MATERIALS Q&A BYDONBUSH
QUESTION:
Are portable hardness testers
adequate for verifying the
hardness of parts for
compliance to industry
standards such as the NACE
sour service standards?
ANSWER:
There are a number of portable hardness
testing devices on the market. These
devices are attractive to manufacturers
and repair companies for several reasons,
such as:
Portability: The units can be easily
used throughout the shop or in the
field for hardness testing of components that cannot be easily brought
to a bench-top hardness tester.
Large components: They can be
used on components that are too
large to test with a bench-top
tester.
Cost: The devices typically cost less
than a bench-top tester.
ings have been made). Indentations
should be 3-4 mm in diameter (although
ASTM A833 only specifies that the
indentations not exceed 4. 2 mm), and
such large indentations may not be
acceptable on many parts, especially
smaller ones. Finally, the resulting
Brinell hardness value must often be converted to another scale, likely Rockwell B
or C, which can introduce uncertainty in
some situations. However, since ASTM
E140 includes some conversion tables
that include Brinell and Rockwell values,
there is at least a standardized basis for
those conversions for some materials.
ured is superficial, and therefore may not
be representative of the bulk hardness.
Finally, since virtually no specifications
reference Leeb hardness requirements,
the resulting hardness values must be
converted to another scale. ASTM E140
includes no hardness conversion tables
that include Leeb values, so one must rely
on tables provided by the vendor for conversion, which introduces more uncertainty than when using tables published
in ASTM E140.
Leeb Testers
Leeb testers, covered by ASTM A956,
bounce an indenter off of a part and use
the difference in the inbound and outbound velocities to determine the hardness. The value measured is a Leeb hard-
Ultrasonic Contact Impedance (UCI) Testers
Ultrasonic contact impedance testers,
covered by ASTM A1038, utilize an
ultrasonically-excited, spring-loaded
Vickers diamond indenter and a device to
measure the frequency shift that occurs
when the indenter penetrates the specimen. The change in frequency is used to
determine the size of the indentation,
There are several popular portable
devices on the market. Three common
devices are described below:
Portable hardness testers have their place in
production and field applications, as well as in
laboratory environments. However, like all hardness
testers, they need to be used with careful
consideration of their capabilities and limitations.
Comparison Hardness Testers
Comparison hardness testers, covered by
ASTM A833, feature a device that incorporates a Brinell ball and a square cross-section bar of known Brinell hardness.
A hammer is used to strike the device
against the part to be measured. Indentations are produced in the part and in the
known bar. An equation or slide-rule
device is used to determine the Brinell
hardness of the part based on the indentation diameters in the known bar and
the specimen and hardness of the known
bar.
Drawbacks of Comparison Testers:
Comparison hardness testing requires the
use of calibrated, consumable bars of
known hardness (i.e., the bar must be
replaced after a certain number of read-
ness number. Generally, this Leeb
hardness number is then converted to
some other scale (such as Rockwell or
Brinell) using conversion charts to determine if the material meets some specified
requirement.
Drawbacks of Leeb Testers: Leeb testing requires that the part have a certain
minimum weight, ranging from 1. 5 kg ( 4
lbs) to 15 kg ( 40 lbs) depending on the
device type and the thickness of the part.
There are minimum surface finish
requirements, ranging from 0.4 μm ( 16
μin) Ra (corresponding to 500 grit emery
paper) to 7 μm (250 μin) (corresponding
to 65 grit emery paper) depending upon
the impact device. The hardness meas-
which is then used to determine the Vickers hardness.
Drawbacks of UCI Testers: UCI testing requires the part have a minimum
weight of at least 300 g (0.7 lbs). There
are restrictions regarding minimum wall
thickness and part stiffness to ensure the
part itself is not excited by the ultrasonic
vibration. There are minimum surface finish requirements that are dependent upon
the test load and the hardness of the specimen. The accuracy of the measurement is
dependent upon the surface finish. For
example, calibration blocks are required
to be finished to 0.4 μm ( 16 μin) or better
for accuracy. The hardness measured is
superficial, and may not be representative
