In the forming process of fasteners, cold heading (extrusion) technology is a main processing technology. Cold heading (extrusion) belongs to the category of metal pressure processing. In production, at room temperature, external forces are applied to the metal to form it in a predetermined mold. This method is usually called cold heading. In fact, the forming of any fastener is not only achievable through cold heading as a deformation method. During the cold heading process, in addition to upsetting deformation, there are also various deformation methods such as forward and backward extrusion, composite extrusion, punching and rolling. Therefore, the term "cold heading" in production is just a customary term, more precisely, it should be called "cold heading (extrusion)". Cold heading (extrusion) has many advantages and is suitable for mass production of fasteners. Its main advantages can be summarized as follows:
High utilization rate of steel. Cold heading (extrusion) is a less cutting and non cutting processing method, such as machining hexagonal head bolts and cylindrical head hexagon screws for rods. By using cutting processing method, the utilization rate of steel is only 25% to 35%. However, using cold heading (extrusion) method, its utilization rate can reach up to 85% to 95%, which is only some process consumption of material head, material tail, and cutting hexagonal head edges.
High productivity. Compared with general cutting processing, the efficiency of cold heading (extrusion) forming is several tens of times higher.
c. Good mechanical performance. The parts processed by cold heading (extrusion) method have much superior strength than those processed by cutting because the metal fibers are not cut off.
d. Suitable for automated production. Fasteners (including some special-shaped parts) suitable for cold heading (extrusion) production are basically symmetrical parts, suitable for production using high-speed automatic cold heading machines, and are also the main method of mass production.
In short, the cold heading (extrusion) method for processing fasteners and special-shaped parts is a highly cost-effective processing method, which is widely used in the fastener industry and an advanced processing method widely used and developed domestically and internationally. Therefore, the purpose and purpose of this chapter is to fully utilize and improve the plasticity of metals, master the mechanism of metal plastic deformation, and develop a scientific and reasonable cold heading (extrusion) processing technology for fasteners.
Basic concepts of metal deformation
1.1 Deformation
Deformation refers to the total relative displacement of the small particles that make up a metal when subjected to external or internal forces, while maintaining its own integrity.
1.1.1 Types of deformation
a. Elastic deformation
Metal undergoes deformation under external forces, and when the external force is removed, it has the ability to restore its original shape and size. This deformation is called elastic deformation.
The quality of elasticity is measured by the elastic limit and proportional limit.
b. Plastic deformation
Metal undergoes permanent deformation under external forces (referring to the deformation that cannot be restored to its original state after removing the external force), but the integrity of the metal itself will not be destroyed, which is called plastic deformation.
The quality of plasticity is expressed by elongation, reduction of area, and yield limit.
1.1.2 Evaluation method of plasticity
In order to evaluate the plasticity of metals, a numerical indicator commonly used is called plasticity index. The plasticity index is expressed by the plastic deformation at the moment when the steel sample begins to fail. In practical production, the following methods are usually used:
(1) Tensile test
Elongation rate for tensile testing δ And the reduction in area ψ To represent. The plastic deformation ability of steel specimens under uniaxial tension is a commonly used plasticity index in metal material standards. δ and ψ The value of is determined by the following formula:
(Formula 36-1)
(Formula 36-2)
In the formula: L0, Lk - the length of the original gauge length and the gauge length after failure of the tensile specimen.
F0, Fk - The cross-sectional area of the original and fracture points of the tensile specimen.
(2) Upsetting test, also known as flattening test
It is to make the specimen into a cylindrical shape with a height Ho of 1.5 times the original diameter Do of the specimen, and then flatten it on a press until the first visible crack appears on the surface of the specimen, which is the degree of compression at this time ε C is the plasticity index. The value can be calculated using the following formula:
(Formula 36-3)
In the formula, Ho - the original height of the cylindrical specimen. Hk - The height of the specimen when the first visible crack appears on the side surface during flattening.
(3) Torsion test
The torsion test is expressed by the angle or number of turns of the specimen when it is twisted and broken on the torsion machine. The most commonly used tests in production are tensile testing and upset testing. Regardless of the testing method, it is relative to a specific stress state and deformation condition. The plasticity index obtained from this is only a relative comparison, indicating the plasticity of a certain metal under certain deformation conditions.
1.1.3 Main factors affecting metal plasticity and deformation resistance
The concept of plasticity and deformation resistance of metals: The plasticity of metals can be understood as the ability of metals to stably change their shape under external forces without breaking the connections between particles. The force exerted by the metal during deformation on the applied external force in the mold is called deformation resistance.
The main factors affecting the plasticity and deformation resistance of metals include the following aspects:
The influence of metal structure and chemical composition on plasticity and deformation resistance
The metal structure is determined by the chemical composition of the metal, the lattice type of its main elements, the properties, quantity, and distribution of impurities. The fewer constituent elements, the better the plasticity. For example, pure iron has high plasticity. Carbon, as a solid melt in iron, also has good plasticity, while as a compound, its plasticity decreases. The compound Fe3C is actually very brittle. Generally, an increase in the composition of other elements in steel can also reduce its plasticity.
The resistance index of steel increases with the increase of carbon content( б B б P б S and so on all increase, while plasticity index( ε,ψ All decrease. During cold deformation, for every 0.1% increase in carbon content in steel, its strength limit б S increases by approximately 6-8 kg/mm2.
