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Measurement of
hardness
Hardness is the property of a
material that enables it to resist plastic deformation, usually
by penetration. There are elastic and dynamic processes to
measure hardness. The usual
method to achieve a hardness value is to measure the depth or
area of an indentation left by an indenter of a specific shape,
with a specific force applied for a specific time. According to
the test force: macro hardness (F force > 30 N), small hardness
load (3-30 N) and micro hardness (< 0,5N).There are three
principal standard test methods for expressing the relationship
between hardness and the size of the impression, these being
Brinell, Vickers, and Rockwell. For practical and calibration
reasons, each of these methods is divided into a range of scales,
defined by a combination of applied load and indenter geometry.
There are available the DIN 50150 to
compare the obtained results according to one of the selected
measuring methods.
Brinell Hardness test
HB.
An iron ball of tempered steel (HBS) or a hard metal ball (HBW)
with a known diameter is pressed vertically with a test force F
in an isolated testing surface. Test force is exerted during a
defined time (from 2 to 8 seconds); incidence time must be from
10 to 15 seconds. Brinell hardness is calculated by the pressure
diameter and the test force. The
hardness measurement developed by Brinell is used for soft and
medium hard metals, non-alloyed construction grade steel,
aluminium, wood and work materials with irregular structures
such as cast iron. An iron ball or one made of a hard metal is
struck with a defined force for measurement against the surface
of the material
Vickers Hardness test
HV
The Vickers hardness test method consists of indenting
the test material with a diamond indenter, in the form of a
right pyramid with a square base and an angle of 136 degrees
between opposite faces subjected to a load of 1 to 100 kgf. The
full load is normally applied for 10 to 15 seconds. The two
diagonals of the indentation left in the surface of the material
after removal of the load are measured using a microscope and
their average calculated. The area of the sloping surface of the
indentation is calculated. The Vickers hardness is the quotient
obtained by dividing the kgf load by the square mm area of
indentation. The Vickers hardness
testt measures hard work materials but also the hardness of
materials and thin walls or marginal areas.
Rockwell Hardness test
The Rockwell hardness test method
consists of indenting the test material with a diamond cone or
hardened steel ball indenter. For C and A versions (hard and
very hard materials) as well as for N version (for thin walls) a
diamond ball is used. For B and F versions (medium-hard and soft
materials) or T (for thin walls) we use a steel ball.
The
permanent increase in depth of penetration, resulting from the
application and removal of the additional major load is used to
calculate the Rockwell hardness number.
The abbreviations for Rockwell are the following: HRC,
HRA, HRB, HRF, HR- 15N, HR15T, HR30N, HR30T, HR45N, HR45T. The
hardness value goes before teh abbreviation: 45 HRC; 76
HR45N. DIN 50103.
Shore hardness test
Testers of Shore hardness for rubber and similar work materials
are composed of a spring loaded point which rebounds when it
penetrates during testing, serving as a measurement of the
corresponding Shore hardness, which is indicated in a scale from
0 Shore (2.5 millimeters of penetration) up to 100 Shore (0
millimeters of penetration)
Impact hardness test
Dynamic process to measure hardness. The calculation basis
is the diameter of a ball pressure produced by a hammer (Poldi
hammer) or by a winding spring. With the Poldi hammer the ball
is pressed in a known test hardness bar.
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