Guide
When is a Leeb hardness test acceptable on a machined part?
A Leeb hardness test is a portable rebound test in which a spring fires a small impact body at the surface and the instrument compares its speed before and after impact; the ratio is reported as a Leeb number such as HLD. Short answer: it is acceptable for large, heavy or fixed steel and cast iron parts, and for sorting and field checks; it is a poor acceptance test for thin, light, small or curved parts unless they are coupled to a heavy base, and a converted Rockwell or Brinell value from it is only approximate.
By Saola Manufacturing engineering team · Published · Updated
Leeb testers are popular because they are fast, portable and leave a tiny mark. The catch is that the reading depends on how the part responds to the impact as a whole, not just on the surface hardness. This guide explains when a Leeb reading can stand in for a bench test, which part sizes and shapes cause false readings, how to write a Leeb requirement on a drawing, and how to accept the result. The bench methods themselves are compared in the Rockwell vs Brinell guide.
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How does a Leeb hardness test work, and what does HLD mean?
The tester measures how much speed the impact body loses when it bounces off the part, and a harder surface returns more of the energy. HLD is the Leeb value measured with the standard D impact device; other letters, such as HLC, HLDL or HLG, name other impact devices and are not interchangeable.
The scope of ASTM A956/A956M-22 covers Leeb hardness testing of steel, cast steel and cast iron, plus verification of the instruments and calibration of the test blocks. Its significance section calls the Leeb test a dynamic, or rebound, test and a superficial determination that only measures the condition of the surface contacted. The international counterpart is ISO 16859, which the instrument maker Screening Eagle lists alongside ASTM A956 as the method standards its Leeb probes comply with.
Because the result is a rebound ratio, anything that absorbs energy lowers it: a part that flexes, rocks or vibrates under the impact, a rough or oily surface, or a soft surface layer. That is the root of every limit in the sections below.
How heavy and thick must a part be for a Leeb test?
Per the maker's summary of ISO 16859 and ASTM A956, a compact part tested with the D probe needs to be heavy and thick, or it must sit on a solid support or be coupled to a heavy plate. Below those limits the reading is typically too low.
Most machined parts are lighter than the free-standing limit, which is why the coupling method exists. The part is bonded to a heavy, flat base with a thin layer of coupling paste so that the two react to the impact as one body. Coupling needs a flat contact face on the part and a base of adequate mass; a part that rocks on its base gives low, scattered readings.
| Condition | D, DC, DL, E, S probes | C probe (lower energy) | G probe (higher energy) |
|---|---|---|---|
| Compact part, free standing | 5 kg | 1.5 kg | 15 kg |
| Part on a solid support | 2 kg | 0.5 kg | 5 kg |
| Part coupled to a heavy plate | 0.05 kg | 0.02 kg | 0.5 kg |
| Minimum thickness, uncoupled | 25 mm | 15 mm | 70 mm |
| Minimum thickness, coupled | 3 mm | 1 mm | 10 mm |
| Minimum surface-layer thickness | 0.8 mm | 0.2 mm | not listed |
Why do thin, small and curved parts read wrong?
Thin and light parts flex or move under the impact and absorb energy, so they read softer than they are. Curved surfaces, such as small shafts and bores, change the contact and the rebound path, and need a support ring matched to the radius.
The same goes for a thin hard layer on a soft core. The Leeb impact affects material well below the visible dent, and the maker's table above gives a minimum surface-layer thickness for each probe. On a shallow case-hardened, nitrided or plated part, the reading mixes the layer and the core, so a Leeb value does not prove case hardness. Those parts need a microhardness traverse or a superficial Rockwell scale.
Surface condition matters as much as size. The maker's guide lists a minimum roughness class of N7 for the D probe and a finer class for the C probe, which in practice means a fine-turned, ground or locally sanded test spot. Scale, decarburization, paint and plating must be removed at the test point, or the reading describes the coating, not the steel.
Can Leeb numbers be converted to Rockwell or Brinell?
Yes, but only approximately and only for the material class the conversion was developed for. If the drawing limit is in HRC or HB and the test is Leeb, the report must say it is a converted value.
