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Steel Making

Drop Forging Dies
The kind of steel used in the die of course influences the he...

Cutting-off Steel From Bar
To cut a piece from an annealed bar, cut off with a hack saw,...

Heating Of Manganese Steel
Another form of heat-treating furnace is that which is used ...

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

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

Process Of Carburizing
Carburizing imparts a shell of high-carbon content to a low-...

Highly Stressed Parts
The highly stressed parts on the Liberty engine consisted of ...

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

Hints For Tool Steel Users
Do not hesitate to ask for information from the maker as to t...

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

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

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

Air-hardening Steels
These steels are recommended for boring, turning and planing...

Hardening High-speed Steel
In forging use coke for fuel in the forge. Heat steel slowly ...

The Influence Of Size
The size of the piece influences the physical properties obta...

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

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

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

Steel Worked In Austenitic State
As a general rule steel should be worked when it is in the a...

SILICON is a very widespread element (symbol Si), being an es...

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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