The black annealed steel tube production line utilizes welded pipes as base material, achieving performance optimization through annealing and blackening processes. The core workflow is as follows:

The pretreatment system employs shot blasting machines or alkaline degreasing equipment to remove surface oil, weld slag, and scale from the steel pipes, ensuring surface cleanliness. Some production lines incorporate acid washing (e.g., 10-15% sulfuric acid solution) for additional rust removal and enhanced oxide film adhesion. Pre-treated pipes are conveyed via roller conveyors to the annealing stage.

In the annealing furnace system, pipes enter a pit furnace or bell furnace. They are held at 700-900°C for 2-8 hours (specific parameters adjusted based on pipe diameter and material) to eliminate welding stresses and soften the material through recrystallization. During annealing, a dense black oxide film of iron(III) oxide (Fe₃O₄) naturally forms on the pipe surface in an oxidizing atmosphere, with a thickness of approximately 0.5-2μm.

Cooling and Straightening Post-annealing, pipes cool gradually with the furnace to room temperature, stabilizing the oxide film structure. Straightness is then adjusted to 0.15-0.4mm/m using two-roll straighteners (e.g., JY series), while variable frequency speed control (10-30m/min) ensures dimensional accuracy.

Cutting to Length and Surface Treatment Steel tubes are cut to customer-specified lengths using high-precision flying saws. Some production lines incorporate a soap treatment process (immersion in 80-100°C soap solution) to form an iron stearate protective layer on the oxide film surface, further enhancing rust resistance. Products exceeding bending tolerance limits are ultimately rejected through manual or automated inspection.