HARDMEKO 2004

Hardness Measurements Theory and Application in Laboratories and Industries

11-12 November, 2004, Washington, D.C., USA

EQUOTIP – REBOUND HARDNESS TESTING AFTER D. LEEB

M. Kompatscher

PROCEQ SA, Schwerzenbach ZH, Switzerland

Abstract Hardness tests have always been important local, regional level. Within the last decades also several
to conclude on specific mechanical properties of materials in improvements in standardization on a global level could be
a fast and economic manner, i.e. in a (quasi) non destructive achieved.
test. This paper has its focus on the most recent test method, Standardized hardness tests according to the static
the dynamic rebound hardness test method after D. Leeb. methods of Brinell, Vickers and Rockwell and to the
The EQUOTIP1, Leeb’s original instrument, is fully dynamic method of Leeb, respectively, are the most
discussed and newest improvements are presented. So far, frequently used. Each method allows for presentation of
PROCEQ SA, Swiss manufacturer of the instruments, results in several, different scales like Rockwell C or B,
maintained and protected the constancy of the L-Value over HV5 or HV100. The scale definition depends on method
the last 30 years. Recent round robin results show the way, and test parameters applied (indenter type and shape, test
where standardization work can be improved to hold the force application and quantity). Other hardness test methods
high level of reliability of the measuring base, the L-Value. conclude to these values indirectly by comparison
measurements.
Keywords: EQUOTIP, Dynamic Hardness, Portable The classic, stationary hardness tests after Brinell,
Hardness Testing Vickers and Rockwell are well established. However, as
soon as it was required to test very large pieces or pieces
1. INTRODUCTION that are unwieldy to be tested in the usual types of testing
machines no alternative was available. Testing parts of fixed
Hardness tests are important means in materials research structures, or testing under any conditions which require that
and industrial production as mechanical characteristics of the indentation force be applied in a direction other than
materials are retrieved quickly and economically. For vertical, made the search for portable solutions
example, in industrial production specific material indispensable. Comparators working according to the UCI
treatments (alloying, cold and hot working, heat treating …) (ultrasonic contact impedance) test method or according to
have a strong influence on the final conditions of products the Pin- and Tele-Brinell test method were used at first, but
and their mechanical properties. Hardness tests provide a basically dynamic hardness testing showed its practical use
convenient and reliable mean to optimize material properties due to fast and easy use and high accuracy [2-4]. The use of
or the production process and to quantify the defined those portable hardness testers was initially limited to field
specifications in a final quality control. tests where the test piece couldn’t be brought to the testing
The term “hardness” may be defined as the ability of a instrument. However, meanwhile many different portable
material to resist permanent indentation or deformation hardness testers proved there excellence even for laboratory
when in contact with an indenter under load. Also the more use.
general variant “Hardness is the response of a material to
stressing by penetration exerted by a body harder than itself” 2. EQUOTIP
is used, especially for dynamic hardness testing [1].
Generally, a hardness test consists of pressing an indenter of 2.1. Leeb’s invention
known geometry and mechanical properties under pre- Dietmar Leeb studied in the early 70ties the different
defined conditions into the test material. hardness test methods with main focus on portable solutions
Different hardness test methods were developed and like the method of Baumann-Steinrück, Schmidt and Shore
adopted in a big variety of test problems. The main concern [5]. The most frequently used portable hardness testers
is always to compare test results on an unequivocally base, where of dynamic type, where the test load is applied
an accepted reference scale. As hardness depends on the test impulsively. The question arose, how can - without loss in
method and the definition of the application parameters, measurement accuracy - the common hardness range be
hardness is not a fundamental (physical) property of extended and the operation become more flexible, e.g.
materials. Thus, it is highly important of clearly defining the independent of test direction, faster, user independent and
instrument and the test parameters. Several decades ago comfortable.
already first agreements and standards where developed on a The genius result was the invention and successful
production of the EQUOTIP in 1975 (Fig. 1), a dynamic
1
EQUOTIP is a registered trademark of PROCEQ SA. hardness test method and instrument [6]. This method is also
known in connection with the inventors name as Leeb
hardness test method or rebound hardness test after D. Leeb.
The EQUOTIP is nowadays the most frequently used
portable hardness tester. Users appreciate especially it’s
flexibility, ease of use and fast and accurate measurement
over a long period of time. Due to it’s usefulness – even in
overhead position – and the sound measurements very
frequently the whole series production is controlled by
EQUOTIPs where now also the need for an automated tester
has grown up.

