As the portable Leeb hardness test celebrates its 35th anniversary, Leeb is the most widely used portable hardness testing method.
As seen in this application, Leeb testers are often used on huge parts. Source: Proceq
the portable Leeb hardness test celebrates its 35th anniversary, Leeb is the
most widely used portable hardness testing method. It has become one of the top
four hardness tests alongside the traditional methods of Rockwell, Brinell and
Hardness testing is one of the oldest and probably the most commonly applied
quantitative inspection method for metallic materials. One reason for this is
that hardness itself is a useful material parameter. Another is that there are
more or less direct correspondences between hardness and other important
material properties that are much more difficult to measure directly, such as tensile
strength, yield strength or fatigue behavior. Finally, unexpected hardness
values also may indicate undesired changes of microstructure, for instance,
close to welds.
Variants of Leeb testers provide great flexibility. In the field of macro hardness testing, the Leeb method is now third after Rockwell C and Brinell. Source: Proceq
Traditional Hardness Tests
The property hardness of a metallic material is vaguely defined as the resistance of a material to permanent deformation. This means that hardness is not a physical quantity in the strictest sense, but rather the outcome of a well-defined deformation process. At the beginning of the 20th century, several practical methods were designed to quantify metal hardness, the most popular being Rockwell, Brinell and Vickers. In all of these, hard test tips are pressed into the material under defined loads, and deformation parameters are measured, such as the indentation depth or the diameter of the indentation mark. Usually, a simple formula correlates these deformation parameters with a hardness unit, such as HRC, HBW10/3000 and HV30.
Automated data recording and even remote controlled hardness tests enhance
efficiency and minimize the risk of human errors. Source: Proceq
Invention of Leeb Hardness
In 1975, Dietmar Leeb and Dr. Marco Brandestini invented an alternative hardness test method and hardness tester. An impact body of mass m with a hard-ball tip is accelerated by a spring toward the material surface and hits the material at velocity vI, corresponding to the kinetic energy ½mvI². The impact body bounces back with velocity vR, which is lower than vI due to the loss of kinetic energy used for the plastic deformation of the material (½mvR² = ½mvI² - Wpl, where Wpl is the plastic deformation work). The harder the material, the smaller the loss of kinetic energy and the greater the velocity ratio vR/vI. The velocities of the impact body shortly before (vI) and shortly after impact (vR) are measured inductively. The scaled ratio HL = 1000 · vR/vI produces the Leeb hardness number.
Benefits of Leeb Tests
Leeb method has become popular over the years. In the field of macro hardness
testing, it is now third after Rockwell C and Brinell.
The most important reason behind the spread of Leeb is the size of the Rockwell,
Brinell and Vickers benchtop machines. Specimens need to be taken to the
traditional benchtop testers, whereas Leeb testers are small, portable
instruments that can be used directly on the work piece on the factory floor or
With traditional hardness testers, the objects under test must be small and
plane. In practice, samples must often be cut off from work pieces. In
contrast, Leeb testers can measure hardness on large objects with complicated
geometries and curved surfaces. Also, modern Leeb testers permit measurements
not only vertically downwards, but in any direction. This adaptability means
that Leeb hardness testing remains virtually nondestructive, while a Rockwell
or Brinell test requires scrapping the object after the test.
and accuracy of the genuine Leeb testers is comparable to high-end benchtop
testers. For example, the high repeatability can easily be observed when
measuring in a small area on a test block that has homogeneous hardness. The
highest accuracy is found when the hardness is reported in the original Leeb
Major users such as those in the steel-making and the automotive industries use
the HL units more and more frequently to eliminate the additional uncertainty
that can arise when converting to other scales. The measurement uncertainty of
genuine HL results can be calculated much more easily and is much smaller than
for converted hardness numbers.
However, conversion functions from HL to other scales become handy when
customers specify the work piece hardness, for example, in Rockwell’s HRC
scale. Some Leeb testers offer a large number of such conversions. Each
conversion function is applicable to a group of materials that exhibit similar
physical parameters, such as a similar Young’s modulus and creep behavior.
Provided the material group is selected correctly, conversion errors will not
normally exceed ±2 HRX for Rockwell scales, and ±10% for Brinell hardness
number (HB) and Vickers pyramid number (HV). (HRX in this case stands for
Rockwell hardness scales in general, where X is the wildcard. This means that
in general, for all 30 Rockwell scales, the maximum error for conversion is 2.)
In most cases, the conversion error is significantly lower.
If higher accuracy is required, or if the alloy under test is not covered by
one of the built-in conversions, high-end testers provide a variety of methods
to generate material-specific conversions.
Probes Provide Flexibility
types of impact devices suit specific fields of
The D probe often is seen as the general-purpose impact device and thus most
widely used. Its impact body carries a tungsten carbide ball with a
3-millimeter diameter that hits the surface with an impact energy of 11.5
millijoules (mJ). The D probe works best when the surface roughness is Ra ≤ 2
microns (N7). The DC and DL devices are essentially identical, but have shorter
and longer nozzles, respectively, so that confined spaces can be accessed, such
as in narrow vessels or bore holes. The S and the E devices are equipped with
especially hard tips for testing on extremely hard surfaces, such as
The applicability of the Leeb devices is limited by the dynamic measuring
principle, which requires the sample to have a minimum mass and thickness. For
D, DC, DL, S and E devices, specimen dimensions ideally exceed 5 kilograms and
25 millimeters. However, they can be tested down to 0.1 kilogram and a few
millimeters when special precautions are taken.
In addition, the C probe is available, with an impact energy 3 mJ. Work pieces down
to 1.5 kilograms and 10-millimeter thickness can be tested easily, while
lighter samples and sheets down to 1-millimeter thickness are measurable with
precautions. For the C probe, data scatter increases once
Ra ≥ 0.4 micron.
In contrast to the C device, the G impact device applies an impact energy of 90
mJ with a test tip of 5 millimeters in diameter. It is designed for rougher (Ra
≤ 7 microns, N9) and more inhomogeneous surfaces as found on cast steel parts.
Parts should ideally have greater than 15 kilograms and greater than
Standardized Hardness Test
1975, this test method also has been standardized in international bodies under
the name Leeb/Equotip hardness test, and guidelines for users are available.
As with traditional hardness test methods, standards require verification of
the instruments on test blocks. Before each working shift, the instrument
should be checked on a test block with hardness not far from the hardness of
the work piece. If desired, block and instrument calibrations are available
traceable to the German national institute.
Thirty-five years after its introduction, Leeb has become one of the top
hardness tests. The instruments and test methods are still popular, as
customers seek to increase testing efficiency and reach new applications. Q
testing is one of the oldest and probably the most commonly applied
quantitative inspection method for metallic materials.
- The property hardness of a metallic material is vaguely defined as the
resistance of a material to permanent deformation.
In addition, unexpected hardness values may indicate undesired changes of
microstructure, for instance, close to welds.