Steel surface treatment: application of new environmental protection blackening process

What is blackening?

Blackening is a common method of chemical surface treatment. The principle is to produce an oxide film on the metal surface to isolate the air and achieve the purpose of rust prevention. Blackening treatment can be used when the appearance requirements are not high. The surface blackening treatment of steel parts is also called bluing.

Common method

The commonly used methods for blackening treatment are the traditional alkaline heating blackening and the late normal temperature blackening. However, the blackening process at room temperature is not very effective for low carbon steel. Alkaline blackening is subdivided, and there is a difference between blackening once and blackening twice. The main components of blackening liquor are sodium hydroxide and sodium nitrite. The temperature required for blackening is relatively large, and a good surface can be obtained between 135-155°C, but the time required is somewhat long.


  • (1) Blackening is safe without electricity, and 100% electricity is required for alkaline high-temperature blackening.
  • (2) Improve work efficiency: 1-2 hours in total.
  • (3) The blackening cost is low, the equipment is simple, and the operation is convenient; the blackening time is strictly controlled.
  • (4) Strong process adaptability: The problem that nodular cast iron cannot be blackened is solved.

A bluing and rust-proof treatment process for the chamfers and ribs of bearing rings. After the heat treatment, the bearing ring is degreasing and degreasing, and then discolored in a mixed solution of sodium hydroxide, sodium nitrite, sodium nitrate and water. The ratio is 2-3:1, and the rest is added with water; the amount of water added is based on temperature It is suitable at 135℃-145℃; time 15-25 minutes; take out the ferrule and rinse it with clean water and then passivate it. Use potassium dichromate solution as the filling liquid for passivation treatment, the concentration of which is 12%-18%; 1-2 minutes at room temperature; after passivation treatment and drying, put it in engine oil or anti-rust oil at 105°C-120°C for 1-3 minutes, take it out after the bubbles disappear completely, and check after leaving for 10-15 minutes. The oxide film produced by this process has stable performance, can protect the chamfer of the ferrule for a long time at room temperature, and the ribs will not rust, and the appearance quality of the bearing is significantly improved.
A3 steel is better with alkaline blackening.
In actual operation, it is necessary to pay attention to the quality of derusting and degreasing before blackening of the workpiece, and the passivation immersion oil after blackening. The quality of blackening often changes due to these processes.

Process steps

  • 1. Cleaning;
  • 2. Degreasing: The workpiece must be completely immersed in the degreasing solution; the concentration of the degreasing solution is 12-14, the treatment time is 10-30min, shaking up and down several times every 3-5 minutes, and the degreasing powder is supplemented when the concentration of the chemical solution is lower than ph12;
  • 3. Wash with water;
  • 4. Pickling: the pH value of the pickling solution is 2-4, and the treatment time is 5-10min;
  • 5. Water washing;
  • 6. Blackening: the pH value of the pool solution is 12.5-13.5, and the treatment time is 10-12min;
  • 7. Water washing;
  • 8. Blow dry
  • 9. Oiling.

Application of new environmental protection blackening process

The alkaline bluing process has a history of more than 30 years and has played an important role in the rust prevention of mechanical parts. However, with the development of modern environmental protection heat treatment industry, the “three wastes” produced by this traditional bluing process pollute water and soil. The irritating odor has a stimulating effect on the skin and respiratory mucosa, and the problems of low production efficiency have become more prominent.
In order to realize “green manufacturing”, through collection, investigation and analysis, it is believed that the steam bluing is environmentally friendly, but the heating temperature is high, 60% of the number of parts are not applicable, and the waste heat blackening technology is environmentally friendly and has wide applicability. Further research on its application is of great significance.

Test process

The principle of the blackening of iron and steel waste heat is to use the tempering waste heat during the heat treatment process to form a layer of black polymer material protective film on the metal surface according to the thermal polymerization reaction mechanism to achieve the purpose of blackening, rust and corrosion prevention of metal workpieces, Two blackening agents produced by a certain company are selected to test the reliability and applicability.

Test plan

Select five typical parts for the blackening test, and the scheme is shown in Table 1.

Program Blackening temperature/℃ Experimental parts Experimental steps Blackening agent Remarks
1 420
1) Pin shaft
Manufacturing route: sawing material → quenching and tempering → finishing → bluing
2) Connectors and plugs for reducers
Manufacturing route: sawing material → quenching and tempering → finishing → bluing
3) Bolts
Manufacturing route: sawing → upsetting cap → finishing → quenching and tempering (quenching) → bluing, the rod part is the machined surface
1) The blackening agent is mixed with water to a concentration of 10%
2) Heat preservation 1-1.5h
3) Dip into the blackening liquid, and the remaining temperature of the black liquid will be 70-90℃
4) Dry or dry at 120-150℃
5) Oil seal, curing for 24h
1) Dry the pins after turning black, and dry the other parts
2) Supplementary experiment of Scheme 50 and Scheme 3
2 280 2#
3 560 1#,2#
4 360
5 560 1#

Test results

(1) Brightness

360℃, 420℃ blackened parts are bright and beautiful in appearance, dark black, 1# blackening agent is brighter than 2#, as shown in Figure 1a~d, while alkaline bluing is gray-black and low gloss , As shown in Figure 1e. The blackening at 280°C is light and the surface is flowery, and the blackening at 560°C is deep, but there is a little film peeling off in some parts, as shown in Figure 1f.

