What is blackening？

Introduction

In machining, blackening (also known as black oxide or bluing) is a surface treatment process used on metal parts to form a thin, protective black or blue-black oxide layer. The primary purpose of this treatment is to improve corrosion resistance, enhance appearance, and in some cases, provide slight lubricity.

Common methods Common methods of blackening include traditional alkaline heating blackening and later room temperature blackening. However, the room temperature blackening process is not very effective for low carbon steel. Alkaline blackening is further divided into single blackening and double blackening. The main components of blackening liquid are sodium hydroxide and sodium nitrite. The temperature required for blackening has a wide tolerance. A good surface can be obtained between 135-155℃, but the time required is somewhat longer.

Effect

(1) Blackening is safe and does not require electricity. Alkaline high temperature blackening requires 100% electricity.

(2) Improve work efficiency: a total of 1-2 hours.

(3) Blackening cost is low, equipment is simple, and operation is convenient; blackening time is strictly controlled.

(4) Strong process adaptability: It solves the problem that ductile iron cannot turn black. A blue anti-rust treatment process for chamfering and flange of bearing rings. After heat treatment, the bearing rings are degreased and then color-changed in a mixed solution of sodium hydroxide, sodium nitrite, sodium nitrate and water in a ratio of 2-3:1, and the rest is water; the amount of water added is suitable for a temperature of 135℃-145℃; the time is 15-25 minutes; the rings are taken out and rinsed with clean water and then passivated. The passivation treatment uses potassium dichromate solution as the filling liquid with a concentration of 12%-18%; 1-2 minutes at room temperature; after passivation treatment and drying, put it in 105℃-120℃ engine oil or anti-rust oil for 1-3 minutes, take it out after the bubbles completely disappear, and park it for 10-15 minutes before inspection. The oxide film generated by this process has stable performance and can protect the chamfering and flange of the ring from rust for a long time at room temperature, and the appearance quality of the bearing is significantly improved.

A3 steel is better blackened with alkaline.

In actual operation, it is necessary to pay attention to the quality of rust removal and oil removal before blackening, as well as the passivation oil immersion after blackening. The quality of blackening often varies due to these processes.

Process steps

1. Cleaning;

2. Degreasing: The workpiece must be completely immersed in the degreasing liquid; the degreasing liquid concentration pH value is 12-14, the processing time is 10-30min, and it is shaken up and down several times every 3-5 minutes. When the liquid concentration is lower than pH12, add degreasing powder;

3. Water washing;

4. Pickling: the pickling liquid concentration pH value is 2-4, and the processing time is 5-10min;

5. Water washing;

6. Blackening: the pool liquid concentration pH value is 2.5-3.5, and the processing time is 10-12min;

7. Water washing;

8. Blow dry

9. Oiling.