Cold heading processing is a kind of processing method with high comprehensive economic benefits for fasteners and special-shaped parts. It is a processing method commonly used in the fastener industry, and it is also a kind of advanced and widely used in domestic and foreign. processing methods. Therefore, how to make full use of and improve the plasticity of metal and master the mechanism of metal plastic deformation is the purpose and purpose of this paper.
First, the advantages of cold heading processing are summarized:
1. High steel utilization rate
Cold heading is a kind of processing method with less cutting and no cutting, such as hex head bolts for machined rods and hexagon socket head cap screws. The cutting method is used, the steel utilization rate is only 25% to 35%, and the cold heading processing method is used. Its utilization rate can be as high as 85% to 95%, which is only the process consumption of the material head, the tail and the hex head.
2, high productivity
Compared with general-purpose cutting, cold heading is more than tens of times more efficient.
3, good mechanical properties
Parts processed by the cold heading method are much more powerful than cutting because the metal fibers are not cut.
4, suitable for automated production
Fasteners (also including some special-shaped parts) suitable for cold heading method are basically symmetrical parts, suitable for high-speed automatic cold heading machine production, and also the main method of mass production.
Second, the type of metal deformation:
Deformation refers to the sum of the relative displacements of the fine particles that make up the metal itself under the condition of maintaining its own integrity (external force, internal force); in the process of cold heading, because of the material or external factors of the metal The types of formation will be different.
1, elastic deformation
The metal is deformed by an external force, and when the external force is removed, the original shape and size are restored. This deformation is called elastic deformation. The strength of elasticity is measured by the elastic limit and the proportional limit.
2, plastic deformation
Under the action of external force, the metal produces permanent deformation (referring to the deformation that cannot be restored after the external force is removed), but the deformation of the metal itself is not destroyed, which is called plastic deformation. The quality of plasticity is expressed by elongation, reduction of area, and yield limit.
Third, the evaluation method of metal plasticity:
In order to assess the quality of metal plasticity, a numerical index is commonly used, called plasticity index. The plasticity index is expressed by the amount of plastic deformation at the moment when the steel sample begins to break. In actual production, the following methods are usually used:
1. Tensile test
The tensile test is expressed by elongation δ and reduction in area ψ. It indicates the plastic deformation ability of steel samples in uniaxial tension and is a plasticity index commonly used in metal material standards. The values ​​of δ and ψ are determined by the following formula:
δ= [(Lk-Lo)/Lo]*100%
ψ=[(Fo-Fk)/Fo]*100%
Where: L0, Lk——the original gauge length of the tensile specimen and the length of the gauge length after the failure; F0, Fk——the cross-sectional area of ​​the original and broken part of the tensile specimen.
2. Upsetting test (flattening test)
It is a cylindrical shape in which the sample height is 1.5 times the original diameter Do of the sample, and then flattened on a press until the first visible crack of the naked eye appears on the surface of the sample. The degree εc is a plasticity index. Its value can be calculated as follows:
Εc =[(Ho-Hk)/Ho]*100%
Where: Ho - the original height of the cylindrical specimen.
3. Torsion test
The torsion test is expressed by the torsion angle or the number of torsion turns when the sample is twisted on the twisting machine. The most commonly used in production are tensile tests and upsetting tests.
Regardless of the test method, it is relative to a specific stress state and deformation condition. The plasticity index thus obtained is only relatively comparative, and only shows the plasticity of a certain metal under which deformation conditions.
Fourth, the degree of cold deformation of metal :
Metal deformation refers to the ratio of the amount of compression of the blank to the forged section to the original height, or the ratio of the cross-sectional area of ​​the blank to the cross-sectional area of ​​the upset forging and the original cross-section. The representation of the degree of deformation is:
1. Upsetting ratio (S)
Namely: S=h0/d0
Where: h0 - the original height of the upset portion; d0 - the original diameter of the upset portion.
The upsetting ratio can determine the difficulty of upsetting, the smaller the upsetting ratio, the smaller the deformation, and the easier the deformation. The larger the upsetting ratio is, the more difficult the deformation is, the irregular flow of metal fibers, and some fibers are bent to form a longitudinal bending phenomenon.
2, upsetting rate (ε)
Namely: ε=[(ho-h)/ho]*100%; ε=[(F-Fo)/F]*100%
Where: ho, Fo - the original height and cross-sectional area of ​​the head material before upsetting; h, F - the height and cross-sectional area of ​​the workpiece after upsetting.
Fourth, the determination of the number of ups and downs :
In cold heading, the product usually has to be formed by more than two upsettings. The number of upsetting is determined reasonably. The degree of allowable deformation of the metal will be fully utilized to improve the service life of the mold and ensure the quality of the product. Consider the following factors:
1. Upsetting ratio
The billet needs the ratio of the length to the diameter of the deformed part. If the ratio is too large, the longitudinal bending phenomenon will occur in one upsetting, and after the flattening, the interlayer will appear, as shown in the following figure.
To avoid these defects in upset forging, the number of upsets must be increased. That is, the billet is first pre-twisted and then refined until the desired shape is reached.
The number of upsets is generally determined by the following data:
• When ≤2.5, one-time upsetting;
• When 2.5< or ≤4.5, upset twice;
• Upset for three times when 4.5< or ≤6.5.
2. The ratio of the diameter D of the workpiece head to the height H
As shown in the figure, it is a large-diameter thin flat-headed rod with a large head diameter and a small height. The required blank h 0 /d0 is more than 2 large-head thin rod parts. Cracks are formed at the edges of the head. Similar workpieces, only increased the number of upsets, using a stepwise forming method.
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