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Ref. No. : ISO/R 726-1968 (E)
IS0
I N T E RN AT I ON A L O R G AN I z AT I ON F O R s TA N DARD I ZAT I O N
IS0 RECOMMENDATION
R 726
CALIBRATION OF STANDARDIZED BLOCKS
TO BE USED FOR BRINELL HARDNESS TESTING MACHINES
1st EDITION
May 1968
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The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 726, Calibration of standardized blocks to be used for Brinell
hardness testing machines, was drawn up by Technical Committee ISO/TC 17, Steel, the Secretariat
of which is held by the British Standards Institution (BSI).
Work on this question by the Technical Committee began in 1961 and led, in 1965, to the
adoption of a Draft IS0 Recommendation.
In March 1966, this Draft IS0 Recommendation (No. 926), replacing Draft IS0 Recommend-
ation No. 525, was circulated to all the IS0 Member Bodies for enquiry. It was approved, subject to
a few modifications of an editorial nature, by the following Member Bodies :
Argentina Israel Sweden
Australia
Italy Switzerland
Austria
Japan Turkey
Belgium Korea, Rep. of U.A.R.
Canada Netherlands United Kingdom
Czechoslovakia New Zealand U.S.A.
Denmark U.S.S.R.
Norway
Finland Poland Yugoslavia
France Romania
Germany South Africa,
Hungary Rep. of
India Spain
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council
Y
which decided, in May 1968, to accept it as an IS0 RECOMMENDATION.
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ISO/R 726-1968 IE
IS0 Recommendation R 726 May 1968
CALIBRATION OF STANDARDIZED BLOCKS
TO BE USED FOR BRINELL HARDNESS TESTING MACHINES
INTRODUCTION
Blocks standardized according to this IS0 Recommendation are adequate for the verification of ail
Brinell hardness testing machines. Any machine verified by these blocks may be employed, without
any additional verification, for tests on a modified Brinell scale, for example using a tungsten carbide
ball.
1. SCOPE
This IS0 Recommendation applies to the calibration of standardized blocks for the indirect veri-
fication of hardness testing machines as described in IS0 Recommendation R 156 (2nd edition),
Venfcation of Brinell hardness testing machines. It does not necessarily apply to the calibration of
test blocks which are used in the routine checking of the testing machine by the user, but it is not
intended to preclude the use of blocks calibrated in accordance with this IS0 Recommendation for
the routine checking of a machine.
NOTE. - Test blocks that are used in routine checking of the testing machine may be calibrated using the load
duration specified in clause 4.2 of IS0 Recommendation R 79 (2nd edition), Brinell hardness test for steel.
2. MANUFACTURE
Each metal block to be standardized should be of a thickness not less than
2.1
16 mm (5/8 in) for 10 mm balls,
12 mm (1/2 in) for 5 mm balls,
6 mm (1/4 in) for smaller bails.
2.2 The block should be specially prepared and the attention of the manufacturer is drawn to the
need to use a manufacturing process which will give the necessary homogeneity, stability of
structure and uniformity of surface hardness. It is recommended that the fineness and
regularity of grain and the uniformity of the metallurgical structure be verified by micro-
scopical examination. A microscopical examination may also be made by the standardizing
authority.
The standardized blocks should not be magnetized. It is recommended that the manufacturer
2.3
should ensure that the blocks, if of steel, have been demagnetized at the end of the manufac-
turing process.
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ISO/R 726-1968 (E
2.4 The flatness of the test surface and the uniformity of the thickness of the standardized block
should be in accordance with Table 1 below.
Maximum error Maximum change
Diameter of ball
in flatness of test surface in thickness
mm mm mm/50 mm
10 0.05 0.05
5 0.025 0.025
less than 5 0.005 0.0 1 O
2.5 The lower surface of the standardized block should have a ground finish.
2.6 The test surface (upper surface) should be polished and its surface roughness should not exceed
0.0003 mm (centre line average) for 10 mm balls and 0.000 15 mm (centre line average) for
smaller balls. It should also be free from scratches which interfere with the measurement of the
indentations.
2.7 To permit checking that no material is subsequently removed from the standardized block, its
thickness at the time of standardization should be marked on it to the nearest 0.1 mm
(0.004 in). or an official mark should be made on the test surface (see section 13).
3. STANDARDIZING PROCEDURE
The standardized blocks should be calibrated in a standardizing machine complying with the
requirements of sections 4 to 8, at a temperature of 20 ?r 2 "C in temperate climates and 27 f 2 "C in
tropical climates, using the general
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