What pretreatment is required for stainless steel welded pipes before bright annealing

The pretreatment of stainless steel welded tubes before bright annealing is a crucial step to ensure the quality of annealing and avoid defects such as black spots. It mainly includes the following steps:

1. Surface oil contamination removal

Remove the rolling oil, cutting fluid, fingerprint grease, etc. on the surface of the welded tubes. Common methods include:

Solvent cleaning: Soak and wipe with alcohol, acetone or special metal cleaning agents to dissolve non-polar oil stains;

Alkali washing: Spray or immerse in a 5%-10% sodium hydroxide solution (temperature 60-80℃) to remove polar oils and some oxides;

Ultrasonic cleaning: For complex pipe fittings, high-frequency vibration is used to detach oil stains from the surface, especially suitable for weld seams and other dead corners.

2. Removal of weld seams and surface impurities

Mechanical cleaning: Use stainless steel wire brushes or grinding wheels to grind the weld seam area to remove welding spatter, oxide scale and unmelted metal particles;

Pickling treatment: Pickle the entire or part (such as weld seams) of the welded tubes, commonly using a mixture of nitric acid and hydrofluoric acid (volume ratio 5:1) to remove the black oxide layer produced by high-temperature welding. After pickling, thoroughly rinse with clean water to avoid acid residue.

3. Removal of moisture and residual cleaning agents

After cleaning, the tubes should be dried by hot air (temperature 80-120℃, time 10-30 minutes) or blown dry with compressed air to ensure the surface moisture content is ≤0.1%, preventing water evaporation into the protective gas system during annealing and causing oxidation black spots.

4. Pipe fitting shaping and inspection

Correct the deformation and bending of the welded tubes to avoid abnormal local heating due to uneven contact with the furnace rollers;

Visually or with fluorescent inspection to check for cracks, dents and other defects on the surface. Defective pipe fittings need to be repaired before entering the annealing process to prevent impurities from accumulating at the defect sites.

5. Environmental control before loading into the furnace

The pretreated pipe fittings should be stored in a clean and dry environment (relative humidity ≤50%) to avoid secondary contamination;

Use stainless steel racks when loading into the furnace to avoid contact with carbon steel and other dissimilar materials to prevent iron ion contamination of the surface. 

These pretreatment steps can effectively reduce the risks of impurity carbonization and oxidation during the annealing process, providing a fundamental guarantee for the surface quality after bright annealing.