How to make a better and higher quality bolt

How to make a better and higher quality bolt

1. first understand the bolt, a brief introduction to the next bolt bolt:

mechanical parts,Nutof cylindrical threaded fasteners. A type of fastener consisting of a head and a screw (a cylinder with external threads) must be matched with a nut to fasten and connect two parts with through holes.

 

There are various types of bolts.

According to the stress mode of connection, there are common bolts and bolts for reaming holes. According to the shape of the head: hexagonal head, round head, square head, countersunk head and so on. According to the length of the thread, it is divided into two categories: full thread and non-full thread.

According to the thread tooth type, it is divided into coarse tooth and fine tooth. The coarse tooth type is not displayed in the mark of the bolt. Bolts are divided into eight grades according to their performance grade: 3.6, 4.8, 5.6, 5.8, 8.8, 9.8, 10.9 and 12.9. Bolts above grade 8.8 (including grade 8.8) are made of low carbon alloy steel or medium carbon steel and are heat treated (quenched and tempered). They are commonly known as high-strength bolts, and those below grade 8.8 (excluding grade 8.8) are commonly known as ordinary bolts. Ordinary bolts can be divided into three grades A, B and C according to the production accuracy. Grade A and B are refined bolts and Grade C is crude bolts. For steel structure with connecting bolts, unless otherwise specified, generally for ordinary crude grade C bolts. Of course, there are other special-shaped bolts and special purpose bolts, which will not be introduced. Let's take grade 10.9 high-strength bolts as an example to talk about bolts.

2. you want to make a bolt, you need to design a bolt first.

If you want to do a good job in something, you must first define its characteristic requirements. Here is the technical requirements. How to define the technical requirements is actually quite rare. You must know the connection between the functional requirements and the technical requirements. The technical requirements come from the functional requirements, and the functional requirements come from the requirements of the user or the use environment or other external conditions. Functional requirements analysis can be analyzed using quality function deployment (also known as quality house) techniques, which are not described here.

How can the functional requirements be converted into technical requirements? This requires a certain amount of technical accumulation. To give two simple examples, if you want the bolt to have better fatigue resistance, you must reasonably design the transition fillet of the bolt head, the fillet of the bolt root, the transition between the thread and the screw, and then reflect the designed information on the design drawings or in the technical requirements. For another example, in practical application, bolts are required not to be corroded for a long time in a relatively vicious corrosive environment. You need to select appropriate surface treatment or bolt materials to meet the requirements of the application environment according to the corresponding standards in the corrosive environment. In addition, the performance level of bolts, thread tolerance level, dimensional accuracy level, surface treatment method and so on are all factors to be considered in the design, and even the sequence of some processes needs to be defined in the design (such as thread rolling and tempering treatment, which has a certain influence on the fatigue strength of threads after the sequence is reversed).

Three, to make the bolt to meet the application needs, the material needs to be selected

In fact, when designing bolts, the materials should have been selected. I say it again here separately because the author thinks that the materials are really important. Most bolts are ordinary carbon steel or alloy steel, which is a common situation. However, bolts may be used in highly corrosive environments or environments with large temperature changes, or extreme temperature environments, so ordinary carbon steel or alloy steel is not suitable, for example, ordinary carbon steel materials will become brittle under extreme low temperature conditions, and it is easy to produce brittle fracture without warning. If it is applied to high temperature environment, ordinary carbon steel will produce creep, that is, in the case of thread is not loose, the bolt preload will be reduced, easy to produce loose or fatigue failure.

In addition, some bolts also require high creep resistance, good stress corrosion resistance, oxidation resistance and low notch sensitivity.

 

4. design materials are ready, process sequence is important

How are bolts made? Generally, in mass production, the whole plate of wire rod is used and then cut into the required length after straightening. Some processes can be changed according to the layout of the manufacturer's production equipment, but some processes cannot be adjusted. If the process is adjusted, the product quality may be affected or the product performance may be reduced.

An important basis for determining the process sequence is to improve production efficiency and reduce production costs without affecting product performance or ensuring product quality.

Which process sequence adjustments will affect product quality, such as heat treatment and rolling threads. If the bolt has no special anti-fatigue performance requirements, there is no problem with rolling threads after heat treatment or rolling threads before heat treatment, but if the fatigue strength requirements are high, the threads must be rolled after heat treatment.

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V. Heading Bolt Head

The cold heading process is the preferred process for the forming of the fastener head. The cold heading process has the characteristics of high productivity, high material utilization, high surface quality and high internal quality. When the steel rod is cut into the required length, the head can be upset. The head can be cold formed or hot formed. In general, relatively small bolts are formed by cold heading, and large bolts are formed by hot heading. Forged bolt head, metal fiber continuous, high strength, better fatigue resistance.

