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Carbon Tool Steel
Heat to a bright red, about 1,500 to 1,550 deg.F. Do not ham...

Steel For Chisels And Punches
The highest grades of carbon or tempering steels are to be re...

Open Hearth Process
The open hearth furnace consists of a big brick room with a l...

Shrinking And Enlarging Work
Steel can be shrunk or enlarged by proper heating and cooling...

Care In Annealing
Not only will benefits in machining be found by careful anne...

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

Refining The Grain
This is remedied by reheating the piece to a temperature slig...

The Thermo-couple
With the application of the thermo-couple, the measurement of...

Heavy Forging Practice
In heavy forging practice where the metal is being worked at...

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

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

A Chromium-cobalt Steel
The Latrobe Steel Company make a high-speed steel without tun...

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

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

Lathe And Planer Tools
FORGING.--Gently warm the steel to remove any chill, is parti...

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

A Satisfactory Luting Mixture
A mixture of fireclay and sand will be found very satisfactor...

Uses Of The Various Tempers Of Carbon Tool Steel
DIE TEMPER.--No. 3: All kinds of dies for deep stamping, pres...

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

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

Hardness Testing


The word hardness is used to express various properties of metals,
and is measured in as many different ways.

Scratch hardness is used by the geologist, who has constructed
Moh's scale as follows:

Talc has a hardness of 1
Rock Salt has a hardness of 2
Calcite has a hardness of 3
Fluorite has a hardness of 4
Apatite has a hardness of 5
Feldspar has a hardness of 6
Quartz has a hardness of 7
Topaz has a hardness of 8
Corundum has a hardness of 9
Diamond has a hardness of 10

A mineral will scratch all those above it in the series, and will
be scratched by those below. A weighted diamond cone drawn slowly
over a surface will leave a path the width of which (measured by
a microscope) varies inversely as the scratch hardness.

Cutting hardness is measured by a standardized drilling machine,
and has a limited application in machine-shop practice.

Rebounding hardness is commonly measured by the Shore scleroscope,
illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter,
3/4 in. in length, and weighing about 1/12 oz. is dropped a distance
of 10 in. upon the test piece. The height of rebound in arbitrary
units represents the hardness numeral.

Should the hammer have a hard flat surface and drop on steel so hard
that no impression were made, it would rebound about 90 per cent
of the fall. The point, however, consists of a slightly spherical,
blunt diamond nose 0.02 in. in diameter, which will indent the steel
to a certain extent. The work required to make the indentation
is taken from the energy of the falling body; the rebound will
absorb the balance, and the hammer will now rise from the same
steel a distance equal to about 75 per cent of the fall. A permanent
impression is left upon the test piece because the impact will
develop a force of several hundred thousand pounds per square inch
under the tiny diamond-pointed hammer head, stressing the test
piece at this point of contact much beyond its ultimate strength.
The rebound is thus dependent upon the indentation hardness, for
the reason that the less the indentation, the more energy will
reappear in the rebound; also, the less the indentation, the harder
the material. Consequently, the harder the material, the more the

Indentation hardness is a measure of a material's resistance
to penetration and deformation. The standard testing machine is
the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter,
is forced into the test piece with a pressure of 3,000 kg. (3-1/3
tons). The resulting indentation is then measured.

While under load, the steel ball in a Brinell machine naturally
flattens somewhat. The indentation left behind in the test piece is
a duplicate of the surface which made it, and is usually regarded
as being the segment of a sphere of somewhat larger radius than
the ball. The radius of curvature of this spherical indentation
will vary slightly with the load and the depth of indentation.
The Brinell hardness numeral is the quotient found by dividing the
test pressure in kilograms by the spherical area of the indentation.
The denominator, as before, will vary according to the size of the
sphere, the hardness of the sphere and the load. These items have
been standardized, and the following table has been constructed
so that if the diameter of the identation produced by a load of
3,000 kg. be measured the hardness numeral is found directly.

Diameter of Ball Hardness Number Diameter of Ball Hardness Number
Impression, mm. for a Load of Impression, mm. for a Load of
3,000 kg. 3,000 kg.
2.0 946 4.5 179
2.1 857 4.6 170
2.2 782 4 7 163
2.3 713 4.8 156
2.4 652 4.9 149
2.5 600 5.0 143

2.6 555 5.1 137
2.7 512 5.2 131
2.8 477 5.3 126
2.9 444 5.4 121
3.0 418 5.5 116

3.1 387 5.6 112
3.2 364 5.7 107
3.3 340 5.8 103
3.4 321 5.9 99
3.5 302 6.0 95

3.6 286 6.1 92
3.7 269 6.2 89
3.8 255 6.3 86
3.9 241 6.4 83
4.0 228 6.5 80

4.1 217 6.6 77
4.2 207 6.7 74
4.3 196 6.8 71.5
4.4 187 6.9 69

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