In the industrial manufacturing field, especially for high-strength steel fasteners such as Flanged Hexagon Head Bolt Screws, Hexagon Head Screws with Washers, and various Hexagon Head Fasteners with Built-in Washers, surface blackening is a common and economical rust prevention and decorative process.
Blackening (also known as bluing or oxidation treatment) involves a chemical reaction to create a dense black or bluish-black oxide film on the steel surface, improving the metal's corrosion resistance, aesthetics, and assembly performance. Based on different temperatures and chemical reaction principles, blackening processes are mainly divided into three types: alkaline high-temperature blackening, room-temperature blackening, and residual heat blackening.
Alkaline High-Temperature Blackening Technology
Alkaline high-temperature blackening is a traditional and mature method for steel surface treatment. The principle is that in a strong alkaline solution containing an oxidant (such as sodium nitrate or sodium nitrite), at a high temperature of 135–145°C, steel reacts with the oxidant to form a black magnetic iron oxide film (Fe₃O₄).
This film is typically 0.5–0.8 μm thick, bonds firmly to the metal substrate, and has a bright, uniform surface. It is suitable for various hexagon head flange bolts, bolts with flanges and hexagon heads, and other high-strength fasteners.
Advantages:
The film is hard and has good wear resistance.
The process equipment is simple and low-cost.
It does not change the surface precision and dimensions of the bolt.
Disadvantages:
High energy consumption; operating temperature reaches up to 150°C;
Contains harmful substances (such as nitrites);
Wastewater requires strict treatment, resulting in a significant environmental burden.
Therefore, this process is still widely used in medium- and high-strength fasteners, but it is gradually being replaced by more environmentally friendly room-temperature and waste-heat blackening processes.
Room Temperature Blackening Technology for Steel Fasteners
With the development of green manufacturing concepts, room temperature blackening technology has been rapidly promoted due to its energy-saving and low-pollution advantages. This technology is suitable for small to medium-sized fasteners such as Hex Head Screws with Washer Assembly and Hexagon Head Screws with Integrated Washer.
The main film-forming systems of room temperature blackening solutions include:
Copper-Selenium system
Copper-Sulfur system
Copper-Manganese-Sulfur system, etc.
1. Copper-Selenium System Blackening Mechanism
The iron substrate reacts with copper ions to precipitate copper. The surface copper reacts with selenides to form a black copper selenide film (Cu₂Se), which is then oxidized to form a dense film layer with a uniform deep black color.
2. Non-Selenide System Mechanism
A film is formed through black coatings such as Cu₂O and CuS, resulting in good anti-corrosion and decorative effects.
3. Room Temperature Blackening Process
Degreasing → Water Washing → Rust Removal → Blackening Treatment → Water Washing → Passivation/Sealing → Rust-Inhibiting Oil Immersion
The film thickness is approximately 0.2μm, with a uniform and glossy surface. Post-treatment processes (such as passivation or water-based sealing) are crucial for improving rust prevention performance.
Advantages:
Low energy consumption, operating temperature at room temperature;
Simple process, wide applicability;
Uniform film color, strong decorative effect.
Disadvantages:
Thin film, moderate wear resistance;
The stability and environmental adaptability of the blackening solution need to be controlled.
Residual Heat Blackening Technology
Residual heat blackening is a rapidly developing new process in recent years, particularly suitable for fasteners such as Hexagonal Head Screws Featuring Washers and Hexagon Flange Head Machine Screws, which retain a relatively high temperature after heat treatment.
This technology utilizes the residual heat from the workpiece's tempering (160–500℃) to react with organic film-forming substances in the blackening solution, forming an organic polymer composite film. This film offers excellent corrosion protection and decorative effects.
Main Features:
Thin film (≤0.2μm), yet with excellent rust prevention;
It can be combined with the tempering process for continuous, automated production.
No wastewater discharge, environmentally friendly, and energy-saving;
Suitable for carbon steel, alloy steel, and heat-treated parts.
Precautions:
Workpieces must be kept at an appropriate temperature and dry.
Small-sized bolts will have poorer blackening effects if the heat is insufficient.
The wear resistance of the film is slightly lower than that of Fe₃O₄ films.
Innovation and Development Trends of Blackening Technology
For high-strength fasteners such as Flamed Hex Head Bolts and Hex Head Screws with Washers, future blackening technology is developing in the following directions:
Improving film adhesion and corrosion resistance, extending service life;
Standardizing and environmentally friendly blackening solutions to reduce harmful substances;
Developing automated, continuous blackening production lines, linking them with heat treatment, cleaning, and testing
Introducing nanofilm formation and composite protection technologies to enhance film density and aesthetics.
With the advancement of green manufacturing and lean production, modern blackening processes will continue to play a crucial role in the manufacturing of Hexagon Head Screws with Washers, Hexagon Flange Head Machine Screws, and other high-precision fasteners.
Conclusion
Blackening technology is not only a surface protection method but also a key process for improving quality and competitiveness in fastener manufacturing. From alkaline high-temperature to room temperature, and then to waste heat blackening, different technical routes adapt to different product structures and performance requirements.
In the production of products such as Flamed Hexagon Head Bolt Screw, Hexagon Head Flange Bolt Screw, and Washer-equipped Hexagon Head Screws, the appropriate selection of blackening processes can not only improve corrosion resistance but also enhance appearance consistency and assembly smoothness, providing solid technical support for high-end manufacturing.
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