Materials Engineering Lab

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03.ColdRolling.ppt
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ME 251
MATERIALS ENGINEERING LAB
Fall 2018

Cold Rolling

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Metal Fabrication Techniques

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Cold Rolling

Cold Rolling is a process which passes metal through rollers at temperatures below its recrystallization temperatures. This increases the yield strength and hardness of the metal. This is done by introducing defects into the crystal structure of the metal creating a hardened microstructure which prevents further slip.

Recrystallization Temperature

The temperature at which atomic mobility can repair the damage caused by the working process

Metal Recrystallization temperature (°C)
Lead, Tin 10°C below room temperature
Zinc 25
Magnesium, Aluminum 150
Gold, Copper & Silver 200
Iron, Low alloy steels 450
Tungsten 1400

Basic Definition

  • Anisotropic: A material not possessing the same properties in all directions.
  • Isotropic: A material possessing the same properties in all directions
  • Work Hardening/Strain Hardening: An increase in hardness and strength caused by plastic deformation at temperature below the plastic deformation range

The above bar has anisotropic properties because it is ductile on the right side of the bar and brittle on the left: notice geometry.

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Metal Forming

  • Large group of manufacturing processes in which plastic deformation is used to change the shape of metal work-piece.
  • The tool, usually called a die, applies stresses that exceed yield strength of metal.
  • The metal takes a shape determined by the geometry of the die.

Hot Working

  • Hot working is plastically deforming of the metallic material at a temperature above the recrystallization temperature.

No Strain Hardening

Usually performed at elevated temperatures

Lead and Tin are exceptions (low melting point)

Lower limit of the hot working temperature: 60% of the melting temperature

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Hot Working Processes: Rolling

Hot Rolling: A material is passed through two rolls that rotate opposite directions and the gap is smaller than the material thickness.

Before Deformation

Deformation produced elongated grains

Forging

Forging is mechanically working or deforming a single piece of a metal; this may be accomplished by the application of successive blows or by continuous squeezing.

Bulk Deformation Processes: Extrusion

For extrusion, a bar of metal is forced through a die orifice by a compressive force that is applied to a ram; the extruded piece that emerges has the desired shape and a reduced cross-sectional area.

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Drawing

Drawing is the pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side. A reduction in cross section results, with a corresponding increase in length

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Hot Working: Advantages Over Cold Working

  • Work-piece shape can be significantly altered.
  • Lower forces and power required.
  • Metals that usually fracture in cold working can be hot formed.
  • Strength properties of product are generally isotropic.
  • No strengthening of part occurs from work hardening.

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Hot Working: Disadvantages

  • Lower dimensional accuracy.
  • Higher total energy required (due to the thermal energy to heat the work piece).
  • Work surface oxidation (scale), poorer surface finish.
  • Shorter tool life

Cold Working

Plastic deformation of metals below the re-crystallization temperature.

  • The process is usually performed at room temperature, but mildly elevated temperatures
  • Initially the dislocations can move through the metal structure.
  • As the working continues, however, the movement of the dislocations becomes more difficult
  • The metal becomes less malleable and ductile
  • Squeezing/Cold Rolling
  • Pin a material between to solid objects and cause plastic deformation.
  • Bending
  • Is the plastic deformation of materials about a linear axis with little or no change in the surface area.
  • Shearing/Blanking
  • Is the mechanical cutting of materials without the formation of chips or the use of burning or melting.
  • Drawing
  • Sheet metal
  • The forming of parts where plastic flow occurs over a curved axis.
  • Wire/Rod/Tubing
  • A process of reducing the cross section of the material by pulling it through a die.

Cold Working Processes

Cold Working: Advantages Over Hot Working

  • No heating required.
  • Strength, fatigue and wear properties are improved through strain hardening.
  • Superior dimensional control is achieved.
  • Better surface finish is obtained.
  • Directional properties can be imparted.
  • Contamination problem are minimized.

Cold Working: Disadvantages

  • Higher forces are required to initiate and complete the deformation.
  • Less ductility is available.
  • Heavier and more powerful equipment is required
  • Metal surfaces must be clean and scale-free
  • Undesirable residual stresses may be produced.

  • Imparted directional properties may be detrimental.
  • In some cases, metal may not be ductile enough to do cold work.
  • Intermediate anneals may be required to compensate for the loss of ductility that accompanies strain hardening

Strain Hardening

  • Increasing the amount of cold working will increase the YS, UTS, and reduce the ductility.
  • When plastically deforming a material dislocations are increased making it more difficult for plastic deformation to occur.
  • A material becomes more brittle as it is cold worked.

References

  • Callister Jr., W.D. and Rethwisch, D.G. “Materials Science and Engineering: An Introduction,” 9th Edition, John Wiley & Sons, 2013.
  • Smith, W.F. and Hashemi, J. “Foundations of Materials Science and Engineering,” McGraw-Hill, 2004.

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Stress

% cold work

Strain