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NOBLE” POLDI HARDNESS
TESTER |
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MODEL : “NP” |
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DIRECTIONS
FOR USE OF POLDI HARDNESS TESTER MODEL “NP”: |
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Insert the tapered end of the standard test bar after
cleaning into the space provided between the ball and the plunger
and slide it further beyond the tapered position. The test bat will
thus be firmly gripped between the ball and the plunger pressed by
a spring. The specimen to be tested be properly ground or filed and
polished at the surface where it is to be tested to ensure accurate
readings, With standard test bar inserted place the tester vertically
on the specimen such that the ball touches the polished surface. Give
a blow on the top of the plunger, with a hammer. |
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The blow should not be very hard and should
as vertical as possible. The blow will cause two indentations, one
on the standard test bar and the other on the specimen. The diameters
of these two indentations should be measured accurately with magnifier
having, lease count of 0.1 mm provided with the machine. If the indentations
so obtained are slightly elliptical, average diameter should be found
out. The distance between the centers of any two indentations on standard
test bar should be less than 15mm. If the bar is fully utilized it
has to be replaced. The limiting indentation diameter on the standard
test bar is 4.2mm. |
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SPECIFICATION OF POLDI TESTER |
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MATERIAL |
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ALLOY STEEL EN-8 OR EN-9 |
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DIA |
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30 MM |
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LENGTH |
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85 MM |
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TUNGSTON CARBIDE BALL |
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DIA |
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10 MM |
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POLDI BAR |
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MATERIAL |
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ALOOY STEEL EN-8 OR EN-9 OR CARBEN STEEL- WPS |
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SIZE |
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12mm x 12mm x 150mm |
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EYE PIECE |
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8 X |
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BRINELL MICROSCOPE |
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25 X (OPTIONAL AT EXTRA COST) |
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Tensile strength and
Brinell Hardness Numbers of various metals can be found as under : |
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STEELS |
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The booklet contains two tables, one for
steel in natural of annealed state and the other for steels in hardened
or hardened and tempered state. It is necessary to know the condition
of specimens before and It, however it is not known or cannot be determined
then first tables for natural or annealed steels be used for specimens
having Brinell hardness of up to 360. When the hardness of the specimens
exceeds 360, the other tables are to be used. |
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OTHER METALS |
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Separate Tables have been provided for
other metals, e.g. Brass, Copper, Aluminium, Bronze and Cast Iron.
These tables do not show the tensile strength of these metals but
only the Brinell hardness number because the relation between the
Brinell hardness & the corresponding tensile strength has not
yet established with as much reliability as in the case of steels. |
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PROCEDURE |
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After measuring the actual dia of indentation
on standard test bar the column “Dia of indentation on standard
test bar” should be referred for the same value and for actual
dia of indentation on work piece the raw “Dia of indentation
on work piece” should be referred. Where the horizontal and
vertical columns intersect the tensile strength in tones per sq. in
(the upper figures) and the Brinell Hardness Numbers (the lower figures)
can be read. To convert tensile strength from tons / sq. in to Kg.
/ sq. mm use a multiplying factors of 1.575. |
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To take care of the varieties in the tensile
strength and the hardness of standard test bars coming out of production,
each test bar is properly tested and a multiplying factor is inscribed
on it, at one end. |
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For determining the exact tensile strength
and hardness of specimens, multiply, the reading found from the tables
by this multiplying factor. |
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EXAMPLE |
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Diameter of ball indentation - |
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1) On the test bar 1.8 mm.
2) On the specimen (steel in natural condition) 1.7 mm.
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Tensile strength founds from Tables 50.8
tone/sq. inch after multiplying with 1.575 to convert in to kg/sq.
mm comes to 80 Kg./sq.mm. |
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Brinell Hardness Number found from tables
225 BHN. |
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Multiplying factor
inscribed on standard test bar: 1.14. Therefore, the tensile strength
of specimen is 80 x 1.14 = 91.20 Kg/sq. mm in and the Brinell hardness
= 225 x 1.14 = 256.5 BHN. When after some use, the protruding end
of the plunger gets deformed by blows, it may be reconditioned and
rounded off again or simply replaces. |
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Poldi type Impact Hardness Tester is not
designed to replace the tensile strength testing machine or standard
hardness tester. The accuracy of this hardness tester may very sometimes
to the extent
+/- 10% as against the more accurate table type standard Hardness
testers. However, the Poldi Impact Hardness Tester has the advantage
of easy handling and greater flexibility. This is very important,
especially for testing heavy specimens who cannot be taken to table
type hardness are extremely costly eompared to Poldi tester and require
greater skill to operated properly.
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