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Steel Making

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

Heavy Forging Practice
In heavy forging practice where the metal is being worked at...

Oil-hardening Steel
Heat slowly and uniformly to 1,450 deg.F. and forge thorough...

Nickel may be considered as the toughest among the non-rare a...

Heating Of Manganese Steel
Another form of heat-treating furnace is that which is used ...

Tempering Colors On Carbon Steels
Opinions differ as to the temperature which is indicated by t...

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

Pickling The Forgings
The forgings were then pickled in a hot solution of either ni...

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

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

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

Brown Automatic Signaling Pyrometer
In large heat-treating plants it has been customary to mainta...

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

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

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

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

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

Quenching The Work
In some operations case-hardened work is quenched from the bo...

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

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

The Theory Of Tempering


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

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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