What are the key process steps for manufacturing BA-grade stainless steel welded pipes

The production of BA-grade stainless steel welded tubes requires precise control through multiple processes to achieve high surface cleanliness and brightness. The key process steps are as follows: 

1. Raw Material Selection and Pretreatment

Ultra-low carbon stainless steel strips (such as 316L, C ≤ 0.03%) are selected to ensure that the content of impurity elements (S, P) is ≤ 0.015%, avoiding the precipitation of contaminants after annealing.

The surface of the steel strip is cold-rolled to Ra ≤ 0.1 μm to remove oxide scale and rolling defects, and then ultrasonically cleaned with a neutral cleaner. After drying, the dew point is controlled below -40°C to prevent secondary contamination. 

2. Forming and Welding Control

Precision roll bending forming is adopted to ensure the butt joint gap of the tube blank is no more than 0.1mm, avoiding spatter during welding due to excessive gap.

Automatic TIG welding is used, with 99.999% pure argon gas back protection inside the tube (flow rate 8-12L/min), the welding current is stabilized at 120-150A, and the arc voltage is controlled at 10-12V to prevent oxidation or spatter adhesion on the inner surface. 

3. Online bright annealing

The annealing temperature is controlled at 1050-1100℃, and the holding time is adjusted according to the wall thickness (usually 1-3 minutes) to ensure the complete dissolution of carbides.

High-purity argon gas (oxygen content ≤ 10 ppm, dew point ≤ -60℃) is introduced into the furnace, and rapid cooling (cooling rate ≥ 50℃/s) is adopted to prevent the precipitation of chromium and the formation of chromium-depleted zones. 

4. Precision Straightening and Surface Finishing

Multi-roll straightening machines are used to control the straightness to ≤ 0.5mm/m, avoiding surface scratches caused by mechanical stress.

Wool or nylon wheels are used for non-abrasive polishing to remove welding marks and minor imperfections, ensuring a bright and uniform surface (gloss ≥ 800GU). 

5. Final Inspection and Clean Packaging

The inner surface is inspected by endoscope (no impurities or oxidation points larger than 0.05mm are allowed), and the outer surface is measured for roughness (Ra ≤ 0.05μm) using a white light interferometer.

Sealed packaging with polyethylene film is carried out in a Class 100 clean room to prevent fingerprints and dust from adhering during transportation, ultimately achieving a highly clean and bright surface that meets BA grade standards. 

These steps must be carried out in a clean environment throughout the process, and the parameter fluctuations at each stage need to be controlled within ±2% to ensure that the welded tubes meet the strict requirements of high-end fields such as semiconductors and biomedicine.