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

Non-shrinking Oil-hardening Steels
Certain steels have a very low rate of expansion and contract...

Protective Screens For Furnaces
Workmen needlessly exposed to the flames, heat and glare from...

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

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

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

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

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

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

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

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

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

Critical Points
One of the most important means of investigating the properti...

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

Impact Tests
Impact tests are of considerable importance as an indication ...

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

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

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

Protectors For Thermo-couples
Thermo-couples must be protected from the danger of mechanica...

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



Hardness Testing






Category: ALLOYS AND THEIR EFFECT UPON STEEL

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
rebound.

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.

TABLE FOR BRINELL BALL TEST
------------------------------------------------------------------------
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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Previous: Fatigue Tests



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