Cold upsetting forming technology of fasteners
Cold upsetting (extrusion) technology is a main processing technology in the forming process of fasteners. Cold upsetting (extrusion) belongs to the category of metal pressure processing. In production, under normal temperature, the metal is applied with external force to form the metal in the predetermined die. This method is usually called cold upsetting. In fact, the forming of any fastener can be realized not only by cold upsetting, but also by forward and backward extrusion, compound extrusion, punching, rolling and so on. Therefore, the term “cold heading” in production is just a habitual term, more precisely, it should be called cold heading (extrusion). Cold upsetting (extrusion) has many advantages, it is suitable for the mass production of fasteners. Its main advantages are summarized as follows:
- a. The utilization rate of steel is high. Cold upsetting (extrusion) is a kind of less and no cutting method, such as machining hexagon head bolts and cylindrical head hexagon screws. Using cutting method, the utilization rate of steel is only 25% ～ 35%, while the utilization rate of cold heading (extrusion) method can be as high as 85% ～ 95%, which is only the process consumption of head, tail and cutting hexagon head edge.
- b. High productivity. Compared with the general cutting process, the efficiency of cold upsetting (extrusion) is tens of times higher.
- c. Good mechanical properties. The strength of the parts processed by cold upsetting (extrusion) is much better than that by cutting because the metal fiber is not cut off.
- d. It is suitable for automatic production. The fasteners (including some special-shaped parts) suitable for cold heading (extrusion) are basically symmetrical parts, which are suitable for high-speed automatic cold heading machine production, and are also the main method for mass production.
In a word, cold upsetting (extrusion) is a processing method with high comprehensive economic benefits, which is widely used in the fastener industry and an advanced processing method widely used and developed at home and abroad. Therefore, how to make full use of and improve the plasticity of metal, master the mechanism of metal plastic deformation, and develop a scientific and reasonable cold upsetting (extrusion) process of fasteners is the purpose and purpose of this chapter.
Basic concepts of metal deformation
Deformation refers to the sum of relative displacements of fine particles composed of itself under the condition of keeping its integrity when the metal is under stress (external force and internal force).
Types of deformation
a. Elastic deformation
When the external force is removed, the metal can recover its original shape and size. This deformation is called elastic deformation.
Elasticity is measured by elastic limit and proportional limit.
b. Plastic deformation
Under the action of external force, the metal produces permanent deformation (refers to the deformation that can not be restored to its original state after removing the external force), but the integrity of the metal itself will not be damaged, which is called plastic deformation.
The plasticity is expressed by elongation, reduction of area and yield limit.
Evaluation method of plasticity
In order to evaluate the plasticity of metals, a numerical index is often used, which is called plasticity index. The plasticity index is expressed by the plastic deformation at the moment when the steel sample begins to destroy. In practice, the following methods are usually used:
(1) Tensile test
The tensile test is expressed by elongation δ and reduction of area ψ. It represents the plastic deformation ability of steel specimen under uniaxial tension, which is a plastic index commonly used in metal material standards. The values of δ and ψ are determined by the following formula:
Where: l0, LK — the length of original gauge length and gauge length after failure of tensile specimen.
F0, FK — the cross-sectional area of the original and broken parts of the tensile sample.
(2) Upsetting test is also called flattening test
It is to make the sample into a cylinder whose height ho is 1.5 times of the original diameter do of the sample, and then flatten it on the press until the first visible crack appears on the surface of the sample. At this time, the compression degree ε C is the plasticity index. The value can be calculated as follows:
Where ho is the original height of the cylindrical sample. HK — the height of the sample when the first visible crack appears on the side surface of the sample during flattening.
(3) Torsion test
The torsion test is expressed by the torsion angle or the number of turns when the sample is twisted on the twisting machine. Tensile test and upsetting test are commonly used in production. No matter what kind of test method, it is relative to a particular stress state and deformation condition. The plasticity index obtained from this is only a relative comparison, which only indicates the plasticity of a certain metal under what deformation conditions.
Main factors affecting metal plasticity and deformation resistance
The concept of plasticity and deformation resistance of metal: the plasticity of metal can be understood as the ability of metal to change its shape stably under the action of external force, while the connection between particles is not destroyed. And the metal in the deformation of the reaction on the external force of the mold force is called deformation resistance.
The main factors that affect metal plasticity and deformation resistance include the following aspects:
a. Influence of metal structure and chemical composition on plasticity and deformation resistance
The structure of a metal depends on the chemical composition of the metal, the lattice type of the main elements, the nature, quantity and distribution of impurities. The less the constituent elements, the better the plasticity. Pure iron, for example, has a high plasticity. Carbon in iron as a solid melt also has a good plasticity, while as a compound, the plasticity decreases. For example, compound Fe3C is actually very brittle. Generally, the increase of other elements in steel will also reduce the plasticity of steel.
With the increase of carbon content in the steel, the resistance indexes of steel (such as бббббббббббббббббббббб. When the carbon content in the steel increases by 0.1%, the strength limit of the steel increases by 6-8 kg / mm2.