20200803085354 71884 - Steel surface treatment: application of new environmental protection blackening process

Figure.1 Brightness comparison of blackening agent

(2) Firmness
The adhesion between the deposition layer of the blackened parts and the substrate is required to have firmness. After the surface protective film is dry, wipe it with cotton yarn. The blackened and blued parts do not show metallic luster. They are qualified products, but blackened parts The firmness is high, as shown in Figure 2a and b.

20200803085913 30115 - Steel surface treatment: application of new environmental protection blackening process

Fig.2 Bolt after blackening

(3) Rust resistance
The rust resistance and corrosion resistance of the workpiece are mainly affected by the coating and sealing of the organic deposition film on the surface. Choose one black bolt and one blue bolt at 420℃, and pass through the glass after immersing in tap water for 24 hours. It was observed that the two parts were not rusty. After 48 hours, the parts were picked up and dried for observation. The blackened parts had no rust, and the blue ones had slight rust, as shown in Figure 2c.
Through the test, it can be seen that the waste heat blackening process is reliable, and the use temperature is completely suitable for the tempering of our company’s parts at 360~520℃. The process has no irritating smell, does not need pickling or washing, and the process is environmentally friendly and safe.

Batch test to verify the feasibility

Process flow

The first process suitable for finishing the parts that are directly blackened: material preparation → cleaning → heating → blackening → drying (drying) → oil seal. It is suitable for parts that require quenching and quenching and tempering treatment technology after finishing. Because of tempering above 260℃, surface oil can be removed, and cleaning is not necessary after quenching. Process 2: material preparation → cleaning → quenching → tempering (pre-cooling) → Blackening → drying (drying) → oil seal.

Process configuration

For parts that are blackened according to process two, the furnace must strictly ensure compliance with the carbon potential during quenching and heating to prevent oxidation. Our company has multiple controllable atmosphere multi-purpose furnaces and their matching tempering furnaces, which only need to be equipped with blackening liquid tank and Its cooling and circulation system ensures that the operating temperature is ≤45℃, and one oil tank (with oil control bracket) forms an assembly line operation with the multi-purpose furnace to realize the blackening process operation. Its advantages are simplified configuration and improved efficiency, as shown in Figure 3.

20200803090412 73457 - Steel surface treatment: application of new environmental protection blackening process

Fig.3 Blackening process configuration

Blackening temperature

According to process one, heat the black parts to 400~460℃ for 60~90min to blacken; according to process two, the parts that are blackened will be pre-cooled to 400~500℃ according to the amount of heat storage, and the temperature will return to black. It can be blackened directly during fire. The principle of temperature selection is: use a higher temperature for small parts and a lower temperature for large parts, adjust the tempering temperature with the best blackening effect, and select the 1# blackening agent with excellent brightness for the test.

Blackening liquid concentration

Water and blackening agent are proportioned at a concentration of 10%. During use, the blackening liquid concentration is judged by observing the surface conditions of the blackened parts. If the blackening degree is light and there is slight rust, the concentration is low, and the blackening agent needs to be added; If there are mottling or uneven thickness of the film, the concentration is too high and water needs to be added. If the effect of the first blackening is not good, the second blackening can be done.

Drying, soaking in oil

Film formation is an important part of the blackening process. Only when the blackening film is formed and cured can the protective and decorative effects be truly achieved. The residual temperature of the black liquor from the parts is controlled to be 70~90℃. Due to the dry environment in the north, there is no water or parts with little water. Place them in a ventilated environment or blow them with an axial fan. The principle is that the ones that can be dried naturally will not be dried. Parts that cannot be dried can be dried at 120~150℃, and then immersed in N10 total loss system oil. The whole basket of parts are placed on the oil control rack for 24 hours to complete the oil sealing process, which increases the surface brightness and film firmness, and reduces residual oil .

Furnace installation method

Pin shafts and stud bolts must be separated by wire mesh and have a certain gap to avoid white spots or white lines due to a large amount of accumulation. For single-head bolts, the furnace is loaded in bulk, the furnace load is controlled to be less than 2/3 of the height of the material frame, and the higher outlet water temperature is controlled to facilitate the evaporation of water. This is an innovation in the furnace loading method.
The above process parameters were formulated, and more than 5000 pieces were trial-produced in batches. The effect is shown in Figure 4.

20200803091102 82430 - Steel surface treatment: application of new environmental protection blackening process

Figure.4 Parts after trial production

Promote application and realize “green manufacturing”

After the successful batch trial production of the waste heat blackening process, four problems were solved during the promotion and application process, thereby replacing the alkaline bluing and realizing “green manufacturing”.

