Understanding welding defects - lamellar cracks

As the most harmful type of welding defects, welding cracks seriously affect the performance and safety reliability of welded structures.

-----Laminar Cracking

01

Non-metallic inclusions, steel plates in the rolling process will be some non-metallic inclusions in steel, etc. (such as sulfides, silicates) rolled parallel to the rolling direction of the strip, resulting in the variability of the mechanical properties of steel, inclusions are potential factors in the production of lamellar tears in welded structures, but also the main reason for lamellar tears.

02

Constraint stress, due to the role of welding thermal cycle, the welded joint will appear constraint force, for a given rolled thick plate T and cross joints, under the conditions of constant welding parameters, there is a critical constraint stress or bending constraint strength, when greater than this value is prone to laminar tearing.

03

Hydrogen diffusion, hydrogen is a fracture promoting factor, due to the diffusion of hydrogen and combined into molecules resulting in a sharp increase in local stress, when the hydrogen gathered in the end of the inclusions to promote non-metallic inclusions and metal debonding, and will pull off the adjacent inclusions of metal, in the fracture of hydrogen fracture characteristics.

04

Parent material properties, although inclusions are the main cause of lamellar tears, the mechanical properties of the metal also have a very important effect on lamellar tears. The poor plastic toughness of the metal, the more easily the crack is extended, that is to say, the resistance to laminar tearing is poor.

To prevent the generation of lamellar cracks, the main measures in the design and construction process are to avoid Z-directional stress and stress concentration, as follows.

1. Improve the design of the joint to reduce the constraint strain. Specific measures such as: the arc plate end extension of a certain length, there is the effect of preventing the initiation of cracking; change the layout of the weld to change the direction of the weld shrinkage stress, the vertical arc plate into a horizontal arc plate, change the position of the weld, so that the total force direction of the joint and the rolling layer parallel, can greatly improve the performance of the resistance to laminar tearing.

2. Adopt suitable welding method, using low hydrogen welding method is beneficial, such as gas shielded welding, submerged arc welding cold cracking tendency is small, which is conducive to improving the performance of resistance to lamellar tearing.

3. The use of low-strength matching welding materials, weld metal with low yield point, high ductility, easy to make the strain concentrated in the weld and reduce the strain in the heat-affected zone of the parent material, can improve the performance of the resistance to lamellar tearing.

4. In the use of welding technology, the use of surface overlay isolation layer; symmetric application of welding, so that the strain distribution is balanced, reducing strain concentration.

5. In order to prevent lamellar tearing caused by cold cracking, some measures to prevent cold cracking should be adopted as far as possible, such as appropriate increase in preheating, control of interlaminar temperature, etc.; in addition, stress relief methods such as intermediate annealing can also be adopted.

6. We can also control the size of the weld seam by using a small welding foot, multi-pass welding welding process.