cold crack

Weld cracks that occur when the welded joint is cooled to lower temperatures (for steel, below the temperature at which martensite transformation begins). Commonly known as welding cold crack. Welding cold cracks are further divided into three types: delayed cracks, thermal stress cracks, and layered tearing. All three types of cracks exhibit characteristics of transgranular, transgranular and intergranular mixed types, or intergranular cracking. Delayed cracks in weld metal are welding cold cracks that arise as a result of the combined effects of plasticity reserves, stress conditions, and the hydrogen content in the weld metal (the delay time can range from a few minutes or seconds to several months). This type of crack mainly occurs in low-alloy high-strength steels. Its occurrence is related to the interaction of the following three factors: (1) the hydrogen content in the weld metal; (2) the tensile stress exerted on the metal of the welded joint; (3) the plasticity reserve of the metal, determined by the hardening tendency of the steel. The critical hydrogen content and critical stress value determine whether delayed crack formation occurs or not. When the hydrogen content is below the critical level, as long as the tensile stress is below the strength limit, the incubation period will be infinitely long, and no delayed cracks will actually occur; similarly, when the tensile stress is below the critical value, the incubation period will also be infinitely long, with no delayed cracks arising in practice. For steel with a low tendency to harden, it has a high plasticity reserve; therefore, the incubation period for delayed cracks is long, and its tendency to develop cracks is low. Conversely, for steels with a high hardening tendency, due to their low plastic deformation capacity, stress concentration tends to occur at defects within the metal, which can lead to the formation of delayed cracks. Not only is the critical stress value low, but the critical hydrogen content is also low, resulting in a high tendency to crack formation. Thermal stress cracks are primarily caused by the high strain and stress concentration resulting from cooling contraction during welding, which intensifies the stress conditions and leads to cracks when they exceed the material’s strain capacity. It usually occurs at the weld root and takes the form of a through-crystal growth. In thick plate welded structures, due to high restraint forces and high residual welding stresses, cracks with a layered and stepped macroscopic morphology parallel to the rolling direction occur in the heat-affected zone or areas near it. This is caused by certain non-metallic inclusions (such as sulfides, complex silicates, and aluminum oxides) that are distributed in bands or chains within planes parallel to the rolling direction. (Xu Zuze)

It’s really good to visit forums; I learn new things there.

The explanation on the 2nd floor makes a lot of sense! !

The post-owner is referring to post-welding cold cracks? When welding thicker plates, failure to preheat them in advance can lead to cold cracks; cracks that appear after welding are generally not cold cracks.

Post-welding cracks are mainly due to heat treatment issues