Principle of Bluing (or Darkening)
To enhance the rust resistance of steel components, the surface is oxidized using a strong oxidizing agent to form a dense and smooth layer of magnetite (Fe3O4). This Fe3O4 layer effectively protects the internal steel from oxidation. At high temperatures (approximately 550°C), the formed Fe3O4 is a sky blue color and is commonly referred to as bluing treatment. At low temperatures (approximately 130°C), the formed Fe3O4 is dark black and is commonly referred to as blackening treatment.
Bluing treatment is commonly used in the arms industry.
Industrial production is commonly used in industrial production.
The key to converting iron into dense and smooth Fe3O4 lies in selecting a strong oxidizing agent. The strong oxidizing agent is composed of sodium hydroxide (NaOH), sodium nitrite (NaNO2), and sodium phosphate (Na3PO4). When blue, treat the workpiece with the molten liquid; when black, treat the workpiece with the aqueous solution.
At a density of 4g/cm³, the oxidation layer of iron in this solution is as follows: Fe → Ma2FeO2 → Na2Fe2O4 → Fe3O4
Practical production experience demonstrates that to achieve a bright and dense Fe3O nitrate, the ratio of nitrate to sodium hydroxide in the oxidizing solution should be maintained between 1:3 to 3.5.
No.2
Process of Blue (Black) Development:
Workpiece clamping → degreasing → cleaning → pickling → oxidation → cleaning → saponification → hot water washing → inspection.
(1) Workpiece Clamping
According to the shape, size, and design of the workpiece, jigs or hoists should be used. The purpose is to ensure adequate spacing between workpieces, preventing any contact, so that each workpiece can be fully immersed in the oxidation solution for oxidation.
(II) Oil Removal
The purpose is to remove oil污 from the workpiece surface. After the frame operation (blue and black are subsequent processes), oil污 on the workpiece surface is inevitably left, particularly rust preventative oil. Any oil污 will severely affect the generation of Fe3O4, which must be removed before the blue and black processes.
The degreasing solution is heated to 800-900°C, and the workpiece is immersed for approximately 30 minutes. If there is excessive oil污, the degreasing time must be extended to ensure complete removal.
(3) Pickling
The purpose of pickling is to remove rust from the workpiece surface. Rust can hinder the formation of a dense Fe3O4 layer. Even if the workpiece does not rust, it should be pickled as it further removes oil污. Pickling increases the activation energy of surface molecules, leading to oxidation in the next process, producing a thicker layer of oil污Fe3O4. The pickling solution is typically a 10% to 15% sulfuric acid solution, with a temperature range of 70℃ to 80℃. The workpiece is immersed in the sulfuric acid solution for 30 minutes; for workpieces with little rust, about 20 minutes are sufficient, while those with more rust require over 40 minutes.
(4) Oxidation
Oxidation is the primary process for producing blue and black finishes. The density, smoothness, and thickness of the Fe3O4 coating depend on the oxidation stage. The blue temperature is 550°C. The temperature of the black solution ranges from 130 to 145°C. The immersion time is between 50 to 80 minutes. High-carbon steel with high carbon content oxidizes quickly and requires a shorter immersion time. Low-carbon steel with low carbon content oxidizes slowly and requires a longer immersion time. Alloy steel, especially high-alloy steel, has a certain amount of residual austenite on the workpiece surface, which is highly resistant to alkali solutions and is less prone to the formation of Fe3O4. Due to the longer immersion time, Table 3 lists the solution temperature and immersion time for oxidation (black).
The normal color of the solution is white. If the solution is red or brown, it indicates an excessive concentration of sodium nitrite, and the solution composition should be adjusted promptly. If the solution is green, it signifies an overly high concentration of iron ions, and the solution should be refreshed immediately.
(L) Saponification
Saponification involves soaking the workpiece in a soap solution at a certain temperature. The purpose is to form a stearic acid iron film to enhance the workpiece's corrosion resistance. The common concentration of the saponification solution is 30-50g (soap) per liter. Heat the saponification solution to 80-100°C and soak the oxidized workpiece for about 10 minutes.
(Six) Inspection
After oxidation, the workpieces must be inspected to see if the Fe3O4 layer is qualified. Randomly select three pieces, soak them in a 2% copper sulfate solution for 20 seconds; those that do not discolor are considered qualified. If one or more of the three pieces are不合格, the entire batch of workpieces is deemed不合格, and the oxidation process must be repeated to increase the thickness of the Fe3O4 layer.
