Blackening treatment of steel pipes

What is the blackening treatment of steel pipes?

Blackening is a common means of metal heat treatment, the principle is to produce a layer of oxide film on the metal surface, in order to isolate the air, to achieve the purpose of rust prevention. Appearance requirements are not high when you can use black treatment, steel parts of the surface black treatment, also known as blue.

Bluing treatment is a chemical surface treatment, its main role is to form a layer of dense oxide film on the surface of the workpiece, prevent corrosion and rust on the workpiece, improve the wear resistance of the workpiece, it is only a surface treatment, will not have any impact on the internal organization, it is not heat treatment, and quenching has a fundamental difference.

Blackening treatment is now commonly used in the traditional alkaline heating blackening and the emergence of late two kinds of normal temperature blackening. But the darkening process at room temperature is not good for low carbon steel. The alkaline blackening is subdivided, and there is a difference between once blackening and twice blackening.The main ingredients of black liquor are sodium hydroxide and sodium nitrite. The tolerance of the temperature required for blackening is larger, about 135 to 155℃ can get a good surface, but the time required is a little longer.

What is the process of blackening treatment?

Process: metal “bluing” liquid medicine

Adopt alkaline oxidation method or acid oxidation method; The metal surface to form a layer of oxide film, in order to prevent corrosion of the metal surface, this process is called “bluing”.

Ferrous metal surface after the “blue” treatment of the formation of oxide film, its outer layer is mainly ferric oxide, the inner layer is ferrous oxide.

Alkaline oxidation method “blue” pharmaceutical solution

1. Formula: sodium nitrate 50 ~ 100 g sodium hydroxide 600 ~ 700 g sodium nitrite 100 ~ 200 g water 1000 g

2. Method: ACCORDING to the formula after measurement, under the condition of stirring, add each material in turn, dissolve, mix evenly.

3. Description:

1) The metal surface must be washed and dried before the “hair basket” treatment.

2) The “bluing” treatment conditions of metal devices are related to the carbon content in the metal. The temperature of the “bluing” liquid and the treatment time of metal devices can be referred to the table below. Carbon content in metal % working temperature (℃) treatment time (min) start to terminate.

3) Analyze the content of sodium nitrate, sodium nitrite and sodium hydroxide in the solution every other week or so, so as to supplement the relevant ingredients in time. Generally, the whole solution should be replaced after half a year of use.

4) Metal “blue” treatment, it is best to rinse with hot soapy water for a few minutes, and then rinse with cold water. Then, rinse with hot water and blow on.

Acid oxidation method “blue” pharmaceutical solution

1. Formula: Phosphate 3 ~ 10 grams of calcium nitrate 80 ~ 100 grams of manganese peroxide 10 ~ 15 grams of water 1000 grams.

2. Method: After measuring according to the formula, add phosphoric acid, manganese peroxide and calcium nitrate in turn under constant stirring condition, dissolve and mix evenly.

3. Description:

1) Metal devices can be cleaned and dried before being “blued”.

2) The protective film obtained by this method is black, its main component is composed of calcium phosphate and iron oxides, its corrosion resistance and mechanical strength are more than the protective film obtained by alkaline oxidation method.

4. “Blue” working temperature is 100℃, processing time is 40 ~ 45 minutes. In the treatment of carbon steel, the content of phosphoric acid in the liquid is controlled at 3 ~ 5 g/L. When treating alloy steel or cast steel, phosphoric acid content should be controlled at 5 ~ 10 g/L. Attention should be paid to periodic analysis of the content of phosphate in the liquid medicine.

5. The cleaning method of metal devices after “bluing” treatment is the same as above.

1) Cleaning;

2) Degreasing: the workpiece must be completely immersed in degreasing liquid; The pH value of degreasing solution is 12-14, the treatment time is 10-30min, and the degreasing powder is added when the concentration of degreasing solution is lower than PH12;

3) Wash;

4) Pickling: acid wash concentration pH value 2-4, treatment time 5-10min;

5) Wash;

6) Blackening: pH value of pool liquid concentration 2.5-3.5, treatment time 10-12min;

7) Wash;

8) Blow dry;

9) Oil.

The purpose of blackening treatment mainly has the following three points:

One is the rust prevention effect on the metal surface.
The second is to increase the aesthetic luster of the metal.
Third, partially eliminate the residual stress of the workpiece.

Blackening treatment of steel pipes