 

 

VI. Heat Treatment

One of the important processes in the manufacture of heat-treated high-strength bolts is to improve the comprehensive mechanical properties of fasteners.FastenersIn particular, the internal quality of high-strength bolts has a very important impact.

With the development of the times, the continuous heat treatment production line with protective effect has occupied a dominant position after the 1990 s. In addition to excellent sealing performance, the device can accurately control the temperature and process parameters by computer, and has the function of equipment failure alarm and display. In 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 the 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 ℃.

Decarburization is a failure factor that bolts need to be prevented in the thermal nursing stage. Decarburization will cause local strength reduction, and micro cracks are easy to produce in the bolt force, and then develop into fatigue failure. The increase of carbon on the surface of the thread will increase the strength and reduce the plasticity, resulting in delayed cracking or fracture. Gas control is not good, but also cause screw decarburization error. During the cold upsetting of high-strength bolts, the annealed decarburized layer of the raw material not only exists, but also squeezes to the top of the high-strength bolt thread. At this time, the surface of the high-strength bolt that needs to be quenched does not need the pre-designed hardness, and the mechanical properties of the high-strength bolt will be greatly reduced.

In a broad sense, the carbon potential of the furnace gas refers to the state of the furnace gas in which the chemical reaction on the phase interface between the furnace atmosphere and 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 parts do not undergo oxidation and decarburization 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 sections, that is, the carbon potential can be increased as much as possible at the beginning, because the carbon absorption ability of the workpiece is very strong, it is not easy to produce carbon black, and the high carbon potential can make the concentration gradient of the surface of the seepage layer large, which is conducive to diffusion and improve the seepage 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 penetration depth, when 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, and the carbon potential in the furnace can be further reduced to make the carbon concentration gradient of the surface layer and the depth of the seepage layer meet the requirements of the process.

7. rolling thread

Before rolling the thread, the part of the thread to be rolled needs to be turned to the thread pitch diameter size. Rolling The thread is shaped by wire plate (roll die) pressure. The advantage of the thread formed by the rolling process is that the plastic metal fiber streamline of the threaded 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 produce the thread outer diameter of the final product, the required thread blank diameter is different, because it is limited by factors such as thread accuracy and whether the material has coating or not. For example, if the thread needs to be hot-dip galvanized surface treatment in the future, the influence of surface treatment on the final size of the thread should be considered during thread processing, and the thread needs to be rolled in advance.

Rolling (rubbing) pressure thread refers to the use of plastic deformation to shape the thread tooth processing method. It uses a rolling (rolling plate) die with the same pitch and tooth shape as the thread to be processed, squeezes the cylindrical blank while rotating the blank, and finally transfers the tooth shape on the rolling die to the blank to form the thread.

The common point of rolling (rubbing) thread processing is that the number of rolling revolutions does not need to be too much. If it is too much, the efficiency will be low, and the thread surface is prone to separation or disorderly buckling. On the other hand, if the number of revolutions is too small, the thread diameter is easy to lose round, and the pressure at the beginning of rolling is abnormally increased, resulting in shortened die life.

Common defects of rolled threads: cracks or scratches on the surface of the threaded part; disorderly buckling; the thread part is out of round. If these defects occur in large quantities, they will be found in the processing stage. If the number of occurrences is small, the production process does not notice these defects will flow to the user, causing trouble. Therefore, the key issues of processing conditions should be summarized, and these key factors should be controlled in the production process.

 

VIII. Surface Treatment

There are many materials used to produce bolts, such as carbon steel, alloy steel, and stainless steel. However, the most commonly used 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 anti-corrosion requirements. Different surface treatment methods have different anti-corrosion levels, and the characteristics of anti-corrosion coatings are also different, and the use temperature of different coatings is also different. Here are several commonly used bolt surface treatment methods.

 

9. lubrication

Different lubricating materials coated on the same thread surface friction coefficient is not the same, even if the same lubricating material coated on different surface treatment of the thread surface, the friction coefficient will be different. And the working temperature of different lubricating materials is not the same, beyond the working temperature range, it will lose lubrication.

General grease can play a role in lubrication, but also has a certain anti-corrosion effect, but if the ambient temperature is higher than 120 degrees Celsius, the grease will play. 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 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 the coefficient of friction when different lubricating materials are matched with different thread surface treatments.

 

Therefore, only the design, material selection, heat treatment, rolling thread, surface treatment, lubrication and other important processes are done, it is possible to make good quality.Bolt.