Sulfur exists in steel as iron sulfide and manganese sulfide. Iron sulfide is brittle, and manganese sulfide becomes filamentous and elongated in the process of pressure processing, thus reducing the mechanical index in the transverse direction perpendicular to the fiber. Therefore, sulfur is a harmful impurity in steel, the less the content, the better.
Phosphorus increases the deformation resistance and decreases the plasticity in steel. The steel containing more than 0.1% – 0.2% phosphorus has cold brittleness. Generally, the phosphorus content of steel should be controlled within 0.0%.
The distribution of other impurities such as low melting point in the metal matrix has a great influence on the plasticity.
In a word, the more complex the chemical composition in steel, the greater the influence on the resistance and plasticity of steel. This explains the reason why it is difficult for some high alloy steels to be cold heading (pressing).
b. Influence of deformation velocity on plasticity and deformation resistance
The deformation velocity is the relative displacement volume per unit time:
The deformation velocity should not be confused with the movement speed of deformation tools, and the deformation velocity should be distinguished from the moving speed of particles in the deformable body.
Generally speaking, with the increase of deformation speed, the deformation resistance increases and the plasticity decreases. In cold deformation, the effect of deformation velocity is not as significant as that of hot deformation, which is due to the elimination of hardening. However, when the deformation speed is very high, the heat (i.e. thermal effect) produced by plastic deformation should not be lost. The increase of temperature will increase the plasticity and reduce the deformation resistance.
c. Influence of stress state on plasticity and deformation resistance
Under the action of external force, the internal force of metal is generated, and the strength per unit area is called stress. The stressed metal is in a state of stress.
A small element cube is separated from the deformation body, and the stress of unknown size but direction is acted on the cube. The diagram of the number and symbol of principal stress on the point is called principal stress diagram. There are nine kinds of principal stress diagrams, four of which are three-dimensional principal stress diagrams, three are plane principal stress diagrams, and two are unidirectional principal stress diagrams, as shown in Figure 36-1.
Figure 36-1 principal stress diagram
If the principal stress is caused by tensile stress, it is a positive sign; if the principal stress is caused by compressive stress, it is a negative sign.
In metal pressure processing, the three-dimensional principal stress diagram with the same sign and different sign is often encountered. Among the three-dimensional principal stress diagrams with different signs, the principal stress diagram with two compressive stresses and one tensile stress is the most common.
In the three-dimensional compressive stress diagram of the same number, when the compressive stresses in all directions are equal (б 1 = б 2 = б 3), and there is no porosity and other defects in the metal, the plastic deformation can not be produced in theory, only the elastic deformation.
The deformation processes of unequal three-dimensional compressive stress diagram include: Volume die forging, upsetting, closed punching, forward and backward extrusion, plate and profile rolling, etc.
In actual production, it is rarely detoured to the three-dimensional tensile stress diagram. Only in the tensile test, when necking occurs, the stress line at the necking point is the principal stress diagram of three-dimensional tension, as shown in figure 36-2
Figure 36-2 principal stress diagram of triaxial tension
Figure 36-3 three dimensional compressive stress diagram during plastic compression
During upsetting, due to friction, a three-dimensional compressive stress diagram is also shown, as shown in Fig. 36-3.
In a word, in the stress state of the stressed metal, the compressive stress is beneficial to the increase of plasticity, while the tensile stress will reduce the plasticity of the metal.
d. Effect of cold deformation hardening on plasticity and deformation resistance of metals
The mechanical, physical and chemical properties of metals are changed by cold plastic deformation. In addition, with the increase of the ratio of thermal conductivity, ductility and ductility, the ultimate strength of the metal is increased The sum of these property changes is called cold deformation hardening, or hardening for short.
e. Influence of additional stress and residual stress
In the deformed metal, the stress distribution is not uniform. The larger the stress distribution is, the smaller the stress distribution is. Due to the integrity of the deformed metal itself, there is a balance of internal forces in the metal, which is called additional stress. When the deformation is terminated, these balanced stresses will exist in the deformation body, forming residual stress, which will affect the plasticity and deformation resistance of the deformed metal in the subsequent deformation process.
Technological measures to improve metal plasticity and reduce deformation resistance
In view of the main factors affecting the plasticity and deformation resistance of metals, combined with the actual production, it is completely possible to take effective technological measures to improve the metal plasticity and reduce its deformation resistance
a. Billet condition
In addition to the chemical composition and structure uniformity and no metal inclusion, the raw materials for cold heading should be softened and annealed. The purpose is to eliminate the residual stress in the metal during rolling, make the structure uniform and reduce the hardness. The hardness of the metal before cold heading is required to be HRB ≤ 80. Spheroidizing annealing is generally adopted for medium carbon steel and alloy steel, the purpose of which is not only to relieve the stress and make the structure uniform, but also to improve the cold deformation plasticity of the metal.
b. Improving the smoothness of die and improving the lubrication condition of metal surface
These two measures are to reduce the friction between the deformation body and the working surface of the die, and to reduce the tensile stress caused by friction as much as possible.
c. Select appropriate deformation specification
In the cold upsetting (extrusion) process, there are few products formed by upsetting at one time, which usually needs two or more upsetting. Therefore, it is necessary to make reasonable distribution of each deformation, which is not only conducive to making full use of the cold deformation plasticity of metal, but also conducive to metal forming. Such as cold heading, cold extrusion compound forming, twice reducing diameter of bolt, big material and small deformation of nut are used in production.
