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Rate Of Absorption
According to Guillet, the absorption of carbon is favored by ...

Pyrometers For Molten Metal
Pyrometers for molten metal are connected to portable thermoc...

Hardening
The forgings can be hardened by cooling in still air or quen...

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

Heat Treatment Of Gear Blanks
This section is based on a paper read before the American Gea...

Steel Worked In Austenitic State
As a general rule steel should be worked when it is in the a...

Heat Treatment Of Punches And Dies Shears Taps Etc
HEATING.--The degree to which tools of the above classes shou...

The Influence Of Size
The size of the piece influences the physical properties obta...

Shrinking And Enlarging Work
Steel can be shrunk or enlarged by proper heating and cooling...

Properties Of Steel
Steels are known by certain tests. Early tests were more or l...

Annealing Work
With the exception of several of the higher types of alloy s...

Sulphur
SULPHUR is another element (symbol S) which is always found i...

Quality And Structure
The quality of high-speed steel is dependent to a very great ...

Heat-treating Department
The heat-treating department occupies an L-shaped building. ...

Instructions For Working High-speed Steel
Owing to the wide variations in the composition of high-speed...

Composition And Properties Of Steel
It is a remarkable fact that one can look through a dozen tex...

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

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

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

Tensile Properties
Strength of a metal is usually expressed in the number of pou...



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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