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How to make a better and higher quality bolt

First of all, understand the bolts, and briefly introduce the following bolts:

Mechanical parts, cylindrical threaded fasteners with nuts. A kind of fastener composed of head and screw (cylinder with external thread), which is used to connect two parts with through-hole by matching with nut.

There are many types of bolts.

  • According to the stress mode of connection: there are ordinary bolts and bolts with reaming holes.
  • According to the head shape: there are hexagonal head, round head, square head, sink head and so on.
  • According to the length of thread, it can be divided into full thread and non full thread.
  • According to the thread profile, it can be divided into two types: coarse and fine. The coarse profile is not shown in the bolt mark.
  • According to the performance grade, bolts can be divided into 8 grades: 3.6, 4.8, 5.6, 5.8, 8.8, 9.8, 10.9 and 12.9. Among them, bolts above grade 8.8 (including grade 8.8) are made of low-carbon alloy steel or medium carbon steel and heat-treated (quenching + tempering). They are generally called high-strength bolts, and those below grade 8.8 (excluding grade 8.8) are generally referred to as ordinary bolts.
  • According to the manufacturing accuracy, ordinary bolts can be divided into three grades: A, B and C. grade A and B are refined bolts and grade C are rough bolts. For steel structure connecting bolts, unless otherwise specified, they are generally ordinary coarse grade C bolts. Of course, there are other special-shaped bolts and special-purpose bolts, which will not be introduced any more. Let’s take grade 10.9 high strength bolts as an example to talk about bolts.

To make a good bolt, we need to design a bolt first.

If you want to do a good job, you should first define its characteristic requirements. Here is the technical requirements. How to define the technical requirements is really difficult. You must know the relationship between the functional requirements and the technical requirements. The technical requirements come from the functional requirements, and the functional requirements come from the users, the use environment or other external conditions. Functional requirements analysis can be carried out using quality function deployment (also known as house of quality) technology, which will not be introduced here.
How to convert functional requirements into technical requirements requires a certain amount of technical accumulation. To give two simple examples, if you want to make the fatigue strength of bolts better, you should reasonably design the transition fillet of bolt head, the fillet of bolt root, the transition of thread and screw, and then reflect the designed information to the design drawings or technical requirements. For example, in practical application, bolts are required to be free from corrosion for a long time in a relatively harsh corrosion environment. You need to select the appropriate surface treatment or bolt material according to the corresponding standards according to the corrosion environment to meet the requirements of the application environment.
EN 14399 4 2005 463x348 1 - How to make a better and higher quality bolt

In addition, the performance grade, thread tolerance grade, dimensional accuracy grade and surface treatment method of bolts are all factors to be considered in the design, and even the sequence of some processes needs to be well defined in the design (such as thread rolling and quenching and tempering treatment, the reverse of the sequence has a certain impact on the fatigue strength of the thread).

In order to make the bolt meet the application needs, the material should be selected well.

In fact, the material should have been selected for bolt design. The reason for this is that the author thinks that materials are really important. Most bolts are ordinary carbon steel or alloy steel, which is common. However, bolts may be used in high corrosive environment, environment with large temperature change, or extreme temperature environment, so ordinary carbon steel or alloy steel is not suitable, For example, ordinary carbon steel materials in extreme low temperature will become brittle, easy to produce brittle fracture without warning. If it is applied to high temperature environment, ordinary carbon steel will produce creep, that is, if the screw thread is not loose, the pre tightening force of bolt will be reduced, which is easy to loose or fatigue failure.
In addition, some bolts require high creep resistance, good stress corrosion resistance, oxidation resistance and low notch sensitivity.

The design material is ready, the process sequence is very important.

How are bolts made? In general, in mass production, the whole plate of wire rod is used, and then it is cut to the required length after straightening. Some processes can be changed according to the manufacturer’s production equipment layout, but some processes can not be adjusted. If the process is adjusted, it may affect the product quality or reduce the product performance.
An important basis for determining the process sequence is to improve the production efficiency and reduce the production cost without affecting the product performance or ensuring the product quality.
Which process sequence adjustment will affect the product quality, such as heat treatment and rolling thread. If the bolt has no special fatigue performance requirements, the rolling thread after heat treatment or the rolling thread before heat treatment is no problem, but if the fatigue strength requirement is high, the thread must be rolled after heat treatment.
Disk element straightening annealing skin peeling pier heat treatment thread rolling surface treatment lubrication packaging

Upsetting bolt head

Cold heading process is the preferred process for the head forming of fasteners. The cold heading process has the characteristics of high productivity, high material utilization, high surface quality and internal quality. When the steel bar is cut to the required length, it can be upset. The heading can be cold forming or hot forming. Generally, the smaller bolt is cold upsetting, and the larger bolt is hot upsetting.
The forged bolt head has continuous metal fiber, high strength and better fatigue resistance.

