【2026 Mechanical Parts】High-strength bolts are commonly treated with blackening, while hot-dip galvanizing is less common
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This post was last edited by The one on 2026-6-9 at 16:44; heat-dip galvanizing is rarely used for high-strength bolts of grade 8.8 and above； Most conventional finished products with a strength of 10.9 or 12.9 grades on the market are in their natural black color; when strict corrosion resistance is required, Dacromet coating is preferred. Both serve the purpose of corrosion and rust prevention, and hot-dip galvanizing is widely used – so why can’t high-strength bolts be used casually? There is only one core reason: the high-temperature process of hot-dip galvanizing directly leads to a decrease in the mechanical strength of high-strength bolts, resulting in a significant decline in their performance and failure to meet the design requirements.
This post was last edited by The one on 2026-6-9 15:51. I. Strength of high-strength bolts: The excellent properties of bolts are not inherent; they are achieved through specialized heat treatment processes. • Molding process: Quenching + high-temperature tempering heat treatment is employed. • Internal structure: It forms a tempered sorbite microstructure, this metastable special structure endows the bolt with excellent strength and toughness.
II. The high temperatures used in hot-dip galvanizing directly reduce the grade of bolts. The temperature during hot-dip galvanizing is typically between 450°C and 500°C, and this range falls within the critical zone for the properties of high-strength bolts. Three types of damage occur as a result, and these damages are irreversible: 1. The microstructure of the steel becomes coarser, leading to a significant decrease in strength. When the temperature exceeds the bolt’s original tempering temperature, carbon compounds within the steel continue to precipitate and grow, causing the grains to become larger over time. The hardness, yield strength, and tensile strength of the bolts all decrease simultaneously; bolts originally rated at 10.9 grade no longer meet the mechanical requirements of that grade after hot-dip coating.
This post was last edited by The one on 2026-6-9 15:08. 2. Surface decarburization leads to a significant reduction in fatigue performance; in high-temperature environments, the carbon elements in the surface layer of bolts are oxidized and lost, resulting in a lower carbon content in that surface layer. Bolts lose their fatigue resistance, wear resistance, and resistance to galling; under long-term exposure to vibrational stresses, they are highly prone to cracking and thread failure.
3. It hides a serious safety risk of hydrogen embrittlement fracture: Pre-galvanizing requires pickling to remove rust, and this process generates a large amount of hydrogen atoms that penetrate into the steel of high-strength bolts. Ultra-high strength steels of grades 10.9 and 12.9 are extremely sensitive to hydrogen embrittlement; they suffer from delayed fracture, experiencing sudden breaks without any warning, which causes serious safety accidents in engineering equipment.
This post was last edited by The one on 2026-6-9 15:51. III. Comparison of performance loss in high-strength bolts across different temperature ranges ✅ Safe range: ≤400°C – The impact of heat is minimal, and the performance can recover to over 80% after cooling.
⚠️ The critical temperature range is 400°C to 600°C: mechanical properties deteriorate sharply at this temperature, with the strength dropping to only 70%–78% of its value at room temperature; the grade falls below 8.8.
❌ The failure range is above 600℃: the structure loses its functionality essentially, and at temperatures of 700℃ the residual strength is only 3%–6%, rendering it unusable.
When carrying out anti-corrosion treatment on high-strength bolts, high-temperature processing methods should be used with caution, as this is also a way to avoid the risks of microstructural damage and structural failure.
IV. Reasons why high-strength bolts available on the market often appear blackened. In daily market circulation, bolts of grade 10.9 and 12.9 mostly have a blackened appearance, as this is the standard anti-corrosion method used in the industry. The blackening treatment process is carried out at normal or low temperatures throughout, without causing any damage to the microstructure of the material after quenching and tempering; it preserves the bolt’s original mechanical strength level intact. It has a low cost and fast processing time; it does not affect the accuracy of the threads or the clamping force. It can meet the basic rust prevention requirements for indoor drying, assembly with conventional equipment, and connection of ordinary steel structures. Combining practicality with good cost-effectiveness, it has become a standard component in the regular shipment of high-strength bolts. The disadvantages are also obvious: the black coating has weak rust-resistant properties, its service life is short in humid, outdoor, or corrosive environments, and it is not suitable for applications requiring high levels of corrosion protection.
The performance of high-strength bolts relies on a specific microstructural pattern; high-temperature environments can irreversibly damage this internal structure, leading to a decrease in strength and creating safety hazards.