Heat Treating Article

To All:
I read a heat treating article in Cutting Tool Engineering this week
that I thought gave a good overview of the processes.
There doesn't seem to be a direct link to the article, but if
interested you can go here, click on the PDF and I believe it's the
second article in a group of articles.
formatting link

Here are some excerpts:
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Heat treating
By Dr. Laroux K. Gillespie
For any specific material, the results
of heat treating depend upon the temperature
and the amount of time the
material is held at that temperature. The
cooling rate for a specific metal, such as
steel, determines its microstructure and
properties.
Isothermal transformation is the key
to understanding what the material
properties will be after cooling. For example,
if the material is cooled at 1,000°
F/sec. until it is below 200° F, a martensite
structure is produced, which is
magnetic and hard. At a slower cooling
rate, such as 60° F/sec., some pearlite,
which is relatively soft, will develop, but
some of the material will have a martensite
structure. If cooled at a rate of 1°
to 30° F/sec. or slower, almost all of the
material will be pearlite.
Steel is in an austenite (?) phase when
heated to 1,333° F. Austenite is a soft
material. If quenched rapidly, the steel
stays in that phase until it reaches about
400° F and then starts turning into
martensite. Once the temperature falls
to about 200° F, the transformation to
martensite is completed.
Martensite?s hardness is 53 to 67
HRC, and pearlite is about 15 HRC.
Bainite is up to 34 HRC but is less
ductile than pearlite. Because tensile
strength is almost directly proportional
to hardness, a faster cooling rate produces
stronger steels?ones that have
transformations similar to AISI 1080.
By raising the temperature and holding
it at just below 400° F for 1 to 2
hours, the steel becomes ?tempered,? attaining
a more uniform phase throughout
the material. Higher temperatures
are also used to temper some of the retained
austenite to bainite and cementite,
or iron carbide. But for tempering
temperatures above 400° F, the steel
starts to soften. The higher the temperature,
the softer the steel will be. In
contrast to making a more uniform and
less brittle steel, annealing softens steel.
To provide a soft pearlitic structure,
1040 steel, for example, is annealed at
1,450 to 1,600° F for 1 hour and then
cooled to 1,200° F.
Dropping the material into water
results in a quench rate of about 300°
F/sec. In contrast, the quench rate in oil
is 30° F/sec. and in air is 1° F/sec.
Tempering the martensitic steel reduces
its hardness and tensile strength.
As the figure shows, heating AISI 1040
martensitic steel to 1,200° F and holding
it there reduces its tensile strength from
130,000 to less than 100,000 psi, but
the material is more ductile with 28 percent
elongation rather than 20 percent.
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BottleBob
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