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Principle and process flow of Bluing (blackening) on steel surface

Bluing (blackening) principle

In order to improve the anti rust ability of steel parts, the surface of steel parts is oxidized to dense and smooth ferric oxide (Fe3O4) with strong oxidant. The Fe3O4 thin layer can effectively protect the steel parts from oxidation.

20201204134000 37520 - Principle and process flow of Bluing (blackening) on steel surface

Fe3O4 oxidized at high temperature (about 550 ℃) is sky blue, which is called bluing treatment. Fe3O4 oxidized at low temperature (about 130 ℃) is dark black, which is called blackening treatment. In the ordnance industry, bluing is commonly used; in industrial production, blackening is commonly used.
Whether iron can be oxidized into dense and smooth Fe3O4 depends on the selection of strong oxidant. The strong oxidant is composed of sodium hydroxide (NaOH), sodium nitrite (NaNO2) and trisodium phosphate (Na3PO4). When bluing, their molten solution is used to treat the workpiece; when blackening, their aqueous solution is used to treat the workpiece. The common components of blackening solution are shown in Table 1.

Table 1

Name of raw material

NaOH

NaNO2

Na3PO4

H2O

Quality composition (%)

33

10

2

55

The concentration of Fe → ma2feo2 → na2fe2o4 → Fe3O4 is 1.4g/cm3 and the boiling point is 130 ℃. Under the action of this solution, the oxidation process of iron is as follows:
The production practice shows that in order to obtain bright and compact Fe3O4 film, the ratio of sodium nitrite to sodium hydroxide should be kept between 1:3 and 3.5.

Operation process of Bluing (blackening)

Workpiece clamping → degreasing → cleaning → pickling → oxidation → cleaning → saponification → hot water boiling → inspection.

Workpiece clamping

Special fixture or spreader should be designed according to the shape and size of the workpiece. The purpose is to leave enough gap between the workpieces and not to contact each other, so that each workpiece can be completely immersed in the oxidation solution to be oxidized.

Degreasing

The purpose is to remove the oil stain on the surface of the workpiece. After the frame work (bluing and blackening are the last process), it is inevitable that oil stains will not be left on the surface of the workpiece, especially when antirust oil is used to prevent rust between working procedures. Any oil pollution will seriously affect the formation of Fe3O4, so it must be removed before bluing and blackening. The commonly used oil removal solution formula is shown in Table 2.

Table 2

Name of raw material

NaOH

NaNO2 

Na3PO4

Na2SiO3 

H2O 

Quality composition (%)

7

3

3

2

85

Pickling

Heat the degreasing solution to 80-90 ℃, and then immerse the workpiece for about 30 minutes. If there is more oil, the oil removal time must be extended, subject to the thorough degreasing.
The purpose of pickling is to remove rust on the surface of workpiece. Because rust will prevent the formation of a dense Fe3O4 layer. Even if there is no rust on the workpiece, acid washing should be carried out, because it can further remove the oil stains. Moreover, acid pickling will increase the activation energy of the molecules on the surface of the workpiece. Due to the oxidation of the next process, thicker Fe3O4 layer can be formed.
The pickling solution is usually 10% – 15% sulfuric acid solution, and the temperature is 70-80 ℃. Immerse the workpiece in sulfuric acid solution for about 30min, 20min for less rust and 40min for more rust.

Oxidation

Oxidation is the main process of Bluing and blackening. Whether the Fe3O4 film is compact, smooth and thick depends on the oxidation stage. The temperature of Bluing is 550 ℃. The temperature of blackening solution is 130 ~ 145 ℃. The immersion time was 50-80 min. High carbon steel with high carbon content has faster oxidation rate and shorter immersion time.
For low carbon steel with low carbon content, the oxidation rate is slow and the immersion time needs to be longer. Alloy steel, especially high alloy steel, has a certain amount of residual austenite on the surface of the workpiece, which has strong resistance to alkali solution, and is not easy to form Fe3O4, because the surface immersion time needs to be longer. Table 3 lists the temperature and immersion time of the solution when the steel is oxidized (blackened).

Table 3

Steel grade

Inlet temperature ( ℃ )

Outlet temperature ( ℃ )

Processing time (min)

High carbon steel

120125

125130

40

Low and medium carbon steel

125130

130135

50

Chromium silicon steel

125130

130140

60

High alloy steel

130135

135145

70

high speed steel

135140

140150

80

Saponification

The normal color of the solution is white. If the solution is red or brown, it means that the concentration of sodium nitrite is too high, the composition of the solution should be adjusted in time. If the solution is green, it means that the iron ion concentration is too high, and the solution should be updated in time.
Saponification is to soak the workpiece with soap solution at a certain temperature. The purpose is to form a layer of iron stearate film to improve the corrosion resistance of the workpiece. The commonly used saponification solution concentration is 30 ~ 50g (soap) / L. The saponification solution is heated to 80-10 ℃, and the oxidized workpieces are soaked for about 10 minutes.

Inspection

After oxidation, the workpiece should be inspected to see whether the Fe3O4 layer is qualified. Take three pieces randomly and soak them in copper sulfate solution of 2% concentration for 20s. Those who do not turn black are qualified. If more than one of the three pieces is unqualified, the whole groove workpiece is regarded as unqualified and needs to be oxidized again to deepen the thickness of Fe3O4.

Source: Network Arrangement – China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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