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Annealing Work
With the exception of several of the higher types of alloy s...

Pyrometers For Molten Metal
Pyrometers for molten metal are connected to portable thermoc...

Crankshaft
The crankshaft was the most highly stressed part of the entir...

Carburizing Material
The simplest carburizing substance is charcoal. It is also th...

Ebbw Vale And The Bessemer Process
After his British Association address in August 1856, Besseme...

Oil-hardening Steel
Heat slowly and uniformly to 1,450 deg.F. and forge thorough...

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

Sulphur
Sulphur is another impurity and high sulphur is even a greate...

The Theory Of Tempering
Steel that has been hardened is generally harder and more br...

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

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

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

Case-hardening Treatments For Various Steels
Plain water, salt water and linseed oil are the three most co...

Gas Consumption For Carburizing
Although the advantages offered by the gas-fired furnace for ...

Short Method Of Treatment
In the new method, the packed pots are run into the case-har...

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

Using Illuminating Gas
The choice of a carburizing furnace depends greatly on the fa...

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

Brown Automatic Signaling Pyrometer
In large heat-treating plants it has been customary to mainta...

Hardness Testing
The word hardness is used to express various properties of me...



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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