Leeb Hardness Tester's Requirements for Measurement
1.Requirements for the surface of the sample
The test surface should have metallic luster, and there should be no oxide scale and other dirt, and the surface roughness should meet the following requirements:
|
Type of impact device |
Specimen Surface Roughness (um) |
|
D 、 DC |
≤1.6 |
|
G |
≤6.3 |
|
C |
≤0.4 |
2.Specimen Weight Requirements
The specimen must have sufficient mass and rigidity to ensure no displacement or bounce during the reconstruction process. The mass should meet the following requirements:
|
Type of impact device |
Specimen weight (Kg) |
||
|
Stable |
fixed or clamped |
need to be coupled |
|
|
D 、 DC |
>5 |
2 ～ 5 |
0.05 ～ 2 |
|
G |
>15 |
5 ～ 15 |
0.5 ～ 5 |
|
C |
>1.5 |
0.5 ～ 1.5 |
0.02 ～ 0.5 |
3.Specimen Thickness Requirements
The sample should have sufficient thickness, and the minimum thickness should meet the following requirements:
|
Type of impact device |
Specimen min thickness (mm) |
|
D 、 DC |
5 |
|
G |
10 |
|
C |
1 |
4. The sample has a surface hardened layer, and the depth of the hardened layer should meet the following requirements:
|
Type of impact device |
Surface hardness layer |
|
D 、 DC |
≥0.8 |
|
C |
≥0.2 |
5. For concave, convex, cylindrical and spherical samples, the surface curvature radius should meet the following requirements:
|
Type of impact device |
Surface curvature radius (mm) |
|
D 、 DC |
≥30 |
|
C |
≥50 |
For specimens with curved surfaces, appropriate support rings should be used to ensure that the positional deviation of the impact head at the moment of impact is within 0.5mm.
6. The sample should not be magnetic.
7. The distance between each measurement point should be greater than 3 ~ 4mm, and the test cannot be repeated on the same point, otherwise it will cause a large error. At the same time, the service life of the sensor will be shortened.
Factors to Affect Test Accuracy
Since the Leeb hardness tester measures the hardness of metals under the action of dynamic force, there are many factors that affect the accuracy of the test results, so these factors should be limited to a certain extent, mainly including: test conditions, test objects, operation techniques and data Several key links such as processing, the following will discuss some specific issues:
1. The influence of the curvature of the specimen on the accuracy
In field work, specimens with curved surfaces are often encountered, and various curved surfaces have different effects on the hardness test results. Under the condition of correct operation, the instantaneous position of the impact body on the surface of the specimen is the same as that of the flat specimen, so it is commonly used. The support ring is sufficient. However, when the curvature is small to a certain size, due to the significant difference between the deformation of the plane condition and the elastic state, the rebound speed of the punch will be low, and the Leeb hardness display value will be low.
2. Errors Caused by Data Conversion
The error when converting the Leeb hardness to other hardnesses includes two aspects: one is the measurement error of the Leeb hardness itself, and the error when the Leeb hardness is converted to other hardnesses includes two aspects, one is the measurement error of the Leeb hardness itself, This involves dispersion when the test is repeated by the same method and error for multiple Leeb hardness testers of the same model. On the other hand, the error caused by comparing the hardness measured by different hardness test methods is due to the lack of a clear physical relationship between various hardness methods, and is affected by the unreliability of measurement in mutual comparison.
3. Errors Caused by Special Materials
The conversion tables stored in the hardness tester may deviate from the following steel grades:
High alloy steel
◆All austenitic steels
◆In high-speed steel, heat-resistant tool steels and ledeburite chromium steels (tool steels) and hard materials (ledeburite carbides such as M7C3 and M6C will cause an increase in the elastic modulus, resulting in a low HL value. This Class steel shall be tested in cross section.
◆ Local cooling and hardening, for example due to cutting or improper specimen preparation, can also cause high HL values.
Magnetic steel
◆When testing the hardness of magnetic materials, the HL value will be low due to the influence of the magnetic field. If the magnetic field is strong, this test method is not recommended.
case hardened steel
◆For surface-hardened materials, especially surface-treated steel, the HL value will be low due to the soft matrix. When the hardened layer is larger than 0.8mm (0.2mm for the C-type impact device), the HL value will not be affected.
For special materials, the following methods can be used to establish a comparison relationship by yourself.
◆The test surface must be carefully prepared
◆If coupling is not carried out, the selected sample size should be as large as possible
◆The hardness of the sample is within the conversion range of the hardness tester
◆Check the accuracy of the static hardness tester with the hardness block of the corresponding measuring range.
◆Measure three points on the sample with a static hardness tester, and measure five values around the indentation with a Richter tester, and take the average value. Comparing the hardness values measured by the two methods gives the error range. A set of different hardness samples can also be used to draw the conversion curve using the above method.
4. Error of Gear Detection
Under normal circumstances, the Leeb hardness tester can guarantee the accuracy of the detection of the gear tooth surface with a modulus greater than 7, but when the gear modulus is less than 7, the test error is relatively large due to the small tooth surface. The corresponding tooling can be designed according to the situation, which will help to reduce the error.
5. Influence of Material Elasticity and Plasticity
In addition to hardness and strength, the Leeb value is also related to the elastic modulus. The hardness value is a characteristic parameter of the hardness and plasticity of the material, because the components of the two must be determined together.
In the elastic part, it is obviously affected by the E-modulus first, in this respect when the static hardness of the material is the same, but the E value is different. Materials with low E values have high HL values.
According to the elastic modulus of the material. Alloy type and heat treatment state can classify various materials.
6. The Hot Rolling Direction Causes Structural Errors
When the workpiece to be tested is formed by hot rolling process, if the test direction is consistent with the rolling direction, the test value will be low due to the large elastic modulus "E", so the test direction should be perpendicular to the hot rolling direction. For example: when measuring the hardness of the section of a cylindrical piece, it should be tested in the radial direction (generally, the hot rolling direction of the cylindrical piece is the axial direction).
7. The influence of Other Factors
The following points should be noted when testing pipe fittings:
◆Pay attention to the stable support of the pipe fittings
◆The test point should be close to the support point and parallel to the support force
◆When the tube wall is thin, put an appropriate core in the tube
In the process of heat treatment, the metal material is sometimes changed (for example, 20Cr steel is carburized-quenched from alloy structural steel to low-alloy tool steel). In this case, attention should be paid to selecting appropriate metal materials.
The hardness dispersion of the workpiece itself also causes the test value error. The hardness distribution should be analyzed according to experience, and the test value error should be reasonably explained. Incorrect operation method, sample preparation, and probe configuration can also cause errors.
Conclusion
All hardness testers are not omnipotent and can not solve all problems! The portable Leeb hardness tester has simple measurement value, small traces, wide hardness value measurement, easy to carry, and is not limited by space and orientation. It is a useful supplement and expansion of desktop hardness testers! It is widely used in molds, rolls, container manufacturing, forging and other industries!
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