hit counter script Annealing Metal

Annealing Metal  (extracts from Machinery's Handbook)

What is Annealing

The purpose of annealing is to soften metal and remove all strains due to rolling or hammering in the manufacturing process.

Annealed metal can then be bent or machined much more easily because it is now softer. In school we often anneal aluminium before we bend it so that it does not fracture.

Process

A common method of annealing is to pack the metal in a cast-iron box containing some material, such as powdered charcoal, charred bone, charred leather, slaked lime, sand, fireclay, etc. The box and its contents are then heated in a furnace to the proper temperature, for a length of time depending upon the size of the metal. After heating, the box and its contents should be allowed to cool at a rate slow enough to prevent any hardening. It is essential, when annealing, to exclude the air as completely as possible while the metal is hot, to prevent the outside of the metal from becoming oxidized.

The temperature required for annealing should be slightly above the critical point, which varies for different metals. Low-carbon steel should be annealed at about 1650 degrees F., and high-carbon steel at between 1400 degrees and 1500 degrees F. This temperature should be maintained just long enough to heat the entire piece evenly throughout. Care should be taken not to heat the metal much above the hardening point.

If only a small piece of steel or a single tool is to be annealed, this can be done by building up a firebrick box in an ordinary blacksmith's fire, placing the tool in it, covering over the top, then heating the whole, covering with coke and leaving it to cool over night. Another quick method is to heat the metal to a red heat, bury it in dry sand, sawdust, lime or hot ashes, and allow it to cool.  

Small sections of Aluminium can be annealed in the workshop by heating them in a brazing hearth. A good indicator that the annealing temperature is reached is to rub its surface with soap. When the correct temperature is reached the soap on the surface of the aluminium turns black. The aluminium then should then be allowed to cool slowly.

When metal is heated above its critical temperature, the grain assumes a definite size for that particular temperature, the coarseness increasing with an increase in temperature. Moreover, if metal that has been heated above the critical point is cooled slowly, the coarseness of the grain corresponds to the coarseness at the maximum temperature; hence, the grain of annealed metal is coarser, the higher the temperature to which it is heated above the critical point