The welding of stainless steel pipes usually consists of backing welding, filling welding, and capping welding. Backing welding of stainless steel pipes is the most critical part of stainless steel pipe welding. It is not only related to the quality of the project but also to the progress of the project. At present, stainless steel backing is divided into two processes: back argon filling and non-argon filling. Back argon filling protection is divided into two types: solid welding wire + TIG process and solid welding wire + TIG + water-soluble paper process; back non-argon filling protection is divided into flux-cored welding wire backing welding and welding rod (flux-coated welding wire) backing TIG welding.


1. The method of using a plugging plate to seal the back for ventilation protection (i.e. solid welding wire + TIG)
When prefabricating stainless steel pipes, the welds can usually be rotated for welding, and ventilation is very easy. At this time, a plugging plate is usually used to seal the ventilation protection on both sides of the weld in the pipe for backing welding, and the outside is sealed with adhesive cloth. When welding, the process of early ventilation and delayed gas stop should be adopted. The outer adhesive cloth should be torn off while welding. Since the plugging plate is composed of rubber and tinplate and is not easy to be damaged, this welding method can well ensure that the inside of the weld is filled with argon and its purity, thereby effectively ensuring that the metal inside the weld is not oxidized and ensuring the quality of the weld base welding.


2. The method of sealing and ventilation protection using only soluble paper or combining soluble paper with a plugging plate (i.e. solid welding wire + TIG + water-soluble paper)
When installing and welding the fixed port of stainless steel pipes, it is difficult to ventilate the inside, and some sides are easier to block. In this case, water-soluble paper + plugging plate can be used for blocking. That is, the side that is easy to ventilate and easy to remove is blocked with a plugging plate, and the side that is not easy to ventilate and difficult to remove the plugging plate is blocked with water-soluble paper, and the outside is blocked by sticking the weld with adhesive cloth.
When welding stainless steel fixed ports, in many cases, both sides of the weld cannot be ventilated. At this time, how to ensure argon protection inside the weld becomes a problem. In actual on-site construction, we use water-soluble paper to seal both sides of the weld, ventilate from the center of the weld, and seal with adhesive tape on the outside, which successfully solves the above problem.
When using water-soluble paper to seal ventilation, since ventilation is from the center of the weld, the ventilation pipe should be quickly unplugged in the final sealing link, and the remaining argon gas inside should be used for protection, and the bottom should be quickly applied and the mouth should be sealed.
When using this method, it should be noted that the water-soluble paper should be double-layered and must be pasted well, otherwise it is easy to cause the water-soluble paper to be damaged or fall off, causing the inner weld to lose the protection of argon gas, resulting in oxidation, resulting in the weld cutting and re-welding, which can not only fail to ensure the welding quality, but also seriously affect the construction period. Therefore, strict inspection should be carried out before welding, and the water-soluble paper should be pasted well.
In many construction sites, we use this welding method for base laying, and its quality can be effectively guaranteed. At the same time, it is also difficult to construct, so careful and skilled welders should be selected for this work.


3. No argon protection is carried out on the back, and flux-cored wire + TIG process is used
This method has been used in my country for several years. E308T1-1, E308LT1-1, E309T1-1, E309LT1-1, 347T1-1, E316T1-1, E316LT1-1, and other flux-cored wires have been produced and applied to on-site welding, achieving good economic benefits.
Since the back is not filled with argon, its advantages are obvious, mainly manifested in high efficiency, simplicity, low cost, and suitability for installation on construction sites. However, due to its structural characteristics, flux-cored welding wire has high requirements for welders during operation. It has fast wire feeding speed and high wire feeding accuracy, which 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 applied this method and successfully solved the problem that argon gas could not be passed through the joint and repair port.


4. No argon gas protection is performed on the back, and flux-coated welding wire (self-protected flux-cored welding wire) + TIG process is used
The protection mechanism of the stainless steel base welding wire + TIG process is that the back weld is protected by the slag generated by the melting of the welding wire and the metallurgical reaction of its alloy elements, and the front weld is protected by argon gas, slag, and alloy elements.
When using this process, the following operating points should be noted: During welding, the welding torch, welding wire, and weldment should maintain the correct angle. The ideal welding torch nozzle tilt angle is 70°-80°, and the angle between the welding wire and the weldment surface is 15°-20°; Correctly control the molten pool temperature by changing the angle between the welding torch and the weldment, changing the welding speed, etc. to change the molten pool temperature, so as to ensure the beautiful weld formation (consistent width and no defects such as concave or over-convex); During operation, the current should be slightly larger than when welding solid wire, and the welding torch should be slightly swung to make the iron The water and the melted coating are separated quickly, which is convenient for observing the molten pool and controlling whether the welding is thorough. 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 concave. During the welding process, the welding wire should be regularly fed in and taken out, and the welding wire should always be protected by argon gas to prevent the end of the welding wire from being oxidized and affecting the welding quality. Pay attention to the welding quality at the arc starting and arc ending. The spot welding point should be polished into a 45° gentle slope at the arc starting point. When closing the arc, attention should be paid to the generation of defects such as arc pits and shrinkage holes. When using the coating welding wire for base welding, there is no need to pass argon gas inside the weld. The welder is easy and fast to operate, with the characteristics of high efficiency and low cost. At the same time, it can also well ensure the welding quality, which is worth promoting. 


The above four stainless steel base welding methods have their own advantages and disadvantages. In actual construction, we should consider the construction cost, welding quality, and construction progress according to the specific conditions of the site, and reasonably select the construction process.