1. Details of
stainless steel pipe welding
When welding stainless steel pipes in petrochemical construction, the weld surface and heat-affected zone are prone to oxidation and discoloration. To ensure the corrosion resistance of stainless steel, the weld 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 for general welding processes and construction measures to ensure the welding quality of the back weld and heat-affected zone, so it is necessary to improve the welding process and take measures to prevent back oxidation and discoloration.
2. Analysis of the reasons why the back and surface of stainless steel pipes are prone to oxidation
1) When the back is filled with argon gas for protection, the air moves in a vortex in the steel pipe and is difficult to be completely exhausted. 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, and the oxygen content in the inner mouth is high at this time, so 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 purity of argon gas is low, and the high oxygen content makes the protection effect of the 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 the welding process of stainless steel pipe
1) The oxygen in the protective gas at the inner mouth of the stainless steel pipe is the main cause of oxidation and discoloration of the weld and heat-affected zone, so the oxidation problem can be solved by eliminating the oxygen in the protective gas. After the process is improved, high-purity argon gas with a purity of 99.999% is used as welding gas, and mixed gas (5% H2+95% Ar) is used as back protective gas, and the chemical reaction of hydrogen and oxygen at high temperature is used to reduce the oxygen content in the steel pipe.
2) The density of argon gas is greater than that of air. When replacing the air in the 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 attenuation, advanced gas supply, and delayed gas stop functions. When the arc is just struck, the burning arc is used to burn the oxygen in the protective 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 and use a nozzle of φ10~φ12mm to 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 or use wet cotton cloth wrapped on both sides of the weld to cool down and reduce the high-temperature residence time of the weld.
8) The first three layers are welded with argon arc welding with small heat input, and argon gas is continued to be 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, the better while ensuring the welding quality, thereby reducing the welding heat input and avoiding oxidation due to excessive weld temperature.
4. Summary
In the construction of a chemical plant, the stainless steel pipe could not be pickled and passivated after welding, and the inner surface of the weld was not allowed to be oxidized during welding. The surface color of the weld and the heat-affected zone must be silver-white. After the above process measures were adopted during the construction, the requirements were met and the corrosion resistance of the stainless steel pipe was guaranteed.
Threeway Steel is known as a professional supplier engaged in manufacturing and distributing a wide range of steel pipe, and our headquarter located the central part of China – Hunan and six associated factories throughout China.
Threeway Steel is known as a professional supplier engaged in designing, manufacturing and distribution of a wide range of steel products with the headquarter located the central part of China – Hunan and six associated factories throughout China.
Threeway Steel is known as a professional supplier engaged in designing, manufacturing and distribution of a wide range of steel products with the headquarter located the central part of China – Hunan and six associated factories throughout China.
Threeway Steel is known as a professional supplier engaged in designing, manufacturing and distribution of a wide range of steel products with the headquarter located the central part of China – Hunan and six associated factories throughout China.
Threeway Steel is known as a professional supplier engaged in designing, manufacturing and distribution of a wide range of steel products with the headquarter located the central part of China – Hunan and six associated factories throughout China.
