With the rapid growth of the petrochemical industry, the use of stainless steel materials has increased and higher demands have been placed on the welding of pipes and plates.
The traditional method of stainless steel arc welding backing has become obsolete and has been replaced by argon arc welding backing.
Argon arc welding backing is faster and offers higher cleanliness than the previous method, however, it also comes with some challenges.
During the welding process, the back of the stainless steel in argon arc welding is susceptible to oxidation, which can result in defects. Hence, proper back protection measures must be implemented to preserve the mechanical properties and corrosion resistance of the weld.
Therefore, it is crucial to take effective measures to protect stainless steel during welding.
Today, several common back protection methods for stainless steel welding are introduced in this article.
1. Back argon protection
Common shielding gases can be classified into two types: pure argon shielding and mixed gas shielding.
A mixture of argon and nitrogen is more suitable for welding austenitic stainless steel, while some inert gases are not used due to their high cost.
The argon-filled method is a more conventional approach to back protection, known for its effective back protection, ease of use, high cleaning efficiency, and high success rate.
This method is further divided into three subtypes: protective cover argon filling protection, local argon filling protection, and direct argon filling protection in the crater.
1. Protective cover filled with argon
The protective cover connects to a metal pipe and is connected to an argon hose.
Open the argon valve to fill the protective cover with argon.
Another person is needed to hold the metal pipe as a handle, so that the protective cover can slide on the back molten pool and be synchronized with the welding on the outside of the plate or pipe.
In this manner, the back is effectively protected.
The feature of this method is that the protection is centralized.
Argon does not need to be opened excessively, resulting in minimal argon waste.
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. Fill the pipe with argon by inserting the argon hose from one end. Make a small hole in the seal at the other end of the pipe (no sponge is needed). This helps with the final backing welding joint and avoids depression due to high internal pressure.
When welding, to avoid a large loss of argon from the weld, tear the sealing tape of the weld and weld one section at a time. This reduces argon loss and protects the weld effectively.
This method is characterized by being wasteful, having slow argon filling, and being costly.
3. Direct argon filling protection method for welded junction
argon filling is inefficient, the quality cannot be ensured, and the cost of the project is too high.
To reduce costs, the method of direct argon filling at the welded junction can be implemented.
Method of making plugs on both sides of weld
Transform the sponge into a stopper with a slightly larger diameter than the pipe, and secure the two sponges together with iron wire at a distance of 300-400mm, forming a double stopper.
Attach a longer piece of steel wire to one end of the stopper.
When joining by butt welding, insert the stopper 150-200mm on both sides of the weld seam.
The long iron wire at one end of the stopper should be longer than the length of the pipe at one end of the weld seam, exposing the end of the pipe.
Flatten one end of a small metal pipe and connect the other end to an argon hose.
Insert the flattened end into the aligned weld seam and fill it with argon. It is preferable to insert the pipe in the upward direction, so that it can be removed before the final backing welding, and the welding can be completed using the residual gas in the pipe.
After welding, remove the stopper using the iron wire.
Water soluble paper protection method
Before assembly, apply water-soluble paper measuring 150-200mm on both sides of the weld crater as a plug.
After alignment, conduct the same gas-filled welding process as with the sponge plug.
During the pipeline hydrostatic test, the water-soluble paper will dissolve and be flushed out with the water.
4. Argon protection judgment
The effectiveness of argon protection can be determined by examining the color of the inner weld. This allows the operator to adjust the argon flow to achieve optimal protection, as indicated by the color.
The ideal color is white or golden yellow. On the other hand, gray or black indicate poor protection.
5. Precautions for stainless steel back protection
(1) Before starting Argon arc welding, it is important to fill the back of the weldment with Argon and set the flow rate appropriately higher. The flow rate will then gradually decrease after the air has been discharged. The pipe must be continuously filled with Argon during welding. After completing the welding, the Argon hose should be removed to protect the weld. It’s worth noting that welding should only commence once all the air has been completely discharged; otherwise, the Argon filling protection will be impacted.
(2) The Argon flow rate should be set appropriately. If the flow rate is too low, the protection will be poor, and the back of the weld is prone to oxidation. On the other hand, if the flow rate is too high, it can result in defects such as concave weld roots and negatively impact the welding quality.
(3) The Argon inlet should be placed as low as possible in the enclosed section, and the air outlet should be positioned slightly higher. This is because Argon is heavier than air, so filling from a lower position ensures a higher concentration and better protection effect.
(4) To reduce the loss of Argon from the gap in the butt joint, protect the weld and minimize cost, tape can be applied along the gap before welding. This will allow only the length of one continuous welding for the welder and the tape can be removed while welding.
2. Welding wire protection
The self-shielded welding wire with a coating is a type of welding wire. During the welding process, the protective coating contributes to the overall protection of both the front and back of the molten pool, forming a dense protective layer that prevents oxidation on the back of the weld bead. After cooling, the protective layer will peel off and be removed through purging and pressure testing.
The usage of this welding wire is similar to that of a regular argon arc welding solid wire, and the weld metal produced meets the necessary requirements. The self-shielded welding wire is not restricted by various welding conditions, making the welding preparation process faster and simpler.
However, the coating on the surface of the welding wire may pose some difficulties for the welding personnel. If the welding method is not suitable for coated wire or if the personnel are not skilled enough, there may be defects such as concavity. Thus, there are specific requirements for the welding personnel’s operating skills and methods.
Due to its high cost, self-shielded welding wire is best used for priming. There are numerous brands of self-shielded welding wires available in the market, and their suitability varies. Foreign imports typically have the best quality and suitability.