How to prevent backside oxidation during stainless steel pipe welding

1. Introduction

During welding of stainless steel pipes in petrochemical construction, the weld surface and heat-affected zone (HAZ) are susceptible to oxidation and discoloration. To ensure the corrosion resistance of stainless steel pipes, the weld seam is pickled and passivated after welding to form a dense oxide film on the surface. However, the inner wall of the stainless steel pipe is often not suitable for pickling and passivation, which severely reduces the corrosion resistance of the inner wall. Conventional welding processes and construction measures make it difficult to ensure the quality of the backside weld bead and HAZ. Therefore, it is necessary to improve the welding process and implement measures to prevent backside oxidation and discoloration.

2. Analysis of the Causes of Susceptibility to Oxidation on the Backside and Surface of Stainless Steel Pipes

1) When argon gas is applied to the backside of the pipe, air vortices continuously flow through the pipe, making it difficult to completely expel it. Even with extended replacement time, the oxygen content can reach <0.01% (volume fraction) when the inner end is fully sealed. However, once the groove is opened and welding begins, the oxygen content rises to 0.05% (volume fraction) or even higher. This high oxygen content ultimately causes oxidation on the backside of the weld bead, discoloring it blue or purple. 2) During on-site construction, welders typically use simple argon arc welding torches that strike the arc with a scratch. This type of torch causes the hot weld bead to instantly lose its argon protection after the arc is broken, leading to oxidation and discoloration of the joint at each arc closure.

3) Each time the arc is struck, air enters the weld bead through the open weld groove, where the oxygen content is high, causing localized oxidation of the hot weld bead.

4) During filler and cap layer welding, high welding currents, high heat input, or high interpass temperatures can also cause oxidation and discoloration on the back of the weld bead.

5) Low argon purity and high oxygen content impair weld protection.

6) Stainless steel pipes have poor thermal conductivity, only about one-third that of steel. Heat cannot be dissipated during welding, resulting in higher temperatures in the weld bead and heat-affected zone, making them more susceptible to oxidation.

3. Improvements to Stainless Steel Pipe Welding Processes

1) Oxygen in the shielding gas at the weld bead is the primary cause of oxidation and discoloration in the weld bead and heat-affected zone. Therefore, eliminating oxygen from the shielding gas can resolve the oxidation problem. The improved process uses 99.999% high-purity argon as the welding gas, and a mixed gas (5% H₂ + 95% Ar) as the back shielding gas. The chemical reaction between hydrogen and oxygen at high temperatures reduces the oxygen content within the pipe.

2) Argon is denser than air, so when displacing air within the pipe, a low-in-high-out principle is adopted, and the filling and replacement time is appropriately extended.

3) Welding machines and high-frequency arc ignition torches with current ramp-up, current decay, pre-gas supply, and delayed gas shut-off functions are used. During initial arc ignition, the burning arc first consumes the oxygen in the shielding gas, preventing oxidation at the weld joint. After arc extinguishing, the high-temperature weld remains effectively protected by the argon gas, preventing localized oxidation and discoloration at the joint.

4) The nozzle diameter is increased to 10-12 mm to extend the protection range.

5) Use tin foil tape with good sealing properties for bevel sealing; do not use paper tape. 6) Reduce the argon gas flow rate during the final joint closing. It is best to maintain the argon gas flow constant while opening one side for optimal welding results.

7) Control the interpass and interlayer temperature to no more than 60°C. During welding, you can also use cooling copper pipes with circulating water around the weld or wrap damp cotton cloths around the weld bead to reduce the time the weld remains at high temperatures.

8) Use argon arc welding with low heat input for the first three layers while continuing to fill the inner opening with argon shielding. Once a certain thickness is reached, use rod welding. Whether using argon arc welding or rod arc welding, keep the current as low as possible, and use a linear or slightly oscillating arc motion. While ensuring weld quality, weld speed should be as high as possible to reduce heat input and prevent oxidation caused by excessive weld temperature.

4. Summary

In the construction of a chemical plant, it was impossible to perform pickling and passivation on the inside of a stainless steel pipe after welding. However, oxidation of the weld bead's inner surface was not permitted during welding, and the weld bead and heat-affected zone (HAZ) surface color must be silvery white. After adopting the above-mentioned welding process measures during construction, the design and owner's requirements were met, and the corrosion resistance of the stainless steel pipe was ensured.