1. Upset bolt head
Cold heading process is the preferred process for fastener head forming. The cold heading process has the characteristics of high productivity, high material utilization, high surface quality and high internal quality. When the steel bar is cut to the required length, it can be heading. , Heading can be cold forming can also be hot forming. In general, relatively small bolts are formed by cold heading, and large bolts are formed by hot heading.
The forged bolt head has continuous metal fibers, high strength and better fatigue resistance.
2. Heat treatment process
Heat treatment is one of the important processes in the manufacture of high-strength bolts. Its purpose is to improve the comprehensive mechanical properties of fasteners. The heat treatment process has a very important impact on the internal quality of fasteners, especially high-strength bolts.
With the development of the times, the continuous heat treatment production line with protective effect after the 1990s occupies a dominant position. In addition to excellent sealing performance, the device can precisely control temperature and process parameters through a computer, and has equipment fault alarm and display functions. During the whole process of quenching and tempering heat treatment, some oxidizing gas will be generated outside the furnace. Through a series of chemical changes and corrosion on the surface of high-strength bolts, high-strength bolt materials will decarburize themselves. According to technical personnel, the decarburization of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700 and 800 °C.
Decarburization is a failure factor that needs to be prevented in the thermal care stage of bolts. Decarburization will cause local strength reduction, and micro-cracks are prone to occur when the bolt is stressed, and then develop into fatigue failure.
Carburizing the thread surface will increase the strength and reduce the plasticity, resulting in delayed cracking or fracture.
Poor gas control will also result in poor screw decarburization. During the cold upsetting process of high-strength bolts, the annealing and decarburization layer of the raw material not only exists, but also extrudes to the top of the high-strength bolt threads. At this time, the surface of the high-strength bolt that needs to be quenched does not need a pre-designed hardness, and the mechanical properties of the high-strength bolt will be greatly reduced at this time.
In a broad sense, the carbon potential of the furnace gas refers to the state of the furnace gas in which the chemical reaction between the atmosphere in the furnace and the interface of the steel with a certain carbon content reaches equilibrium at a certain temperature. The carbon potential of the furnace gas is determined by the composition of the furnace gas itself, the temperature of the furnace gas, and the catalysis and catalyst conditions in the furnace. For the protective gas, the carbon potential of the furnace gas should be equal to or slightly higher than the carbon content of the steel itself, so that the steel will not be oxidized and decarburized during the heating process.
In order to make the carbon potential have a favorable comprehensive effect on the carburizing process, the carbon potential of the furnace gas can be controlled in stages, that is, the carbon potential can be increased as much as possible in the initial stage, because the workpiece has a strong carbon absorption capacity at this time, and it is not easy to produce carbon black, carbon The high potential can make the concentration gradient on the surface of the permeation layer large, which is conducive to diffusion and increases the permeation rate. In the second stage, the carbon potential should be appropriately reduced, because the surface layer has established a high concentration gradient and reached a certain depth of infiltration layer. At this time, the carbon absorption capacity of the workpiece surface has begun to decline, so the carbon potential should not be too high to avoid carbon black. The third stage is the diffusion period, the carbon potential in the furnace can be further reduced, so that the carbon concentration gradient of the surface layer and the depth of the infiltrating layer can meet the technical requirements.
3. Rolled thread
Before rolling the thread, it is necessary to turn the part to be rolled to the pitch diameter of the thread.
Rolling is formed by the pressure of the wire plate (rolling die) to form the thread. The advantage of the thread formed by the rolling process is that the plastic metal fiber flow line of the thread part is not cut off, so the bolt strength is increased, the precision is high, and the quality is uniform, so the rolling process is widely used.
In order to make the outer diameter of the thread of the final product, the required thread blank diameter is different, because it is limited by the thread accuracy, whether the material has a coating or not, for example, if the thread is to be hot-dip galvanized surface treatment in the future, when the thread is processed It is necessary to consider the effect of surface treatment on the final size of the thread, and it is necessary to advance the rolling thread.
Rolling (rubbing) threading refers to a processing method that uses plastic deformation to form thread teeth. It uses a rolling (screwing plate) die with the same pitch and tooth shape as the thread to be processed, while extruding the cylindrical screw blank, while rotating the screw blank, and finally transferring the tooth shape on the rolling die to the On the screw blank, the thread is formed.
The common point of rolling (rubbing) thread processing is that the number of rolling revolutions does not need to be too many. If it is too much, the efficiency will be low, and the surface of the thread teeth will be prone to separation phenomenon or random buckle phenomenon. On the contrary, if the number of revolutions is too small, the diameter of the thread is easy to be out of round, and the pressure at the initial stage of rolling increases abnormally, resulting in a shortening of the life of the die.
Common defects of rolled threads: surface cracks or scratches on the threaded part; random buckles; out-of-round threaded parts. If these defects occur in large numbers, they will be found in the processing stage. If the number of occurrences is small, the production process does not notice these defects and will circulate to users, causing trouble. Therefore, the key issues of processing conditions should be summarized, and these key factors should be controlled in the production process.
4. Surface treatment
There are many materials used to produce bolts, such as carbon steel, alloy steel, and stainless steel. But the most common and economical material is carbon steel. Carbon steel itself has no anti-corrosion ability, so it needs to be properly treated on its surface to meet the requirements of anti-corrosion.
5. Lubrication
Different lubricating materials are coated on the same thread surface with different friction coefficients. Even if the same lubricating material is coated on thread surfaces with different surface treatments, the friction coefficient will be different. And different lubricating materials are suitable for different working temperatures, beyond the working temperature range, the lubrication effect will be lost.
Generally, grease can play a lubricating role and also has a certain anti-corrosion effect, but if the ambient temperature is higher than 120 degrees Celsius, the grease will play a role. Grease cannot be used in a vacuum environment.
Graphite is a good thread lubricant, but graphite must be mixed with fluid medium, and the working temperature of graphite is affected by the working temperature of the medium. Molybdenum disulfide is the most commonly used lubricant,
Molybdenum disulfide has a very good low temperature wear reduction effect. When the temperature is higher than 390 ℃, it becomes trisulfide, which will play a role in increasing wear. The following are the possible ranges of friction coefficients for different lubricating materials and different thread surface treatments.
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