Detailed explanation of the mechanism behind welding porosity

Thread Content

Pores refer to the holes formed by gases in the weld. Porosity is a defect commonly found in welds; its presence reduces the effective cross-sectional area of the weld, leads to stress concentration, undermines the density of the weld, and results in the failure of the welded component. There are surface pores in the weld, as well as internal or root pores, such as the internal pores in electroslag welds; there are also through-pores ； In terms of distribution, there are single stomata as well as clusters of densely packed stomata. The gases that form pores include hydrogen, nitrogen, carbon monoxide, and water vapor, among others. Hydrogen and nitrogen gases dissolve in metals at high temperatures; during the solidification process, their solubility in the liquid-solid phase drops suddenly, and they do not have time to escape, remaining trapped within the metal ； Carbon monoxide gas and water vapor are gases formed in metal metallurgical reactions that are insoluble in metals. Crystallization rate in the molten pool: when the crystallization rate is low, bubbles have sufficient time to escape. 1. Hydrogen pores: In the welds of low-carbon steel and low-alloy steel, hydrogen pores have a screw-shaped cross-section; they mostly appear on the surface of the weld (though in some cases they can also be found inside), are funnel-shaped, and have smooth inner walls surrounding them. In aluminum-magnesium alloys, it often appears inside the welds. In manual arc welding, hydrogen mainly comes from the organic substances in the electrode flux, crystalline water or adsorbed water, as well as oil contaminants on the surface of the base metal and welding wire, and water vapor in the air. The way hydrogen dissolves into metal: Under the protection of slag, the slag itself has a certain capacity to dissolve hydrogen; the hydrogen dissolved in the slag exists mainly in the form of OH–. OH– and Fe2+ generate hydrogen atoms that enter the liquid metal through electron exchange ； During gas shielded welding, hydrogen in the gas dissolves directly into the metal in the form of atoms or protons upon contact with it. Baidu Baike: Also known as bound water. Crystal water are water molecules bound within a compound; they are not liquid water. Many crystals contain crystalline water, but not all crystals do. When a solute crystallizes out of a solution, a certain number of water molecules are bound within the crystal; such water molecules are called crystal water. The amount of hydrogen that dissolves is related to temperature and the state of the metal. The relationship between hydrogen solubility and temperature is such that in solid iron the solubility is less than 0.6 mL/100 g; at 1350°C it is 10.1 mL/100 g. When transitioning from a solid to a liquid state, the solubility of hydrogen increases sharply, reaching a maximum of 43 mL/100 g at 2400°C. The solubility of hydrogen also varies in metals with different lattice types; its solubility in metals with a face-centered cubic lattice is greater than that in those with a body-centered cubic lattice. When liquid iron transforms into δ-Fe, its solubility drops sharply, from 32 mL/100g to 10 mL/100g. If a large amount of hydrogen has been absorbed in the molten pool and the cooling rate is relatively fast, hydrogen diffusion from the solid phase to the liquid phase occurs during solidification, causing the hydrogen concentration in the liquid phase to reach a supersaturated level; this creates the necessary conditions for the formation of hydrogen pores. During the crystallization process in the molten pool, when both solid and liquid phases coexist, hydrogen accumulates at the crystal growth front due to the difference in solubility between these two phases. This accumulation occurs especially in the valleys between adjacent dendritic crystals. As the volume of the liquid phase decreases, the concentration at the bottom of the molten pool increases; once this concentration becomes too high to maintain a supersaturated state, bubbles are formed. It can be seen from this that hydrogen pores are formed during the crystallization process, nucleating in the valleys between dendrites. Growth and upward movement at these locations are hindered by both the dendrites and viscosity; as a result, pores in the shape of a trumpet that are larger at the top and smaller at the bottom are formed, and they often appear on the surface of the weld. 2. Nitrogen pores: The formation process of nitrogen pores is generally considered to be similar to that of hydrogen pores. These pores are also found on the surface of the weld, often appearing in clusters, resembling honeycombs. Fracture analysis revealed that the inner surface of the pores had a rough, uneven texture. However, nitrogen pores rarely appear in the weld during normal welding; they only form when the arc is too long and proper protection is not provided. 3. Carbon monoxide pores CO are primarily formed by the reaction of FeO or other oxides with C at high temperatures. The reaction is as follows: +====CO +====CO+ +10====CO+ +2====2CO+. CO gas is generated in the later stages of metallurgical reactions; as crystallization proceeds, a certain amount of CO is produced. Due to the decrease in temperature, the increase in liquid viscosity, and the fact that the formation of CO is an endothermic reaction, the CO produced cannot escape in time and becomes trapped between the dendritic grains. Since CO bubbles rise more slowly than hydrogen bubbles, they tend to form inside the weld, appearing in worm-like shapes with an oxidized color on their inner walls. Appearance of carbon monoxide pores in the electron beam welded joint of high-carbon tool steel SK5. It should be noted that the gases in various bubbles are not single, but several gases coexist simultaneously. It can be considered that under certain conditions, one of the gases plays a dominant role in the formation of pores, while under the combined effect of various gases, bubbles can develop and grow rapidly.