With the rapid development of petrochemical industry, stainless steel materials have been widely used, and higher requirements have been put forward for the welding of pipes and plates.
The previous stainless steel arc welding backing method has been gradually eliminated, while argon arc welding backing welding has been adopted.
The argon arc welding backing is faster than the arc welding backing with high cleanliness, and there are also some problems.
During the welding process, the back of stainless steel argon arc welding backing is easy to be oxidized to produce defects, so back protection measures must be taken to ensure the mechanical properties and corrosion resistance of the weld.
Therefore, effective protection must be taken when welding stainless steel.
Today, several common back protection methods for stainless steel welding are introduced in this article.
1. Back argon protection
Common shielding gases can be divided into pure argon shielding and mixed gas shielding.
A certain proportion of argon nitrogen mixed gas is more conducive to austenitic stainless steel welding, and some inert gases are not used because of the high cost.
Argon filled method is a more traditional method of back protection, which is characterized by good back protection, easy to grasp, high cleaning and high qualification rate.
It is divided into protective cover argon filling protection method, local argon filling protection method, and crater direct argon filling protection method.
1. Protective cover filled with argon
This method is often used in stainless steel welding of plates and large-diameter pipes.
The protective cover connects a metal pipe and connects an argon hose.
Open the argon valve to fill the protective cover with argon.
Another person needs to hold the metal pipe as a handle, so that the protective cover slides on the back molten pool and synchronizes with the welding outside the plate or pipe.
In this way, the back is effectively protected.
The feature is that the protection is centralized.
Argon does not need to be opened too much, and argon waste is small.
2. Local argon filling protection
For pipes with small local space and short size, it is easy to use local protection.
Seal the pipe welded junction with tape (to prevent gas leakage), seal both ends of the pipe with sponge, rubber, paper shell, etc., insert the argon hose from one end to fill it with argon, and make a small hole at the seal of the other end of the pipe (sponge is not required), which is conducive to the final backing welding joint, and will not cause depression due to high internal pressure, etc.
During welding, in order to prevent a large amount of argon from being discharged from the weld, the sealing tape of the weld should be torn to weld one section at a time, so as to reduce more argon loss and effectively protect the weld.
It is characterized by waste, slow argon filling and high cost.
3. Direct argon filling protection method for welded junction
For the pipeline with too long pipeline and slightly large pipe diameter, partial argon filling is too wasteful, the quality cannot be guaranteed, and the project cost is too high.
In order to save cost, the method of direct argon filling at welded junction can be used.
Method of making plugs on both sides of weld
Process the sponge into a plug with a slightly larger pipe diameter and connect the two sponges with iron wire at a distance of 300-400mm to form a double plug.
One end of the plug is connected with a longer length of steel wire.
When butt jointing, plug the plug at 150-200mm on both sides of the weld seam.
The long iron wire at one end is longer than the length of the pipe at one end of the weld seam and exposes the pipe end.
One end of the small metal pipe is flattened and the other end is connected to the argon hose.
The flattened end is inserted into the aligned weld seam, argon shall be filled, and the insertion direction shall preferably be the uppermost part, so that the small pipe can be pulled out before the final joint of backing welding and the welding can be completed by the residual gas of the pipe.
Pull out the plug with iron wire after welding.
Water soluble paper protection method
Before assembly, paste water soluble paper 150-200mm on both sides of the weld crater as a plug.
After alignment, the same gas filled welding as the sponge plug.
When the pipeline hydrostatic test is conducted, the water soluble paper will be dissolved and 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 is white and golden yellow, and gray black is the worst.
5. Precautions for stainless steel back protection
(1) Before argon arc welding, the back of the weldment shall be filled with argon in advance, and the flow shall be appropriately larger.
After the air is discharged, the flow will gradually decrease.
The pipe shall be filled with argon continuously during welding.
After welding, the argon hose can be pulled out to protect the weld.
In addition, it should be noted that welding can only be carried out after the air is completely discharged, otherwise the protection effect of argon filling will be affected.
(2) Argon flow shall be appropriate.
The flow is too small, the protection is poor, and the back of the weld is easy to be oxidized;
The excessive flow will cause defects such as concave weld roots and affect welding quality.
(3) The argon inlet shall be placed as low as possible in the closed section, and the air outlet shall be placed slightly higher in the closed section.
Because argon is heavier than air, filling argon from a lower position can ensure higher concentration, and the argon filling protection effect is better.
(4) In order to reduce the loss of argon in the pipe from the gap of the butt joint, affect the protection effect, and increase the cost, tape can be pasted along the gap of the weld joint before welding, leaving only the length of one continuous welding for the welder, and remove the tape while welding.
2. Welding wire protection
The back self shielded welding wire is a kind of welding wire with a coating.
During welding, the protective coating will participate in the complete protection of the front and back of the molten pool, forming a dense protective layer, so that the back of the weld bead will not be oxidized.
After cooling, the protective layer will automatically fall off, and will be cleaned with purging and pressure test.
The use method of this welding wire is basically the same as that of the ordinary argon arc welding solid wire, and the performance of the weld metal can meet the requirements.
The self shielded welding wire is not limited by various welding conditions, which makes the welding preparation faster and simpler.
However, due to the coating on the surface of the welding wire, there will be some inadaptability during the operation of the welding personnel.
Due to the inadaptability and the welding method is not suitable for the coated wire, there will sometimes be defects such as concavity.
Therefore, there are certain requirements for the operating skills and methods of the welding personnel.
Self shielded welding wire is best used for priming due to its high cost.
In addition, there are many brands of self-protection welding wires available in the market, and their applicability is different.
The quality and applicability of foreign imports are the best.