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

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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