Sulfur exists in steel as iron sulfide and manganese sulfide. Iron sulfide is brittle, while manganese sulfide becomes filamentous and elongated during pressure processing, resulting in a decrease in mechanical index in the transverse direction perpendicular to the fibers. So sulfur is a harmful impurity in steel, and the lower the content, the better.
Phosphorus in steel increases deformation resistance and reduces plasticity. Steel with phosphorus content higher than 0.1% to 0.2% has cold brittleness. The phosphorus content of general steel is controlled at just over zero percent.
The distribution of low melting point impurities in the metal matrix has a significant impact on plasticity.
In short, the more complex and abundant the chemical composition in steel, the greater the impact on its resistance and plasticity. This is precisely why some high alloy steels are difficult to perform cold heading (pressing) processing.
b. The influence of deformation speed on plasticity and deformation resistance
The deformation speed is the relative displacement volume per unit time:
(Formulas 36-4)
The deformation speed should not be confused with the movement speed of the deformation tool, and should also be conceptually distinguished from the movement speed of particles in the deformation body.
Generally speaking, as the deformation rate increases, the deformation resistance increases and the plasticity decreases. During cold deformation, the influence of deformation speed is not as significant as during hot deformation, due to the absence of hardening elimination process. But when the deformation speed is particularly high, the heat generated by plastic deformation (i.e. thermal effect) must not be dissipated. Increasing the temperature itself will increase plasticity and reduce deformation resistance.
c. The influence of stress state on plasticity and deformation resistance
Under external forces, internal forces are generated within a metal, and the strength per unit area is called stress. The metal under stress is in a state of stress.
A small primitive cube is separated from the deformed body, and a stress of unknown magnitude but known direction is applied on the selected cube. This diagram representing the number of principal stresses and their symbols at a point is called the principal stress diagram. There are nine types of principal stress diagrams that represent the stress state of metals, among which four are three-dimensional principal stress diagrams, three are planar principal stress diagrams, and two are unidirectional principal stress diagrams, as shown in Figure 36-1.
The principal stress caused by tensile stress is a positive sign, while the principal stress caused by compressive stress is a negative sign.
In metal pressure processing, the most commonly encountered are the three directional principal stress diagrams of the same and different numbers. In the three-dimensional principal stress diagram of different signs, the most common one is the principal stress diagram with two compressive stresses and one tensile stress.
When the compressive stresses in all directions are equal in the three directional compressive stress diagram of the same number( б 1= б 2= б 3) Moreover, under the condition of no looseness or other defects inside the metal, theoretically plastic deformation cannot occur, only elastic deformation can occur.
The deformation processes included in the unequal triaxial stress diagram include volume forging, upsetting, closed punching, forward and backward extrusion, plate and profile rolling, etc.
In practical production, it is rare to detour to the triaxial tensile stress diagram. Only in tensile tests, when necking occurs, the stress line at the necking point is the principal stress diagram of triaxial tension.
During upsetting, due to the effect of friction, a three-dimensional compressive stress diagram is also presented.
In summary, in the stress state of a metal under stress, compressive stress is beneficial for increasing plasticity, while tensile stress will reduce the plasticity of the metal.
d. The influence of cold deformation hardening on the plasticity and deformation resistance of metals
Metal undergoes cold plastic deformation, causing changes in its mechanical, physical, and chemical properties. As the degree of deformation increases, all strength indicators (elastic limit, proportional limit, flow limit, and strength limit) are improved, and the hardness is also improved; The plasticity indicators (elongation, reduction in area, and impact toughness) have decreased to some extent; Increased resistance; The corrosion resistance and thermal conductivity decrease, and the magnetic properties of the metal are changed. In plastic deformation, the sum of these changes in the properties of the metal is called cold deformation hardening, abbreviated as hardening.
e. The influence of additional stress and residual stress
The stress distribution in deformed metals is uneven, and larger deformation is desired in areas with higher stress distribution, while smaller deformation is desired in areas with lower stress distribution. Due to the integrity of the deformed metal itself, a mutually balanced internal force is generated within it, known as additional stress. After the deformation terminates, these balanced stresses exist inside the deformation body, forming residual stresses that affect the plasticity and deformation resistance of the deformed metal in subsequent deformation processes.
1.1.4 Process Measures for Improving Metal Plasticity and Reducing Deformation Resistance
In response to the main factors affecting the plasticity and deformation resistance of metals, combined with production practice, effective process measures can be taken to improve metal plasticity and reduce its deformation resistance. The commonly adopted process measures in production include:
a. Condition of raw materials
For cold heading raw materials, in addition to requiring uniform chemical composition and structure without metal inclusions, softening and annealing treatment is generally required to eliminate residual stress inside the metal during rolling, make the structure uniform, and reduce hardness. The hardness HRB of the metal before cold heading is required to be ≤ 80. For medium carbon steel and alloy steel, spheroidizing annealing is generally adopted to improve the cold deformation plasticity of the metal in addition to stress relief and uniform microstructure.
b. Improving the smoothness of molds and improving the lubrication conditions of metal surfaces
Both of these measures are aimed at reducing the friction between the deformable body and the working surface of the mold, and minimizing the tensile stress caused by friction during deformation.
c. Choose appropriate deformation specifications
In the cold heading (extrusion) process, there are very few products that are formed by one forging, and generally require two or more forging processes. Therefore, it is necessary to achieve a reasonable allocation of deformation amount each time, which is not only conducive to fully utilizing the cold deformation plasticity of metals, but also conducive to the forming of metals. For example, in production, cold heading, cold extrusion composite forming, double reduction of bolts, and small deformation of nuts are used.
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