Fundamentally speaking, the measures to prevent the formation of pores in the weld are to limit the dissolution or generation of gas in the molten pool and to exclude the gas present in the molten pool.
(1) Eliminate gas sources:
①Surface treatment Before welding the
steel pipe weldment, the oxide film, rust, and oil stain on the surface of the weldment and welding wire should be carefully cleaned.
② Moisture-proof and drying of welding materials All kinds of welding materials should be packed and stored in moisture-proof. Electrodes and flux should be dried according to the specified temperature and time before welding, and should be stored in a special oven or heat preservation tube after drying, and can be taken as needed.
③Enhanced protection The purpose is to prevent nitrogen pores caused by air intrusion into the molten pool. Attention should be drawn to the following aspects. The arc often cannot get good protection. When the low-hydrogen electrode arc is easy to produce pores, it is because the gas-generating substance CaCO3 in the coating fails to decompose in time to generate sufficient CO2 protection. If the coating falls off during the welding process, the flux or protection If the gas is interrupted, the normal protection will be destroyed. When gas-shielded welding, it must be protected from the wind. The flow velocity of the protective gas at the front end of the welding torch nozzle is generally about 2m/s. If the wind velocity exceeds this value, the protective gas flow will not be stable and become a turbulent state, losing its protective effect. The practice has shown that, except for vacuum welding, the protective effects of existing welding methods are not ideal. For example, some low-carbon steel products are welded with E4303, E4301, and E4315 electrodes or H08A+HJ431 submerged arc welding, and X-ray flaw detection is uniform. No porosity was found, but a single needle-like tiny porosity visible to the naked eye was found when polished. Research has determined that the needle-like micropores are entirely due to the action of nitrogen in the air. To prevent such pores, in addition to effective mechanical protection, nitrogen should also be fixed by alloying elements. CO2 welding of H04Mn2SiTiA or H04MnSiALTiA can eliminate the above-mentioned micropores (small pinholes).
(2) Correct selection of welding materials The selection of welding materials must consider the matching requirements with the base metal. For example, low-hydrogen electrodes have poor rust resistance and cannot be used for welding rusty components, while iron oxide electrodes have good anti-corrosion properties. Rust. In submerged arc welding, if high alkalinity sintered flux is used since the alkalinity is allowed to increase to more than 3, the O2- activity has been reduced, which is different from the commonly used alkaline electrodes, and the sensitivity to rust is significantly reduced.
In gas-shielded welding, from the perspective of preventing the generation of hydrogen holes, the nature of the protective atmosphere is to choose active gas rather than an inert gas. Because the active gas O2 or CO2 can reduce the partial pressure of hydrogen to limit hydrogen dissolution, and at the same time reduce the surface tension of liquid metal and increase its mobility, which is conducive to the discharge of gas.
In addition to matching requirements with the base metal, the composition of the welding wire must also consider the combination of flux (submerged arc welding) or shielding gas (gas-shielded welding), and adjust the composition of the molten pool or weld metal according to different metallurgical reactions. In many cases, it is desirable to create conditions that are sufficiently deoxygenated to suppress the formation of reactive pores. Low carbon steel CO2 welding can prevent porosity by using H08Mn2Si or H08Mn2SiA with reduced carbon content as much as possible and increased deoxidizing elements. Deoxidizing elements that are often used are Mn, Si, Ti, Ai, Zr, and rare earths.
(3) Control welding process conditions The purpose of controlling welding process conditions is to create favorable conditions for the escape of gas in the molten pool, and at the same time, it should also help limit the fusion of peripheral gases into the molten metal. For reactive gases, we should first focus on creating favorable discharge conditions, that is, appropriately increasing the existence time of the molten pool in the liquid state. It can be seen that both increased heat input and proper preheating are beneficial. For hydrogen and nitrogen, only when the gas escape conditions are improved more than the gas fusion conditions, can the porosity be reduced. Therefore, the welding process parameters should have optimal values, rather than simply increasing or decreasing Small. Under horizontal or overhead welding conditions, due to unfavorable gas discharge conditions, pores will be more likely to occur than in flat welding. Upward vertical welding has fewer pores, while downward vertical welding has more pores because the molten metal is easy to fall downward at this time, which is not conducive to gas removal. And there is the possibility of being involved in air.
