Protection method for back side of weld seam of argon arc welding of stainless steel pipeline
1. The tube is filled with argon for protection
For argon arc welding of stainless steel pipes in petrochemical plant engineering, the method of filling the pipe with argon is generally used to prevent oxidation on the back of the weld. The argon filling methods mainly include argon filling as a whole and partial argon filling. In order to save project costs and meet construction requirements, the method of internal argon filling should be used flexibly in combination with the specific construction conditions of the pipeline during the construction process.
A. Whole argon filling method
B. Local argon filling method
C. Reverse protection box method to fill argon
D. Judgment of argon protection effect
Precautions for argon protection:
(1) During argon arc welding, air should be supplied to the back of the weld in advance, and the flow should be appropriately increased. After the air is discharged, the flow will gradually decrease. Argon should be filled into the tube without interruption during welding. When the welding is stopped, the gas is stopped behind, so that the welding seam is fully protected. In addition, special attention should be paid to welding only after the air is exhausted, otherwise the protective effect of argon filling will be affected.
(2) The flow of argon gas should be appropriate. If the flow rate is too small, the protection is not good, and the back of the weld is easy to oxidize; if the flow rate is too large, eddy currents will be generated during welding to bring air into it, and the protection effect will be deteriorated.
(3) The argon inlet should be placed as low as possible in the closed section, and the air exhaust hole should be placed at the highest position in the closed section. Because argon is heavier than air, filling argon from a lower position is easier to ensure a higher concentration, and the protection effect of argon filling is better.
(4) In order to reduce the loss of argon gas in the pipe from the gap between the ports, which will affect the protection effect and increase the cost, tape can be pasted along the gap of the weld before welding, leaving only the length of the welder’s continuous welding at a time, and remove the tape while welding.
2. Self-protected stainless steel TGF series welding wire on the back
The back side self-protected stainless steel TGF series welding wire is a welding wire with a special coating. When welding, the protective coating will penetrate to the back of the molten pool to form a dense protective layer, so that the back of the weld bead will not be oxidized. After cooling, this layer of slag shell will automatically fall off, and it is easy to remove with compressed air or water. . The use method of this kind of welding wire is basically the same as that of ordinary argon arc welding solid wire, the coating will not affect the arc and molten pool shape on the front, and the performance of the weld metal can meet the requirements.
The use of self-shielded welding wire is not limited by pipe specifications, welding position and other factors, and the operation is flexible, and the preparation work for argon filling is reduced. However, due to the thinner coating on the surface of the welding wire, there are some incompatibility in the welding operation, and defects such as recesses are often present, so the operation level of the welder is higher. The self-shielded welding wire is suitable for the bottoming of the weld seam, and is not suitable for the weld bead above the second layer, otherwise it is easy to cause slag inclusion and the weld seam is not beautiful. When full argon arc welding is used for multi-layer and multi-pass welding, it should be used in conjunction with solid core welding wire. In addition, there is basically no domestic self-shielded stainless steel welding wire with stable performance in the market. Imported self-shielded welding wire is more expensive and costly, so its application is subject to certain restrictions.
3. Application of protective agent for welding without argon filling
In recent years, an argon-free welding protective agent has been used in the welding of stainless steel pipes (furnace tubes) in petrochemical and thermal power construction projects. The protective agent is in powder form when supplied, and needs to be mixed with methanol or a special binder when used. As a backside protective agent, a protective coating is formed on the backside of the weld during the welding process, which can effectively prevent the oxidation of the backside of the weld.
How to use protective agent:
(1) First, remove the oil stains on the back of the weld.
(2) Take out an appropriate amount of protective agent, put it in a container with an appropriate amount of methanol (or special adhesive) and mix in proportion, stir the two evenly, and coat them on the weld protection surface of the inner wall of the pipe (width 5-10mm) . It should be noted that after coating, a continuous adhesive film will be formed on the surface of the coating, and the powder will be spread under the film, so the coating cannot be scratched to prevent the adhesive film from breaking and causing the powder Leakage, thereby affecting the protection effect.
(3) Leave it for 5 minutes after coating, and then perform welding after drying. After welding, an insoluble, chemically inert film is formed on the back of the weld, which adheres tightly to the surface of the weld and has no effect on radiographic inspection.
The protective agent has good applicability and can be welded with normal welding current, or with a current that is 10% larger than normal welding. The protective agent has no effect on the welding operation. It can not only prevent the back surface from being oxidized, but also support the molten metal on the back of the weld, resulting in an excellent bottoming bead without weld bead, undercut or deep depression The first pass rate of welding is high.
Argon-free welding protective agent is simple to operate, just apply and weld, saving time and improving efficiency. When applied to small pipe diameter welding, its cost is slightly higher than that of argon-filled protection; but it is worth popularizing and applying in fixed port welding or large pipe diameter welding. At present, the application of argon-free protective agent is in its infancy in China.
Post time: Oct-29-2020