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Silicon
Silicon prevents, to a large extent, defects such as gas bubb...

Liberty Motor Connecting Rods
The requirements for materials for the Liberty motor connecti...

Brown Automatic Signaling Pyrometer
In large heat-treating plants it has been customary to mainta...

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

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

Care In Annealing
Not only will benefits in machining be found by careful anne...

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

Detrimental Elements
Sulphur and phosphorus are two elements known to be detrimen...

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

Placing The Thermo-couples
The following illustrations from the Taylor Instrument Compan...

Hardening Operation
Hardening a gear is accomplished as follows: The gear is tak...

Manganese
MANGANESE is a metal much like iron. Its chemical symbol is M...

Using Illuminating Gas
The choice of a carburizing furnace depends greatly on the fa...

Vanadium
Vanadium has a very marked effect upon alloy steels rich in c...

An Automatic Temperature Control Pyrometer
Automatic temperature control instruments are similar to the ...

Effects Of Proper Annealing
Proper annealing of low-carbon steels causes a complete solu...

Refining The Grain
This is remedied by reheating the piece to a temperature slig...

Preparing Parts For Local Case-hardening
At the works of the Dayton Engineering Laboratories Company, ...

For Milling Cutters And Formed Tools
FORGING.--Forge as before.--ANNEALING.--Place the steel in a ...

Suggestions For Handling High-speed Steels
The following suggestions for handling high-speed steels are ...



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.)





Next: Temperatures To Use

Previous: Quenching Tool Steel



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