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Steel Making

Carburizing By Gas
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The Leeds And Northrup Potentiometer System
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High Speed Steel
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Carbon Steels For Different Tools
All users of tool steels should carefully study the different...

Bessemer Process
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Heat-treating Department
The heat-treating department occupies an L-shaped building. ...

Annealing To Relieve Internal Stresses
Work quenched from a high temperature and not afterward tempe...

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

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

Pyrometry And Pyrometers
A knowledge of the fundamental principles of pyrometry, or th...

Silicon
Silicon prevents, to a large extent, defects such as gas bubb...

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

Heating
Although it is possible to work steels cold, to an extent de...

Gears
The material used for all gears on the Liberty engine was sel...

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

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

Hardening Carbon Steel For Tools
For years the toolmaker had full sway in regard to make of st...

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

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

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



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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