crystallization process to form segregation. The strength after solidification is also low. When the welding stress is large enough , It will pull apart the liquid interlayer or the solid metal that has just solidified to form cracks. In addition, if there are low melting point eutectics and impurities on the grain boundaries of the base material, they will be pulled apart when the welding tensile stress is large enough. In short, the occurrence of thermal cracks is the result of the combined effect of metallurgical and mechanical factors.
In view of its causes, the preventive measures for ERW welded pipes are as follows:
(1) Limit the content of easily segregated elements and harmful impurities in base materials and welding materials (including welding rods, welding wires, fluxes and shielding gases). In particular, the content of sulfur and phosphorus should be controlled and the carbon content should be reduced. Generally used in welding steel The content of sulfur should not be greater than 0.045%, and the content of phosphorus should not be greater than 0.055%; in addition, the more the carbon content of the steel, the worse the welding performance. Generally, when the carbon content in the weld is controlled below 0.10%, the hot crack sensitivity can be Greatly reduced.
(2) Adjust the chemical composition of the weld metal, improve the weld structure, refine the weld grains to improve its plasticity, reduce or disperse the degree of segregation, and control the harmful effects of low melting point co-products.
(3) Use alkaline electrodes or fluxes to reduce the impurity content in the weld and improve the degree of segregation during crystallization.
(4) Appropriately increase the shape factor of the weld, and adopt a multi-layer and multi-pass welding method to avoid centerline segregation and prevent centerline cracks.
(5) Use reasonable welding sequence and direction, adopt smaller welding line energy super, overall preheating and hammering method, and fill the arc crater when closing the arc.
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