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:
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.
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
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 )
|1/8" - .125" (3mm)
|3/8" -.387" (10mm)
|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.