TEMPERING OR DRAWING
The object of tempering or drawing is to reduce the brittleness in hardened steel and to remove the internal strains caused by the sudden cooling in the quenching bath. The tempering process consists in heating the steel by various means to a certain temperature and then cooling it. When steel is in a fully hardened condition, its structure consists largely of martensite. On reheating to a temperature of from about 300 to750 degrees F., a softer and tougher structure known as troosite is formed. If the steel is reheated to a temperature of from 750 to 1290 degrees F., a structure known as sorbite is formed which has somewhat less strength than troosite but much greater ductility.
Tempering Temperature: If steel is heated in an
oxidizing atmosphere a film of oxide forms on the surface which changes color as
the temperature increases. These oxide colors have been used extensively in the
past as a means of gaging the correct amount of temper; but since these colors
are affected to some extent by the composition of the metal, the method is not
dependable.
The availability of reliable pyrometers in combination with
tempering baths of oil, salt or lead make it possible to heat the work uniformly
and to a given temperature within close limits.
Tempering in Oil: Oil baths are extensively used for tempering tools (especially in quantity), the work being immersed in oil heated to the required temperature, which is indicated by a thermometer. It is important that the oil have a uniform temperature throughout and that the work be immersed long enough to acquire this temperature Cold steel should not be plunged into a bath heated for tempering, owing to the danger of cracking it. The steel should either be preheated to about 400 degrees F., before placing it in the bath, or the latter should be at a comparatively low temperature before immersing the steel, and then be heated to the required degree. A temperature of from 650 to 700 degrees F., can be obtained with heavy tempering oils; for higher temperatures, either a bath of nitrate salts or a lead bath is generally used.
In tempering, the best method is to immerse the pieces to be tempered in the oil before starting to heat the latter. They are then heated with the oil. After the pieces tempered are taken out of the oil bath, they should immediately be dipped in a tank of caustic soda, and after that in a tank of hot water. This will remove all ail which might adhere to the tools.
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