Talk about the application of blackening process o

Talking about the application of steel blackening process at room temperature

1 preface

Steel Blackening at room temperature has the characteristics of energy saving, high efficiency, simple equipment, convenient operation and basically no environmental pollution. It can be applied to workpieces with large area, complex shape and different steel grades. However, due to the difficulty of pretreatment, all kinds of blackening agents have different disadvantages, and the promotion of this technology is very uneven. Based on many years of production practice, the author talks about the process control and improvement measures in production application for reference

2 process control and improvement measures

2.1 process flow

generally, it is chemical degreasing, hot water washing, clean water washing, strong corrosion, secondary clean water washing, chemical degreasing, hot water washing, clean water washing, weak corrosion, secondary clean water washing, blackening at room temperature, secondary clean water washing, soaking dehydrated oil for inspection

2.2 pretreatment

the pretreatment of iron and Steel Blackening at room temperature is mainly oil and rust removal. Chemical oil removal and hydrochloric acid corrosion are generally used, and wet sand blasting is also used [1]. The specific process has been introduced in detail by the author before [2]. The blackening solution at room temperature is acidic. Before blackening, the surface of the workpiece requires full moistening water to effectively ensure the firmness of the black film. Otherwise, no matter how good the blackening agent is, it cannot produce qualified products. Years of practice has proved that whether using degreaser alone, the working principle of the experimental machine single chip microcomputer drives the chip L298N. The square wave signal with continuously adjustable output frequency and duty cycle acts on both ends of the DC electromechanical or the degreaser tank with low concentration of three alkali added degreaser, it should be heated at about 50 ℃ to speed up the degreaser. During operation, if the cross operation of oil and rust removal is carried out in a short time (3 ~ 5min), the work efficiency will be significantly improved

2.3 room temperature blackening

since the domestic development in the mid-1980s, the market commodities have gradually increased. The effect of production and application is generally that the adhesion of the membrane is not as good as that of high-temperature blackening, and the stability of the solution is poor [3]

hh902 iron and steel room temperature blackening agent has made great breakthroughs in these two aspects. The production practice in the factory where the author works for several years has proved that the firmness of the film can obtain a black film with good adhesion without waiting for 24h after effect, which is completely comparable to high-temperature blackening. In terms of solution stability, the working solution produces less white precipitation, and it is not easy to affect the quality of the film, so it does not need to be adjusted frequently; The mother liquor is stored for more than two years without precipitation, and the effect remains unchanged

zt blackening agent acrylic paint is added to the blackening agent, and the two processes are combined into one to simplify production. The adhesion and corrosion resistance of the small film are good, but the blackness of the black film is light blue

solution maintenance and adjustment when blackening at room temperature, the surface of steel is activated to produce Fe2 +, and excessive Fe2 + reacts with se2- to produce white precipitates; In the presence of oxidant, Fe2 + is oxidized to Fe3 + and precipitates are also generated in the reaction, which needlessly consumes se2- in the solution and shortens the service life. The following measures should be taken in production:

(1) the first two clean water tanks in the production line design blackening tank should be made of hard plastic plate or steel plate tank lined with soft plastic plate, so as to prevent the iron tank from rusting and polluting the clean water, and Fe2 + will be brought into the blackening tank in production

(2) blackening utensils, plastic baskets for small batch production, titanium baskets, titanium frames, titanium drums for batch production, etc., to eliminate the production of Fe2 + and consumption of blackening agent during blackening. Although titanium utensils have a large one-time investment, they are acid-base resistant and do not react with blackening agents. Their service life is more than 10 years. Comprehensive calculation also significantly reduces production costs

(3) when the blackening tank is covered and shut down, the solution is tightly covered with plastic to prevent oxygen in the air from entering, slow down the generation rate of Fe3 + and reduce the formation of sediment

regularly check the pH value of the solution with precision test paper, and adjust it if it is higher than 2.6. The simpler way is to filter the solution by natural precipitation first, add mother liquor to make the working solution return to the range of ph2.4, and then continue to blacken

if it is not mass production, the preparation of blackening solution is based on the quantity of parts in each batch and the principle of using up the blackening solution at one time, which not only ensures the uniformity of the black film and good adhesion, but also avoids self consumption due to the storage of the solution

2.4 post treatment

(1) conventional technology requires that dehydrated oil be used for sealing

(2) the corrosion resistance of the film is required to be high. Passivation treatment can be added, and then soaked in dehydrated oil

(3) the requirements of corrosion resistance and wear resistance are higher than the standard of conventional blackened film. After blackening, immerse in acrylate lotion curing agent, and slightly dry naturally and then immerse in dehydrated oil

(4) the film layer requires no oil seal with temperature resistance of 400 ℃, and can be soaked with strong sealant, with good surface brightness and high hardness

in order to reduce the production cost in production, simple post-treatment process can be adopted as far as possible according to the surface technical requirements of blackened parts

2.5 key points in production

(1) pretreatment must achieve full wetting of the workpiece, and no residual oil, rust and oxide skin are allowed

(2) when washing with clean water before blackening, strictly clean the hydrochloric acid solution inside and outside the workpiece, otherwise it is easy to precipitate

(3) the blackening time of the new tank solution at room temperature is controlled to 2 ~ 3min, and the old tank solution can be extended to 5 ~ 10min. Generally, it is better to take the workpiece out of the groove in black and gray (turning to black with time). When the workpiece is out of the groove in black, the film is loose and the adhesion is not ideal

(4) water soluble acrylate lotion should be selected for sealing the organic film. After blackening, it can be directly dipped and coated after cleaning, and the process is relatively simple. Prepare the solution according to the instructions. The lotion and water must be stirred evenly and adjusted until there is no flow mark on the film of the workpiece before it can be put into production

(5) immerse dehydrated oil to get rid of the traditional habit of immersing in oil (immerse slightly in the tank and then out of the tank). There is a process of dehydrating first and immersing the oil phase into the workpiece, and the time is controlled within 3 ~ 8min

3 production cost

the author has made detailed statistics on the batch production of HH902 blackening agent in his factory for several years. The results show that the blackening at room temperature is lower than that at high temperature due to the equipment investment in tank allocation. The daily production cost is 1.93 yuan/m2 for normal temperature blackening and 4.17 yuan/m2 for high temperature blackening, which is reduced by 54%. Considering that some post-treatment workpieces need to be passivated or sealed with acrylate lotion, the total cost should also be reduced by more than 30%

4 discussion on film quality

from the film forming mechanism, it can be seen that the blackening of steel at room temperature is the formation of cuse, Cu2O and other coatings on the surface of steel, which is different from the quality of mature electroless nickel plating and copper plating. Because the film-forming mechanism is not mature and the process parameters are based on experience, it is impossible to produce commercial blackening agents. There are loose and porous films, and high temperature differences in adhesion and wear resistance. Therefore, in order to ensure the quality of blackened film at room temperature, the oil and rust of the workpiece must be completely removed before blackening, so that the bonding force can be guaranteed. After blackening, the process of high-temperature blackening cannot be used. Instead, dehydrated oil that can remove water from the film and has the effect of rust prevention can be used. Wetting and sealing can achieve the corrosion resistance effect of high-temperature blackened film. The abrasion resistance of the commercial blackening agent is still unsatisfactory. Adding a sealing layer of organic film after blackening [4] can meet the wear resistance requirements and improve the corrosion resistance of the film