The Library of Thread Rolling - 5/9

The basics of metallurgy

Specialist topic: An exciting overview of thread rolling... The basics, processes, tools, and applications of rolled high-tech threads

Cold forming

Thread rolling is classified as cold forming (also known as cold forging). To make it easier to understand the next chapters, this chapter takes a closer look at the prerequisites and influences on the plasticity of metals.

Definition

In metallurgy, cold forming is defined as a forming process in which the material is neither warmed up (the microstructure remains unchanged) nor heated (the microstructure changes). The small increase in temperature caused by the forming process itself (self-heating) is not covered by this definition of the term. The changes to the material properties caused by cold forming are therefore permanent.

The process of cold forming is always suitable when special material properties are required. As a rule, cold forming is used to achieve increased strength, good surface quality without reworking, but also high dimensional accuracy and good material utilization.

The generic term “cold forming” covers several processes. In this technology blog, however, cold forming is only considered in connection with the thread rolling processes described later.

Requirement

The first requirement for every type of cold forming is the basic ability of the material to change its shape, i.e., its plasticity. A workpiece can only be deformed if the external shear stress applied to the material exceeds the critical internal shear stress. However, it is possible that the breaking strength (or shear strength) is not reached at every point on the workpiece. The change in shape can therefore vary from one point to another. If the conditions at one point are already critical, then the base material will not show any change at all at another point.

Cold forming can be used for solid semi-finished products, as well as flat products (e.g., sheet metal), whose width is much greater than their thickness. In most cases, though, the result depends largely on the type of semi-finished product. It is always a three-dimensional process.

Description of cold formability

The behavior of a material during cold forming cannot be described by a single characteristic. Several properties of the material, the influence of the forming tool and the forming process itself must be taken into account.

Dimensional stability and dimensional flexibility

The cold formability of a material can basically be characterized by two properties: the dimensional stability and the dimensional flexibility. Dimensional stability is the mechanical stress required for plastic flow, which is why it is also referred to as yield stress or yield point. The yield stress can be used to estimate the force required for forming and the expected material properties after forming, but not the expected form.

This upper limit of the material's plasticity is referred to as the dimensional flexibility. The first external signs are cracks or even a fracture. The well-known tensile test is a good illustration. After the yield point is exceeded (transition from elastic to plastic behavior of the material), and after a certain amount of stretching, there is a reduction in the cross-section (necking) and then a fracture occurs. In the case of tensile loading, the elongation at breaking point determined in the tensile test is therefore used as the decisive parameter for plasticity.

The plasticity of a material is primarily influenced by the type of stress state, the material temperature, and the rate of deformation. This material property must therefore be determined specifically for each cold forming process by carrying out tests.

Yield curve

The yield curve describes the relationship between the yield stress and the degree of deformation. To put it somewhat simplistically, the degree of deformation is the extent of the change in shape. You can speak of an ideal yield curve if the speed of the change in shape and the workpiece temperature remain constant during forming. But in all real forming processes, however, the two parameters are constantly changing. A real yield curve takes into account the practical conditions of forming, which always depend on the extent of the forming process. It always differs from the ideal yield curve.

Influences on cold formability

Decisive material properties

There are three material properties that are crucial for the cold forming process: low dimensional stability (low stress for plastic flow), good dimensional flexibility (no cracks during forming), and good surface quality after forming.

Influence of the structure

It is true to say, without going into too much detail, that the material structure has a significant influence on the dimensional stability and dimensional flexibility. Since the surface quality depends on the manufacturing process itself, these two properties can be optimized by selecting the appropriate chemical composition of the base material or by adjusting the most favorable material structure through heat treatment.

Influence of the lattice structure

Another material property that has an influence on cold formability is the slip within the single crystals, which enables permanent changes in shape during forming. We mention this for the sake of completeness, but won't cover it any great depth. The lattice structure therefore also has a significant influence on cold formability. This is because the number of preferred slip planes and the slip directions depend on it.

Forming technology

In cold forming, it is the friction between the tool and the workpiece that determines the stress state. In turn, friction is decisively influenced by the tool geometry and lubrication. Even though a large number of test methods have been developed to simulate the characteristics of various cold forming processes, these methods are generally only valid for the workpieces and the forming process used. Great care must be taken when transferring the characteristic values obtained to other workpiece dimensions or forming processes.

Success through experience

It is precisely because of these variable influencing factors and their effects that experience is key in the success of thread rolling, if not its most important cornerstone. For example, to determine whether the material parameters are within the permissible range, the temperature must be felt, and the rolling noise checked. Special attention has to be paid to ensure that the process temperature remains low. If this is not successful, control over the flow process is lost. The result is that the thread, especially ones with small core diameters, will burst.

Material behavior during cold forming

Self-heating

The self-heating of a blank generally has a positive effect on the flow behavior, but the dimensional stability of the thread deteriorates. Precision becomes practically uncontrollable in the case of strong self-heating.

The rate of deformation has a significant influence on self-heating. However, this is difficult to estimate, let alone quantify. But this influence can be neglected with very slow forming.

Abb. 14-1

Fig. 1: Dependence of Vickers hardness and yield stress on the effective strain

Solidification

In most cases in cold forming, the increase in strength values is a favored property. This hardening – also known as the hardening curve – is proportional to the local change in shape, as shown in Figure 1 for two different types of steel.

The degree of hardening also determines the properties of the workpiece produced. The change in shape and the resulting increase in strength can vary a lot from one point to another in a cold-formed workpiece.

Changed properties of the material

As the degree of forming increases, the yield strength and tensile strength increase, while the uniform elongation (elastic elongation), elongation at break (plastic elongation) and constriction at break decrease, until finally the uniform elongation disappears completely.

