The main causes of welding porosity

The main causes of welding porosity are:

1. The gas generated in arc welding contains excessive hydrogen and carbon monoxide, which is caused by it;

2. Excessive sulfur content in the base material steel;

3. The properties of the flux and the baking temperature are not high enough;

4. The cooling speed of the welding area is too fast;

5. The welding area is caused by oil stains, paint, rust, water, or galvanized layer;

6. There is too much moisture and wind in the air;

7. Arc blowing occurs.

During CO2 arc welding, due to the lack of slag covering on the surface of the molten pool and the strong cooling effect of the CO2 gas flow, the metal in the molten pool solidifies relatively quickly. However, when the gas in the molten pool cannot escape in time, it is easy to produce pores in the weld seam.

There are three main types of pores that may occur: carbon monoxide pores, hydrogen pores, and nitrogen pores.

1. Carbon monoxide pores

The main reason for the formation of CO pores is the reduction reaction between FeO and C in the molten pool, as follows: FeO+C==Fe+CO

This reaction occurs more violently when the molten pool is at the crystallization temperature. As the molten pool has already begun to solidify, CO gas is not easy to escape, resulting in the formation of CO pores in the weld seam.

If the welding wire contains sufficient deoxygenation elements Si and Mn, as well as limiting the carbon content in the welding wire, the above reduction reaction can be suppressed, effectively preventing the generation of CO pores. So in CO2 arc welding, as long as the welding wire is selected appropriately, the possibility of producing CO pores is very small.

2. Hydrogen pore

If a large amount of hydrogen gas is dissolved in the molten pool at high temperatures and cannot be fully discharged during the crystallization process, it will remain in the weld metal and form pores.

The hydrogen in the arc zone mainly comes from the oil stains and rust on the surface of the welding wire and workpiece, as well as the moisture contained in the CO2 gas. Oil pollution is composed of hydrocarbons, while rust contains crystalline water, which can decompose into hydrogen gas under high arc temperatures. Reducing the dissolution of hydrogen in the molten pool can not only prevent hydrogen porosity, but also improve the plasticity of the weld metal. So, on the one hand, it is necessary to properly remove the oil and rust on the surface of the workpiece and welding wire before welding, and on the other hand, CO2 gas with low moisture content should be used as much as possible. The moisture in CO2 gas is often the main cause of hydrogen pores.

In addition, hydrogen is dissolved in the molten pool in the form of ions. When the DC polarity is reversed, the molten pool is the negative electrode, which emits a large number of electrons, causing the hydrogen ions on the surface of the molten pool to recombine into atoms, thereby reducing the number of hydrogen ions entering the molten pool. So when the DC polarity is reversed, the hydrogen content in the weld seam is 1/3 to 1/5 of that in the positive polarity, and the tendency to produce hydrogen pores is also smaller than that in the positive polarity.

3. Nitrogen stomata

The source of nitrogen: firstly, air intrusion into the welding area; The second is the impure CO2 gas. Experiments have shown that adding nitrogen with a diameter of 3% to CO2 gas during short-circuit transition, and adding nitrogen with a diameter of 4% to CO2 gas during jet transition, still does not produce nitrogen pores. However, normal gases contain very little nitrogen, with a diameter of ≤ 1%. From the above, it can be inferred that the possibility of nitrogen porosity caused by impure CO2 gas is not high, and the weld seam