Fig. 1. First two models of EQUOTIP hardness testers. Above:

1975 series; below: series of red hexagonal EQUOTIP, produced
between 1978 and 1990 (courtesy PROCEQ SA, Switzerland).

2.2. The EQUOTIP principle Fig. 2. Schematic view of the EQUOTIP impact device D.
An impact device (Fig. 2) fires an impact body
containing a permanent magnet and the very hard indenter thus, the impact body containing the magnet. The induced
sphere itself towards the surface of the test material. The induction signal (Fig. 3) is recorded in an electronic
velocity of the impact body is recorded in three main test indicator device and the peak induction voltages are further
phases; processed to give Leeb’s hardness number, the L-value. The
i) the pre-impact phase, where the impact body is shape of the induction signal is unique for each device type.
accelerated by spring force towards the surface of the test
piece.
ii) the impact phase, where the impact body and the test
piece are in contact. The hard indenter tip deforms the
test material elastically and plastically and is deformed
itself elastically. After the impact body is fully stopped,
elastic recovery of the test material and the impact body
takes place and causes the rebound of the impact body.
iii) the rebound phase, where the impact body leaves the test
piece with residual energy, not consumed during the
impact phase.
Leeb’s genius idea was to measure the velocity of the
impact body contact-free via the induction voltage generated
by the moving magnet trough a defined induction coil Fig. 3. Typical induction voltage signal generated by the
mounted on the guide tube of the device. The induced permanent magnet inside the impact body during the main three
voltage is directly proportional to the velocity of the magnet, test phases of an EQUOTIP hardness test.
 2.3. The L-value The complex interaction of all these parameters during
The L-value, also known as Leeb-number or Leeb- the whole hardness test can be simply summarized in the
hardness (HL), is simply described as equal to the ratio of term “overall elasticity” (OE). This accounts for the overall
the rebound velocity vr to the impact velocity vi of the behaviour of the impact body as a whole during the impact.
impact body, multiplied by 1000; The main critical components are Young’s-modulus of the
individual parts, geometry and the interconnection of these
Aˆ vr parts. This explains also, why serious manufacturers don’t
L 1000 1000 . (1) offer questionable repair tools to allow for an uncontrolled
Bˆ vi
indenter change.
The point of velocity measurement is determined by the
The peak induction voltages are proportional to the position of the induction coil on the guide tube of the
impact and rebound velocities and are measured at a defined indicating device. At this point, the induction voltages are
position from the test surface. measured. In case of device type D this position is about 1
A fully elastic rebound (vr = vi) would cause L = 1000, mm from the test surface, thus, in principle the velocity of
all energy is elastically recovered and no plastic deformation the impact body is measured at a defined position from the
takes place. On the other hand L will be lowered with test surface. If the measurement point is too near, the
decreasing material hardness, where less resistance to plastic reproducibility of the measurement suffers, because the
deformation is present and a lot of energy is consumed signal is often slightly disturbed shortly after the impact. On
during the indentation. The measurement procedure also the other hand when the distance becomes too long then
explains the naming of the EQUOTIP, where EQUO = internal friction may alter the measurement of rebound
Energy QUOtient. speed vr. Also the width of the measured signal curve plays
The symbol for the hardness scale, L or HL, is followed a role, because it determines, how good the proportionality
by a suffix character, representing the type of impact device between minimum value B and rebound vr velocity is.
used for the test, e.g. LD, LS, LG, … . As in all hardness In summary, the specified impact body represents by
tests, the instruments and the key parameters influencing the way of impact velocity, impact body mass and overall
hardness reading need to be defined within close tolerances. elasticity (vi, m, OE) the penetration stressing and in form of
The main instrument parameters influencing the L-value are; the rebound velocity vr the materials response to this
a) the impact energy penetration. Thus, all information on hardness is available
b) the impact body and the L-value is a suitable and direct measure for materials
c) the point of velocity measurement hardness.
The impact energy (kinetic energy of the impact body
when impacting the surface of the test object) is determined
by its mass m and its velocity vi in the moment of the
impact;
2
m vi
Ei . (2)
2

