How to prevent back oxidation during stainless steel pipe welding

1. Introduction

During the welding of stainless steel pipes in petrochemical construction, the weld surface and heat-affected zone are prone to oxidation and discoloration. In order to ensure the corrosion resistance of stainless steel, the weld of the stainless steel pipe is pickled and passivated after welding to form a dense oxide film on its surface, but the inner wall of the stainless steel pipe is often unable to be pickled and passivated, which seriously reduces the corrosion resistance of the inner wall of the stainless steel pipe. However, it is difficult to ensure the welding quality of the back weld and heat-affected zone with general welding technology and construction measures, so it is necessary to improve the welding technology and take measures to prevent back oxidation and discoloration.

2. Analysis of the reasons for the easy oxidation of the back and surface of stainless steel pipe welding

1) When the back is filled with argon gas for protection, the air moves in a vortex in the stainless steel pipe and it is difficult to completely exhaust it. Even if the replacement time is extended, the oxygen content can reach <0.01% (volume fraction) when the inner mouth is fully sealed, but after the groove is opened and welding begins, the oxygen content will rise to 0.05% (volume fraction), or even higher. The higher oxygen content eventually causes the back surface of the weld to oxidize and turn blue or purple.

2) During on-site construction, welders generally use a simple argon arc welding torch that scratches and strikes the arc. This type of torch causes the high-temperature weld to instantly lose the protection of argon gas after the arc is broken, resulting in oxidation and discoloration of the joints at each arc closure.

3) Each time the arc joint is struck, air enters the inner mouth through the open welding groove. At this time, the oxygen content in the inner mouth is high, and the high-temperature weld is prone to local oxidation.

4) When welding the filling and capping layers, the welding current is large, the welding heat input is large, or the interlayer temperature is high, and the back of the weld will also oxidize and discolor.

5) The low purity of argon gas and the high oxygen content make the protection effect of the stainless steel pipe weld worse.

6) Stainless steel has poor thermal conductivity, which is 1/3 of that of steel. During welding, the heat cannot be dissipated, resulting in high temperatures in the weld and heat-affected zone, making it more susceptible to oxidation.

3. Improvement of welding process

1) Oxygen in the inner shielding gas is the main cause of oxidation and discoloration of the weld and heat-affected zone, so eliminating oxygen in the shielding gas can solve the oxidation problem. After the process is improved, high-purity argon with a purity of 99.999% is used as welding gas, and mixed gas (5% H2+95% Ar) is used as back shielding gas. The chemical reaction of hydrogen and oxygen at high temperatures is used to reduce the oxygen content in the stainless steel pipe.

2) The density of argon is greater than that of air. When replacing the air in the stainless steel pipe, the principle of low filling and high exhaust is adopted, and the filling and replacement time is appropriately extended.

3) Use welding machines and high-frequency arc welding guns with current slow rise, current decay, advance gas supply, and delayed gas stop functions. When the arc is just struck, the burning arc is used to burn the oxygen in the shielding gas first, avoiding oxidation of the weld when the arc joint is struck. After the arc is broken, the high-temperature weld is still under the effective protection of argon gas, preventing local oxidation and discoloration of the joint.

4) Increase the nozzle diameter, use a nozzle of φ10~φ12mm, and increase the protection range.

5) Use tin foil tape with good sealing performance to seal the groove, not paper tape.

6) Reduce the argon filling flow rate when closing the final joint. It is best to keep the argon filling flow rate unchanged and open one side to exhaust for the best welding effect.

7) The temperature between passes and layers should not exceed 60℃. During welding, you can also use cooling copper pipes with circulating water wrapped on both sides of the weld of the stainless steel pipe or use wet cotton cloth wrapped on both sides of the weld to reduce the high temperature residence time of the weld.

8) The first three layers are welded with argon arc welding with low heat input and argon gas is continuously filled for protection of the inner mouth. After reaching a certain thickness, welding rods are used for welding. Whether it is argon arc welding or electrode arc welding, the current should be as small as possible, and the arc operation method of linear walking or micro-swinging should be adopted. The faster the welding speed is, the better it is under the premise of ensuring the welding quality, thereby reducing the welding heat input and avoiding oxidation due to excessive weld temperature.

4. Conclusion

During the construction of a chemical plant, it was impossible to pickle and passivate the stainless steel pipe after welding, and oxidation of the inner surface of the weld was not allowed during welding. The surface color of the weld and the heat-affected zone must be silver-white. After welding using the above process measures during construction, the requirements of the design and the owner were met, ensuring the corrosion resistance of the stainless steel pipe.