Summary of fastener cold heading process knowledge

Cold heading (extrusion) technology is essential in the fastener-forming process. Cold heading (extrusion) belongs to the category of metal pressure processing. In production, at room temperature, an external force is applied to the metal to form the metal in a predetermined mold, usually called cold heading. Today we will take a comprehensive look at fasteners' cold heading forming process.

Forming any #fastener is not only a deformation method of cold heading that can achieve. It is in the cold heading process. In addition to upsetting deformation, it is accompanied by forward, reverse extrusion, composite extrusion, punching, rolling, and other deformation methods.

Therefore, the name of cold heading in production is just a habitual name. More precisely, it should be called cold heading (squeeze).

The advantages of cold heading (extrusion) are many and are suitable for the high-volume production of fasteners. The main benefits are summarized in the following areas:

a. High utilization rate of steel. Cold heading (extrusion) is a less, no-cutting processing method, such as processing rod hexagonal head bolts and cylindrical head hexagon screws. Using cutting processing methods, the steel utilization rate is only 25% ~ 35%. With the cold heading (extrusion) method, its utilization rate can be as high as 85% ~ 95%, with only some process consumption of the material head, material tail, and cutting hexagonal head.

b. High productivity. Compared with general machining, the efficiency of cold heading (extrusion) molding is more than dozens of times higher.

c. Good mechanical properties. The parts are processed by the cold heading (extrusion) method because the metal fiber is not cut. The strength is much superior to that of cutting processing.

d. Suitable for automated production. Fasteners suitable for cold heading (extrusion) method production (including some special-shaped parts), basically symmetrical parts ideal for high-speed automatic cold heading machine production, is also the primary method of mass production.

In short, the cold heading (extrusion) method to process fasteners and special-shaped parts is a processing method with high comprehensive economic benefits. That is a processing method commonly used in the fastener industry and is also an advanced processing method widely used and developed at home and abroad.

Therefore, how to fully use 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 is the purpose and purpose of the research.

First, the basic concept of metal deformation.

Variation

Deformation refers to the sum of the relative displacements of the fine particles that make up the metal itself while maintaining its integrity when it is subjected to forces (external and internal).

1 Types of deformations

a. Elastic deformation

An external force deforms the metal, and the ability to restore its original shape and size is called elastic deformation when the external force is removed.

The elastic limit, the proportional limit, measures the quality of elasticity.

b. Plastic deformation

Under external force, the metal produces permanent deformation (refers to the deformation that cannot return to its original state after removing the external power). Still, the integrity of the metal itself will not be destroyed, which is called plastic deformation.

The quality of plasticity is expressed by elongation, cross-section shrinkage, and yield limit.

?2. Plasticity assessment method

The quality of plasticity is expressed by elongation, cross-section shrinkage, and yield limit.

A numerical index called the plasticity index is commonly used to evaluate the quality of metal plasticity.

The plasticity index is expressed by the amount of plastic deformation when the steel sample begins to fail. In the actual production, the following methods are usually used:

(1) Tensile test

Tensile tests are expressed in terms of elongation δ and section shrinkage ψ. It indicates the plastic deformation ability of steel specimens during unidirectional stretching and is a commonly used plastic index in metal material standards. The following formula determines the values of the δ and ψ:

3 Main factors affecting metal plasticity and deformation resistance

The plasticity and deformation resistance of metals: Metals can be understood as the ability of metals to stably change their shape under the action of external forces without destroying the connections between particles. And the strength of the metal acting on the mold applying external pressure when the metal is deformed is called deformation resistance.

The main factors affecting 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 is determined by the chemical composition of the constituent metal, the lattice class of its main elements, and the nature, quantity, and distribution of impurities: the fewer constituent elements, the better the plasticity.

Pure iron, for example, has a high plasticity.

Carbon in iron as a solid melt also has good plasticity, and in the form of compounds, the plasticity is reduced.

For example, the compound Fe3C is very brittle. Generally, the increase in the composition of other steel elements will also reduce steel's plasticity.

With the increase of carbon content in steel, the resistance index of steel (БB, БP, БS, etc.) increased, while the plasticity index (ε, ψ, etc.) decreased. During cold deformation, for every 0.1% increase in carbon content in steel, its strength limits бs growth by about 6~8kg/mm2.

Sulfur exists in steel as #iron sulfide and manganese sulfide. Iron sulfide is brittle, and manganese sulfide becomes filamentous and elongated during pressure processing, thus reducing the mechanical index in the transverse direction perpendicular to the fiber. Therefore, sulfur is a harmful impurity in steel, and the less the content, the better.

Phosphorus in steel increases deformation resistance and reduces plasticity. Steel containing phosphorus higher than 0.1%~0.2% has cold brittleness. Generally, the phosphorus content of steel is controlled at 0.0 percent. Others, such as low melting point impurities, significantly influence plasticity in the metal matrix.

In short, the more complex the chemical composition and the more content in steel, the greater the impact on the resistance and plasticity of steel. This explains why some high-alloy steels are difficult to cold heading (pressing).

b. Effect of deformation speed on plasticity and deformation resistance

4 Measures to improve metal plasticity and reduce the deformation resistance

The main factors affecting metal plasticity and deformation resistance, combined with the actual production, can take effective process measures to improve mental plasticity and reduce its deformation resistance. In production, often taken process measures are:

a. Blank condition

Raw materials for cold heading, in addition to the requirement of uniform chemical composition, structure, no metal inclusions, etc., are generally softened and annealed the raw materials. The purpose is to eliminate the residual stress inside the metal during metal rolling, make the structure uniform, reduce the hardness, and require the hardness of the metal before cold heading HRB ≤80.

For medium carbon steel, alloy steel generally adopts spheroidization annealing. The purpose is to eliminate stress and make the structure uniform, but also to improve the cold deformation plasticity of the metal.

b. Improve mold smoothness and improve metal surface lubrication conditions

Both measures aim to reduce the friction between the deformed body and the mold's working surface and reduce the tensile stress caused by friction in the deformation as much as possible.

c. Select the appropriate deformation specification

In the cold heading (extrusion) process, only some products are formed at one time and generally have to go through two or more upsetting.

Therefore, it is necessary to achieve a reasonable distribution of each deformation, which is not only conducive to making full use of the cold deformation plasticity of the metal but also conducive to forming the metal.

For example, cold heading and cold extrusion composite forming are used in production, the two diameter reduction of #bolts , and the slight deformation of large nuts materials.