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Care In Annealing
Not only will benefits in machining be found by careful anne...

Typical Oil-fired Furnaces
Several types of standard oil-fired furnaces are shown herew...

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

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

Tensile Properties
Strength of a metal is usually expressed in the number of pou...

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

Refining The Grain
This is remedied by reheating the piece to a temperature slig...

Manganese
MANGANESE is a metal much like iron. Its chemical symbol is M...

Instructions For Working High-speed Steel
Owing to the wide variations in the composition of high-speed...

Hints For Tool Steel Users
Do not hesitate to ask for information from the maker as to t...

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

Application To The Automotive Industry
The information given on the various parts of the Liberty eng...

William Kelly's Air-boiling Process
An account of Bessemer's address to the British Association w...

Heat Treatment Of Steel
Heat treatment consists in heating and cooling metal at defin...

Air-hardening Steels
These steels are recommended for boring, turning and planing...

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

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

Corrosion
This steel like any other steel when distorted by cold worki...

Optical System And Electrical Circuit Of The Leeds & Northrup Optical Pyrometer
For extremely high temperature, the optical pyrometer is lar...

The Forging Of Steel
So much depends upon the forging of steel that this operation...



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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