Back Shielding Welding of Stainless Steel
The rapid development of the petrochemical industry has a higher requirement for welding of stainless steel pipes and plate, the early stainless steel welding backing to be being washed out gradually and now more using argon arc welding backing welding, with more cleanliness and higher efficient. At the same time there are also appeared some problems, namely, the welding process with argon arc welding of stainless steel base back be oxidized easily and produce defects so on the back protection measures must be taken, so maintaining the weld mechanical properties and corrosion resistance, etc., today here we introduced several kinds of commonly used stainless steel welding back shielding methods:
Back shielding with Ar
The commonly used shielding gas can be pure argon and mixed gas. In fact, a specific proportion of argon and nitrogen mixed gas is more conducive to the welding of Austenitic stainless steel. Some inert gas is not used because of the high cost. The argon filling is the most commonly used back shielding method, which is characterized by good effect, easy operation, high cleaning and high qualified rate. It can be divided into protective cover filling argon shielding, local filling argon shielding, welded junction filling argon shielding, etc.
Protection cover filling argon
Used in stainless steel sheet and large diameter pipe welding. A metal shield connected pipe and argon hose, make the shield filled with argon gas, welder handheld metal pipe as handle make the shield on the back of molten pool slide and plate or pipe welding together, such making that the back got effective protection, greatly reduce the waste of argon.
Local filling argon
Used in locally small space or short size of the pipeline. The welding joint of the pipeline should be sealed with adhesive tape (to prevent air leakage), and both ends of the pipeline should be sealed with sponge, adhesive tape or paper, etc. One end of the argon hose should be filled with argon. It is better to make a small hole at the other end of the pipe (sponge is not required), which is conducive to the final backing welding joint and will not sag due to excessive internal pressure. Disadvantages are slow filling argon and costly.
Welded junction filling argon
For too long and large pipe diameter pipelines, the cost of local argon filling is high and the quality can not be guaranteed, so the welding junction filled argon methods can be directly used. The argon shielding can be judged according to the color of the inner weld joints, and welders can adjust Argon according to the color to achieve the best protection. White and gold are the best, while gray and black are the worst. But in the process of operation, there are some tips for stainless steel back shielding:
(1) Before argon arc welding, the welding parts can be protected by filling argon with a large flow on the back in advance, and the flow gradually decreases after the air is discharged. During the welding process, fill the pipe with argon continuously and stop after the welding is complete. In addition, the welding can only be carried out after the air is cleared, otherwise, the protection effect of argon filling will be affected.
(2) Argon gas flow should be appropriate. Too small flow is not good protection, the back of the weld is easy to oxidize; Excessive flow will cause concave defects at the root of the weld and affect the welding quality.
(3) The argon inlet should be placed as low as possible in the closed section, and the air outlet should be placed slightly higher. Because argon is heavier than air, charging it from a lower position ensures a higher concentration and provides better protection.
(4) In order to reduce the argon leakage from the joint gap, adhesive tape can be used along the welding gap before welding, leaving only the length of a continuous welding for the welder, and the adhesive tape can be removed while welding.
Self-shielded welding wire
The back self – shielding wire is a kind of welding wire with a flux-cored coating. During welding, the shielding coating will penetrate into the weld pool to form a dense protective layer, so that the back of the weld bead will not be oxidized. After cooling, the protective layer will fall off automatically and will be cleaned up with the purge pressure test.
The self-shielding welding stainless steel wire is not restricted by various welding conditions, and the operation is quick and simple. But because the flux cored coating may appear smoke and poison gas, also sag and other defects, so there are certain requirements for welders. Self – shielded wire is suitable for backing welding due to high cost. The method of this welding wire is basically the same as that of ordinary solid cored argon arc welding wire, and the weld metal can meet the use requirements in performance.