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

Annealing In Bone
Steel and cast iron may both be annealed in granulated bone. ...

Protective Screens For Furnaces
Workmen needlessly exposed to the flames, heat and glare from...

Quenching Tool Steel
To secure proper hardness, the cooling of quenching of steel ...

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

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

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

Classifications Of Steel
Among makers and sellers, carbon tool-steels are classed by g...

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

Correction By Zero Adjustment
Many pyrometers are supplied with a zero adjuster, by means ...

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

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

Effect Of A Small Amount Of Copper In Medium-carbon Steel
This shows the result of tests by C. R. Hayward and A. B. Joh...

Annealing Method
Forgings which are too hard to machine are put in pots with ...

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

Temperature Recording And Regulation
Each furnace is equipped with pyrometers, but the reading an...

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

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

Optical System And Electrical Circuit Of The Leeds & Northrup Optical Pyrometer
For extremely high temperature, the optical pyrometer is lar...

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



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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