The Theory Of Tempering

: HARDENING CARBON STEEL FOR TOOLS
: The Working Of Steel

Steel that has been hardened is generally

harder and more brittle than is necessary, and in order to bring

it to the condition that meets our requirements a treatment called

tempering is used. This increases the toughness of the steel, i.e.,

decrease the brittleness at the expense of a slight decrease in

hardness.



There are several theories to explain this reaction, but generally

it is only n cessary to remember that in hardening we quench steel

from the austenite phase, and, due to this rapid cooling, the normal

change from austenite to the eutectoid composition does not have

time to take place, and as a consequence the steel exists in a

partially transformed, unstable and very hard condition at atmospheric

temperatures. But owing to the internal rigidity which exists in

cold metal the steel is unable to change into its more stable phase

until atoms can rearrange themselves by the application of heat.

The higher the heat, the greater the transformation into the softer

phases. As the transformation takes place, a certain amount of heat

of reaction, which under slow cooling would have been released in

the critical range, is now released and helps to cause a further

slight reaction.



If a piece of steel is heated to a certain temperature and held

there, the tempering color, instead of remaining unchanged at this

temperature, will advance in the tempering-color scale as it would

with increasing temperature. This means that the tempering colors

do not absolutely correspond to the temperatures of steels, but the

variations are so slight that we can use them in actual practice.

(See Table 23, page 158.)