What is the most afraid of welding? Crack, because it will seriously affect the mechanical properties and structural integrity of welded components. In fact, there are many types of cracks, such as hot cracks, cold cracks, and reheat cracks. Compared with other cracks, the occurrence of cold cracks is more dangerous. Because it is not produced immediately, but only a few hours, days or even longer after the welding is completed. If the defect is not found before the weldment is put into use, it usually leads to disastrous consequences. Since such cracks often occur at room temperature, we call them cold cracks.
So, how do cold cracks form?
First, the moisture and hydrogen-containing compounds in the vicinity of the welding arc are decomposed into hydrogen atoms under the action of the high temperature of the arc. Hydrogen atoms are well soluble in the liquid bath. During the cooling and solidification of the molten pool, hydrogen atoms exist in the weld metal in the form of supersaturation. Due to the small size of hydrogen atoms, the supersaturated hydrogen atoms in the weld tend to migrate to the heat affected zone. And the defects and discontinuities in the crystal lattice are sought and gathered by diffusion. The residual internal stress caused by welding deformation works together with the aggregated hydrogen to expand the discontinuity of the lattice to the crack size. With the crack propagation, the crack tip is far away from the hydrogen accumulation point, and the hydrogen further diffuses to the crack tip to promote the crack propagation.
Therefore, high diffusible hydrogen content, high hardness microstructure and high welding residual stress are important factors affecting the occurrence of cold cracks. The diffusible hydrogen content is largely affected by the welding shielding gas.
At present, one of the most commonly used MIG welding methods is solid wire gas shielded welding (GMAW), and the main gas used is argon(Ar)+ carbon dioxide (CO2) mixture. Through a series of experiments, we found that for GMAW, with the increase of carbon dioxide (CO2) content in the welding gas, the diffusion hydrogen content of the deposited metal decreases. The main reason is that as the carbon dioxide content in the welding shielding gas increases, oxygen atoms enter the arc atmosphere by decomposition. The oxygen content in the arc atmosphere can remove the monatomic hydrogen in the arc atmosphere and effectively reduce the hydrogen partial pressure in contact with the liquid weld metal.