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

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

Sulphur
SULPHUR is another element (symbol S) which is always found i...

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

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

Phosphorus
PHOSPHORUS is an element (symbol P) which enters the metal fr...

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

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

Heat Treatment Of Punches And Dies Shears Taps Etc
HEATING.--The degree to which tools of the above classes shou...

Complete Calibration Of Pyrometers
For the complete calibration of a thermo-couple of unknown e...

Carburizing Low-carbon Sleeves
Low-carbon sleeves are carburized and pushed on malleable-ir...

Manganese
Manganese adds considerably to the tensile strength of steel,...

Surface Carburizing
Carburizing, commonly called case-hardening, is the art of pr...

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

Steel Can Be Worked Cold
As noted above, steel can be worked cold, as in the case of ...

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

Connecting Rods
The material used for all connecting rods on the Liberty engi...

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

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

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



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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