I. Materials and production technology of high-strength bolts
1? Material of high strength bolt
Commonly used steel types for high-strength bolts are as follows:
8? Grade 8 bolts are made of 35 steel, 45 steel and ML35 steel.
10? 35CrMo and 40Cr for grade 9 bolts
12? 35CrMo and 42CrMo for grade 9 bolts.
2? Production technology of high strength bolts
The production process of high-strength bolts is as follows:
1 cold heading bolt: spheroidizing annealing → cold drawing → cold heading forming → thread processing → tempering → surface treatment.
2 Hot upsetting bolts: cold drawing → hot upsetting forming → hexagonal forming → thread processing → tempering → surface treatment.
2. Preheating treatment
1? Microstructure of cold heading steel
The original structure of cold heading bolt steel will directly affect the formability during cold heading. In the process of cold heading, the plastic deformation of local area can reach 60% ~ 80%, so the cold heading bolt steel must have good plasticity.
When the chemical composition of steel is certain, metallographic structure is the key to determine plasticity. Generally, coarse flaky spheres are not conducive to cold heading, while fine pearlite can significantly improve the plastic deformation ability of steel. Therefore, for medium carbon steel and medium carbon alloy steel with more pearlite content, spheroidizing treatment must be carried out before cold heading to obtain uniform and detailed spheroidized bodies.
2? Spheroidizing annealing process
When steel is spheroidized and annealed, the heating temperature is usually around the critical point Ac 1 of steel, and the spheroid is spheroidized after long-term heat preservation.
For medium carbon steel, the heating temperature should not be too high, and the temperature should be kept at 7 15 ~ 725℃ for 2 ~ 4 hours? Ordinary spheroidizing annealing. The hardness should be controlled at 76 ~ 79 HRB, otherwise serious decarbonization will produce three cementite precipitates along the grain boundary, which will worsen the cold heading performance of steel and cause cold heading cracking.
For medium carbon alloy steel, the spheroidizing annealing temperature is selected to be below Ac 1,/kloc-0 ~ 30℃, about 760 ~ 770℃, holding for 2 ~ 3 hours, cooling to 690 ~ 7 10℃, and isothermal 1 ~ 2 hours? Spheroidizing isothermal annealing, cooling to below 550℃ with the furnace, and discharging. The hardness is controlled at 8 1 ~ 84 HRB. When annealing at higher temperature, the decarburized layer will be too deep and too thick.
Through spheroidizing annealing, the metallographic structure of steel will change from coarse to fine, from flake to spherical, and the cracking rate of cold heading will be greatly reduced.
3? Decarburized layer after annealing
The mechanical performance requirements of bolts are mainly manifested in the threaded end. Because the surface of the cold heading bolt is no longer processed after cold drawing and cold heading, the decarburized layer of steel will remain on the surface of the part all the time, and when the thread of the bolt is rolled, the decarburized layer will be squeezed to the top of the thread, which will lead to the decrease of thread strength and the phenomenon of tripping during use. Light will lead to early elongation and deformation, and heavy will lead to fatigue fracture. Therefore, the depth of decarburized layer must be strictly controlled during spheroidizing annealing.
Thirdly, final heat treatment.
High-strength bolts must be quenched and tempered at high temperature to obtain uniform tempered sorbite structure, so as to obtain high strength, sufficient toughness and good fatigue resistance.
1? medium carbon steel
The quenching heating temperature of medium carbon steel can be determined by formula AC3+30℃ ~ 50℃. The chemical composition of 35 steel, 45 steel and ML35 steel is different in addition to carbon content, but it is important that Mn and Si elements are different, which directly reflects the difference of quenching performance caused by heat treatment process. But it must be pointed out that it is also restricted by the quenching medium and the size and shape of the parts. Therefore, the determination of quenching heating temperature varies with size factors and medium temperature. The principle is that the length-diameter ratio of bolts is less than 7, the lower limit is small ratio, and the upper limit is large ratio.
The Ac3 of 45 steel hot heading bolt is 780℃, which is very sensitive to quenching cracks. The forging stress of external hexagon head after hot upsetting without annealing treatment is large, so the lower limit temperature is selected in principle.
For medium carbon steel 35 steel and ML35 steel, the mass percentage of carbon content is generally controlled at 0? Below 35%, most of the lower limit is 0? 32%~0? 33%。 Therefore, the gradient distribution of hardness value after quenching? Large parts are steep, that is, the depth of hardened layer is shallow. Therefore, the tempering heating temperature of bolts with the same material and different diameters is different. Like 8? Grade 8 bolts M 10 and M 16 are also made of 35 steel. The tempering temperature of the former is 5 10 ~ 530℃, while that of the latter is only 480 ~ 500℃.
2? Medium carbon alloy steel
10? Bolts above grade 9 bear large load and complex stress state. They are mainly made of 35CrMo, 40Cr and 42CrMo steels. The quenching heating temperature is 50 ~ 80℃ above Ac3, generally 850 ~ 870℃. This steel will cause decarbonization when spheroidizing annealing.
10? Grade 9 requires that the total decarburization depth at the top of the bolt should not be greater than1/3 of the thread height; 12? Grade 9 requirements shall not be greater than 1/4 of the thread height. Therefore, in addition to protecting the thread tip from decarburization during quenching and heating, decarburization process should be adopted for bolts whose raw materials have been decarburized.
Tempering heating temperature is selected according to bolt strength grade. 10? Grade 9 adopts 480 ~ 510℃; 12? Grade 9 adopts 430 ~ 460℃, and the time is generally 60 minutes.