However, even if absolute energy is kept constant,

different combinations of mass and velocity would give a
different response of the material tested and consequently a
different L-value. Thus, both, mass and velocity need to be
precisely defined. It was Leeb’s intention to make gravity
and friction effects on impact velocity as small as possible,
thus, the flight path is kept as short as possible, the
measurement point is close to the surface, spring force is
used to accelerate the impact body and magnetic and eddy
current influences are controlled.
The impact body is of paramount importance for the L-
value. His specific characteristics result from the
combination of the individual structural components;
- the material, form and shape of the indenter, e.g. the
spherical diamond test tip of impact device E.
- the material, orientation and dimensions of the
permanent magnet and its position in the impact body.
- the material, stiffness, form and dimension of the impact
body as a whole.
- the way of combining these different elements with each
other. Fig. 4. Family of seven EQUOTIP impact devices (courtesy
PROCEQ SA, Switzerland).
 2.4. Different impact devices surface hardened components (case hardened) and
Depending on application out of a whole family of coatings with a min. layer thickness of 0.3 mm (0.01 in.),
impact devices (Fig. 4) can be selected. as well as walled or impact sensitive components (small
i) Impact device D is the universal unit for most hardness measuring indentation). Measurements can be made on
measurements with a wide measuring range, testing steel and cast steel, cold worked tool steel, and cast
components up to a maximum hardness of 68 HRC. It is aluminium alloys. Better surface finish than impact
recommended for steel and cast steel, cold work tool device D is required.
steel, stainless steel, cast iron (lamellar and nodular
graphite), cast aluminum alloys, brass, bronze, wrought To extend the application range of the EQUOTIP
copper alloys (low alloyed). instruments also for small, light weight and thin materials,
ii) If high volume measurements are performed and the the use of a static hardness probe, developed also by Mr.
wear of impact body D is reached fast, the newly Leeb according to the Rockwell method, is also possible.
developed impact device S is recommended. Impact The probe R5, also known as EQUOSTAT, is fully
device S is has a 10times longer durability than impact compatible with the EQUOTIP indicating device (Fig. 5,
device D and will most probably become the standard upper image). Thus, EQUOTIP has become a complete
rebound hardness tester of the future. The same materials portable hardness test system for all kinds of metals.
as with impact device D are tested but with slow wear of
the impact body’s test tip. 3. APPLICATIONS
iii) Impact device E utilizes a synthetic diamond test tip
(approx. 5000 HV), testing components up to a 3.1. Applications
maximum hardness of 72 HRC. The same materials as The EQUOTIP is a very renowned test method in nearly
standard D unit are tested but preferred at extended all industries where metallic components are fabricated, heat
hardness range, i.e. above 800 LD (58 HRC, 690 HV). treated, machined and maintained (see Fig. 5 for typical
Applications for measurement are in the high end range, applications). Big, massive and thick test objects like heavy
such as steel and cast steel, stainless steel, cold work tool rolls, dyes and moulds, rails and tracks on tooling machines,
steel with carbide inclusions and on rolls in the hardness cranes, pressure vessels, pipelines and motor blocks are
range up to 1200 HV. Impact bodies show no fading classic applications.
even at high hardness levels compared to device D. The application range for small and thin test objects is
iv) Impact device DC is a short impact device with the same accessible with the optional static probe R5 (Rockwell low
properties and applications as impact device D, testing load tester). The instrument is best suitable for all kinds of
components up to a maximum hardness of 68 HRC. It is metals, even curved surfaces and hard to access test
suited for special applications in very confined spaces locations can be tested.
such as holes, cylinders, or measurements inside of
assembled machines and constructions.
v) Impact device D+15 tests components up to a maximum
hardness of 68 HRC and has the same range of
applications like D/DC but utilizes a particularly slim
front section, which allows hardness measurements in
holes and grooves and on recessed surfaces by using an
elongated impact body and a coil position elevated 15
mm (0.6 in.).
vi) Impact device DL tests components up to a maximum
hardness of 68 HRC. The range of applications is the
same as D+15, but has the special feature of a slimmer
front section — 4 mm diameter × 50mm (0.16 × 2 in.) —
for use in confined spaces and at the base of grooves,
drill holes, and gears.
vii) Impact device G features increased impact energy
(approx. 9× that of the standard impact device D), testing
components up to a maximum hardness of 646 HB.
