High-strength fasteners must be dephosphorized prior to heat treatment to prevent phosphate residues from affecting the performance of the heat treatment process.

Phosphate coating during phosphating and cold forming:

During the cold forming of steel, parts such as fasteners, steel wire and pipes usually require phosphating and coating lubricants. The phosphate layer acts as a carrier for the lubricant and acts as a barrier between the workpiece and the tool.

The principle is based on the formation of a phosphate layer, which is firmly fixed to the metal surface and carries a special lubricant. On the one hand, combined with the adhesion and separation effect of the carrier layer, on the other hand, combined with the separation and sliding properties of the special lubricant, the metal forming achieves high quality, excellent surface quality, long service life of the auxiliary tool, and moderate pressing performance.

After the metal is formed, there will be a layer of "phosphate-pressed shading" on the surface of the cold-formed workpiece, consisting of zinc or zinc phosphate phosphate coating and a lubricant (such as metal soap, polymer, molybdenum disulfide, graphite).

The phosphorus removal process avoids many problems, and the alkaline stripping process is preferred:

The heat treatment process after the fastener is cold formed may be affected by the phosphate pressed shading and is more prone to cracking and intergranular corrosion. In addition, during the non-oxidative annealing process, residual phosphate on the surface of the workpiece also causes phosphorus to diffuse into the edge regions. This results in a more prone to stress corrosion cracking and increased failure, especially for high tensile fasteners such as Class 12.9 and higher fasteners. In order to avoid these problems, residual phosphate must be removed prior to heat treatment.

According to DIN EN ISO 898-1, carbon steel and alloy steel fasteners do not allow the detection of white areas with a high phosphorus content. In addition, Volkswagen's "VW 60250 high-strength bolts" stipulate that bolts with a strength class of 10.9 and above must be dephosphorized before heat treatment.

Phosphate pressed shading can be removed by mechanical blasting or chemical methods such as acid or alkaline stripping. Due to the risk of brittle fracture due to hydrogen, it is generally prohibited to use an acidic process, so an alkaline stripping process is preferred.

High-performance phosphorus removal process - KeyKote 400:

Effective removal of the phosphate layer prior to heat treatment is critical to the quality of the workpiece being produced. The improvement of modern equipment and quality requires strong and effective phosphorus removal products. Medeex's KeyKote 400 is the latest phosphate removal product that has been tested and tested.

The simplest alkaline phosphorus removal system is based on a mixture of a strong base (i.e., sodium hydroxide or potassium hydroxide) and a complexing agent. The effectiveness of these simple solutions decreases rapidly with contamination. The operating time and solution concentration quickly must therefore be increased to achieve a uniform good result. These solutions tend to foam easily and then solidify and carry the entire system. Therefore, the conventional alkaline cleaning solution does not solve the problem. Since the lubricant accumulates in a large amount in the tank, it tends to solidify or become a gel when cooled to below 65 °C.

The use of the KeyKote 400 is not easy to produce these unnecessary effects, and can also improve the quality of the phosphorus removal process. At the same time, another advantage of the KeyKote 400 is its ability to cope with accumulated lubricants and metal soaps without excessive foaming. In addition, this alkaline system also synchronizes cleaning components.

The metering pump can be remotely controlled to improve process control:

Phosphorus removal processes are typically operated in drum immersion equipment, immersion in a rotating system or Archimedes screw system, operating at temperatures above 65 °C. The type of equipment is highly dependent on the desired output and the surface structure of the workpiece. With drum immersion equipment, the operating time is usually more than 10 minutes, immersion in the rotating system takes 3 – 6 minutes (see Table 1), and the operating time of the screw system is usually 3 – 10 minutes. Immersion in the rotating system is very efficient, but requires high-performance phosphorus removal products such as KeyKote 400, which can remove phosphorus in a short time, with less foam and less impact on wastewater.

The amount of the chemical concentrate added can be remotely operated and controlled by a metering pump. Metering pumps can be adjusted based on productivity, average product usage, and solution concentration analysis. This means that equipment operators are rarely exposed to chemical concentrates, which increases operator safety and improves process control.

The oil can be removed from the solution. Conventional oil slick separators have proven to be very effective in removing oil slicks, and even easier installation of the oil slicker directly into the treatment tank can effectively remove oil.

Application of KeyKote 400 as a liquid concentrate:

The KeyKote 400 supplied by Medes is a liquid concentrate based on a special blend of potassium hydroxide and weak complexing agents that can be processed in commonly used wet chemical wastewater treatment plants. Depending on the requirements, this product is used in conjunction with surfactants such as KeyKote 501C (see Table 2) to increase emulsifying capacity.

KeyKote 400 has excellent absorption capacity for oils, fats and metal soaps at very low concentrations and has a very long solution life. KeyKote 400 also prevents metal soap lubricants from "contaminating" working fluids to solidify (gel) at lower temperatures (downtime). Due to its special formulation, the KeyKote 400 is ideal for rollers, immersion rotary, and even spiral systems.

The operating parameters and system settings depend on the specific equipment and the workpiece to be dephosphorized.

Quality control with "blue test":

Phosphorus removal solutions are typically analyzed by simple acid/base titration. The ratio of metal soap to oil stain can be determined by an "acid decomposition" reaction. The phosphorus removal solution sample was mixed with the inorganic acid, and after the reaction, the floating oily substance was measured and the volume percentage was recorded. This gives the concentration of the contaminants. In addition, the concentration of zinc and complexing agents can be analyzed and added separately as needed to extend bath life.

The phosphorus removal performance was determined by a so-called "blue test". The clearly visible surface of the workpiece is immersed in the chemical solution and the solution reacts with the residual phosphate to form a blue color. The intensity of blue is proportional to the amount of phosphate present (see figure below).

The control of the washing solution is also very important, because the phosphate solution that is taken out contains phosphorus and will also dry on the workpiece, affecting the accuracy of the test.

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