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Steel Making

The Effect
The heating at 1,600 deg.F. gives the first heat treatment w...

Effect Of Different Carburizing Material
[Illustrations: FIGS. 33 to 37.] Each of these different p...

Hardening High-speed Steels
We will now take up the matter of hardening high-speed steels...

Composition And Properties Of Steel
It is a remarkable fact that one can look through a dozen tex...

Quenching Tool Steel
To secure proper hardness, the cooling of quenching of steel ...

Heat Treatment Of Gear Blanks
This section is based on a paper read before the American Gea...

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

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

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

Reheating for machine ability was done at 100 deg. less than ...

Properties Of Alloy Steels
The following table shows the percentages of carbon, manganes...

Annealing Alloy Steel
The term alloy steel, from the steel maker's point of view, r...

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

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

An Automatic Temperature Control Pyrometer
Automatic temperature control instruments are similar to the ...

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

Classifications Of Steel
Among makers and sellers, carbon tool-steels are classed by g...

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

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

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

Temperatures To Use


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.

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
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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