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

Carburizing By Gas
The process of carburizing by gas, briefly mentioned on page ...

Corrosion
This steel like any other steel when distorted by cold worki...

The Forging Of Steel
So much depends upon the forging of steel that this operation...

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

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

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

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

Conclusions
Martien was probably never a serious contender for the honor ...

Annealing Of High-speed Steel
For annealing high-speed steel, some makers recommend using g...

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

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

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

Placing Of Pyrometers
When installing a pyrometer, care should be taken that it re...

Machineability
Reheating for machine ability was done at 100 deg. less than ...

Double Annealing
Water annealing consists in heating the piece, allowing it to...

Placing The Thermo-couples
The following illustrations from the Taylor Instrument Compan...

Annealing Alloy Steel
The term alloy steel, from the steel maker's point of view, r...

Cutting-off Steel From Bar
To cut a piece from an annealed bar, cut off with a hack saw,...

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



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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