Leeb Rebound Hardness

The impact device contains a spring that propels a small mass (the impact body) toward the specimen surface. A permanent magnet embedded in the impact body induces a voltage in a surrounding coil as the body travels. The peak voltage during the forward stroke is proportional to the impact velocity; the peak voltage during the return stroke is proportional to the rebound velocity. The instrument’s electronics compute the ratio (rebound / impact) × 1000, which is the Leeb hardness value. A perfectly elastic surface would return all the impact energy, yielding a high HL value; a soft, plastically deforming surface absorbs more energy, yielding a lower value.

Different impact body types—designated D, DC, DL, D+15, C, G and E—vary in mass, tip material (tungsten carbide or diamond), tip geometry and impact energy. The D-type is the standard reference device and is suitable for the widest range of applications. The DC type uses a diamond tip for very hard materials. The DL type has a longer body for access to narrow spaces. The C and G types are designed for low-mass and very-low-energy applications. Selecting the appropriate impact body depends on the specimen’s size, mass, geometry and expected hardness range.

Taking a Leeb measurement involves preparing the test surface (grinding or polishing to remove scale, paint, rust and roughness), coupling the impact device to the specimen if required, selecting the correct impact direction compensation, and triggering the impact. The instrument displays the HL value within a fraction of a second. Multiple impacts at the same location are avoided because the first impact work-hardens the surface; instead, each impact is made at a fresh location with sufficient spacing to prevent interaction between impression zones.

Interpreting Leeb values directly is practical for trending and comparison—higher HL means harder material. For specification compliance, the HL value is typically converted to a traditional scale (HRC, HV, HB) using material-group-specific conversion tables stored in the instrument’s firmware. The accuracy of the converted value depends on the correctness of the material-group selection, the quality of the conversion table and the testing conditions. Raw HL values are instrument-traceable; converted values carry additional uncertainty from the empirical conversion. In standards terms, this is the difference between complying with the Leeb method itself under ASTM A956/A956M or ISO 16859-1 and claiming equivalence to a direct Rockwell, Vickers or Brinell result.

• Material and Sample Characteristics: Specimen mass and rigidity are critical. A lightweight or thin specimen absorbs impact energy through movement or flexing rather than through surface deformation, producing a falsely low rebound value.
• Environmental Conditions: Temperature affects the material’s hardness (metals become marginally harder at lower temperatures) and the instrument’s electronics. Most instruments are rated for operation between −10 °C and +50 °C.
• Instrument and Fixture Parameters: Impact body tip wear is the primary maintenance concern. A tungsten-carbide tip that has been flattened by repeated impacts on hard materials no longer creates the intended contact geometry, shifting the rebound ratio.
• Operator Technique and Procedure: The impact device must be held firmly against the specimen to prevent recoil, which would absorb energy and produce a low reading; a hesitant or angled hold introduces scatter that no amount of surface preparation can remove.
• Impact Direction and Coupling: Two variables set the Leeb method apart from static indentation, and both bias the reading if ignored. Because gravity adds to or subtracts from the impact body’s velocity, a measurement taken vertically down, vertically up or horizontally needs the matching direction compensation selected — instruments that auto-detect orientation remove this error, but a wrong manual setting shifts every reading. And a specimen below the device’s minimum mass returns the impact energy by moving or flexing rather than by deforming at the surface, reading falsely low until it is coupled to a heavy backing mass with paste or clamped solid.

Leeb testing is used for heat-treatment verification (confirming quench-and-temper results on forgings, castings, machined parts), weld inspection (checking heat-affected-zone hardness against code limits), material identification (sorting mixed stock by hardness), and in-service assessment (detecting degradation in pressure vessels, turbine components and structural steels). Its portability enables testing at the point of manufacture, in the field and on installed equipment.

A common misinterpretation involves trusting converted hardness values without verifying that the correct material group has been selected in the instrument. Applying a carbon-steel conversion table to a stainless steel specimen produces an inaccurate converted value, even though the raw HL reading is correct. The instrument’s material-group setting must match the specimen. Another is assuming that a converted HRC, HV or HB value automatically satisfies a laboratory-method requirement; if the specification is written around ASTM E18, ASTM E92, ASTM E10 or their ISO equivalents, that acceptance decision must be explicit in the governing procedure.

Another frequent error is testing on inadequately prepared surfaces. Scale, paint, decarburisation layers and surface roughness all reduce the measured rebound, producing a lower HL value—and, by conversion, a lower HRC or HV value—than the material’s true bulk hardness. Investing the time to prepare the surface properly is essential for meaningful results.

• Metal Hardness — the method in context and where Leeb fits among the metal hardness scales.
• Portable Metal Hardness Testing — field testing on installed components, structures and assemblies.
• Metal Hardness Scale Conversions — how HL converts to Rockwell, Vickers and Brinell, and the uncertainty involved.

If Leeb rebound testing matches your metal-hardness application, the next decision is usually whether you need a portable field instrument, clearer scale-conversion logic, or both.

• Choose a Portable Metal Hardness Tester helps you decide whether a Leeb-based instrument suits your field conditions and reporting needs.
• Select the Right Hardness Scale for Your Metal helps when the harder problem is the reported scale, conversion limits or specification logic.