Basic law of metal plastic deformation
Law of minimum resistance
In the process of metal deformation, the particle of deformed body may move in all directions. The movement of deformation mass point is along the direction of its minimum resistance, which is called the law of minimum resistance.
In the multi position cold upsetting of hexagon head bolt, the metal flows to the opening of the upper and lower die and forms flash during the second upsetting, which is the embodiment of the minimum resistance law. Figure 36-4 shows that when the blank is upset in the die, it flows around the gap formed by the upper and lower dies while filling the upper and lower die cavities. Only when the resistance of the flow to the flash is greater than that in other parts of the die cavity, it is possible for the metal to fill the die cavity. In the downward movement of the upper die, the metal flow resistance on the flash increases with the decrease of the flash thickness, so that the upper and lower die cavities can be filled finally.
Fig. 36-4 flow diagram of metal according to the law of minimum resistance during upsetting
Law of invariance of volume
In the plastic deformation of metal, the density change is very small and can be ignored. The volume of plastic deformation object remains unchanged, and the volume of metal blank before plastic deformation is equal to that after deformation.
The volume invariance law is to calculate the volume according to the shape and size of the product, and then determine the specific size of the blank.
The law of minimum resistance is the most important basis for determining the number of metal deformation, how to distribute each deformation, and how to determine the structural shape of tools and dies.
Main factors affecting metal flow in deformation
a. Effect of friction
In the deformation process, there is friction force on the contact surface between the die and the blank. Because of the friction force, the characteristics of metal flow are changed. As shown in Fig. 36-5, when upsetting the bad rectangular material between plates, due to the friction force, the resistance in each direction is different, and the cross section can not keep rectangular during deformation. According to the law of minimum resistance, it will gradually become circular. If there is no friction force, the blank is in an ideal uniform deformation state, and the geometric shape is still similar before and after deformation.
Fig. 36-5 metal flow diagram of damaged parts with rectangular section during upsetting between plates
Figure 36-6 upsetting diagram of annular blank
Fig. 36-6 is the upsetting diagram of annular blank. When there is no friction, the annular part is compressed in height. According to the condition of constant volume, the diameter of metal increases in both outer and inner layers, that is, all metals flow radially outward. Due to the existence of friction, the flow is hindered. The closer to the inner layer, the greater the resistance of outward flow of metal, which is greater than that of inward flow, thus changing the flow direction. As shown in the figure, there is a flow interface (DN) in the annular part.
b. Influence of tool and die shape
Due to the different shapes of tools and dies, the force applied to the blank and the friction force between the die and the blank are not the same, which leads to the difference of metal flow resistance in all directions and the distribution of metal flow volume in each direction.
c. The influence of uneven properties of metals
The uneven nature of metal reflects the uneven composition, structure and internal temperature of metal during deformation. Due to the non-uniformity of these properties, there are additional stresses in the metal which are balanced with each other. Due to the existence of internal force, the resistance of metal flow is different, and the deformation first occurs in the part with the least resistance.
Metal cold upsetting (extrusion) process
Cold heading and cold pressing
At room temperature, the blank is placed in the die of automatic cold heading machine or press, and pressure is applied to the die. The relative motion of upper and lower dies is used to deform the blank in the die cavity, reduce the height and increase the cross-section. Such pressure processing method is called cold heading for automatic cold heading machine and cold pressing for press.
In actual production, the cold forming process of fasteners is often accompanied by extrusion in the process of cold heading. Therefore, the cold upsetting process of fastener products is actually a composite processing method of both cold heading and extrusion.
Deformation mode of cold heading (extrusion)
- a. Blanking separates a part of the blank from the body. Such as wire cutting, nut punching, head trimming of hexagon head bolt, etc.
- b. Upsetting can shorten the height of blank and increase the cross-section, such as upsetting ball of nut, pre upsetting and finish upsetting of bolt head forming.
- c. When the blank is deformed in the lower die, the flow direction of metal is consistent with that of the upper die in cold upsetting. The diameter reduction of thick bar in cold heading bolt and cylindrical head hexagon screw is a kind of forward extrusion.
- d. The flow direction of the metal is opposite to that of the upper die in the deformation of the back extrusion blank. The forming of the head of cylindrical head socket head screw belongs to back extrusion.
- e. The flow direction of metal is the same as that of the upper die and the other is opposite. In other words, there are both forward extrusion and reverse extrusion in deformation. For example, when the cylindrical head hexagon screw is deformed at the same working position, there are both rod necking (forward extrusion) and head forming (reverse extrusion).
Source: China Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading flange 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 [email protected]
Please notice that you might be interested in the other technical articles we’ve published:
- Failure analysis and preventive measures of stainless steel flange bolt connection
- What are flange bolts
- What’s the difference between high strength bolt and common bolt