-
-
-
-
Honor
-
-
-
-
-
Contact
-
Talk about the ubiquitous, flickering welding holes.
Release time:
2021-08-30
Author:
Source:
The so-called pores refer to the holes formed by the gas in the weld. Porosity is a common defect in welds, which reduces the effective cross-sectional area of welds, causes stress concentration, destroys the density of welds, and causes weldments to fail. There are surface pores in welds, internal or root pores, such as internal pores in electroslag welding welds, and penetrating pores. From the distribution point of view, there are single pores and piles of dense pores. Gas forming pores include hydrogen, nitrogen, carbon monoxide gas and water vapor. Hydrogen, nitrogen gas is dissolved in the metal at high temperature, in the process of solidification, hydrogen, nitrogen gas in the liquid-solid stage solubility occurs suddenly.
The so-called pores refer to the holes formed by the gas in the weld. Porosity is a common defect in welds, which reduces the effective cross-sectional area of welds, causes stress concentration, destroys the density of welds, and causes weldments to fail.
There are surface pores in welds, internal or root pores, such as internal pores in electroslag welding welds, and penetrating pores. From the distribution point of view, there are single pores and piles of dense pores.
Gas forming pores include hydrogen, nitrogen, carbon monoxide gas and water vapor. Hydrogen and nitrogen gas is dissolved in the metal at high temperature. During the solidification process, the solubility of hydrogen and nitrogen gas in the liquid-solid stage suddenly decreases, and it is too late to escape and remain in the metal. Carbon monoxide gas and water vapor are formed by metal metallurgical reactions and are insoluble in metal gases.
The process of bubble floating
The crystallization speed of the molten pool, when the crystallization speed is small, the bubbles can have sufficient time to escape.
1. Hydrogen hole
In low-carbon steel and low-alloy steel welds, the section of the hydrogen hole is screw-like, most of which appear on the surface of the weld (in some cases, it will also appear inside), which is bell-shaped, and there is a smooth inner wall around the hole. For aluminum magnesium alloy often appear in the internal weld.
For manual arc welding, hydrogen mainly comes from organic matter, crystal water or adsorbed water in electrode flux, oil stains on the surface of base metal and welding wire, and moisture in the air. The way hydrogen dissolves into the metal: under the condition of slag protection, the slag itself has a certain ability to dissolve hydrogen. The hydrogen dissolved in the slag mostly exists in the form of OH-, and OH-and Fe2 + generate hydrogen atoms into the liquid metal by exchanging electrons. During gas shielded welding, the hydrogen in the gas directly dissolves in the form of atoms or protons after contacting the metal.
baidu encyclopedia:Also known as combined water. Crystal water is water molecules that are bound in compounds and are not liquid water. Many crystals contain crystal water. But not all crystals contain water of crystallization. When a solute crystallizes out of solution, a certain number of water molecules are bound in the crystal, so thatThe water molecule is called crystal water.
The amount of hydrogen dissolved is related to the temperature and the state of the metal.
The relationship between the solubility of hydrogen and temperature is that the solubility in solid iron is less than 0.6mL/100g and 10.1mL/100g at 1350 ℃. when the solid is changed into liquid, the solubility of hydrogen rises suddenly, and the maximum is 43mL/100g at 2400 ℃. The solubility of hydrogen in metals of different lattice types is also different, and the solubility of hydrogen in metals of face-centered cubic lattices is greater than that of body-centered cubic lattices.
When liquid iron is converted to δ-Fe, the solubility drops suddenly, from 32mL/100g to 10mL/100g. If more hydrogen has been absorbed in the molten pool and the cooling rate is relatively fast, the solidification process will inevitably be accompanied by the diffusion of hydrogen from the solid phase to the liquid phase, and the hydrogen in the liquid phase will reach a supersaturated state, thus creating the necessary conditions for the generation of hydrogen holes.
In the crystallization process of the molten pool, when the solid-liquid two phases coexist, due to the difference in solubility of the solid-liquid two phases, hydrogen will accumulate at the crystallization front, especially at the valley between adjacent dendrites, the concentration at the bottom of the molten pool increases continuously with the decrease of the liquid phase, and bubbles will be generated when the concentration cannot maintain the supersaturated state.
It can be seen that hydrogen holes are formed during the crystallization process, nucleating at the valley part between the dendrites, and the growth and floating at this part will be hindered by the dendrites and the resistance of viscosity. Therefore, the upper and lower trumpet holes are formed, and they are often presented on the surface of the weld.
2. Nitrogen hole
The formation process of nitrogen holes is generally considered to be similar to that of hydrogen holes, which are also distributed on the surface of the weld, and most of them appear in piles, similar to honeycomb. The fracture analysis found that the inner surface of the pores was concave and convex. However, there are few nitrogen holes in the weld during normal welding, and only when the arc is longer and the protection is not good, the nitrogen holes will be generated.
3. Carbon monoxide stomata
CO is mainly produced by FeO or other oxides and C at high temperatures. The reaction is as follows:
[C] [O]====CO
[FeO] [C]====CO [Fe]
[MnO]十[C]====CO [Mn]
[SiO2] 2[C]====2CO [Si]
CO pores are generated in the later stage of metallurgical reaction, and with the progress of crystallization, the generated CO reaches a certain amount. Due to the decrease of temperature, the increase of liquid viscosity, the generation of CO is the endothermic reaction and other reasons, the CO produced is too late to escape, and is trapped in the branches of the grain. Since CO bubbles float slower than hydrogen bubbles, they are mostly formed inside the weld, showing a worm-like shape with oxidized color on the inner wall. Such as high carbon tool steel SK5 electron beam welding seam carbon monoxide pore morphology.
It should be noted that the gas in the various bubbles is not a single gas, but several gases exist at the same time. It can be considered that under certain conditions, one of the gases plays a leading role in the formation of pores, and under the joint action of various gases, the bubbles can develop and grow rapidly.
Reprinted from welding and cutting online, infringement must be deleted
Key words:
Welding, porosity, weld, gas, metal, solubility, crystallization, co, formation, bubble