Heat treatment

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. 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 has occupied a dominant position since 1990s. In addition to excellent sealing performance, the device can also accurately control the temperature and process parameters by computer, and has the function of equipment fault alarm and display. During the whole process of quenching and tempering heat treatment, some oxidation gas will be produced outside the furnace. Through a series of chemical changes and corrosion on the surface of high strength bolt, the material of high strength bolt will decarburize by itself. According to the technical personnel, decarburization of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700 ~ 800 ℃.
Decarburization is a failure factor that needs to be prevented in the thermal nursing stage. Decarburization will cause local strength reduction. When the bolt is stressed, it is easy to produce microcracks and then develop into fatigue failure.
Carburizing on the surface of screw thread can increase the strength and decrease the plasticity, resulting in delayed crack or fracture.
If the gas control is not good, the decarburization of screw will be out of tolerance. In the cold upsetting process of high strength bolt, the annealing decarburization layer not only exists, but also extrudes to the top of high strength bolt thread. At this time, the hardness of the surface of the high strength bolt need not be designed in advance, and the mechanical properties of the high strength bolt will be greatly reduced.
In a broad sense, the carbon potential of furnace gas refers to the equilibrium of chemical reaction at the interface between furnace atmosphere and steel with certain carbon content at a certain temperature. The carbon potential of the furnace gas depends on the composition of the gas itself, the temperature of the gas, the catalyst conditions in the furnace and so on. For the protective gas, the carbon potential of furnace gas should be equal to or slightly higher than the carbon content of steel parts, so as to prevent oxidation and decarburization of steel parts during heating.
In order to make the carbon potential have a favorable comprehensive effect on the carburizing process, the carbon potential of furnace gas can be controlled by stages, that is, the carbon potential can be increased as much as possible in the initial stage, because at this time, the carbon absorption capacity of the workpiece is very strong, it is not easy to produce carbon black, and the high carbon potential can make the concentration gradient on the surface of the carburizing layer large, which is conducive to the diffusion and increase the carburizing speed. In the second stage, the carbon potential should be appropriately reduced, because the surface layer has established a higher 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. Therefore, 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 surface carbon concentration gradient and the depth of the infiltration layer meet the requirements of the process.

Rolling thread

Before rolling the thread, it is necessary to turn the part to be rolled to the pitch diameter of the thread. The thread is formed by the pressure of the wire plate (rolling die). The thread formed by rolling process has the advantage that the plastic metal fiber streamline 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 screw accuracy, whether the material has coating or not. For example, if the screw 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 in the thread processing, and the rolling thread needs to be advanced.
Rolling (rubbing) thread is a processing method that uses plastic deformation to form thread teeth. It uses a rolling die with the same pitch and profile as the processed thread, extrudes the cylindrical screw blank and makes the screw blank rotate at the same time, and finally transfers the tooth shape on the rolling die to the screw blank to form the thread.
The common point of rolling (rubbing) thread processing is that the rolling speed is not too much, if too much, the efficiency is low, and the thread surface is easy to produce separation phenomenon or random thread phenomenon. On the contrary, if the number of revolutions is too small, the thread diameter is easy to be out of roundness, and the pressure in the initial rolling stage will increase abnormally, which will shorten the die life.
Common defects of rolling thread: surface crack or scratch of thread part; random thread; out of roundness of thread part. If these defects occur in large numbers, they will be found in the processing stage. If the quantity is small and the production process does not notice these defects, it will flow to users and cause trouble. Therefore, the key problems of processing conditions should be summarized and these key factors should be controlled in the production process.

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 has no anti-corrosion ability, so it needs proper treatment on its surface to meet the anti-corrosion requirements. Different surface treatment methods have different anti-corrosion grades, and the characteristics of anti-corrosion coating are also different, and the use temperature of different coating is also different. Several common bolt surface treatment methods are introduced below.

Lubrication

The friction coefficient of different lubricating materials on the same thread surface is not the same, even if the same lubricating material is coated on the thread surface with different surface treatment, the friction coefficient will be different. Moreover, the working temperature of different lubricating materials is not the same. If the working temperature exceeds the range, the lubrication effect will be lost.
General grease can play the role of lubrication, also has a certain anti-corrosion effect, but if the ambient temperature is higher than 120 ℃, the grease will play a role. Grease cannot be used in vacuum. Graphite is a good thread lubricant, but graphite must be mixed with fluid medium. The working temperature of graphite is affected by the working temperature of medium.
Molybdenum disulfide is the most commonly used lubricant. Molybdenum disulfide has a good effect on reducing wear at low temperature. When the temperature is higher than 390 ℃, it becomes trisulfide, which can increase the wear. The following is the possible range of friction coefficient when different lubricating materials match different thread surface treatments.
Therefore, only when the design, material selection, heat treatment, thread rolling, surface treatment, lubrication and other important processes are done well, can the bolt with good quality be made.

Source: Network Arrangement – China Bolts Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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