2. Further treatments
page 9
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Photo of annealing
Picture 2.3 A roll of steel sheet is annealed by heating it and then letting it cool inside a dome.
Annealing, hot working and quenching
Annealing
Work hardened metals can be made more malleable and ductile again by a process called annealing.

The work hardened metal is heated to a temperature which is about half its melting point. The grains within the structure re-crystallise into many fine grains. The temperature at which the new grains begin to grow is called the re-crystallisation temperature. In the new structure dislocations can move more easily. The metal therefore becomes softer and more malleable and ductile.

When a metal is being shaped by cold working, it may be necessary to anneal it several times during the process.

Photo of hot rolls
Picture 2.4 A bar of hot steel passes between heavy, shaping rolls.
Use the links below to see video clips of the processes.

Hot rolling (short, high qual - 536k)
Hot rolling (short, low qual - 180k)
Hot rolling (long, high qual - 1.1 M)
Hot rolling (long, low qual - 534k)

Hot working
When a metal is hot worked, it is shaped while it is above its re-crystallisation temperature. In these circumstances, annealing takes place while the metal is worked rather than being a separate process. The metal can therefore be worked without it becoming work hardened. Hot working is usually carried out with the metal at a temperature of about 0.6 of its melting point.

In hot rolling, the metal is forced between two rolls which have a narrow gap between them.

In forging, the metal is pounded by hammers or squeezed between a pair of shaped dies.

Quenching
Quenching describes the sudden immersion of a heated metal into cold water or oil. It is used to make the metal very hard.
When is quenching used?
Quenching is usually used with metals that are alloyed with small amounts of other metals. At high temperature the alloying metals are dissolved in the base metal. If the material is cooled slowly, the alloy elements have time to precipitate out separately. If the metal is quenched, however, the alloying metals are trapped within the crystal grains which makes them harder. The precipitates also reduce the movement of dislocations which contributes to the hardness of the material.

Quenching is an important process that is used in the production of steel cutting tools. Steel used for this purpose contains nearly 1% carbon. (This is a high carbon steel).

Photo of steel structure
Picture 2.5 Photomicrographs of martensite, showiung the grain structure. It is very difficult for dislocations to move through this.

What happens in quenching?
At a temperature of around 750 °C, iron has a body centred cubic structure. This type of iron is called ferrite. The carbon atoms can easily be held within this less tightly packed structure in what is called a solid solution.

If the steel is cooled slowly, the iron ions rearrange into a face centred cubic structure called austenite. The iron ions are more tightly packed in this arrangement and can't hold as many carbon atoms within the structure. The remaining carbon forms a compound with iron called iron carbide or cementite. Some regions of the material are therefore made up of layers of ferrite and cementite. These regions are known as pearlite.

If the same steel is quenched rather than being cooled slowly, the carbon atoms do not have time to form cementite. They are trapped within a ‘frozen’ austenite structure in an arrangement called martensite. Movement of dislocations is very difficult in this structure so the metal becomes very hard and brittle.

Question 2-3.
a) What temperature is steel heated to when it is annealed?

b) What temperature is hot working carried out at?

c) Which of the following are alternate layers of ferrite and cementite known as:
(i) austenite
(ii) martensite
(iii) pearlite


Summary                   Close
  • annealing involves heating a metal to about half its melting point to make it more malleable and ductile
  • in hot rolling, the metal gets annealed whilst it is being rolled
  • quenching produces very hard steel