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Correction For Cold-junction Errors
The voltage generated by a thermo-couple of an electric pyrom...

Calibration Of Pyrometer With Common Salt
An easy and convenient method for standardization and one whi...

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

Plant For Forging Rifle Barrels
The forging of rifle barrels in large quantities and heat-tre...

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

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

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

Heat-treating Equipment And Methods For Mass Production
The heat-treating department of the Brown-Lipe-Chapin Company...

S A E Heat Treatments
The Society of Automotive Engineers have adopted certain heat...

Carbon Steels For Different Tools
All users of tool steels should carefully study the different...

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

Open Hearth Process
The open hearth furnace consists of a big brick room with a l...

Highly Stressed Parts
The highly stressed parts on the Liberty engine consisted of ...

Impact Tests
Impact tests are of considerable importance as an indication ...

High-carbon Machinery Steel
The carbon content of this steel is above 30 points and is ha...

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

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

Preventing Decarbonization Of Tool Steel
It is especially important to prevent decarbonization in such...

Crucible Steel
Crucible steel is still made by melting material in a clay or...

High-chromium Or Rust-proof Steel
High-chromium, or what is called stainless steel containing f...



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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