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A Satisfactory Luting Mixture
A mixture of fireclay and sand will be found very satisfactor...

Lathe And Planer Tools
TO FORGE.--Gently warm the steel to remove any chill is parti...

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

An Automatic Temperature Control Pyrometer
Automatic temperature control instruments are similar to the ...

Steel Before The 1850's
In spite of a rapid increase in the use of machines and the ...

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

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

The Penetration Of Carbon
Carburized mild steel is used to a great extent in the manufa...

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

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

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

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

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

Flange Shields For Furnaces
Such portable flame shields as the one illustrated in Fig. 1...

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

Furnace Data
In order to give definite information concerning furnaces, fu...

Protectors For Thermo-couples
Thermo-couples must be protected from the danger of mechanica...

Liberty Motor Connecting Rods
The requirements for materials for the Liberty motor connecti...

Application Of Liberty Engine Materials To The Automotive Industry
The success of the Liberty engine program was an engineer...

Hardening High-speed Steel
In forging use coke for fuel in the forge. Heat steel slowly ...

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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