High-strength bolts are called high-strength bolted joints in production, and are generally not simply referred to as high-strength bolts. According to the installation features are divided into: large hexagon bolts and torsion shear bolts. The torsional shear type is only used in class 10.9. According to the performance grade of high-strength bolts, it is divided into 8.8 and 10.9. Among them, the 8.8 grade has only large hexagonal high-strength bolts. In the marking method, the number before the decimal point indicates the tensile strength after heat treatment; the number after the decimal point indicates the ratio of the yield ratio to the measured value of the ultimate tensile strength and the measured value of the ultimate tensile strength. . Grade 8.8 means that the tensile strength of the bolt rod is not less than 800 MPa and the yield ratio is 0.8; the magnitude of 10.9 means that the bolt rod has a tensile strength of not less than 1000 MPa and a yield ratio of 0.9. Do you want to know more about the manufacturing process steps of high-strength bolts?
First, the selection of high-strength bolt materials
The correct selection of fastener materials in the manufacture of fasteners is an important part because the properties of fasteners are closely related to their materials. Cold heading steel is a highly interchangeable steel for fasteners produced by cold heading. Because it is formed by metal plastic processing at room temperature, the deformation amount of each part is large and the deformation speed is high. Therefore, the performance requirements of cold heading steel raw materials are very strict.
(1) If the carbon content is too high, the cold forming performance will be lowered, and if it is too low, the mechanical properties of the parts cannot be met.
(2) Manganese can improve the permeability of steel, but excessive addition will strengthen the matrix structure and affect the cold forming properties.
(3) Silicon can strengthen ferrite to promote cold forming performance and material elongation.
(4) Although the boron element has a significant effect of improving the permeability of the steel, it also causes an increase in the brittleness of the steel. Excessive boron content is highly detrimental to bolts, screws and studs that require good overall mechanical properties.
(5) Other impurity elements, their existence will cause segregation along the grain boundary and cause grain boundary embrittlement, and the mechanical properties of the damaged steel should be reduced as much as possible.
Second, high strength bolt spheroidizing annealing
When the hexagonal cylinder head bolt is produced by the cold heading process, the original structure of the steel material directly affects the forming ability during cold heading processing. Therefore, the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor determining the quality of the plastic. It is generally considered that the coarse flaky pearlite is not conducive to cold heading, and the small spherical pearlite can significantly improve the plastic deformation of the steel. For medium carbon steel and medium carbon alloy steel with high strength fasteners, spheroidizing annealing is performed before cold heading to obtain uniform and fine spheroidized pearlite to better meet the actual production needs.
Third, high-strength bolt shelling and descaling
The process of removing the iron oxide plate from the cold-rolled steel wire rod is to peel off the descaling, and there are two methods of mechanical descaling and chemical pickling. The chemical pickling process that replaces the wire rod by mechanical descaling improves both productivity and environmental pollution. The descaling process includes a bending method, a spray method, etc., and the descaling effect is good, but the residual iron scale cannot be removed. In particular, when the scale of the iron oxide is very strong, the mechanical descaling is affected by the thickness, structure and stress state of the iron, and is used for carbon steel wire rods for low-strength fasteners. After the mechanical descaling of the wire rod for high-strength fasteners, the descaling is performed by removing all the scales and then performing the chemical pickling process. For low carbon steel wire rods, the iron scale left by mechanical descaling is likely to cause uneven wear of the grain draft. When the die draft hole is adhered to the iron sheet due to the outer temperature of the wire rod friction, the surface of the wire rod wire is longitudinally grained, and the wire rod is cold-twisted with a flange bolt or a cylinder head screw, and the head is slightly cracked.
Fourth, high-strength bolt drawing
The purpose of the drawing process is to resize the raw materials, and the second is to obtain the basic mechanical properties of the fasteners through deformation strengthening. If the distribution of the reduction rate of each pass is not suitable, the wire rod will also produce torsion cracks during the drawing process. This crack along the longitudinal distribution of the steel wire is exposed during the cold rolling process of the wire. In addition, if the lubrication is not good during the drawing process, the cold-drawn wire rod wire may have a transverse crack regularly. When the wire rod exits the wire die, the tangential direction is not the same as that of the wire drawing die, which will cause the wear of the single-edge hole type of the wire drawing die to be intensified, and the inner hole is out of round, resulting in uneven drawing deformation of the wire in the circumferential direction, so that the wire is made The roundness of the roundness is too poor, and the cross-sectional stress of the steel wire is uneven during the cold rolling process, which affects the pass rate of cold heading.
5. High-strength bolt cold heading forming
Usually, the forming of the bolt head is formed by cold heading plastic processing. The cold head forming process includes cutting and forming, single-station clicking, double-clicking cold heading and multi-station automatic cooling. An automatic cold heading machine performs multi-station processes such as stamping, upsetting, extrusion and reduction of diameter in several forming concave molds.
(1) Cutting a blank with a semi-closed cutting tool, the simple method is to use a sleeve type cutting tool.
(2) During the transfer of the short-size blank from the previous station to the next forming station, the fasteners with complicated structure are processed to improve the precision of the parts.
(3) A punch returning device shall be installed at each forming station, and the die shall be provided with a sleeve type topping device.
(4) The structure of the main slider rail and the process component can ensure the positioning accuracy of the punch and the die during the effective use period.
(5) The terminal limit switch must be installed on the baffle that controls the material selection. Attention must be paid to the control of the forging force.
Sixth, high strength bolt thread processing
Bolt threads are generally cold worked, which is limited by factors such as thread accuracy and material plating. Rolling thread refers to a processing method in which a thread is formed by plastic deformation. It uses a rolling mold with the same pitch and shape as the thread to be machined, and squeezes the cylindrical blank while rotating the screw blank, and finally transfers the tooth shape on the rolling mold to the screw to make the thread Forming. The common point of rolling thread processing is that the number of rolling revolutions does not have to be too much. If too much, the efficiency is low, and the surface of the thread is prone to separation or disorder. On the other hand, if the number of revolutions is too small, the diameter of the thread is easily rounded, and the initial pressure during the rolling is abnormally increased, resulting in a shortened life of the mold.
Seven, high-strength bolt heat treatment
High-strength fasteners are tempered according to technical requirements. Heat treatment and quenching and tempering is to improve the comprehensive mechanical properties of the fastener to meet the tensile strength and yield ratio specified by the product. The heat treatment process has a crucial impact on high-strength fasteners, especially its intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology must be available. 
Editor in charge: Hu Na
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