With the rapid development of the petrochemical industry, stainless steel materials have been widely used, but also put forward higher requirements for pipeline and plate welding, gradually eliminated the previous stainless steel arc welding bottom method, and the use of argon arc welding bottom welding.
Argon arc welding primer is faster than arc welding primer with high cleanliness, but there are also some problems.
During the welding process, because the back of the stainless steel bottom is easy to be oxidized to produce defects, it is necessary to take protective measures on the back, which can ensure the mechanical properties and corrosion resistance of the weld, so it is necessary to take effective protection when welding stainless steel.
Today, several common stainless steel welding back protection methods are introduced:

01
Back argon shield
Common protective gases can be divided into pure argon gas protection and mixed gas protection, a certain proportion of argon-nitrogen gas is more conducive to austenitic stainless steel welding, some inert gases are not used due to high cost.
Argon filling method is a traditional back protection method, which has the characteristics of better back protection, easy to master, high cleaning and high qualification rate. It is divided into protective cover filled with argon protection, local filled with argon protection, direct charge of welding, argon joint protection and so on.

1 Protective cover filled with argon protection
This method is often used in the stainless steel welding of plates and large diameter pipes. The protective cover is connected to a metal pipe and an argon hose. The argon valve is opened to fill the protective cover with argon.
Another person is required to hold the metal tube as a handle so that the protective cover slides over the back pool in sync with the plate or tube exterior welding.
In this way, the back has been effectively protected, the characteristics, the protection is concentrated, the argon does not need to be opened too much, and the argon waste is less.

2 Local argon filled protection
It is easy to use local protection for pipelines with small local space and short size.
Method: The welding joint of the pipeline is sealed with adhesive tape (to prevent air leakage), the two ends of the pipeline are sealed with sponge, rubber, paper shell, etc., the argon hose is inserted from one end and filled with argon, and the other end of the pipeline is best to make a small hole (sponge is not needed), which is conducive to the final bottom welding joint, and will not sag due to high internal pressure.
During welding, in order to prevent a large amount of argon from the weld, the weld sealing tape should be torn off and welded one section at a time, which can reduce more loss of argon and effectively protect the weld. Features waste, slow argon filling, high cost.

3 Direct argon filling of welding joint
For pipelines with too long pipelines and slightly larger pipe diameters, local argon filling is too wasteful, the quality cannot be guaranteed, and the project cost is too high. In order to save costs, direct argon filling of welding joints can be used.

Method of making both sides of weld plugs
The sponge is processed into a pipe with a slightly larger diameter plug and the two sponges are connected with a wire in the middle, a distance of 300-400mm, to make a double plug, and one end of the plug is connected to a longer section of steel wire.
When matching, plug the plug on both sides of the weld 150-200mm, one end of the long wire should be longer than the length of the weld end of the pipe and expose the end of the pipe, the end of the metal tube is flattened, the end of the metal tube is connected to the argon hose, and the end of the flattened end is inserted into the good weld, filled with argon, the insertion direction is best at the top. In this way, the small tube is pulled out before the final joint of the bottom welding and the residual gas welding is completed. Pull out the plug with wire after welding.

Water soluble paper protection law
Before the pair, apply water-soluble paper at 150-200mm on both sides of the welding joint as a plug, and weld the same gas-filled welding as the sponge plug. When the pipeline hydraulic test is conducted, the water-soluble paper will dissolve and be discharged with water.

4 Argon protection judgment
The argon protection effect can be judged according to the color of the inner weld, so that the operator can adjust the argon according to the color to achieve the best protection effect. Its color to white, golden is the best, gray black worst.

5 Stainless steel back protection precautions

(1) Before argon arc welding, the back of the weldment should be filled with argon protection in advance, and the flow rate should be appropriately large, and the flow rate will gradually decrease after the air is discharged. The tube should be filled with argon continuously during welding. Only after welding can the argon hose be removed, so that the weld is well protected.
In addition, special attention should be paid to the fact that welding can only be carried out after the air is discharged, otherwise the protection effect of argon filling will be affected.

(2) Argon flow rate should be appropriate. The flow rate is too small, the protection is not good, and the back of the weld is easy to oxidize; Excessive flow will cause defects such as concave weld root and affect welding quality.
(3) The argon inlet should be placed at the lowest possible position in the closed section, and the air discharge hole should be placed at a slightly higher position in the closed section. Because argon is heavier than air, filling it from a lower position guarantees a higher concentration and better protection.
(4) In order to reduce the loss of argon in the tube from the opposite gap, affecting the protection effect and increasing the cost, tape can be affixed along the welding gap before welding, leaving only the length of a continuous welding by the welder, and remove the tape while welding.

02
Self-protecting wire protection
The backside self-protecting welding wire is a welding wire with a coating. When welding, the protective coating will participate in the complete protection of the front and back of the weld pool, forming a dense protective layer, so that the back of the weld pass is not oxidized, this protective layer will automatically fall off after cooling, and will be cleared with the purge pressure test.
The use method of this welding wire is basically the same as that of ordinary argon arc welding solid core wire, and the weld metal can meet the requirements in terms of performance.
The self-protected wire is not restricted by various welding conditions, making welding preparation faster and simpler. However, due to the coating on the surface of the welding wire, there will be some inadaptability in the operation of the welding personnel.
Due to the incompatibility and welding techniques are not suitable for coated wire, sometimes there will be defects such as concave, so there are certain requirements for the operation skills and techniques of welding personnel. Self-protecting wire is best used as a base due to high cost.
In addition, there are many brands of self-protective welding wire on the market to choose from, and the applicability is not the same.