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Correction By Zero Adjustment
Many pyrometers are supplied with a zero adjuster, by means ...

Non-shrinking Oil-hardening Steels
Certain steels have a very low rate of expansion and contract...

The Effect
The heating at 1,600 deg.F. gives the first heat treatment w...

Compensating Leads
By the use of compensating leads, formed of the same materia...

The Thermo-couple
With the application of the thermo-couple, the measurement of...

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

The Quenching Tank
The quenching tank is an important feature of apparatus in c...

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

Fatigue Tests
It has been known for fifty years that a beam or rod would fa...

Heat Treatment Of Lathe Planer And Similar Tools
FIRE.--For these tools a good fire is one made of hard foundr...

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

Hardening High-speed Steels
We will now take up the matter of hardening high-speed steels...

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

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

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

Chromium
Chromium when alloyed with steel, has the characteristic func...

Tempering Round Dies
A number of circular dies of carbon tool steel for use in too...

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

Annealing To Relieve Internal Stresses
Work quenched from a high temperature and not afterward tempe...

Temperature For Annealing
Theoretically, annealing should be accomplished at a tempera...



Temperatures To Use






Category: HARDENING CARBON STEEL FOR TOOLS

As soon as the temperature of the steel reaches
100 deg.C. (212 deg.F.) the transformation begins, increasing in intensity
as the temperature is raised, until finally when the lower critical
range is reached, the steel has been all changed into the ordinary
constituents of unhardened steels.

If a piece of polished steel is heated in an ordinary furnace, a
thin film of oxides will form on its surface. The colors of this
film change with temperature, and so, in tempering, they are generally
used as an indication of the temperature of the steel. The steel
should have at least one polished face so that this film of oxides
may be seen.

An alternative method to the determination of temper by color is
to temper by heating in an oil or salt bath. Oil baths can be used
up to temperatures of 500 deg.F.; above this, fused-salt baths are
required. The article to be tempered is put into the bath, brought
up to and held at the required temperature for a certain length
of time, and then cooled, either rapidly or slowly. This takes
longer than the color method, but with low temperatures the results
are more satisfactory, because the temperature of the bath can
be controlled with a pyrometer. The tempering temperatures given
in the following table are taken from a handbook issued by the
Midvale Steel Company.

TABLE 23.--TEMPERING TEMPERATURES FOR STEELS
----------------------------------------------------------------------------
Temperature Temperature
for 1 hr. for 8 min.
--------------- Color --------------- Uses
Deg. F.Deg. C. Deg. F.Deg. C.
-----------------------------------------------------------------------
370 188 Faint yellow 460 238 Scrapers, brass-turning tools,
reamers, taps, milling cutters,
saw teeth.
390 199 Light straw 510 265 Twist drills, lathe tools,
planer tools, finishing tools
410 210 Dark straw 560 293 Stone tools, hammer faces,
chisels for hard work, boring
cutters.
430 221 Brown 610 321 Trephining tools, stamps.
450 232 Purple 640 337 Cold chisels for ordinary work,
carpenters' tools, picks, cold
punches, shear blades, slicing
tools, slotter tools.
490 254 Dark blue 660 343 Hot chisels, tools for hot
work, springs.
510 265 Light blue 710 376 Springs, screw drivers.
----------------------------------------------------------------------------

It will be noted that two sets of temperatures are shown, one being
specified for a time interval of 8 min. and the other for 1 hr. For
the finest work the longer time is preferable, while for ordinary
rough work 8 min. is sufficient, after the steel has reached the
specified temperature.

The rate of cooling after tempering seems to be immaterial, and
the piece can be cooled at any rate, providing that in large pieces
it is sufficiently slow to prevent strains.





Next: Knowing What Takes Place

Previous: The Theory Of Tempering



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