Welding material lecture hall | Take you to understand the welding defects-welding blowhole
If you want to weld works with beautiful appearance, smooth transition and no defects, then it is necessary to understand the welding defects existing in the welding process. It will reduce the performance of welding products, shorten the service life of welding products, and even cause accidents.
We used four lessons to lead you to a comprehensive understanding of welding crack defects in welding defects. I believe everyone has mastered it. In addition to crack defects, there are also welding defects such as blowholes, undercuts, weld bumps, and spatters, which need to be mastered gradually.
Today, We will continue to lead you into the welding world and understand the blowholes of welding defects. Welding air holes are holes formed in the weld seam by the gas in the welding molten pool. Generally including carbon monoxide pores, hydrogen pores and nitrogen pores.
Carbon monoxide pores
The reason for the generation of CO pores is mainly that the FeO and C in the molten pool react as follows:
FeO + C = Fe + CO
This reaction proceeds more vigorously when the molten pool is at the crystallization temperature. Because the molten pool has begun to solidify at this time, CO gas is not easy to escape, so CO pores are formed in the weld.
If the welding wire contains sufficient deoxidizing elements Si and Mn, and the carbon content of the welding wire is limited, the above-mentioned reduction reaction can be suppressed and the generation of CO pores can be effectively prevented. Therefore, in CO2 arc welding, as long as the welding wire is properly selected, the possibility of generating CO pores is very small.
If a large amount of hydrogen is dissolved in the molten pool at high temperature and cannot be discharged in time during the crystallization process, it will remain in the weld metal to form pores.
The hydrogen in the arc mainly comes from the oil and rust on the welding wire, the surface of the workpiece, and the moisture contained in the protective gas. Oil stains are hydrocarbons, and rust contains crystalline water, which can decompose hydrogen at high temperature of the arc. Reducing the amount of hydrogen dissolved in the molten pool can not only prevent hydrogen gas holes, but also improve the plasticity of the weld metal. Therefore, on the one hand, it is necessary to properly remove the oil and rust on the surface of the workpiece before welding; on the other hand, a clean protective gas is used, and the moisture in the gas is often the main reason for the hydrogen holes.
In addition, hydrogen is dissolved in the molten pool in the form of ions. When the reverse polarity of DC, the molten pool is the negative electrode, emitting a large number of electrons, so that the hydrogen ions on the surface of the molten pool recombine into atoms, thus reducing the hydrogen ions entering the molten pool. Quantity. Therefore, when the DC polarity is reversed, the hydrogen content in the weld is 1/3 to 1/5 of the positive polarity, and the tendency of generating hydrogen holes is also smaller than that of the positive polarity.
The main reason for the nitrogen hole in the weld is that the shielding gas atmosphere is destroyed, and a large amount of air intrudes into the welding area. The factors causing the effectiveness of the shielding gas atmosphere are: too small gas flow rate, the nozzle is partially blocked by the spatter, the distance between the nozzle and the workpiece is too large, and there is lateral wind at the welding site. Therefore, appropriately increasing the flow of protective gas to ensure the smoothness of the gas path and the stability and reliability of the gas is the key to preventing nitrogen holes in the weld.
In addition, process factors also have an impact on the generation of pores. The higher the arc voltage, the greater the possibility of air intrusion, and the more likely it is to produce blowholes. The welding speed mainly affects the crystallization speed of the molten pool. If the welding speed is slow, the crystallization of the molten pool is slow, and the gas is easy to overflow; if the welding speed is fast and the crystallization of the molten pool is fast, the gas is not easy to be discharged, and it is easy to produce pores.
Fundamentally speaking, the measures to prevent pores in the weld are to limit the melting or generation of gas in the molten pool and to eliminate the gas present in the molten pool.
In order to prevent the generation of welding porosity, the following measures can be adopted:
1.Eliminate gas sources
The oxide film or rust and oil stain on the surface of the workpiece and welding wire should be effectively treated. The weld groove should be thoroughly cleaned, and the electrode should be dried according to the regulations before use.
2.Correct selection of welding consumables
The selection of welding materials is very important, and the matching requirements with the base material must be considered. For example, the low-hydrogen electrode has a poor rust resistance type, and it is easy to generate pores when there is rust, while the acid electrode has good rust resistance.
3.Control welding process conditions
The purpose of controlling the welding process conditions is to create a powerful condition for the gas overflow in the molten pool, and at the same time, it should be beneficial to control the dissolution of the gas surrounding the arc into the molten metal.