Application of Low Carbon Martensite Technology in High Strength Bolts

Application of Low Carbon Martensite Technology in High Strength Bolts

First, low-carbon martensite technology has the following characteristics:
(1) The quenching heating temperature is above Ac3 (80-120 ℃). Considering the effect of quenching strengthening, appropriately increasing the quenching heating temperature is beneficial to the homogenization of austenite, refining grains, improving the hardenability of steel and shortening the heating time. In addition, the low-carbon martensite hardening effect depends not only on the quenching heating temperature, but also on the temperature at which the fastener is quenched.
　　
⑵ Intense quenching cooling method of chilling and deep cooling shall be adopted. Low-carbon martensite can be obtained after intensive quenching (5%-10% NaCl solution quenching or PAG aqueous solution) of low-carbon steel or low-carbon alloy steel. Low carbon, low alloy structural steel with strong quenching will not produce quenching crack. The low carbon medium and high alloy steel due to the high carbon equivalent (>0.45%) quenching cooling should adopt appropriate cooling methods and cooling medium. Such as: water-air, water-oil or oil-cold.
　　
(3) The low carbon martensite can be directly used without tempering after quenching.

The larger the size of the fastener section, the higher the hardness after quenching. The hardness of different sizes of steel after quenching is shown in Table 1.

Table 1 Hardness of steel with different sizes after quenching

The application of low-carbon martensite technology can not only significantly improve the mechanical properties of fasteners and extend the service life of fasteners, but also significantly save steel, save energy and resources, and help protect the environment.

Application of 2. Low Carbon Martensite Technology in High Strength Bolts
With the rapid development of China's national economic construction, there are more and more key projects such as automobile industry, mechanical engineering, large-scale construction projects and bridge engineering. The market demand for high-strength bolts is increasing by 40% per year, and it is developing in the direction of high-strength (13.9 grade, 14.9 grade) and large specifications (above 50mm).

The original design of the bolts on the cylinder head of the automobile engine is made of 40Cr steel, which increases with the engine horsepower. The design requirement of the maximum tension of the bolt is increased from the original 8t to more than 10t, and the 40Cr steel can no longer meet the design requirements. Using 15MnVB steel low carbon martensite strengthening instead of 40Cr steel to make bolts, not only meet the design requirements, but also 15MnVB steel has good cold heading performance, basically no cracking, high production efficiency and low scrap rate.

20MnSi steel high-strength fishtail bolts are used as fasteners for seamless railway lines. They require high strength, good plasticity and toughness, low notch deflection sensitivity and low cold brittle transition temperature. The use of 20MnSi steel low carbon martensite strengthening, the service life of the bolt than the original design of 40B steel bolt 1 times. And the price per ton of 20MnSi steel is 260 yuan lower than that of 40B steel.

Cylinder head bolts were originally quenched and tempered with 45 steel, with a hardness of 24-30HRC. During quenching, about 12% of the bolts often have longitudinal cracks and are scrapped, and the threads are easy to slip. 20 steel is heated at 910 ℃ * 12min, quenched into brine, tempered at 200 ℃ * 2h, with surface hardness of 40-43HRC and core hardness of 30-35HRC. Under the maximum assembly tightening torque, the axial extension is ≤ 0.09, while the quenching and tempering state 45 steel is generally 0.16, the service life is 35%-40% higher than that of 45 steel, and the qualified rate of heat treatment is 100.

1. Application of 15MnVB Steel High Strength Bolt in Automobile Engine
2. 20MnSi steel high strength fishtail bolt
3. 20 steel high strength cylinder head bolt
4. 20MnTiB steel 12.9 grade high strength bolt

M60 large size 12.9 grade high strength bolts for a construction project were originally made of 42CrMo steel, but the comprehensive mechanical properties were unqualified, especially the plasticity and toughness were low. After technical research, 20MnTiB steel is adopted, after strong quenching of low-carbon martensite and low-temperature tempering, the comprehensive mechanical properties meet the technical requirements. Not only high strength, but also plasticity, toughness, give full play to the high strength and toughness characteristics of low carbon martensite.

Source: Golden Spider Fastener Web