1. Introduction To Metallurgy METL 1301
Getting Metals into Usable Form: Cold Working
Lecture7
Spring 2016
Introduction To Metallurgy METL 1301

2. Student Learning Outcomes
Upon completion of this lesson, the student will be able to:
Define forging, swaging, coining, and extrusion
Describe the process of rolling
Label the primary components of a drawing system
List the factors which affect the machining process.

3. Metal Forming Processes
An important property of metals is their ability to deform plastically.
Plastic deformation is the deformation that does or will remain permanent after removal of the load that caused it.
The plastic nature of metals is the basis of all mechanical forming operations.

4. Metal Forming Processes
Forming by tension operations involves pulling or stretching the metal.
Examples of such processes are stretching for purposes of straightening material, stretch-forming operations, and wire drawing.
Many metal forming operations involve bending.

5. Metal Forming Processes
In forming operations, the metal is formed by tension, compression, bending, or shearing, or by some combination of these processes.
All compression operations reduce the thickness of the metal by squeezing it, either slowly as in press operations or more rapidly as in hammer forging operations.
Metal rolling processes for making bars, strips, plates and other shapes are also compression forming operations.

6. Effects of Cold Working
Plastic deformation such that strengthening or hardening occurs is called cold working, strain hardening, or work hardening.
Plastic deformation such that work hardening does not occur is called hot working.
Cold working increases hardness, yield strength and tensile strength, and lowers ductility.
It also increases electrical resistivity.

7. Effects of Cold Working
Cold working constitutes one of the basic methods used to strengthen metals.
The tensile strength can usually be about doubled and the yield strength can be increased by a factor of three to five.
The magnitude of increases vary with the base metal and its alloy content

8. Effects of Cold Working
Intermediate increases in strength can be obtained by adjusting the amount of cold working.
The material is then described as being in a particular temper.
Each metal producing industry has developed a standardized convention of terms to designate various degrees of temper.

9. Effects of Cold Working
In general, the terms are based on the ca proportion of the increase in tensile strength that has been obtained relative to the maximum that can be achieved.
Industry uses cold working to strengthen semi fabricated products such as sheet and wire.
An interesting example is steel wire.

10. Effects of Cold Working
The wire used in steel ropes and cables is made by cold drawing steel of controlled prior composition and microstructure.
It is produced in a range of strengths up to 1380 MN/m2 (200,000 psi).
Wires for the strings of musical instruments (known as piano wire) are made in the same way.

11. Effects of Cold Working
These wires may have tensile strengths of up to 3450 MN/ m2 (500,000 psi), which is the highest strength that has been obtained in metals.
Strength approaching that of the theoretical strength of iron.

12. Effects of Cold Working
The second way in which practical advantage is taken of strain hardening is in strengthening a product during forming.
Many consumer and industrial products are strengthened in this way.
An example is the now-common aluminum beverage can.

13. Effects of Cold Working
The bodies of these cans are produced by a cold drawing process, and it is the work hardening that occurs in the body walls during drawing that enables the walls to be made so thin.
This in turn enables less aluminum to be used, a feature which contributes to the economic success of this type of can.

14. Effects of Cold Working
Aluminum cans are produced at an eye-boggling speed and are a triumph of production and metallurgical engineering.

15. Forming by Compressive Loading
Forging. In hammer forging, the heated metal is struck repeated blows by a hammer or dies.
The forging is deformed by the descending hammer and is turned, either mechanically or by hand, until the desired shape is obtained.
Shaping may be aided by the use of open or closed dies.

16. Forming by Compressive Loading
Cold heading is a cold forging process in which force is developed by one or more strokes (blows) of a heading tool.
The force is employed to upset, or displace, the metal in a portion of a wire or rod blank to form a section of different contour or, of larger cross section than the original.

17. Forming by Compressive Loading
Advantages of the process over machining of the same parts from suitable bar stock is that almost no waste material is generated.
Tensile strength is increased, and grain flow is controlled.

18. Forming by Compressive Loading
The principal use of cold heading is for the production of heads on rivets or on blanks for threaded fastener.
A variety of other shapes can also be successfully and economically formed by the process.
Most cold heading is done on low-carbon steel wire with hardness ranging from Rockwell B 75 to 87.

19. Forming by Compressive Loading
Low-carbon steel wire is the type of material for which most machines are rated.
Copper, aluminum, stainless steel, and some nickel alloys are also cold headed.
Titanium, beryllium, magnesium, and refractory metals are less formable at room temperature and are likely to crack when cold headed.
Those metals are sometimes warm headed.

20. Forming by Compressive Loading
Quality Levels of Steel. Steel wire for cold heading is generally available in five quality levels (pertaining to surface quality).
Quality levels are listed below, in order of increasing quality and cost:
Industrial quality
Cold heading quality
Recessed head or scrapless nut quality
Special head quality
Coil-turned, ground, or shaved wire (seamfree).

