Key points for welding between stainless steel seamless pipes

Stainless steel seamless pipes are long strips of steel with hollow sections and no seams around them. The thicker the wall thickness of the product, the more economical and practical it is. The thinner the wall thickness, the higher the processing cost. Stainless steel seamless pipes have stronger corrosion resistance than ordinary seamless pipes.

Things to note when welding stainless steel seamless pipes
1. Use a vertical external characteristic power supply, with the positive electrode for DC (the welding wire is connected to the negative electrode)
2. Generally suitable for welding thin-walled steel pipes below 6mm, with the characteristics of beautiful weld shape and small welding deformation.
3. The protective gas is hydrogen with a purity of 99.99%. When the welding current is 50~150A, the hydrogen flow rate is 8-10L/min. When the welding current is 150~250A, the hydrogen flow rate is 12-15L/min.
4. The appropriate length of the tungsten electrode extending from the gas nozzle is 4 to 5 mm. The areas with poor shielding such as fillet welding are 2~3mm. The groove depth is 5~6mm, and the distance from the nozzle to the workpiece generally does not exceed 15mm.
5. In order to prevent the occurrence of welding pores, if there is rust or oil on the welding part, it must be cleaned.
6. When welding ordinary steel pipes, the welding arc length should be 2~4mm, and when welding stainless steel pipes, the welding arc length should be 1~3mm. If it is too long, the protection effect will not be good.
7. When making base joints, in order to prevent the back side of the bottom weld bead from oxidation, the back side also needs to be protected by gas.
8. In order to make the hydrogen protect the welding pool well and facilitate the welding operation. The center line of the tungsten electrode at the welding point and the center line of the workpiece should generally maintain an angle of 80~85°. The angle between the filler wire and the surface of the workpiece should be as small as possible, generally around 10°.
9. Windproof and ventilated. In windy places, please take shelter measures and take appropriate ventilation measures indoors.

Determination of stainless steel seamless pipe welding process

Stainless steel comes in many grades. According to alloy composition, it can be divided into chromium stainless steel and chromium-nickel stainless steel. According to the metal structure of stainless steel pipes, it can be divided into austenitic, ferritic, martensite, etc. The most commonly used in construction is austenitic stainless steel seamless pipe. Austenitic stainless steel seamless pipes have good weldability and are relatively easy to weld. The welded joints have high toughness even in the welded state. But compared with ordinary carbon steel pipes, its thermal conductivity is about 1/3 of carbon steel pipes, and its expansion coefficient is 1.5 times larger than carbon steel seamless pipes. Due to the low thermal conductivity and high expansion coefficient of austenitic stainless steel, large deformation and strain will occur during the welding process. Therefore, welding quality mainly depends on whether the welding process is compatible with the base metal.
Selection of welding methods Commonly used welding methods for stainless steel include manual arc welding, gas shielded welding and automatic submerged arc welding. It is mainly determined based on the design medium parameters, construction conditions and operating environment, construction costs, etc. In the construction of process pipelines, due to different pipe diameters and a large number of valves and pipe fittings on the pipelines, the position changes of welded joints are more complicated. Therefore, manual arc welding is generally used. For pipelines transporting flammable, explosive or media with certain cleanliness requirements, hydrogen arc welding is usually used for priming. Manual arc welding is used for surface coverage to improve the internal quality of the weld.

Preparation work before welding stainless steel seamless pipes

Before welding work. Be prepared accordingly. This preparation is an important part of ensuring weld quality. Its content is mainly considered from the following three aspects:

1. Confirmation of skills of welding operators Welders must hold a certificate to engage in welding operations, and must strictly follow the allowed welding items indicated on the work certificate. Welders should preferably have at least two years of experience welding stainless steel or chromium-molybdenum steel.
2. Management of welding materials: The welding rod should be baked according to the instruction manual before use (if there is no regulation, it should be processed at a drying temperature of 150~200°C and a drying time of 1 hour). Baking must use a special oven with temperature control. You can bake as much as you want and use as much as you want. The dried electrode should be placed in an insulating cylinder for use. If the exposure time exceeds 2 hours, it should be re-baked. The number of repetitions should not exceed 3 times.
3. The bevel of stainless steel pipes can be machined or plasma cut. The oxide layer and burrs at the trench should be removed first. In order to facilitate the removal of spatter after welding, powder slurry can be applied within 50mm on both sides of the weld and removed after welding. Since the contact between stainless steel and carbon steel will produce "carburization", special grinding wheels and stainless steel brushes must be used to clean the weld beads and spatter.