How to prevent back oxidation of stainless steel pipe during welding?
During the welding of stainless steel pipe in petrochemical construction, the surface of weld bead and heat affected zone are easy to be oxidized and discolored. In order to ensure the corrosion resistance of stainless steel, pickling and passivation treatment is carried out on the weld after welding to form a dense oxide film on its surface, but the inner wall of stainless steel pipe is unable to be pickled and passivated, which seriously reduces the corrosion resistance of the inner wall of stainless steel pipe. However, general welding process and construction measures are difficult to ensure the welding quality of back weld bead and heat affected zone (see Fig. 1 and Fig. 2), so it is necessary to improve the welding process and take measures to prevent back oxidation and discoloration.
Fig.1 Serious oxidation of heat affected zone of back weld bead
Figure.2 Local oxidation of back weld bead joint
Analysis on the reasons of oxidation on the back and surface of stainless steel pipe welding
- When the back is filled with argon for protection, the air in the pipe is always in the form of eddy current, which is difficult to be completely discharged. Even if the replacement time is extended, the oxygen content in the full sealing state of the inner mouth can be less than 0.01% (volume fraction), but when the groove is opened for welding, the oxygen content will rise to 0.05% (volume fraction), or even higher. The higher oxygen content will eventually lead to the oxidation of the back surface of the weld bead and turn into blue or purple.
- In the field construction, the welder generally uses the simple argon arc welding gun to scratch and strike the arc. This kind of welding gun makes the high-temperature weld bead lose the protection of argon gas instantaneously after the arc breaking, which causes the joint of each arc stopping to be oxidized and discolored easily.
- Each time the arc joint is started, the air enters the inner opening through the open welding groove. At this time, the oxygen content in the inner opening is high, and the high-temperature weld bead is easy to be oxidized locally.
- When filling and covering layer are welded, the back of the weld bead will be oxidized and discolored if the welding current is large, the welding heat input is large, or the interlayer temperature is high.
- The protection effect of low purity and high oxygen content of argon on weld is worse.
- Stainless steel has poor thermal conductivity, which is 1/3 of that of steel. When welding, the heat cannot be dissipated, resulting in higher temperature of weld bead and heat affected zone, so it is easier to be oxidized.
Improvement of welding process
- The oxygen in the inner protective gas is the main cause of oxidation discoloration of weld bead and heat affected zone, so the oxidation problem can be solved by removing the oxygen in the protective gas. After improving the process, 99.999% high purity argon was used as welding gas, 5% H2 + 95% Ar as back shielding gas, and the chemical reaction of hydrogen and oxygen at high temperature was used to reduce the oxygen content in the tube.
- The density of argon is higher than that of air. When replacing the air in the tube, the principle of low charge and high discharge shall be adopted, and the replacement time of inflation shall be appropriately prolonged.
- The welding machine and high-frequency arc welding gun with the functions of current slow rise, current attenuation, air supply in advance and gas stop in delay are used. The oxygen in the protective gas is first burnt by the burning arc when the arc is just started to avoid the oxidation of the weld bead when the arc is started. The high temperature weld bead is still under the effective protection of the argon after the arc is broken, thus preventing the local oxidation and discoloration of the joint.
- Increase the diameter of the nozzle, use a nozzle of φ 10 ~ φ 12mm to increase the protection range.
- The sealing groove adopts the tin foil paper tape with good sealing performance, not the paper tape.
- When closing the final joint, reduce the argon filling flow, the best way is to use the same argon filling flow, and the best welding effect is to open one side to exhaust.
- The temperature between passes and layers shall not be controlled more than 60 ℃. During welding, cooling copper pipes with circulating water on both sides of the weld or wet cotton cloth on both sides of the weld can also be used to cool down, so as to reduce the high temperature residence time of the weld bead.
- The first three layers are welded by argon arc welding with small heat input. At the same time, the inner opening is filled with argon for protection. When the thickness reaches a certain level, the welding rod is used for welding. No matter argon arc welding or electrode arc welding, the current should be as small as possible, and the method of straight walking or micro swinging should be adopted. The faster the welding speed is, the better under the premise of ensuring the welding quality, so as to reduce the welding heat input and avoid oxidation due to too high weld bead temperature.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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