Leeb Testing Factors

Factors Affecting Accuracy and Limitations of Leeb Hardness Testing:

As with any hardness testing be it a heavy bench top unit, or a portable device, an inappropriate sample will give a large measurement error. Preparation of the sample and the test surface area must meet all the conditions of the following basic requirements:

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Short version: Thin walled parts like, tubing, sheet metal, sheet aluminum, smaller die cast parts, machined parts with thin walls and light weight parts are generally considered as not suitable for this test method.

1) MOST IMPORTANT. The Sample must be of sufficient weight and rigidity. The Leeb principle is for the impact "rod" to rebound off the test surface and to measure that rebound. So if the sample lacks weight and rigidity, the sample will absorb much of the impact giving an inaccurate result.  

• Sample weight ≥5kg (10lbs) and does not easily move, can be directly tested and is the ideal sample;

• Sample weight is 2~5kgs, (4-10lbs) the sample should be fixed in place with appropriate clamping;

• Sample weight is 0.05~2kgs, (0.1 -4 lbs) the sample should be coupled with another supporting material before the test;

• Sample weight  ＜0.05kg (0.1lbs), this hardness tester is inappropriate to use.

Coupling method: The test sample’s backside should be prepared to make a plane as a supporting surface with a smooth formation that contacts the whole area. Filling with a little coupling substance (Industrial Grease can be used), users can  now press to the surface of the supporting object (The weight of supporting object should be more than 5 kg, and it can be replaced by test block) to make in effect an integrated sample. IMPORTANT More info on what coupling is in this link click HERE

2) When preparing the sample, users should take care not to alter the sample by cold processing ( e.g.. striking with hammer to cause work hardening ) and thermal processing ( e.g.. grinding excessively in effect causing localized heat treating).

3) A flat or planer sample surface will give the best results. Ideally the test surface must be clean and should have a metallic shine, and not have an oxide layer (rust) or other stains, paint etc.

4) When the testing sample surface is not flat, the curvature radius of testing and nearby surface should be larger than 30mm. An appropriate supporting ring should be selected and installed. The rule is that the tester probe must be as close to perpendicular to the test surface as possible throughout the test procedure.

5) The surface roughness of the sample at all testing points must be Ra ≤ 1.6. For the D type the approximate surface finish can be achieved with a 200 grit wheel or for the G Type which is designed for castings and other rougher surfaces, 60 grit is a good approximation for surface finish.

Types of impact devices

Max surface roughness of sample Ra

D/D+15/DL
G
C

2µm
7µm
0.4µm

 

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Good reference to surface finish. Click here.

6) Samples should be thick enough and with a sufficient surface absorption layer. If a D-type impact device is used, the thickness of sample must be greater than 1" ( 25mm ) if not supported, and the surface absorption layer (surface-hardening layer) should not be less than 0.8mm. To accurately measure the hardness of the material, the best way is to remove the surface-hardening layer by pre-processing. ( Grinding with a portable grinder is suggested, but note 2 above ).

Types of impact devices

Minimum sample thickness ( the parts must be coupled to a heavier base as noted in 1 above )

D/D+15/DL

1/8" - .125" (3mm)

G

3/8" -.387" (10mm)

C

3/64" - .047 (1mm)

7) A sample should not be magnetized. The signal of the impact device would be seriously interfered with by the magnetic field and would likely cause inaccurate test results.

8) The surface temperature of the sample should be less than 250 deg F (120 deg C ).

9) Test location is a minimum of 5mm ( 0.2inch) from the edge of the part. 10mm ( 0.5 inch ) or greater is recommended.

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I have been testing it out on various materials of varying size and thickness. I am impressed that repeat testing on various samples seems to provide very reproducible results. I have tested it on materials quite massive and on materials that were small and relatively thin and got expected results there with small fin parts requiring back up and heavier parts not. Provided with the tester is a good size chunk of polished hardened steel that one uses as a back up for small-ish coupons. It is recommended that one use grease to couple the test sample to the back up block. Coupling does result in significantly higher hardness readings if the coupling is done correctly if not the light coupons will, as expected, test softer than they are.

One of my questions concerning testing of my castings was whether testing in areas (see photo above) that spanned underlying support struts might result in lower readings than tests done right over supporting webs. On my castings the location of testing in that regard does not seem to affect readings as the casting itself is greater than 5kg and is stiff enough to respond as a unit rather than the spanning member springing enough to lower the reading. I am sure there must be a lower limit to the spanning thickness but at roughly 1/4” spanning 4 to 6 inches with a stiffening web underlying, the testing seems unaffected.

