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Steel Making

Protectors For Thermo-couples
Thermo-couples must be protected from the danger of mechanica...

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

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

Pyrometry And Pyrometers
A knowledge of the fundamental principles of pyrometry, or th...

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

Open Hearth Process
The open hearth furnace consists of a big brick room with a l...

Making Steel Balls
Steel balls are made from rods or coils according to size, st...

Carbon-steel Forgings
Low-stressed, carbon-steel forgings include such parts as car...

Robert Mushet
Robert (Forester) Mushet (1811-1891), born in the Forest of D...

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

The Packing Department
In Fig. 56 is shown the packing pots where the work is packe...

Bessemer Process
The bessemer process consists of charging molten pig iron int...

Carbon Steels For Different Tools
All users of tool steels should carefully study the different...

Hardening Carbon Steel For Tools
For years the toolmaker had full sway in regard to make of st...

Composition Of Transmission-gear Steel
If the nickel content of this steel is eliminated, and the pe...

Annealing
There is no mystery or secret about the proper annealing of d...

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

Pyrometers
Armor plate makers sometimes use the copper ball or Siemens' ...

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

Rate Of Cooling
At the option of the manufacturer, the above treatment of gea...



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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