Manufacturing process of high-strength bolts

Manufacturing process of high-strength bolts 1. Selection of high-strength bolt raw materials

The correct selection of fastener materials in fastener manufacturing is an important part, because the performance of fasteners is closely related to its materials. Cold heading steel is a highly interchangeable steel for fasteners produced by cold heading forming technology. Since it is formed by metal plastic processing at room temperature, the deformation of each part is large, and the deformation speed is also high. Therefore, the performance requirements for cold heading steel raw materials are very strict. (1) If the carbon content is too high, the cold forming performance will be reduced, and if it is too low, it will not be able to meet the requirements of the mechanical properties of the parts. (2) Manganese can improve the permeability of steel, but adding too much will strengthen the matrix structure and affect the cold forming performance. (3) Silicon can strengthen ferrite to reduce cold forming performance and material elongation. (4) Although the boron element has the effect of significantly improving the permeability of the steel, it will also increase the brittleness of the steel. Excessive boron content is very unfavorable to workpieces such as bolts, screws and studs that require good comprehensive mechanical properties. (5) Other impurity elements, their existence will produce 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.

2. Spherical annealing of high-strength bolts

When hexagon socket head bolts are produced by cold heading process, the original structure of steel will directly affect the forming ability during cold heading, so the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor that determines the plasticity. It is generally believed that the thick flaky pearlite is not conducive to cold heading, while the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium-carbon steel and medium-carbon alloy steel with a large amount of high-strength fasteners, spheroidizing annealing is performed before cold heading to obtain uniform and fine spheroidized pearlite to better meet actual production needs.

3. Shelling and descaling of high-strength bolts

The iron oxide plate removal process for cold heading steel wire rod is peeling and descaling, and there are two methods: mechanical descaling and chemical pickling. The chemical pickling process of wire rod is replaced by mechanical descaling, which not only improves productivity but also reduces environmental pollution. This descaling process includes bending method, spraying method, etc. The descaling effect is better, but the residual iron scale cannot be removed. Especially when the scale adhesion is very strong, so mechanical descaling is affected by the thickness, structure and stress state of the scale, and it is used for carbon steel wire rods for low-strength fasteners. After mechanical descaling of wire rods for high-strength fasteners, in order to remove all oxide scales, they are then subjected to chemical pickling process, that is, composite descaling. For low-carbon steel wire rod, the iron sheet left by mechanical descaling is likely to cause uneven wear of grain draft. When the grain draft hole adheres to the iron sheet due to the friction of the wire rod against the external temperature, longitudinal grain marks appear on the surface of the wire rod, and when the wire rod cold-heads flange bolts or cylindrical head screws, microcracks appear on the head.

4. High-strength bolt drawing

The purpose of the drawing process is to modify the size of the raw material, and the second is to obtain the basic mechanical properties of the fastener through deformation strengthening. If the distribution of the area reduction rate of each pass is not appropriate, torsional cracks will also occur in the wire rod during the drawing process. This kind of cracks with a certain period of distribution along the longitudinal direction of the steel wire will be exposed during the cold heading process of the steel wire. In addition, if the lubrication is not good during the drawing process, it can also cause regular transverse cracks in the cold-drawn wire rod. The tangent direction of the coiled steel wire exiting the grain wire die is not concentric with the wire drawing die, which will cause the wear of the unilateral pass of the wire drawing die to intensify, make the inner hole out of round, and cause the drawing deformation of the steel wire in the circumferential direction to be uneven, so that the steel wire The roundness of the steel wire is out of tolerance, and the cross-sectional stress of the steel wire is not uniform during the cold heading process, which affects the pass rate of cold heading.

5. Cold heading forming of high-strength bolts

Usually, the forming of the bolt head adopts cold heading plastic processing. The cold heading forming process includes cutting and forming, single-click single-station, double-click cold heading and multi-station automatic cold heading. An automatic cold heading machine performs multi-station processes such as stamping, upsetting, extrusion and diameter reduction in several forming dies. (1) Cut the blank with a semi-closed cutting tool, the easiest way is to use a sleeve-type cutting tool. (2) During the transfer of short-sized blanks from the previous station to the next forming station, fasteners with complex structures are processed to improve the precision of parts. (3) Each forming station should be equipped with a punch ejecting device, and the die should be equipped with a sleeve-type ejecting device. (4) The structure of the guide rail of the main slider and the process components can ensure the positioning accuracy of the punch and the die during the effective use period. (5) Terminal limit switches must be installed on the baffle for controlling material selection, and attention must be paid to the control of upsetting force.

6. High-strength bolt thread processing

Bolt threads are generally cold-worked, which is limited by factors such as thread accuracy and whether the material is coated or not. Thread rolling refers to a processing method that uses plastic deformation to form thread teeth. It uses a rolling die with the same pitch and tooth shape as the thread to be processed. While extruding the cylindrical screw blank, the screw blank is rotated, and finally the tooth shape on the rolling die is transferred to the screw blank to make the thread take shape. The common point of rolling thread processing is that the number of rolling revolutions does not need to be too many. If it is too many, the efficiency will be low, and the surface of the thread teeth is prone to separation or random buckling. Conversely, if the number of revolutions is too small, the diameter of the thread is likely to be out of round, and the pressure at the initial stage of rolling is abnormally high, resulting in shortened die life.

7. Heat treatment of high-strength bolts

High-strength fasteners must be quenched and tempered according to technical requirements. The purpose of heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength and yield ratio of the product. The heat treatment process has a vital impact on high-strength fasteners, especially its intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are necessary. .

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