Processing of Metals: Cold Working

3/17/2011

Alessandro Anzalone, Ph.D. Hillsborough Community College, Brandon Campus

Agenda

1. Introduction 2. Factors in Cold Working 3. Cold Working Processes 4. References

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Introduction

After being hot formed in a steel mill, steel shapes such as bars, sheets, and tubes could be descaled and further shaped by the process of cold forming. Today, cold-formed products range from very fine hypodermic needles to huge pipeline tubes and from tiny hair-size filaments to propeller shafts for ships. Almost any conceivable shape can now be made by one or more processes of cold forming.

Some of the advantages of cold working as compared with hot working are better surface finish, closer dimensional tolerances, better machinability, superior mechanical properties (and consequently better strength-to- weight ratio), and enhanced directional properties. Some of the negative consequences of cold working are that the metal becomes more brittle and less workable, so annealing is often required to continue the process; the metal may contain residual stresses that can cause warping or distortion when machined or welded; more massive and powerful equipment is required; and subsequent heating (e.g., welding) will change the cold- worked structure and reduce its strength.

Factors in Cold Working

Plasticity

One of the most valuable characteristics of metals is known as plasticity, the ability of metals to be deformed permanently in any direction without cracking or splitting. The higher temperatures used in hot working make metals more plastic, but recrystallization at the high temperatures prevents them from increasing in strength. To achieve higher strengths and hardnesses the metal must be deformed at normal temperatures, that is, below the recrystallization temperature. Also, plastic deformation can occur only at stresses above the elastic limit or yield point.

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Factors in Cold Working

Ductility and Malleability

Ductility is the property of a metal that allows it to deform permanently or to exhibit plasticity without rupture while under tension. Any metal that can be drawn into a wire is ductile. The ability of a metal to deform permanently when loaded in compression is called malleability. Metals that can be rolled into sheets or upset cold forged are malleable. Most ductile metals are malleable, but some very malleable metals such as lead are not very ductile and cannot be easily drawn into wire. Some ductile metals are steel, aluminum, gold, silver, and nickel. A few nonferrous alloys such as brass and Monel are also quite ductile, but most alloys of steel are less ductile than plain carbon steel.

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Factors in Cold Working

Elastic Behavior

When a metal is placed under stress within its elastic range it will return to its former shape when the load is removed. If the metal takes on a permanent set by loading it beyond the elastic limit (into its plastic range), it will be permanently deformed but will bounce back to some extent because of its elastic properties. This characteristic of metals, called springback or elastic recovery, is a design consideration for forming and bending dies and fixtures. Parts are thus overbent several degrees, ironed, or bottom beaded to counteract springback.

Cold Working Processes

Coin Making

Aluminum Foil Making

Scuba Tank Manufacture

Aluminum Cans Manufacture

Nuts and Bolts Making

Metal Spinning

Progressive Die Press

Press Brake

Tube Drawing

Large Pipe Making

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References

1. R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, , Modern Materials and Manufacturing Processes, Prentice Hall, 3rd edition, 2003, ISBN: 9780130946980