21. Forming by Compressive Loading
The difference in cost between items 1 and 5 above is usually about 30%.

22. Forming by Compressive Loading
Swaging. Swaging is a process that is similar to hammer forging since metal is deformed in compression.
Rather, a series of rapid blows are used in the operation.
The swaging operation produces parts of circular cross section and will only handle pieces of relatively small diameter.

23. Forming by Compressive Loading
The swaging process is suitable for either hot or cold metal working.
The operation is used to reduce the diameter of wire and small tubes and to point the ends of wire so that it may be threaded through the dies of a wire drawing machine.

24. Forming by Compressive Loading
Coining is a compression operation in which a design or form is pressed or hammered into the surface of a metal part.
A die, bearing the design in relief on its face, is forced into the work and an impression is made in the surface.
Coining is really a closed-die squeezing operation, in which all surfaces of the work are confined or restrained.

25. Forming by Compressive Loading
Coining is also a restriking operation (called, depending on the purpose, sizing, or bottom or corner setting) used to sharpen or change a radius or profile.
In coining, the surface of the workpiece copies the surface detail in the dies with dimensional accuracy that is seldom obtained by any other process.
It is because of the accuracy that the process is used for the minting of metallic coins.

26. Forming by Compressive Loading
In the production of metal currency, a small disc is placed in a die set, one half of which contains the “heads” pattern and the other the “tails.”
The dies are brought together and the metal flows to fill the die cavities.
Since the thickness of the coined piece is not uniform, the metal must flow sideways to fill the dies against very high frictional forces between the metal and the die.

27. Forming by Compressive Loading
In thin sections, the forces necessary to cause the metal to flow are exceptionally high and consequently coining is one of the most severe of all metal working operations.
Decorative items, such as patterned tableware, medallions, and metal buttons, as well as coins, are produced by coining.
When articles with a design and a polished surface are required, coining is the only practical production method to use.

28. Forming by Compressive Loading
Dimensional accuracy equal to that available only with the very best machining practice can often be obtained in coining.
Many automotive components are sized by coining.
Sizing is usually done on semifinished products, frequently effecting significant savings in material and machining costs.

29. Cold Extrusion
The forming of a metal by extrusion involves the squeezing or squirting of metal through a hole in a die.
Low melting and relatively ductile metals like aluminum, tin, and lead may be easily extruded.
More recently, with the introduction of special glass lubricants, some steels are now being extruded.

30. Cold Extrusion
Cold extrusion is so called because the slug or preform metal enters the die at room temperature or at a temperature appreciably below the recrystallization temperature.
Any subsequent rise in temperature is caused by the thermomechanical effects of plastic deformation and friction.

31. Cold Extrusion
Cold extrusion involves backward or forward, or combined backward-and forward, displacement of metal by plastic flow under steady, though not uniform pressure.
Backward displacement from a closed die is in the direction opposite to punch travel.
Workpieces are often cup-shaped and have wall thicknesses equal to the clearance between the punch and die.

32. Cold Extrusion
In forward extrusion, the work metal is forced in the direction of the punch travel.
Sometimes these two basic methods of extrusion are combined so that some of the work metal flows backward and some forward.

33. Cold Extrusion
Metals Cold Extruded. Aluminum and aluminum alloys, copper alloys, low-carbon and medium-carbon steels, modified carbon steels, low alloy steels, and stainless steels are the metals most commonly cold extruded.
The listed metals are in the order of decreasing extrudability.

34. Rolling
In its simplest form the rolling operation consists of passing a bar of metal between two horizontal, parallel steel cylinders which rotate about their axes in opposite directions.
As the metal passes between the rolls, which are separated by a gap narrower than the thickness of the metal being fed in, it is reduced in thickness by the compressive forces exerted on it by the rolls.

35. Rolling
The varying resistance of metals to the compressive forces of the rolls is called the separating force.
The magnitude of the separating force is determined by the plasticity, hardness and thickness of the metal which is being rolled, the size of the rolls, and the friction between the work and the rolls.

36. Rolling
The maximum separating force that the rolls, bearing and framework of the rolling mill can withstand without failure determines the capacity of the machine.

37. Rolling
It should be pointed out, that friction exist between workpieces and dies as the metal flows into the die.
Similarly, in rolling, as the metal is deformed, there is friction between the rolls and the work.
As this friction increases, the energy required to turn the rolls and hold them at the correct roll gap increases.

38. Rolling
Friction is reduced by using rolls with very hard, highly polished surfaces and by proper lubrication.
However, if the friction between the work and the rolls is very small, only small reductions are possible because there must be some frictional forces to draw the work between the rolls.

39. Rolling
In compression loading, the thinner the work, the greater are the forces required to cause further deformation.
As a piece of metal is rolled thinner and thinner the larger the separating force becomes.
In cold rolling, separating force also increases as the metal work hardens.

40. Rolling
A point may ultimately be reached at which no further plastic deformation is possible.
The separating force created by the work may be so great that it elastically deforms the rolls and their housings and the work will pass between the rolls without being plastically deformed.