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Impact Tests
Impact tests are of considerable importance as an indication ...

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

Heating
Although it is possible to work steels cold, to an extent de...

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

Properties Of Alloy Steels
The following table shows the percentages of carbon, manganes...

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 ...

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

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

A Chromium-cobalt Steel
The Latrobe Steel Company make a high-speed steel without tun...

Critical Points
One of the most important means of investigating the properti...

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

The Pyrometer And Its Use
In the heat treatment of steel, it has become absolutely nece...

Carburizing Material
The simplest carburizing substance is charcoal. It is also th...

Highly Stressed Parts
The highly stressed parts on the Liberty engine consisted of ...

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

Cutting-off Steel From Bar
To cut a piece from an annealed bar, cut off with a hack saw,...

Steel For Chisels And Punches
The highest grades of carbon or tempering steels are to be re...

Case-hardening Treatments For Various Steels
Plain water, salt water and linseed oil are the three most co...

Leeds And Northrup Optical Pyrometer
The principles of this very popular method of measuring tempe...



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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