Take you to understand welding defects-layered cracks
As the most harmful type of welding defect, welding cracks seriously affect the performance and safety of the welded structure. Through the study of the previous lessons, we have mastered the knowledge of hot crack, cold crack and reheat crack. Today, we come to understand the last type of cracks-layered cracks.
Lamellar crack is a stepped crack formed along the rolled layer of the steel plate in the welding structure during welding. It generally shows a transcrystalline or intergranular crack trend. The generation of lamellar cracks includes the following reasons:
Non-metallic inclusions, during the rolling process of the steel plate, some non-metallic inclusions (such as sulfides and silicates) in the steel are rolled into ribbons parallel to the rolling direction, resulting in differences in steel mechanical properties Inclusion, inclusions are the latent factors that cause lamellar tearing of the welded structure, and also the main cause of lamellar tearing.
Restraint stress, due to the thermal cycle of welding, the restraint force of the welded joint will appear. For a given rolled thick plate T-shaped and cross joint, there is a critical restraint stress or bending restraint under the condition of unchanged welding parameters Strength, when greater than this value, it is easy to produce lamellar tear.
Diffusion of hydrogen, hydrogen is a promoting factor for cracking. Due to the diffusion of hydrogen and the combination of molecules, the local stress increases sharply. When hydrogen gathers at the end of inclusions, it promotes the debonding of non-metallic inclusions and metals, and will pull off the adjacent inclusions. Metal, showing hydrogen-induced fracture characteristics on the fracture.
The performance of the base material, although inclusions are the main reason for lamellar tearing, but the mechanical properties of the metal also have a very important effect on lamellar tearing. The metal's plastic toughness is poor, and the more easily cracks propagate, that is, the ability to resist lamellar tears is poor.
In order to prevent the occurrence of layered cracks, the Z-direction stress and stress concentration are mainly avoided in the design and construction process. The specific measures are as follows:
1. Improve joint design and reduce restraint strain. Specific measures such as: extending the end of the arc starting plate by a certain length, which has the effect of preventing cracking; changing the layout of the weld to change the direction of the shrinkage stress of the weld, changing the vertical arc starting plate to the horizontal arc starting plate, changing the position of the weld, Making the joint's total stress direction parallel to the rolled layer can greatly improve the resistance to lamellar tearing.
2. Adopting suitable welding methods, low-hydrogen welding methods are advantageous, such as gas shielded welding and submerged arc welding, the cold cracking tendency is small, which is conducive to improving the resistance to lamellar tearing.
3. Using low-strength matching welding materials, when the weld metal has a low yield point and high ductility, it is easy to concentrate the strain on the weld and reduce the strain in the heat-affected zone of the base material, which can improve the resistance to lamellar tearing.
4. In the application of welding technology, the surface surfacing welding isolation layer is adopted; symmetric welding is applied to balance the strain distribution and reduce strain concentration.
5. In order to prevent the layered tear caused by cold cracking, some measures to prevent cold cracking should be adopted as much as possible, such as appropriately increasing preheating, controlling the temperature between layers, etc. In addition, intermediate annealing and other stress relief methods can also be adopted.
6. We can also use small welding feet and multi-pass welding process by controlling the weld size