What is steel blackening (bluing)?
Why do some steel parts turn blue (black) on the surface? What is the processing principle and process flow?
introduction
Oxidation treatment of steel, also known as blackening or bluing treatment, is a surface treatment process for steel. The film color is generally blue black or dark black, depending on the oxidation treatment process conditions and workpiece composition.
The iron oxide formed by oxidation at high temperature (about 550 ℃) appears sky blue, which is called bluing treatment.
The iron oxide formed by oxidation at low temperatures (about 130 ℃) appears dark black and is called blackening treatment.
In the weapons industry, the commonly used treatment is bluing; In industrial production, blackening treatment is commonly used.
Its essence is to generate a thin oxide film on its surface through chemical or electrochemical methods, with a thickness of 1-2 μ m, and the main component of the film is Fe 3 O ₄ (i.e. magnetic iron oxide).
advantage
Enhance rust prevention capability
The presence of oxide film can to some extent inhibit the formation of red rust on the surface of steel (rust that corrodes steel into a broken state).
The protective film of iron oxide is porous, and soaking these pores in rust proof oil can prevent moisture from entering and further improve the rust prevention ability.
In addition, using rust proof oil for post-treatment can keep the oil on the surface of the film, thus improving the sliding performance of cutting tools and machine components.
Not easy to peel off
Blackening treatment is a surface treatment process that triggers a chemical reaction to only change the surface of the component to a different substance (Fe3O4 protective film), which is difficult to peel off.
The dimensional accuracy remains basically unchanged
In the blackening process, the film formation speed is not very fast, so the final thickness of the formed film is only about 1-2 μ m.
The film is formed from the surface of the component towards the interior. And since the processing temperature is below 150 ℃, the temperature is relatively low, so there is no need to worry about thermal deformation of the parts.
Beautiful appearance
Blackening treatment can achieve an appearance similar to dyeing black. The surface roughness remains unchanged before and after treatment. Parts with gloss before treatment will have a gloss similar to black paint, while matte parts will have a dull black surface.
Low price
Due to the low cost of processing solution and the absence of electrodes or fixtures, blackening treatment is cheaper than other surface treatments such as coating and coating treatment.
Even parts of different sizes and shapes can be processed in one go, so when you want to process various different parts at a lower cost, it is recommended to use blackening treatment.
shortcoming
Limited rust prevention ability
This treatment process cannot achieve the same strong rust prevention protection as galvanizing or nickel plating.
Although this process can provide some rust resistance, it is not suitable for long-term outdoor use, parts used in areas with high moisture content, or near the sea.
After the rust proof oil evaporates completely, it is easy to rust
The anti rust effect of the protective film of iron oxide sheet itself is not strong, so as mentioned earlier, it is usually necessary to apply anti rust oil to the surface of the protective film to further prevent rust. But if the subsequent anti rust oil evaporates completely, external moisture and air will seep into the micropores inside the protective film of the iron oxide sheet, causing the formation of red rust.
Some materials are not suitable for blackening treatment
Blackening treatment is a surface treatment process suitable for steel materials. However, parts such as castings that have undergone quenching treatment or wire cutting may sometimes turn red black instead of completely black. Alloy steel with a high content of chromium or nickel will turn gray instead of black.
Thin film thickness
Although the dimensional accuracy has little impact, it also means that its protective ability is relatively limited and cannot provide very strong wear or corrosion resistance.
purpose
After oxidation treatment in the steel industry, the corrosion resistance is improved, but the protection is still poor. Further treatment with soap solution, oil immersion or chromic acid solution is needed to improve the corrosion resistance and lubricity of the film.
Steel oxidation is widely used in mechanical parts, electronic equipment, precision optical instruments, springs, and weapons. It is still necessary to wipe the oil frequently during use.
workmanship
The bluing process of steel is related to temperature. According to the processing temperature, the bluing method of steel can be divided into high-temperature chemical oxidation method and room temperature chemical oxidation method.
High temperature chemical oxidation treatment
High temperature chemical oxidation, also known as alkaline chemical oxidation, is a traditional blueing method.
During the high-temperature oxidation of steel, the use of strong acids and alkalis due to the high process temperature results in poor production conditions and significant environmental pollution.
Blue treatment at room temperature
Steel room temperature chemical oxidation, also known as acidic chemical oxidation or room temperature blue treatment, has the advantages of energy saving, environmental protection, high efficiency (fast oxidation rate, usually 2-4 minutes), low cost, and simple operation compared to high-temperature blue treatment process. At the same time, the obtained film layer has good corrosion resistance and uniformity.
The disadvantage is that the tank solution is unstable and has a short lifespan, so it needs to be prepared as needed, but the adhesion of the coating layer is slightly weaker.
Common Defects and Handling
defectss | Reasons and Handling Methods |
| The oxide film has a reddish-brownish layer of dirt. | (1) If the NaOH content is too high, reduce the concentration of NaOH. (2) If the temperature is too high, lower the temperature. (3) If there is excessive iron content in the solution, dilute the solution to lower the boiling point to around 120℃. Some sodium ferrous acid will hydrolyze into Fe(OH)3 precipitate. Remove the precipitate, then heat and concentrate to raise the boiling point to the process condition. Alternatively, add glycerol to remove the scum. |
| The oxide film has uneven color and is pitted. | (1) The oxidation time is insufficient; (2) The NaOH content is low. Add more NaOH to raise the boiling point of the solution; (3) The oil removal is not thorough. Strengthen the pre-treatment. |
| Poor adhesion of the oxide film | The content of NaN02 is low. Add more NaN02. |
| The oxide film color is very light, and in some cases, no oxide film is formed at all. | The solution is too dilute. Add the components or evaporate the water to raise the boiling temperature. |
| Local area does not form an oxide film or the oxide film at that location falls off. | (1) The workpieces should be frequently turned over during oxidation to ensure close contact between them. (2) If the degreasing is not thorough before oxidation, enhance the pre-treatment and cleaning procedures. |
| There is a yellowish-green frost on the workpiece. | (1) If the temperature of the oxidation solution is too high, add water to lower the boiling temperature of the solution. (2) If the content of NaN02 is too high, adjust the content of NaN02. |
| After the workpiece has oxidized, when it is treated with soap solution, white spots appear on the oxide film. | The water quality of the soap solution is hard, corrosive or does not get thoroughly cleaned after oxidation. Therefore, the post-oxidation cleaning process should be strengthened. |
| White frost appeared on the workpiece during storage | If the oxidation is not thoroughly cleaned after it occurs, then enhance the cleaning process. |
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