Application is in the Brinell range on heavy coarse
grained castings and forgings, steel and cast steel, cast
iron (lamellar and nodular graphite), and cast aluminum.
The increased diameter of the spherical test tip provides
in addition a better averaging of the coarse grained
microstructure. It requires less surface finish than impact
device D for accurate readings.
viii) Impact device C uses reduced impact energy (about
25% of that for impact device D), testing components up Fig. 5. EQUOTIP applications for portable and automatic testers
(courtesy PROCEQ SA, Switzerland).
to a maximum hardness of 70 HRC. Applications are
 3.2. Requirements It is alarming to see, that instruments of different origin
Suitability of one of the EQUOTIP impact devices for a than the EQUOTIP may give considerable different results
specific application depends on the following criteria; at discretion of the well defined Leeb Method. Quite often
- Material (grade, structure, physical properties) the measurements are good in the hardness range of standard
- Mass and thickness of the test piece and support of the reference blocks between 700 and 750 LD. But below and
test location above this hardness level, deviations may be considerably
- Surface properties (roughness, curvature, test area) larger (up to 40 LD). The results by such testers have to be
- Accessibility of the test location considered insignificant and erroneous [7].
- Indentation size The EQUOTIP measurement base, the L-value, was
Generally it is recommended to perform the test on maintained constant for the last 30 years by the developer
objects of sufficient mass and size properly supported and at and manufacturer. Some of the first instruments sold at the
rest. Care should be taken when preparing the surface finish very beginning are still in use and the measured L-value is
as heat or cold working could change the surface hardness. within the specified limits in perfect agreement with results
Each hardness test is a localized test, thus, the result is valid of newly produced EQUOTIP instruments. Other
for a determined zone only. In order to get a representative manufactures, however, may use production criteria which
average for a test object, it is recommended to use statistic have an influence on the original L-value as discussed
aids. Typically the mean value, the standard deviation and above. There are also instruments available using a different
the scattering range of five to ten impacts is used to retrieve method of velocity determination (triple coil technique).
general test results. These instruments need to be calibrated to the original L-
value as defined by the EQUOTIP instrument. The same
4. INTERNATIONAL STANDARDIZATION WORK holds as for conversion relations to static hardness values;
the genuine instrument value is more accurate than any
4.1. EQUOTIP - reliability tuned value, especially, if the whole hardness range is
The widespread acceptance to the industries of the considered.
EQUOTIP principle has led to it’s standardization in 1996
(ASTM 956-96). Test procedure, instrument verification and 4.2. EQUOTIP – conversions
calibration of reference material are described therein. Since At the very beginning of the EQUOTIP hardness test
that time, many companies started to produce variants method, the new hardness number Leeb (HL, L-value) was
similar to the original EQUOTIP. Recently, a round robin not known and conversion had to be supplied into traditional
test has been started by the ASTM with the goal to confirm hardness values like Brinell (HB), Rockwell (HR), Vickers
the EQUOTIP conversion curves. A wide hardness range is (HV) or Shore (HS). As the users had already their
covered by using a set of standard reference plates, all experiences to assess production quality, they wanted to get
calibrated in HB and HRC (Fig. 6). familiar with the new number gradually. In the meantime
Excellent agreement of the EQUOTIP conversion curve more and more the genuine Leeb hardness value according
with the measured values on NIST traceable reference to the EQUOTIP principle is accepted as decisive method to
material is found. The variant, however, shows a constant qualify production processes. It’s significance and reliability
offset of about 10 LD and consequently a remarkable for acceptance tests has been certified throughout many
discrepancy in the conversion chart. This highlights the fact, industries. Industry simply adopted the corresponding
that dynamic hardness testers according to Leeb but of written specifications, thus, it was not necessary to change
different origin than the EQUOTIP, can deviate any working procedure and the relevant production control
considerably from the original defined hardness scale. doesn’t suffer from conversion scattering.
Conversion into other scales has to be considered with
56
EQUOTIP conversion for HRC = f(LD) for material group 1 care in order to avoid problems related to changes in
52 material properties, e.g. due to handling (stress induced
48
hardening for instance). The “real” converted hardness value
may divert from converted curves out of standard materials
44

