1. Characteristics of thin-walled stainless steel pipes
The use of argon-electric welding for thin-walled stainless steel pipe connections has the following advantages over manual arc welding and argon arc welding:
1.1 Good welding quality: According to the welding process assessment, the appropriate welding wire, tungsten electrode, welding process parameters, and shielding gas with a purity that meets the requirements can be selected to achieve a good fusion of the weld root. When performing radiographic flaw detection, the qualified rate is significantly high.
1.2 High efficiency: The same welder uses argon-electric welding and manual arc welding to weld the same weld. The welding efficiency of argon-electric welding is 2 to 4 times that of manual arc welding and 1 to 2 times that of argon arc welding, which significantly shortens the construction period.
1.3 Low cost: After comprehensive measurement, it is found that argon-electric welding can reduce the overall construction cost by 10 to 20% compared with manual arc welding, and can reduce the overall construction cost by 5 to 15% compared with argon arc welding. In addition, the weld is well formed and the rework rate is low, which reduces the overall cost.

2. Scope of application of thin-walled stainless steel pipes
This method is mainly suitable for the thin-walled stainless steel industry and boiler pipeline installation with high welding quality. The welding pipe diameter is DN100mm~DN230mm and the pipe wall thickness is 3~6mm.

3. Process principle of thin-walled stainless steel pipes
Argon arc welding is a welding method that uses argon arc welding to weld the bottom of the weld and then uses arc welding to cover the surface. When welding, first determine the angle and position of each welding zone for the circumferential butt weld of the pipe and then determine the parameters of each zone: such as preheating temperature, welding temperature, current, welding pulse, argon gas flow, etc. It combines the advantages of the two welding methods and can better ensure the quality of the project.

4. Process flow and key points of operation of thin-walled stainless steel pipes.
4.1 Process flow: Construction preparation → Pipeline cutting → Groove processing → Cleaning the inner and outer surfaces of the groove → Rod drying → DC welding machine current adjustment → Welding assembly → Positioning spot welding → Bottom welding → Cover welding → Appearance inspection → Flaw detection
4.2 Key points of operation:
4.2.1 Construction preparation:
1) Strictly control material procurement, conduct material testing, and mechanical property re-inspection according to regulations, and do a good job in welding process assessment and grade welder training, technical disclosure, etc.
2) Rod drying: This process uses stainless steel alkaline welding rods, the drying temperature is 350-380℃, the drying time is 1-2 hours, the dried welding rods are placed in a 110-150℃ insulation box, and the welding rods used on site are stored in an insulation barrel.
3) Process parameter selection:
① Control of preheating temperature: Generally, the preheating temperature is controlled at 180-200℃, and the preheating time is 1-3 minutes.
② Welding track inclination: When the inclination is too small, bridging is more likely to occur, and when the inclination is too large, it is easy to produce a cold weld. The track inclination should be controlled between 5°-7°.
③ Welding temperature: The welding temperature should be controlled at 250±5℃.
④ Use the orthogonal test method to find the most ideal parameter value, and refer to the table below for process parameters.
⑤ Welding environment: A windproof shed is set up at the pipeline welding site, and a portable fire extinguisher is arranged in the shed.
4.2.2 Pipe cutting: Use a special pipe-cutting machine to cut the material to ensure that the port quality meets the standard. The gap between the thin-walled stainless steel pipe must be controlled within 0.5mm.
4.2.3 Bevel processing: Due to the large diameter and thin wall of thin-walled pipes, the size and straightness of the bevel are not easy to grasp. Use an LG-400-2 plasma cutting machine, use a V-shaped bevel for the bevel after cutting, and grind the cut pipe mouth to grind out a 30° bevel.
4.2.4 Cleaning of groove surface: Grind the weld spatter, weld scar, and weld slag at the weld joint, and degrease the weld joint with a degreasing machine to remove surface dirt.
4.2.5 Adjust the current of the DC welding machine: Select 60~80A and the upper limit of the plate thickness.
4.2.6 Pipe assembly and positioning spot welding: The length of the spot welding weld should not exceed 10~13mm, the height should not exceed 2/3 of the pipe thickness, and three points should be evenly divided on the circumference. Tungsten electrode argon protection is used for positioning spot welding.
4.2.7 Bottom/cap welding:
Bottom welding: In the welding gun, the diameter of the tungsten electrode is 1.6mm (generally 1-2.5), the short arc is used for manual welding, the angle between the welding gun and the weld is 80-90, the end of the tungsten pole is ground into a cone, and the distance from the weld is 1.5-2.0mm. The tungsten electrode is usually tungsten or thoriated tungsten. Imported tungsten electrodes can also be used to ensure the reliability of arc starting. To ensure the internal quality of the weld, argon gas is delivered 5-10 seconds before arc initiation to remove the air from the pipe being welded. After welding, the power must be turned off within 3-5 seconds until the tungsten electrode and the molten pool area cool down.
Cover welding: The surface slag and spatter should be cleaned before welding. The welding current should be 70-100A, and the welding speed should be fast. Short arc and fast straight welding should be used during welding. No swinging is required to obtain a neat weld surface. Due to the thin wall of the plate, one end of the weldment can be raised at an angle of 15-20 degrees for downhill welding. The weld should be polished to make the weld flat and have a similar gloss to the parent material.
4.2.8 Appearance inspection: Check the surface for defects such as cracks, surface pores, slag inclusions, undercuts, and incomplete penetration.
4.2.9 NDT: High-pressure pipelines must be inspected by X-ray NDT as required and meet Level II standards: no defects such as incomplete penetration and unfused parts, pores, and oxide inclusions less than 2 mm, with a 10 mm × 10 mm field of view as the evaluation field and 1 defect point.