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Hardening
Steel is hardened by quenching from above the upper critical....

Quenching
It is considered good practice to quench alloy steels from th...

William Kelly's Air-boiling Process
An account of Bessemer's address to the British Association w...

Annealing Of Rifle Components At Springfield Armory
In general, all forgings of the components of the arms manufa...

Pyrometers
Armor plate makers sometimes use the copper ball or Siemens' ...

Ebbw Vale And The Bessemer Process
After his British Association address in August 1856, Besseme...

Calibration Of Pyrometer With Common Salt
An easy and convenient method for standardization and one whi...

Tungsten
Tungsten, as an alloy in steel, has been known and used for a...

Lathe And Planer Tools
TO FORGE.--Gently warm the steel to remove any chill is parti...

Carbon Steels For Different Tools
All users of tool steels should carefully study the different...

Typical Oil-fired Furnaces
Several types of standard oil-fired furnaces are shown herew...

Uses Of The Various Tempers Of Carbon Tool Steel
DIE TEMPER.--No. 3: All kinds of dies for deep stamping, pres...

Silicon
SILICON is a very widespread element (symbol Si), being an es...

Judging The Heat Of Steel
While the use of a pyrometer is of course the only way to hav...

Plant For Forging Rifle Barrels
The forging of rifle barrels in large quantities and heat-tre...

Drop Forging Dies
The kind of steel used in the die of course influences the he...

Furnace Data
In order to give definite information concerning furnaces, fu...

Temperatures To Use
As soon as the temperature of the steel reaches 100 deg.C. (...

Steel Can Be Worked Cold
As noted above, steel can be worked cold, as in the case of ...

Surface Carburizing
Carburizing, commonly called case-hardening, is the art of pr...



The Theory Of Tempering






Category: HARDENING CARBON STEEL FOR TOOLS

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 necessary 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.)





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Previous: Quenching Tool Steel



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