40

36 of similar grade. This effect is significant for all conversions

32 20 between scales even for HB HR, HV HR, HRx
HRy etc. (x, y different parameter defining unequivocally
28 15
10
24

20
5
0
the corresponding scale).
16 450 550
LDEQUOTIP
650 750
As the way of material usage is different in different test
12
450 500 550 600 650 700 750
methods and as each test has its own uncertainty, the
EQUOTIP conversion HRC=f(LD) +confidence -confidence
LD-value
correlation of two methods is always more affected
VARIANT EQUOTIP Instr.1 EQUOTIP Instr.2
regarding accuracy than in a specific test itself. In addition,
Fig. 6. First ASTM round robin results of Leeb hardness testers on such conversion curves are always material specific, thus
NIST traceable reference material. The continuous line rapresents depending on the material the correlation can be different.
the common EQUOTIP conversion curve within confidence This holds especially for dynamic rebound hardness values,
intervals. The small image inside the graph shows the deviation where the permanent deformation behaviour, the hardness,
from the LD-values of the original EQUOTIP if a tester of different is basically determined by the elastic response of the
origin (variant) is used. material. Thus, the elastic properties, Young’s modulus and
the yield strength, of the material to test have a dominant EQUOTIP components are fully interchangeable from
effect on the test result. In Fig. 7 a simplified stress-strain instrument to instrument. The EQUOTIP reference
diagram for two materials of equal yield strength R but instrument fulfils of course more stringent requirements
different Young’s-moduli, E1 and E2, illustrates the effect. than the series instrument and was maintained constant
The same amount of total deformation work W used to over all the time by the manufacturer of the EQUOTIP.
penetrate a defined indenter into the material causes ii) The impact devices and impact bodies are designed in
different elastic (Wel) and plastic (Wpl) deformation. The such a way, that a user cannot alter or “adjust” anything.
material with higher E-modulus, E1, will accumulate a This concept helped to guarantee over all the years the
significantly smaller part of the elastic energy and therefore constant quality and absolute comparability of the L-
release less residual energy when the testing body rebounds. value measured by an EQUOTIP instrument. However,
the performance of the instrument can easily be checked
(verified) using a calibrated reference material.
iii) Standard reference blocks are tested by a reference
E1 E2 instrument, a calibration machine. The measured value is
R R
than marked on the reference block, this is called
calibration. Calibration instruments need to conform to
highest quality requirements. The requirements on the
Wpl1 Wpl2 reference material are material, treatment, dimension,
Wel1 Wel2 surface, magnetization and homogeneity over the whole
test area. The calibrated reference blocks are used to
indirectly check the performance of series produced
Strain hardness testers.
Fig. 7. Schematic view of a stress-strain curves from two materials
Once the reference block is consumed, i.e. the calibrated
with different Young’s-modulus but same yield strength. surface area is fully impacted; the functionality of the
block can not be regained by grinding or any other
A correlation between L-value and static hardness is means. The reason is that the required hardness
only possible for specific materials. However, it was found uniformity over the whole surface area is very
that the empirical assessed conversion curves are valid for a production sensitive and unique. Hardness uniformity
broad range of materials, grouped according their common and calibration values are not applicable to subsurface
elastic properties (e.g. 10-20% spread in Young’s modulus). cross sectional areas.
For example in Fig. 6, the EQUOTIP conversion curve
HRC=f(LD) for material group 1 (unalloyed and low The traceability back to national standards is an
alloyed steel and cast steel in hot rolled or forged and important instrument criterion, e.g. “NIST traceability” is a
thermally treated condition) was determined based on commonly found term in the USA. The meaning of it is that
standard reference material (DIN Werkstoff No. 1.2842, i.e. an instrument can be verified on reference material traceable
AISI O2, DIN Werkstoff No. 1.0402, i.e. AISI 1020, DIN to the corresponding national entity, e.g. the NIST. NIST
Werkstoff No. 1.0301, i.e. AISI 1010) traceable to the has the primary hardness machine for the Rockwell C Scale