So far, this unit seems quite reliable and precise.

A few notes:

The bouncing ball is not a loose 2 or 3 mm ball, but is actually a 2 or 3 mm ball embedded in a “dart-like” assembly. I wonder if it uses an air-rifle like mechanism to fire the dart at the sample?

This photo shows the latching end of the "dart" that is held by the trigger mechanism.

This is the ball that actually impacts the sample.


One concern I had was whether I might get dirt and crud into the tube that carries the dart or whether debris might collect on the ball assembly. The unit is supplies with a brush to brush out the tube and the dart can easily be removed and wiped down. So far, no issues with dirt, but, should it occur, not hard to deal with.

It is also possible to select the orientation the probe is held whether vertical---most common---or 45, horizontal, 135, 180 from vertical. This is handy on massive parts not easily positioned. Such would not be possible if gravity alone drove the dart, but with the spring or air gun mechanism it works fine. The dart is a close fit in the tube in which it is fired. If you take the end off the tube, hold it pointing up and drop in the dart, it slowly descends the tube as air seeps between the dart and tube.

Surface finish should be 1.6 micometers and minimal unsupported mass 5kg with support of 2-5kg and grease coupling of .2 to 2 kg parts.

It does readout in Leeb units, Rc, BrinnelB, and other units as selected from a menu on the hand-held display.

My conclusion at this point is that this device will serve my needs well. How durable it may be in practice remains to be seen. But my general i
impression is that build quality is quite good. I may soon have a chance to compare it to a more expensive unit at a foundry that mentors me.

I much appreciate the folks that took their time to comment above. The intent of this post is to "pay back" the forum for its help.

Denis

Well, I bought a Leeb hardness tester. I was reluctant to shell out major money for a tester given the sort of experimental nature of the tool for my application and the lack of a resounding “you’ll be sorry” from users here. The cost range of such testers is from 200+ to 5000 dollars. The one I selected is sold on Amazon:I have been testing it out on various materials of varying size and thickness. I am impressed that repeat testing on various samples seems to provide very reproducible results. I have tested it on materials quite massive and on materials that were small and relatively thin and got expected results there with small fin parts requiring back up and heavier parts not. Provided with the tester is a good size chunk of polished hardened steel that one uses as a back up for small-ish coupons. It is recommended that one use grease to couple the test sample to the back up block. Coupling does result in significantly higher hardness readings if the coupling is done correctly if not the light coupons will, as expected, test softer than they are.One of my questions concerning testing of my castings was whether testing in areas (see photo above) that spanned underlying support struts might result in lower readings than tests done right over supporting webs. On my castings the location of testing in that regard does not seem to affect readings as the casting itself is greater than 5kg and is stiff enough to respond as a unit rather than the spanning member springing enough to lower the reading. I am sure there must be a lower limit to the spanning thickness but at roughly 1/4” spanning 4 to 6 inches with a stiffening web underlying, the testing seems unaffected.So far, this unit seems quite reliable and precise.A few notes:The bouncing ball is not a loose 2 or 3 mm ball, but is actually a 2 or 3 mm ball embedded in a “dart-like” assembly. I wonder if it uses an air-rifle like mechanism to fire the dart at the sample?This photo shows the latching end of the "dart" that is held by the trigger mechanism.This is the ball that actually impacts the sample.One concern I had was whether I might get dirt and crud into the tube that carries the dart or whether debris might collect on the ball assembly. The unit is supplies with a brush to brush out the tube and the dart can easily be removed and wiped down. So far, no issues with dirt, but, should it occur, not hard to deal with.It is also possible to select the orientation the probe is held whether vertical---most common---or 45, horizontal, 135, 180 from vertical. This is handy on massive parts not easily positioned. Such would not be possible if gravity alone drove the dart, but with the spring or air gun mechanism it works fine. The dart is a close fit in the tube in which it is fired. If you take the end off the tube, hold it pointing up and drop in the dart, it slowly descends the tube as air seeps between the dart and tube.Surface finish should be 1.6 micometers and minimal unsupported mass 5kg with support of 2-5kg and grease coupling of .2 to 2 kg parts.It does readout in Leeb units, Rc, BrinnelB, and other units as selected from a menu on the hand-held display.My conclusion at this point is that this device will serve my needs well. How durable it may be in practice remains to be seen. But my general iimpression is that build quality is quite good. I may soon have a chance to compare it to a more expensive unit at a foundry that mentors me.I much appreciate the folks that took their time to comment above. The intent of this post is to "pay back" the forum for its help.Denis

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