Work Hardening and Metallurgical Annealing Made Simple

Also known as cold working or strain hardening, work hardening is a process by which a metal is strengthened due to plastic deformation. This may seem counterintuitive due to the seeming contradiction but you’ll see how this works. If you were to break a metal down to a molecular level, you would get what’s called a crystal structure.

This crystal structure is an arrangement or lattice of a given material’s molecular makeup, and helps to determine its various properties. Typical metals are naturally filled with random dislocations, and when stresses are introduced, these existing molecular dislocations will begin multiplying within the crystal structure. Plastic stress strain curve for ductile materialsdeformation occurs when these dislocations are allowed to travel and propagate within the crystal structure.

However, the dislocations themselves are – when sufficiently numbered – what prevent their own movement. Relatively few dislocations will result in an overall weakening of the material, whereas significant numbers will result in work hardening due to what’s known as pinning points. Pinning points act as points in a crystal structure that impede the movement of dislocations.

A simple example of work hardening is the bending of thin steel wire. When you bend steel wire sharply, say 90 degrees or so, and try to bend it back to its original position, what happens? I’m sure you’ve noticed that that little bend doesn’t want to just bend back. No doubt a section of wire immediately next to the existing bend will begin bending instead. The original bend became work hardened when you bent it sharply thus making it difficult to bend back to its prior shape.

Of course, there is a limit as to how much stronger a material can get by work hardening. Once the material’s grain structure begins breaking down, any further stress will induce necking (for ductile materials), after which the material will fracture. A material will also become more brittle with increased work hardening, which is why it must be controlled so as to achieve the optimal balance. (The stress-strain curve above is an approximate representation of the behavior of a ductile material.)

What is Annealing and how does it Work?

On a related note, annealing is the process by which a metal is heated to a sufficient temperature and then cooled, for the purpose of positively altering its chemical properties. Common reasons for annealing metals is to annealing cylinders to relieve internal stressrelieve internal stresses caused by practices like work hardening. You can think of annealing as a reset of sorts, where a metal can be restored to its original properties after being altered.

Annealing involves heating the metal above the recrystallization temperature so as to accomplish the above aims. After heating, the cooling process is not uniform for all metals. Ferrous metals, or iron-based metals such as steel, must be cooled slowly for proper annealing to occur. Other non-ferrous metals such as brass, copper, and silver, have the option of either being air-cooled like steels, or quenched quickly in water.

The image above shows LPG cylinders being passed through a heat furnace to relieve internal stresses created during the forming and welding phases of manufacture. These particular cylinders are exposed to temperatures of 930 degrees Celsius for a period of time before being allowed to gradually cool down.

  • See Knives and Steel for a more detailed explanation regarding steel; its properties, benefits, and behavior under various stresses and conditions, including work hardening. It starts off basic but quickly moves on to more advanced principles.

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