The welding of stainless steel pipes usually consists of root welding, filling welding and cover welding. The bottom welding of stainless steel pipe is the most critical part of stainless steel pipe welding. It is not only related to the quality of the project, but also related to the progress of the project. At present, the back welding of stainless steel pipe is divided into two processes: back-filling and non-argon filling. Argon-filled back protection is divided into solid wire + TIG process and solid wire + TIG + water-soluble paper process; back without argon-filled protection is divided into flux-cored wire backing and welding rod (coated wire) backing TIG welding.
The bottom welding of stainless steel usually adopts the TIG process. According to the actual situation on site, we can use the following four methods for bottom welding.
01. The method of blocking ventilation and protection by using blocking boards on the back (that is, solid welding wire + TIG)
When the stainless steel pipe is prefabricated, the welding joint can usually be rotated and welded, and the ventilation is very easy. At this time, the blocking plate is usually used to block and ventilate both sides of the welding joint in the pipeline to protect the bottom welding, and at the same time, the outer side is sealed with adhesive cloth. blockage.
When welding, the process of ventilating in advance and stopping the gas later should be adopted. The outer adhesive cloth is torn off while welding. Since the blocking plate is composed of rubber and white iron, it is not easy to be damaged, so this welding method can well ensure the inside of the weld. Filled with argon gas and ensure its purity, so as to effectively ensure that the metal inside the weld is not oxidized, and ensure the quality of the weld backing.
02. Only use soluble paper or a combination of soluble paper and blocking board for blocking and ventilation protection (ie solid welding wire + TIG + water-soluble paper)
When the fixed port of the stainless steel pipe is installed and welded, it is difficult to ventilate the inner side, and some sides are easier to block. In this case, water-soluble paper + blocking plate can be used for sealing. That is, the side that is easy to ventilate and easy to remove is sealed with a blocking board, and the side that is not easy to ventilate and difficult to remove the blocking board is blocked with water-soluble paper.
When welding the stainless steel fixed port, in many cases, there will be no ventilation on both sides of the weld. At this time, how to ensure the protection of argon filling inside the weld becomes a difficult problem. In the actual construction on site, we use water-soluble The method of sealing with paper, ventilating from the center of the weld seam, and pasting the outside with adhesive cloth successfully solved the above problems.
When water-soluble paper is used to seal the ventilation, since the ventilation is from the center of the weld seam, in the final sealing process, the ventilation tube should be pulled out quickly, and the remaining argon inside should be used for protection, and the bottom should be quickly finished and the mouth should be sealed.
With this method, it should be noted that the water-soluble paper should be double-layered, and it must be pasted well, otherwise the water-soluble paper will easily be damaged and fall off, and the inner weld will lose the protection of argon gas, and oxidation will occur, causing the weld to be cut and re-opened. Welding can not guarantee the welding quality, but also seriously affects the construction period, so strict inspection should be done before welding, and water-soluble paper should be pasted.
In many construction sites, we have adopted this welding method for backing, its quality can be effectively guaranteed, and it is also difficult to construct, so careful and skilled welders should be selected for this work.
03. The back side is not protected by argon gas, and the flux cored wire + TIG process is used
This method has been used in our country for several years, and flux-cored welding wires such as E308T1-1, E308LT1-1, E309T1-1, E309LT1-1, 347T1-1, E316T1-1, E316LT1-1 have been produced, and have been applied in the field The welding has achieved better economic benefits.
Since the back side is not filled with argon, its advantages are obvious, such as high efficiency, simplicity, and low cost, and it is suitable for installation on the construction site. However, due to its structural characteristics, flux-cored welding wire has high requirements for welders during operation. Its wire feeding speed is fast and the accuracy of wire feeding is high, so it is difficult to master. Welders should be specially trained and skilled before they can participate in welding. In Nanjing Yangba and foreign construction sites, we have successfully solved the problem that argon cannot be ventilated at the meeting port and repair port by applying this method.
04. The back side is not protected by argon gas, and the coated welding wire (self-protected flux-cored welding wire) + TIG process is used
In the 1990s, Kobelco and other companies in Japan developed bottom welding wires. In recent years, my country has also developed stainless steel bottom welding wires (that is, coated welding wires, such as TGF308, TGF308L, TGF309, TGF316L, TGF347, etc.) , and applied to the actual construction, and achieved good results, we have successfully used this method in the capacity expansion and transformation project of Wupec.
The protection mechanism of the stainless steel backing wire + TIG process is that the back weld is protected by the metallurgical reaction between the slag generated by the welding wire melting and its alloy elements, and the front weld is protected by argon, slag and alloy elements.
When using this process, the following operating points should be paid attention to: During the welding process, the correct angle between the welding handle, welding wire and welding piece should be maintained. The ideal back angle of the welding handle nozzle is 70°-80°, The angle is 15°-20°; correctly control the temperature of the molten pool, change the temperature of the molten pool by changing the angle between the welding handle and the weldment, changing the welding speed, etc., so as to ensure that the weld shape is beautiful (the width is the same, no concave, Convexity and other defects);
During operation, the current should be slightly larger than that of welding solid core wire, and the welding handle should be slightly swayed to accelerate the separation of molten iron and molten coating, which is convenient for observing the molten pool and controlling whether the penetration is complete; when filling the welding wire, it is best to send it to 1/2 of the molten pool, and press it slightly inward to ensure root penetration and prevent indentation;
During the welding process, the welding wire should be fed and taken out regularly, and the welding wire should always be under the protection of argon gas, so as to prevent the end of the welding wire from being oxidized and affecting the welding quality; The spot welding should be ground to a gentle slope of 45°, and attention should be paid to defects such as arc craters and shrinkage cavities when closing the arc.
Covered welding wire is used for bottom welding, and argon gas is not used inside the weld. The operation of the welder is simple and fast, with the characteristics of high efficiency and low cost. This method is used to weld a total of 28 joints and reworked joints, and the passing rate of one-time perspective welding is 100%), which is worthy of our promotion and use.
The above four stainless steel bottom welding methods have their own advantages and disadvantages. In actual construction, we should consider not only the construction cost, but also the welding quality and construction progress according to the specific conditions on the site, and choose a reasonable construction process.
Post time: Mar-15-2023
