Welding methods and weld standards for stainless steel welded pipes

First, what are the welding methods for stainless steel welded pipes

There are many welding methods for stainless steel welded pipes, and the most commonly used methods for stainless steel welded pipes can be divided into the following methods.

1. Argon arc welding: When stainless steel is welded by argon arc welding, due to its good protection, alloy elements are not easy to burn, the transition coefficient is high, the weld is well formed, there is no slag shell, and the surface is smooth. Therefore, the welded joint has high heat resistance and good mechanical properties. At present, manual tungsten inert gas welding is widely used in argon arc welding. It is used to weld 0.5~3mm stainless steel sheets. The composition of the welding wire is generally the same as that of the weldment. The shielding gas generally uses industrial pure argon gas. The welding speed should be appropriately faster during welding, and lateral swing should be avoided as much as possible. For stainless steel with a thickness greater than 3mm, consumable argon arc welding can be used. The advantages of consumable argon arc welding are high productivity, small heat-affected zone of the weld, small deformation of the weldment, good corrosion resistance, and easy automation.

2. Gas welding: Gas welding is convenient and flexible, and can be used to weld welds in various spatial positions. For some stainless steel parts such as thin plate structures and thin-walled tubes, gas welding can sometimes be used when there is no corrosion resistance requirement. To prevent overheating, the welding nozzle is generally smaller than when welding low-carbon steel of the same thickness. The gas welding flame should use a neutral flame. The welding wire is selected according to the composition and performance of the weldment. The gas welding powder uses gas 101. It is best to use the left welding method when welding. When welding, the welding torch nozzle, and the weldment are inclined at 40~50°, the flame core should be no less than 2mm from the molten pool, the end of the welding wire is in contact with the molten pool and moves along the weld with the flame. The welding torch does not swing horizontally, the welding speed should be fast, and interruption should be avoided as much as possible.

3. Submerged arc welding: Submerged arc welding is suitable for welding stainless steel plates of medium thickness or above (6~50mm). Submerged arc welding has high productivity and good weld quality, but it is easy to cause segregation of alloy elements and impurities.

4. Manual welding: Manual welding is a very common and easy-to-use welding method. The length of the arc is adjusted by hand, which is determined by the size of the gap between the electrode and the workpiece. At the same time, when used as an arc carrier, the electrode is also a weld filler material. This welding method is simple and can be used to weld almost all materials. For outdoor use, it has good adaptability and can be used even underwater.

5. MIG/MAG welding: This is an automatic gas-shielded arc welding method. In this method, the arc burns between the current carrier wire and the workpiece under the shield of the protective gas. The wire fed by the machine is used as a welding rod and melts under its arc. Due to the advantages of versatility and specificity of MIG/MAG welding, it is still the most widely used welding method in the world. It is used for materials based on steel, non-alloy steel, low alloy steel, and high alloy, which makes it an ideal welding method for production and repair. When welding steel, MAG can meet the requirements of thin gauge steel plates with a thickness of only 0.6mm. The shielding gas used here is an active gas, such as carbon dioxide or a mixed gas. The only limitation is that when outdoor welding is performed, the workpiece must be protected from moisture to maintain the effect of the gas.

6. TIG welding: The arc is generated between the refractory tungsten welding wire and the workpiece. The shielding gas used here is pure argon, and the wire fed in is not charged. The wire can be fed either manually or mechanically. Some specific uses do not require the wire to be fed in. The material being welded determines whether direct current or alternating current is used. When direct current is used, the tungsten welding wire is set as the negative electrode because it has a deep penetration ability and is very suitable for different types of steel, but it does not have any "cleaning effect" on the weld pool.

Second, what are the standards for the welds of stainless steel welded pipes?

The weld standards for stainless steel welded pipes mainly focus on the appearance, size, chemical composition, mechanical properties, and corrosion resistance of the welds. The following are some common standards:

1. Weld appearance: The welds should be evenly distributed along the circumference of the steel pipe, without obvious welding defects such as cracks, slag inclusions, pores, and undercuts. The weld should be smooth and neat, without burrs and protrusions, and ensure that the outer surface of the steel pipe is smooth.

2. Weld size: The size of the weld (such as height, width, excess height, etc.) should meet the requirements of the manufacturing standards and be within the allowable tolerance range. Different specifications of stainless steel welded pipes have different requirements for weld size, which needs to be determined according to specific specifications.

3. Chemical composition: The chemical composition of the weld should match the parent material to ensure the stable performance of the welded joint. The content of impurity elements in the weld should comply with the relevant standards to avoid adverse effects on the performance of the welded joint.

4. Mechanical properties: The weld should have certain mechanical properties, such as tensile strength, yield strength, elongation, etc., to meet the use requirements. The welds of stainless steel welded pipes for building structures should have plastic deformation properties of tensile and bending resistance.

5. Corrosion resistance: The corrosion resistance of the weld should meet the requirements of relevant standards, including resistance to corrosive media such as acid, alkali, and salt. The weld and heat-affected zone should not tend to intergranular corrosion to ensure the stable corrosion resistance of the welded joint.

6. Nondestructive testing: The weld should be subjected to nondestructive testing, such as radiographic testing, ultrasonic testing, etc., to check whether there are defects inside the weld. The results of non-destructive testing should meet the requirements of relevant standards and specifications to ensure the quality of welded joints.

7. Weld quality level: Different application scenarios have different requirements for weld quality, and weld quality is usually divided into different levels. The weld quality of stainless steel welded pipes for building structures should meet the first-level requirements to ensure the quality and reliability of welded joints.