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The Leeds And Northrup Potentiometer System
The potentiometer pyrometer system is both flexible and subst...

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

Quenching
It is considered good practice to quench alloy steels from th...

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

Carburizing By Gas
The process of carburizing by gas, briefly mentioned on page ...

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

Annealing Method
Forgings which are too hard to machine are put in pots with ...

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

Mushet And Bessemer
That Mushet was "used" by Ebbw Vale against Bessemer is, perh...

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

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

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

Blending The Compound
Essentially, this consists of the sturdy, power-driven separa...

Preventing Cracks In Hardening
The blacksmith in the small shop, where equipment is usually ...

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

The Modern Hardening Room
A hardening room of today means a very different place from ...

Rate Of Absorption
According to Guillet, the absorption of carbon is favored by ...

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

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

The Quenching Tank
The quenching tank is an important feature of apparatus in c...



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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