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Annealing
ANNEALING can be done by heating to temperatures ranging from...

Connecting Rods
The material used for all connecting rods on the Liberty engi...

Gears
The material used for all gears on the Liberty engine was sel...

Process Of Carburizing
Carburizing imparts a shell of high-carbon content to a low-...

Carbon-steel Forgings
Low-stressed, carbon-steel forgings include such parts as car...

Piston Pin
The piston pin on an aviation engine must possess maximum res...

Conclusions
Martien was probably never a serious contender for the honor ...

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

Chromium
Chromium when alloyed with steel, has the characteristic func...

Pickling The Forgings
The forgings were then pickled in a hot solution of either ni...

Crucible Steel
Crucible steel is still made by melting material in a clay or...

Phosphorus
PHOSPHORUS is an element (symbol P) which enters the metal fr...

Gas Consumption For Carburizing
Although the advantages offered by the gas-fired furnace for ...

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

Introduction Of Carbon
The matter to which these notes are primarily directed is the...

Tempering Round Dies
A number of circular dies of carbon tool steel for use in too...

The Care Of Carburizing Compounds
Of all the opportunities for practicing economy in the heat-t...

Heat Treatment Of Lathe Planer And Similar Tools
FIRE.--For these tools a good fire is one made of hard foundr...

High-carbon Machinery Steel
The carbon content of this steel is above 30 points and is ha...

Oil-hardening Steel
Heat slowly and uniformly to 1,450 deg.F. and forge thorough...



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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