The material selection and heat treatment process of high strength bolts are fully analyzed

1. material selection: dual adaptation of performance and scene

The core of high strength bolt performance lies in the cooperative optimization of material selection and heat treatment process. The following are the mainstream material selection solutions and technical points of the industry:

1. Medium carbon alloy steel: the first choice for high strength

o Typical grade: 42CrMo, 35CrMoA, 40Cr and other materials with high hardenability and strength toughness balance ability, become the choice of 10.9 and 12.9 class bolts, especially suitable for wind power, nuclear power and other heavy load scenarios.

o hardenability requirements: The material should ensure that more than 90% of the quenched section is martensitic structure, to avoid the non-martensitic structure in the heart leading to a decline in strength. For example, M48-M64 bolts made of 42CrMoA steel can meet the 10.9 performance index by quenching at 840-860℃ and tempering at 540℃.

2. Low carbon alloy steel: both strength and resistance to delayed fracture

o stands for material: 20MnTiB, 35VB, etc. are suitable for 10.9 class bolts below M24, and their low phosphorus content and high grain boundary purity can reduce the risk of delayed fracture.

3. Stainless steel: A solution for special environments

o Austenitic stainless steel (such as 304, 316) with corrosion resistance, widely used in Marine engineering and chemical equipment; Martensitic stainless steel (such as 410) is tempered to achieve high strength requirements.

4. Material organization control

o spherodizing annealing pretreatment: medium carbon steel needs to be isothermal annealing at 700℃ to convert the flake pearlite into a ball, improve the cold heading plasticity (deformation rate up to 60%-80%), and avoid cold heading cracking.

2. heat treatment process: accurate balance of strength and toughness

Heat treatment is the core link to determine the performance of bolts, and it is necessary to achieve comprehensive mechanical properties improvement through the "quenching + tempering" tempering process.

1. Spheroidizing annealing process

o Process parameters: medium carbon steel is cooled slowly after heating near the critical point (700℃); Medium carbon alloy steel isothermal spheroidization annealing (AC1+20-30℃ heating, 700℃ isothermal) to ensure the uniformity of the structure.

2. Quenching process optimization

o Temperature control: The quenching temperature range of medium carbon alloy steel is 840-870℃, such as 42CrMoA steel using 850-870℃ heating can refine the grain and improve the depth of the quenching layer.

o cooling medium selection: oil quenching (L-AN10~L-AN32) is the main solution, taking into account cooling speed and deformation control; Large specification bolts (such as M30 and above) are quenched with PAG aqueous solution to avoid quenching cracks.

3. Tempering process innovation

o Temperature matching: tempering temperature of 42CrMo steel should be controlled at 500-540℃, which can not only eliminate quenching stress, but also maintain the balance between tensile strength (≥950MPa) and section shrinkage (≥44%).

o brittleness prevention and control: For 12.9 class bolts, water cooling or oil cooling is used to quickly pass the 370-560℃ tempering brittle zone to ensure that the impact toughness meets the standard.

4. Testing and quality control

o Key indicators: hardness (32-38HRC), tensile strength (≥1040MPa), elongation after fracture (≥8%) and metallographic structure (tempered sorbite + trace ferrite) need to be strictly tested.

o Defect avoidance: eliminate phosphorus polarization layer (cold and brittle source) and decarburization layer (the main cause of insufficient hardness), and achieve surface "zero oxidation" through continuous mesh belt furnace protection atmosphere control.

3. industry pain points and localization breakthrough

1. Technical bottleneck

o 12.9 class bolts rely on imports: due to insufficient material purity (high P, S content) and unstable tempering process, domestic products have a gap in impact toughness and delayed fracture resistance compared with imported products.

2. Localization progress

o Large specification bolt breakthrough: By optimizing the quenching temperature (850-870℃) and tempering parameters (500℃×180min) of 35CrMoA steel, enterprises such as Shuihui Jiangtianbao Industrial realized the mass production of M30-M36 specification 10.9 bolts.

o Process upgrade: Introduce a controlled atmosphere continuous heat treatment line, accurately regulate the depth of the decarburization layer (≤0.02mm), and improve product consistency.

4. future trend: intelligent and high-performance two-wheel drive

With the improvement of bolt performance requirements in the field of new energy equipment and aerospace, the industry is accelerating its evolution in two directions:

• Process intelligence: real-time monitoring system of heat treatment parameters based on the Internet of Things to achieve accurate control of hardness fluctuation range ≤2HRC;

• High-end materials: The development of low-alloy ultra-high strength steel (such as 40CrNiMoA), the target impact work ≥60J (-40℃), to promote the full localization of 12.9 bolts.

Conclusion

The material selection and heat treatment of high-strength bolts is the "hidden champion technology" in the field of precision manufacturing, which requires scientific and rigorous process design, but also relies on the collaborative innovation of the whole chain of materials, equipment and testing. With the continuous breakthrough of domestic enterprises in the core process, the "high-strength bolts" made in China are moving from "usable" to "easy to use" and "durable", providing a solid guarantee for the autonomy of high-end equipment.