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

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

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

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

Hardening High-speed Steel
In forging use coke for fuel in the forge. Heat steel slowly ...

Standard Analysis
The selection of a standard analysis by the manufacturer is t...

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

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

Crankshaft
The crankshaft was the most highly stressed part of the entir...

The Modern Hardening Room
A hardening room of today means a very different place from ...

The Leeds And Northrup Potentiometer System
The potentiometer pyrometer system is both flexible and subst...

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

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

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

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

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

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

Application To The Automotive Industry
The information given on the various parts of the Liberty eng...

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

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



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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