Why do high-strength bolts need heat treatment processes?

High-strength bolts, as key connecting components in modern industry, their performance directly affects the structural safety in fields such as Bridges, buildings, and machinery. Heat treatment process is precisely the core means to endow bolts with high strength, high toughness and fatigue resistance.

I. Why must high-strength bolts undergo heat treatment?

At the connection points of the steel box girders of the Hong Kong-Zhuhai-Macao Bridge, each 10.9 grade bolt needs to withstand a tensile force of up to 1,000 mpa - equivalent to hanging 10 small cars over an area the size of a fingernail. Ordinary bolts that have not undergone heat treatment will present the following conditions under this working condition:

Plastic deformation leads to the attenuation of preload (experimental data show that the attenuation rate exceeds 40%).

The risk of stress corrosion cracking increases by 3 to 5 times

The fatigue life is less than one-tenth of the design requirement

Heat treatment precisely regulates the microstructure of metals, enabling bolts to acquire three core properties:

Strength leap: Quenching and tempering treatment increases tensile strength by 200-400%

Toughness guarantee: Tempered sorbite structure enables the impact energy to reach over 54J

Stress balancing: Eliminate residual stress from cold working to prevent delayed fracture

Ii. Four Key Processes of Heat Treatment Technology

Quenching: Creating a "steel body"

Heat to 850-950℃ to achieve complete austenitization

Rapid oil quenching forms martensite (the cooling rate needs to be controlled at 80-120℃/s)

The actual measurement of a certain aerospace fastener enterprise shows that the hardness after quenching has jumped from HRB70 to HRC50

2. Tempering: An art that combines hardness and softness

Temper at 460-540℃ for 2-4 hours

Martensite transforms into tempered sorbite (with hardness stabilized at HRC32-39)

Data from a certain automotive bolt factory: The impact toughness increases by 8 times after tempering

3. Controllable atmosphere protection

Adopt a nitrogen-hydrogen mixed gas (with oxygen content) 0.1%

The surface decarburization layer is controlled within 0.02mm (as required by the GB/T3098.1 standard).

4. Special process upgrade

Isothermal quenching: Obtaining lower bainite structure (special for wind power bolts)

Induction heating: Achieving local strengthening of threaded parts (Application in aviation bolts)

Iii. Technological Innovation of Industry Benchmarks

The intelligent heat treatment production line of a certain fastener company in Shanghai

Adopt PID temperature control system (±3℃ accuracy)

Real-time monitoring by the online hardness tester

Big data analysis optimizes process parameters to enable bolts:

The strength dispersion has decreased from 15% to 5%

Zero hydrogen embrittlement accidents occurred in millions of products

The fatigue life exceeds 500,000 cycles

Four. Five Reasons for Choosing Heat-Treated Bolts

Safety redundancy: The actual carrying capacity exceeds the nominal value by 30%

Long-term stability: 20-year preload retention rate 95%

Environmental adaptability: Stable performance from -60℃ to 150℃

Economic benefits: The full life cycle cost is reduced by 40%

Compliance guarantee: Comply with the ISO898-1/GB/T3098.1 standard