The problem of scale after blackening

After the finishing bolts are blackened, serious oxide scale appears on the surface, which affects the blackened appearance. After reverse inspection, tracking and analysis, for the problem of heating and oxidation caused by low carbon potential in the quenching heating furnace, alkaline blueing must be changed and pickled Remove the traditional concept of oxide scale, strengthen process inspection; to cause secondary pollution to the oxide scale produced by high temperature tempering of other parts, clean the surface oxide scale of the material frame or other high temperature tempered parts before installing the furnace to prevent secondary pollution, Process management must be strengthened. If uncontrollable factors form oxide skin during the heating process, it can be washed with washing oil before turning black.

The problem of raw material corrosion and scale

There are many types of blackening. In addition to the processed surface, there are unprocessed surfaces such as plates, bars, castings, etc. The original rust or oxide skin affects the blackened appearance and anti-rust effect. For parts with low precision, use machinery The shot blasting method is used to clean, and the parts with high precision are removed by mechanical processing.

The problem of water accumulation after blackening

Due to the diversification of the structure of the blackened parts, adopting a reasonable furnace installation method is the key to uniform blackening film and reducing the drying process. The principle of furnace loading is summarized as follows: install the furnace in a way that facilitates the flow of moisture. Pin shafts with holes, the holes are installed downwards, and the moisture can flow out after blacking, and can be dried naturally; the oil plugging holes are stacked neatly downwards; the plate lids are installed vertically to reduce water accumulation and dry in time.

The blackening effect is not ideal

In production applications, it is found that some workpieces have unsatisfactory color after they turn black, and the black film comes off during the trial run of the complete set of products, or the thin flat parts or small screws such as the anti-return pad with a thickness of 3~5mm. Uneven, variegated.
After consulting data and practical verification, reduce the staying time of the workpiece in the groove or shorten the moving distance and shake slowly to promote film formation and improve the curing effect; for a small number of workpieces with thickness or diameter ≤ 4mm, the configuration concentration is 15%~20% The blackening liquid is blackened alone, and the above two measures have achieved good results.

Benefit output

Environmental protection and safety

The waste heat blackening process technology does not need alkaline “black boiling”, no three wastes discharge, no irritating odor, environmentally friendly and safe operation, simplified process flow, strong applicability, can replace alkaline bluing, and realize the “green” of the parts anti-rust technology manufacture”. During the production process, the working fluid tank should be maintained in a timely manner, the aging solution should be filtered, the blackening tank dirt should be cleaned regularly, and the blackening liquid MSDS safety technical requirements should be handled.

Production efficiency

According to statistics, direct blackened parts accounted for 26.2% of the total blackened parts, quenched and tempered parts accounted for 73.8%, 3 shifts a day, output 0.6t/furnace, while alkaline blue production was 0.2t/basket, and the maximum output was 0.8 t/shift, one shift per day; the two tempering furnaces can be used for blackening and tempering. The efficiency is increased by 200% after calculation.

Production cost

Calculated based on the maximum production of 296t/year, the manufacturing cost is 717.3 yuan/t, the blackening is 665.8 yuan/t, which reduces the cost by 51.5 yuan/t and saves 15,000 yuan/year; parts do not need to be transferred from the multi-purpose furnace section to the In the blue section, it is estimated that the transshipment cost will be saved by 10,000 yuan/year, and the above two items will save 25,000 yuan/year.

Concluding remarks

  • 1) The blackened appearance is bright, the color is dark black, the appearance, firmness, and rust resistance are better than alkaline bluing. The process is environmentally friendly and safe. It can replace the alkaline bluing to promote and apply, and realize the “green manufacturing” of anti-rust technology. , The efficiency is increased by 200%, and the cost is reduced.
  • 2) The tempering temperature of the waste heat blackening process technology is completely suitable for all parts of our company. The blackening effect of low temperature tempering (≤300℃) is poor, and the blackening effect of medium temperature tempering is good. The high temperature tempering parts need to take pre-cooling measures, and the temperature is selected It is related to the heat storage of the parts.
  • 3) In actual production, the blackening liquid concentration can be judged by observing the appearance of the blackening parts. The concentration required for small parts is higher. The color can be improved by shortening the residence time of the blackening liquid tank, controlling the movement mode, and appropriately increasing the blackening residual temperature. effect.
  • 4) Install the furnace in a way that is conducive to the outflow of water. If it can be dried naturally, it will not be dried. If there is too much water to dry, it should be dried in time.
  • 5) Use mechanical shot blasting and mechanical processing methods to remove the original surface rust or oxide scale, and improve the blackening and rust resistance.
  • 6) The blackening tank needs to be equipped with cooling and circulation systems, and the blackening liquid tank needs regular maintenance.

Source: China Fasteners Manufcturer – Yaang Pipe Industry (

(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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