Fastener products include bolts, screws, studs or nuts, washers, etc. Some of them choose blackening process, which can not only meet the appearance and rust prevention requirements of fasteners, but also have low cost and high production efficiency; and good blackening process can reduce quenching stress, making fasteners less likely to break under stress conditions.
Blackening treatment principle: The fasteners to be blackened are placed in a solution tank (145±5℃) of sodium hydroxide (NaOH) and sodium nitrite (NaNO2) oxidants for heating and oxidation, so that a layer of magnetic iron oxide (Fe3O4) film is generated on the surface of the metal fasteners, with a thickness of generally 0.6~0.8μm and black or blue-black.
Purpose of blackening treatment
① Anti-rust effect on the surface of fasteners;
② Increase the beauty and gloss of the surface of fasteners;
③ Reduce the stress effect of the quenching process.
Blackening process
Rack mounting → degreasing with boiling alkali aqueous solution → water cleaning → rust removal with hydrochloric acid solution → water cleaning → heating with boiling water → oxidation → water cleaning → saponification → water cleaning → self-drying → hot oil immersion → oil control → inspection → storage.
Key points of blackening process operation:
1. Rack mounting
To ensure the uniform surface quality of the fasteners to be blackened, the fasteners should be in point or line contact.
2. Degreasing with boiling alkali aqueous solution
First add clean water to the alkali solution tank, then add 15g/L water-soluble sodium hydroxide (NaOH) and heat to boiling point, put the fasteners in the tank, and clean for 5-10 minutes. For non-smooth surfaces, increase the time as appropriate.
3. Water cleaning
Wash with boiling water and rinse with cold water to remove the residual alkali solution on the surface of the fasteners.
4. Rust removal with hydrochloric acid solution
After adding clean water to the acid tank, slowly pour hydrochloric acid into the clean water, the concentration is 14%-18% (tested with a Baume concentration meter), the temperature is ≤30℃, put the fasteners to be pickled in the tank, the pickling time is 3-5min, and the cleaning time is increased as appropriate for non-processed surfaces.
5. Water cleaning
Wash with cold water to rinse off the residual acid on the surface of the fasteners.
6. Boiling water heating
Put the rinsed fasteners in boiling water, heat to above 90℃, and put them in the oxidation tank.
7. Oxidation
① The composition ratio in the oxidation tank: add 600g of sodium hydroxide (NaOH), 60g of sodium nitrite (NaNO2), and 20-40g of trisodium phosphate (Na3PO4) containing crystal water per liter of water.
② Preparation method: First put 2/8 volume of water in the oxidation tank, then slowly add the weighed sodium hydroxide (NaOH), and when the solution reaches about 100℃, add trisodium phosphate (Na3PO4) to make an oxidation solution.
③ Oxidation temperature and time: Put the fastener heated by boiling water into the oxidation liquid with oxidation temperature (145±5℃) (tested with 0-200℃ glass thermometer, calibrated with boiling water), oxidation time 30-50min.
8. Water cleaning
Rinse the residual oxidation liquid on the surface of the fastener with clean water and wait for saponification.
9. Saponification
①Put the rinsed fastener into the saponification tank for saponification, so that a layer of iron stearate film is formed on the surface of the fastener. Use 10 civilian soaps, melt them into 100L water, adjust the concentration and volume according to the proportion, and make saponification liquid.
② Heating temperature and saponification time: saponification liquid temperature 80-90℃ (measured with glass thermometer), saponification time 2-3min. Add a soap to the saponification liquid every day according to the size of the saponified fastener workpiece, and add an appropriate amount of water to maintain the concentration of the solution.
10. Water cleaning
Rinse the soap liquid on the surface of the fastener with clean water and heat it with boiling water.
11. Self-drying
Self-drying from water using its own heat.
12. Hot oil immersion
Immerse the self-dried fasteners without water droplets in 80℃ (measured by glass thermometer) L-AN15 full loss system oil for 1-3 minutes.
13. Oil control
After the oil-immersed fasteners leave the oil tank, control the oil droplets on the surface and around the fasteners.
14. Inspection
① Appearance inspection: Place the fasteners under a fluorescent light source, 300mm away from the naked eye, and observe its surface. If the color is uniform and there is no obvious spotting and rust, it is good, and the color is black or blue-black.
② Oxide film looseness determination: Drop a few drops of 3% copper sulfate solution on the degreased fasteners. If it does not show copper color within 30s, it is qualified.
15. Warehousing
Warehousing after passing the inspection according to quality inspection requirements, otherwise rework according to the original process procedure.
