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Heat Treatment Of Gear Blanks
This section is based on a paper read before the American Gea...

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

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

Critical Points
One of the most important means of investigating the properti...

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

Although it is possible to work steels cold, to an extent de...

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

Tungsten, as an alloy in steel, has been known and used for a...

Suggestions For Handling High-speed Steels
The following suggestions for handling high-speed steels are ...

Process Of Carburizing
Carburizing imparts a shell of high-carbon content to a low-...

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

Blending The Compound
Essentially, this consists of the sturdy, power-driven separa...

Molybdenum steels have been made commercially for twenty-five...

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

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

High Speed Steel
For centuries the secret art of making tool steel was handed ...

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

ANNEALING can be done by heating to temperatures ranging from...

Carbon In Tool Steel
Carbon tool steel, or tool steel as it is commonly called, us...

Cyanide Bath For Tool Steels
All high-carbon tool steels are heated in a cyanide bath. Wi...

Temperatures To Use


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.

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
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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