ASTM E140, the hardness conversion standard, includes a table for Leeb D hardness on non-austenitic steels in its 2019 reapproval, and it states that its conversion values are only approximate and valid only for the materials indicated. It also recommends applying conversions to specification limits rather than converting test data where that can be avoided. In practice that means two things for a buyer. First, a Leeb reading converted to HRC on austenitic stainless, aluminum or brass with a steel curve is not a valid number. Second, a part that is just inside an HRC limit by converted Leeb value is not proven to meet the limit.
The cleaner approach is to state the limit in the scale you will test with. If you want Leeb on the shop floor and Rockwell for disputes, write both: a Leeb acceptance range for routine checks, and a Rockwell test as the referee method.
How should a Leeb hardness requirement read on a drawing?
Name the scale and impact device, the limits in that scale, the test location, the number of impacts averaged, and the standard. If the part is below the mass or thickness limits, say how it must be supported or coupled, or call a bench method instead.
A workable note reads: "Hardness 600 to 650 HLD per ASTM A956, average of five impacts at the location marked H, test spot prepared to fine machined finish; part coupled to a base for testing; referee method HRC per ASTM E18." Adjust the numbers to your material. The parts that matter are the impact device letter, which fixes the scale, the location, which keeps the dent off functional surfaces, and the referee method, which settles disagreements.
Avoid three common mistakes. Do not write a Leeb test on a part far below the coupled mass limit. Do not let "hardness check" stand without a scale; the supplier will use whatever tester is nearest. And do not compare a supplier's Leeb readings with your own Rockwell readings without agreeing the conversion first.
How do you accept Leeb readings from a supplier?
Ask for the instrument and impact device type, the daily verification result on a calibrated reference test block, the individual readings with their average, and the support or coupling used. A single number with no method behind it tells you little.
The instrument maker's guide shows daily verification on a test block with an acceptance band based on DIN EN ISO 16859-1, and it states its D probe accuracy as within 4 HL. Readings on the part spread more than that because of surface and support, so look at the scatter: a wide spread between impacts on one spot usually means a support, surface or curvature problem rather than a variable part. For heat-treated parts, a few parts per lot checked by a bench Rockwell tester give a cross-check on the Leeb results.
How is hardness checked on Saola parts?
The lead CNC plant's inspection equipment includes a hardness tester, and the test method and scale for your part are confirmed at drawing review. If your drawing calls Leeb hardness, say which impact device and support you expect.
For turned steel parts, the CNC turning page lists bar materials such as 52100 bearing steel and 12L14, and parts like these are usually small enough that a bench hardness method, or a coupled Leeb test, is the realistic choice. Put the scale, limits and test location on the drawing, and state whether you need hardness results on the first article only or on every lot. The quality page lists the first-article inspection report, the dimensional inspection record on every lot, and material certification on request; ISO 9001 is in progress, not yet held. Send drawings through the quote form.
Frequently asked
Is a Leeb hardness test destructive?
Only slightly. It leaves a small dent, so test on a non-functional spot or on a sacrificial part, and mark the allowed test location on the drawing.
Can a Leeb tester measure aluminum or brass?
Instruments carry curves for some non-ferrous metals, but ASTM A956 covers steel, cast steel and cast iron. For aluminum and brass, agree the method and conversion with the supplier, or use a bench test such as Rockwell B or Brinell.
Why is the Leeb reading different near the edge of a part?
Near an edge the material around the impact has less support, so the rebound drops. Keep impacts away from edges and from other dents.
Does the impact direction matter?
Yes. Gravity changes the impact speed, so instruments correct for direction. Make sure the direction set on the tester matches the way the probe is held.
Can Leeb replace Rockwell for heat-treat certification?
Usually not on its own for small parts. Use Rockwell or Brinell for certification and Leeb for quick sorting, unless the specification names Leeb.
Sources
- ASTM A956/A956M-22, Standard Test Method for Leeb Hardness Testing of Steel Products (scope and significance), ASTM International, 2022
- ASTM E140-12B(2019)e1, Standard Hardness Conversion Tables for Metals (scope), ASTM International, 2019
- Equotip Portable Hardness Testing Selection Guide, Screening Eagle Technologies, 2025