The process and technical requirements for the production of high-strength bolts

1. Material selection
Carbon steel has excellent machinability, can meet a wide range of strength performance combinations, and has a lower cost compared to other fastener materials.
The mechanical properties of carbon steel are sensitive to carbon content, which is usually below 1.0%. We often distinguish low carbon steel (C% ≤ 0.25%), medium carbon steel (0.25%<C% ≤ 0.45%), high carbon steel (C%>0.45%), and alloy steel (adding alloying elements to ordinary carbon steel) based on the carbon content in the carbon steel material.
2. Spheroidization (softening) annealing
(1) When spheroidized (softened) annealed countersunk head screws and hexagonal cylindrical head bolts are produced using cold heading technology, the original structure of the steel directly affects the forming ability during cold heading processing. During the cold heading process, the plastic deformation in local areas can reach 60% -80%. Therefore, it is required that the steel must have good plasticity.
(2) For medium carbon steel and medium carbon alloy steel with a large amount of high-strength fasteners, spheroidization (softening) annealing should be carried out before cold heading to obtain uniform and fine spheroidized pearlite, in order to better meet practical production needs.
3. Wire drawing
Stretch the disc and apply corresponding pulling force to it to achieve the required wire diameter (i.e. diameter) for production.
The fixed wire diameter grades for bulk round plates in steel mills include M8, M10, M12, M16, etc. They cannot be directly used for bolt production. For example, producing a 10mm diameter fully threaded bolt requires pulling the diameter of the disc to 8.78+0.04mm, while producing a 16mm diameter bolt requires a wire diameter of 14.38+0.02mm
4. Cold forging
The forming of the hex bolt head adopts cold forging plastic processing. Compared with cutting processing, the metal fiber (metal wire) is continuous along the product shape without cutting in the middle, thus improving the product strength and obtaining excellent mechanical properties. The cold heading forming process includes cutting and forming, single station single click, double click cold heading, and multi station automatic cold heading.
6. Thread processing
Bolt threads are generally processed by cold working, in which thread blanks within a certain diameter range are rubbed (rolled) by a screw plate (die), and the threads are formed by the pressure of the screw plate (rolling die). The plastic streamline of the threaded part can be obtained without being cut, resulting in increased strength, high precision, and uniform quality of the product, which is widely used.
The common feature of rolling (rubbing) thread processing is that the rolling speed does not need to be too high. If it is too high, the efficiency is low and the surface of the thread teeth is prone to separation or disorderly threading. On the contrary, if the number of revolutions is too low, the thread diameter is prone to lose roundness, and the initial pressure during rolling increases abnormally, resulting in a shortened mold life.
7. Heat treatment
Heat treated high-strength fasteners must undergo quenching and tempering treatment according to technical requirements to improve the comprehensive mechanical properties of the fasteners and meet the specified tensile strength values and yield ratios of the product.
Due to the large production volume and low price of high-strength bolts, as well as the relatively delicate and precise structure of the threaded part, it is required that the heat treatment equipment must have strong production compatibility, high degree of automation, and high heat treatment quality. In order to produce high-quality high-strength fasteners, advanced heat treatment technology equipment is indispensable.