German national calibration standard at MPA Nordrhein- and provides reference material calibrated with this
Westfalen, Dortmund, and confirmed on reference material machine. All Rockwell testers can be checked for
(AISI 4140, i.e. DIN Werkstoff No. 1.7225) traceable to the conformity. Once the instrument conformity is given, this
American national calibration standard at NIST, Rockwell tester, e.g. of an accredited laboratory, can
Gaithersburg. calibrate secondary reference material. This procedure
However, when conversions need to be made they provides traceability back to the primary machine at NIST
should be done with discretion and under controlled or at any other national entity, maintaining the national
conditions. Converted hardness values should be considered primary hardness test machine.
as informative estimation and not as acceptance values. The Hardness testers providing conversion curves to other
important exception is to have direct comparison of results hardness scales like the Rockwell C scale can by this way
between different hardness testing methods on the same indirectly be checked for conformity with the national
material. standard, even, if basically the conversion is verified. Note:
each hardness number is a combination of several factors
3.5. Calibration and Verification which can have a compensating influence on each other.
In EQUOTIP hardness testing, the following distinction Thus, it is generally recommended to verify an EQUOTIP
is made; frequently on the supplied reference test block (indirect
i) An EQUOTIP hardness tester is produced according verification). A direct verification by an authorised body
stringent construction criteria and all parameters should be performed periodically. If traceability to a
influencing the L-value are 100% controlled. At the end national reference machine is requested, this can only be
of the production chain, all instrument components achieved by indirect verification of the EQUOTIP
(impact device, impact body, indicating device) are conversion curve. The international primary EQUOTIP
cross-checked (calibrated) against referenced hardness tester is protected and maintained since 1975 by
components of the reference instrument. Thus, the manufacturer of the EQUOTIP.
 5. CONCLUSION [2] K. Borggreen, D.H. Hansen, J.V. Hansen, P. Auerkari,
“Acceptance Values for Equotip Hardness of some Pressure
Different international associations and standardization Vessel Steels”, Nordtest Technical Report 424 - Part 1,
FORCE Institute, Copenhagen, 1999.
bodies like the DIN (Germany) or the ASTM (USA)
[3] Borggreen, K., Tønder, P., Lorentzen, M.S., Hansen, J.V.,
acknowledge the high testing quality of EQUOTIP and Auerkari, P., “Comparison of Portable Hardness Testers –
realize the widespread, common acceptance of it. However, Performance with Ideal Samples”, Nordtest Technical
to ensure the high reliability of the measurement base, more Report 424 - Part 2, FORCE Institute, Copenhagen, 1999.
international, independent standardization work is needed. [4] Borggreen, K., Tønder, P., Lorentzen, M.S., Hansen, J.V.,
The L-base needs to be assured. Auerkari, P., “Comparison of Portable Hardness Testers –
Regarding the future trends in dynamic hardness testing, Performance with Non-ideal Samples and Cases”, Nordtest
the main efforts are made in providing more durable and Technical Report 424 - Part 3, FORCE Institute,
flexible instruments, ready to communicate with standard Copenhagen, 1999.
documentation facilities. [5] D. Leeb, “Dynamische Härteprüfung”, in “Härteprüfung an
Metallen und Kunststoffen”, eds. W.W. Weiler, D.H. Leeb,
Especially, fully automated testing in production lines is K. Müller and D.M. Rupp., 2nd Edition, Expert Verlag,
on demand. Promising solutions are provided by the Ehningen bei Böblingen, 1990.
automatic version of the EQUOTIP, the so called [6] D.H. Leeb, “New dynamic method for Hardness testing of
EQUOmatic (Fig. 5, bottom right). Interaction with common metallic materials”, VDI-Report No. 308, pp. 123-128, 1978.
programmable logic control (PLC) centres makes a quick [7] M. Tietze, “Hot Rebound Hardness Testing”, Heat Treating
100% control of components feasible. Progress, vol. 2, no. 5, pp. 33-38, 2002.

REFERENCES
Author: Dr. Michael Kompatscher, Product Manager Materials
[1] D. Leeb, “Definition of the hardness value “L” in the Testing, PROCEQ SA, Ringstrasse 2, Postfach 336, CH-8603
EQUOTIP dynamic measuring method”, VDI-Report No. Schwerzenbach/Zürich, Switzerland, +41 (0)43 355 38 00 (phone),
583, pp. 109-133, 1986. +41 (0)43 355 38 00 (fax), michael.kompatscher@proceq.com.