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Steel Making

Placing The Thermo-couples
The following illustrations from the Taylor Instrument Compan...

Temperature For Annealing
Theoretically, annealing should be accomplished at a tempera...

Properties Of Steel
Steels are known by certain tests. Early tests were more or l...

Steel is hardened by quenching from above the upper critical....

Tempering Colors On Carbon Steels
Opinions differ as to the temperature which is indicated by t...

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

Robert Mushet
Robert (Forester) Mushet (1811-1891), born in the Forest of D...

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

The Forging Of Steel
So much depends upon the forging of steel that this operation...

Tungsten, as an alloy in steel, has been known and used for a...

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

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

The forgings can be hardened by cooling in still air or quen...

Separating The Work From The Compound
During the pulling of the heat, the pots are dumped upon a ca...

Preparing Parts For Local Case-hardening
At the works of the Dayton Engineering Laboratories Company, ...

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

Chromium when alloyed with steel, has the characteristic func...

Tool Or Crucible Steel
Crucible steel can be annealed either in muffled furnace or b...

Leeds And Northrup Optical Pyrometer
The principles of this very popular method of measuring tempe...

Nickel may be considered as the toughest among the non-rare a...

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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