Understand Welding Defects Easily - Laminar Cracks

Welding cracks are the most harmful type of welding defects, which seriously affect the performance and safety and reliability of welded structures. Today, I will take you to know one of the types of cracks - layered cracks.

01

Non-metallic inclusions, some non-metallic inclusions (such as sulfides, silicates) in the steel are rolled into strips parallel to the rolling direction during the rolling process of the steel plate, resulting in differences in the mechanical properties of the steel Inclusions are the potential factors and the main cause of laminar tearing in welded structures.

02

Restraint stress. Due to the thermal cycle of welding, there will be a restraint force in the welded joint. For a given rolled thick plate T-shaped and cross joint, under the condition of constant welding parameters, there is a critical restraint stress or bending restraint. Strength, when it is greater than this value, it is easy to produce lamellar tearing.

03

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 the hydrogen accumulates at the end of the inclusion, it promotes the loss of adhesion between the non-metallic inclusion and the metal, and will break the adjacent inclusions. The metal exhibits hydrogen-induced fracture characteristics on the fracture.

04

Base metal properties, although inclusions are the main cause of lamellar tearing, the mechanical properties of the metal also have a significant impact on lamellar tearing. The plastic toughness of the metal is poor, the easier the crack is to expand, that is to say, the ability to resist lamellar tearing 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 ignition plate to a certain length, which has the effect of preventing crack initiation; changing the welding seam layout to change the direction of the shrinkage stress of the welding seam, changing the vertical arc ignition plate to the horizontal arc ignition plate, changing the position of the weld, Making the total stress direction of the joint parallel to the rolling layer can greatly improve the resistance to laminar tearing. 

 2. It is advantageous to adopt suitable welding methods, such as gas shielded welding and submerged arc welding, with low tendency of cold cracking, which is beneficial to improve the resistance to laminar 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 metal, which can improve the resistance to laminar tearing. 

 4. In the application of welding technology, the surface surfacing isolation layer is used; the welding is symmetrical to make the strain distribution even and reduce the strain concentration. 

 5. In order to prevent laminar tearing caused by cold cracking, some measures to prevent cold cracking should be adopted as far as possible, such as appropriate increase of preheating, control of interlayer temperature, etc. In addition, intermediate annealing and other stress relief methods can also be adopted. 6. We can also control the size of the welding seam and adopt the welding process of small welding feet and multi-pass welding.