Since there are stresses present in the semi-finished product and the tool also introduces stresses, there may still be mechanical stresses of varying degrees in cold-formed metals after plastic deformation. Such stresses tend to lead to the tearing open of heavily transformed areas. An experienced cold former will be aware of this.

Limits of the material

Depending on how a material fails, different types of fracture appear. Shear stress fractures that occur when the greatest (locally occurring) shear stress reaches or exceeds the shear fracture strength are much dreaded occurrences. The difference between the shear stress at the start of plastic deformation and the shear or separating fracture strength is therefore important for the formability of a material.

In principle, any metal can be cold formed. In practice, the material must meet certain physical requirements (in particular have sufficient elongation) and it must be possible to comply with the limits of the material. In principle, any metal can be cold formed. In practice, the material must meet certain physical requirements (in particular have sufficient elongation) and it must be possible to comply with the limits of the material.

Cold forming of steel

Cold formability of carbon and alloying elements

Steels with a carbon content of more than 1.5 % are not suitable for cold forming. Additions of, e.g., sulfur, phosphorus, or lead generally impair the plasticity. For example, leaded free-cutting steel is ideal for machining by turning, milling, or drilling, but is only of limited use for cold forming. Alloying or accompanying elements such as manganese, silicon, aluminum and nitrogen have a strengthening effect. That’s why people try to keep their proportion low during melting and casting.

Steels with a low carbon content and without alloy additives are regarded as very malleable. The microstructure of low-carbon and unalloyed steels consists almost exclusively of ferrite grains; the proportion of cementite is low. Up to carbon contents of about 0.35 %, ferrite has a decisive influence on the hardening capacity of unalloyed and low-alloy steels.

Intermediate annealing is necessary to achieve higher degrees of forming because of the hardening that occurs. In addition, depending on the material and heat treatment, the tools can withstand a maximum surface pressure of 2,800 N/mm2. As the surface pressure increases with increasing flow stress, the limit of forming is reached when this value is reached, even if the dimensional flexibility has not yet been exhausted.

Suitable steels

In addition to unalloyed and low-alloy steels, stainless (chemically resistant) steels are also frequently cold-formed. A distinction must be made between ferritic, martensitic, and austenitic steels.

Ferritic and martensitic chromium steels

The crystals of the basic structure of ferritic chromium steels, such as the low-alloy chromium steel type 1.4021, have a body-centered cubic lattice. These steels are only moderately corrosion resistant. Their hardening behavior is similar to that of unalloyed steels. Martensitic chromium steels, which contain additions of cobalt, molybdenum, nickel, or vanadium, and also only have limited corrosion protection, have a tetragonally distorted body-centered microstructure. They are much more difficult to cold form than ferritic chromium steels.

 

medium_2

Fig. 2: Yield curves of two different steels determined from the compression test at 20 °C

Austenitic steels

Austenitic steels, on the other hand, have a significantly better hardening capacity thanks to their face-centered cubic lattice structure with its numerous sliding abilities. As the proportion of alloying elements such as chromium, copper, molybdenum, or nickel increases, the hardening of austenitic steels decreases.

Theory and practice

Figure 2 shows the yield curves of an unstable austenitic and a stable austenitic steel determined during the compression test. Although stable and unstable austenitic steels have approximately the same initial values of yield stress, the yield stress of unstable austenitic steel, which tends to transform, is about 65% higher than that of stable austenitic steel after forming In practice, the results of cold forming are usually very different from those of the compression test.

 

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Trapezgewinde

Fig. 3: Trapezoidal thread according to basic profile DIN 103, flat trapezoidal thread according to DIN 380, multi-start Tr-thread, also left/right-handed

 

 

Literature and sources

The nine blog articles contain excerpts from the Library of Technology, Volume 286, Thread Rolling. This book was compiled with the expert support of Kurt Husistein and published by Verlag Moderne Industrie, ISBN 978-3-937889-30-6.

 

Kübler, Karl-Heinz, Mages Walter J. Handbuch der hochfesten Schrauben, 1. Aufl. Essen: W. Girardet Buchverlag, 1986.

http://www.hp-gramatke.de/ Hans-Peters Mathematisch-Technisch-Algorithmisch-Linguistisches Sammelsurium.Verein Deutscher Eisenhüttenleute (Hrsg.): Werkstoffkunde Stahl, Bd. 1 Berlin: Springer, 1984. Apel, Heinz: Gewindewalzen: Kaltverformen von Präzisionsgewinden und Spindeln, München: Hanser 1952.

 

© 2007 Alle Rechte bei sv corporate media, D-80992 München 
Abbildungen: Nr. 1, 23-25 RWT Rollwalztechnik GmbH, Engen; Nr. 2 Foto Deutsches Museum, München; Nr. 3 Musée du tour automatique et d'histoire de Moutier, Moutier (Schweiz); Nr. 16 Fette GmbH, Schwarzenbek; Nr. 18 Meinrad Plaz, Staufen (Schweiz); Nr. 26 Habegger SA, Court (Schweiz); Nr. 34-36 FBT Fahrzeug- und Maschinenbau AG, Thörigen (Schweiz); Nr. 37, 38 Schleuniger AG, Thun (Schweiz); Nr. 39, 40 Max-Planck-Institut für Physik (Heisenberg-Institut), München; Nr. 41 Saurer AG, Arbon (Schweiz); Nr. 42 Line Tech AG, Glattbrugg (Schweiz); alle übrigen Eichenberger Gewinde AG, Burg (Schweiz). Satz: abavo GmbH, D-86807 Buchloe. Druck und Bindung: Sellier Druck GmbH, D-85354 Freising. Printed in Germany 889030.

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