Leeb hardness testing technology

The Leeb hardness testing technology is a newly developed technology following the hardness of Brinell, Rockwell, Vickers, and Shore in the world, and it has become more and more popular in the world. In China, the Leeb hardness technology has been initially developed. In order to promote this advanced technology, referring to international standards, the Ministry of Machinery Industry has promulgated the "Leech hardness tester technical conditions ZBN7l 010-90", and the National Bureau of Quality and Technical Supervision has issued " Metallic Leeb hardness test method GB / T 17394-1998 ".

1. What is Leeb hardness

The concept of Leeb hardness is proposed by Dr. Dietmar Leeb of Switzerland. It is a dynamic hardness test method. When the impact body of the hardness sensor impacts the workpiece to be measured, the ratio of the rebound speed and the impact speed at 1 mm from the workpiece surface is multiplied by 1000, which is defined as the Leeb hardness value. The formula for calculating the Leeb hardness in HL is as follows:
HL = Vb / Va × 1000
Vb: indicates the rebound speed
Va: indicates the impact speed

Second, the characteristics of the Leeb hardness tester

1. The Leeb hardness tester belongs to the dynamic load test method

Leeb examines the speed of the rebound and impact of the impact body. It can be used in any direction through speed correction, which greatly facilitates the user. Compared with Shore, it belongs to the dynamic load test method, but Shore examines the vertical height of the rebound of the impact body, so it is determined that the Shore hardness tester should be used vertically downward, which will inevitably cause great limitations in actual use;

2. The Leeb hardness tester is small in size and light in weight, which is convenient for on-site hardness testing

The hardness sensor of the Leeb hardness tester is as small as a pen, and can be directly operated by hand, without a work table, whether it is a large or heavy workpiece, or a workpiece with a complex geometric size, it can be easily detected. The commonly used cloth, Luo, Vickers hardness tester. Due to its large size, it is not easy to use on site, especially when testing large and heavy workpieces, because the hardness tester cannot accommodate it, it cannot be detected at all.

3. Conversion between Leeb hardness and other hardness

There is a corresponding relationship between the Leeb hardness value and other hardness values ​​(HRC, HRB, HB, HV, HSD). Therefore, the Leeb value (HL) can be converted into other hardness values. The Leeb hardness tester can be automatically converted by the built-in microcomputer.

4. Classification and comparison of different hardness and testing requirements

Classified from the microscopic deformation, the cloth, Rockwell, and Vickers hardness are the plastic deformation of the material, which is represented by the size or depth of the indentation; the Li and Shore hardness are the elastic deformation of the material, which is represented by the magnitude of the rebound speed or height.

Fifth, the measurement requirements of the Leeb hardness tester

1. Requirements of sample surface

The test surface should have metallic luster, and should not have oxide scale and other dirt. The surface roughness should meet the following requirements:

2. Sample weight requirements
The sample must have sufficient quality and rigidity to ensure that no displacement or springing occurs during the reconstruction process. The quality should meet the following requirements:

3. Sample thickness requirements
The sample should have sufficient thickness, and the minimum thickness should meet the following requirements:

4. The sample has a surface hardened layer, and the depth of the hardened layer should meet the following requirements:

5. For concave, convex, cylindrical and spherical specimens, the radius of surface curvature should meet the following requirements:

6. The sample should not be magnetic.
7. The distance between each measuring point should be greater than 3 ~ 4mm, the test can not be repeated at the same point, otherwise it will cause a large error. At the same time, it will shorten the service life of the sensor.

7. Several issues that affect the accuracy of the test

Because the Leeb hardness tester measures the metal hardness under the action of dynamic force, there are many factors that affect the accuracy of the test results. Therefore, if there are certain restrictions on these factors, they mainly include: test conditions, test objects, operation technology and data Dealing with several key links, the following will discuss some specific issues:
1. The influence of the curvature of the test piece on the accuracy,
In the field work, we often encounter curved surface test pieces. The influence of various curved surfaces on the hardness test results is different. Under the correct operation, the instantaneous position of the impact body on the surface of the test piece is the same as the flat test piece, so it is universal 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 conditions and the elastic state, the rebound speed of the punch will be low, thereby making the value of the Leeb hardness low.
2. Errors caused by data conversion
The error when converting the Leeb hardness to other hardness includes two aspects. On the one hand, it is the measurement error of the Leeb hardness itself, which involves the dispersion when the test is repeated by the same method, and the error for multiple Leeb hardness testers of the same model ; On the other hand, it is to compare the errors produced by different hardness test methods. This is because there is no clear physical relationship between the various hardness methods, and it is affected by the unreliability of the measurement in the comparison.
The hardness conversion of this instrument is done automatically, so the hardness of the hardness tester can be directly determined by the Brinell, Rockwell, Vickers, and Shore hardness standard blocks.
3. Special materials cause errors
The conversion table stored in the hardness tester may deviate from the following steel grades:
(1), high alloy steel
â—† All austenitic steels
â—† In high-speed steel, heat-resistant tool steel and chromite steel (tool steel) hard materials (Leiden carbide, such as M7C3 and M6C will cause the elastic modulus to increase, which makes the HL value low. Steel-like should be tested on the cross-section.
â—† Local cooling and hardening, for example, due to cutting or improper sample preparation will also cause the HL value to be high.
(2), magnetic steel
â—† When testing the hardness of magnetic materials, the HL value will be lower due to the influence of the magnetic field. If the magnetic field is strong, it is recommended not to use this test method.
(3) Surface hardened steel
â—† The hardened materials on the surface, especially the surface-treated steel, will have a low HL value due to the soft substrate. When the hardened layer is greater than 0.8mm (C-type impact device is 0.2mm), the HL value will not be affected.
(4) For special materials, the following methods can be used to establish a comparison relationship by yourself.
â—† The test surface must be prepared carefully
â—† If coupling is not performed, the full sample size should be as large as possible
â—† The hardness of the sample is within the conversion range of the hardness tester
â—† Check the accuracy of the static hardness tester with the hardness block of the corresponding measurement range.
◆ Measure three points on the sample with a static hardness tester, and measure five values ​​around the indentation with a Richter scale, and take the average value. Compare the hardness values ​​measured by the two methods to get the error range. A set of samples with different hardness can also be used to draw the conversion curve using the above method.
4. Error of gear detection
In general, the Leeb hardness tester can ensure the accuracy of the detection of the gear tooth surface with a modulus greater than 7, but when the gear module is less than 7, the tooth surface is small; the test error is relatively large. For this, the user Corresponding tooling can be designed according to the situation, which will help reduce errors.
5. Influence of material elasticity and plasticity
In addition to the 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 jointly determined. In the elastic part, first of all, it is obviously affected by the E modulus. In this respect, when the static hardness of the material is the same, and the E value is different. For materials with a low E value, the HL value is large.
6. Structure error caused by hot rolling direction
When the tested workpiece is formed by the 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 cross section of the cylindrical part, it should be tested in the radial direction (generally the hot rolling direction of the cylindrical part is the axial direction).
7. Influence of other factors
Pay attention to the following points when testing pipe fittings:
â—† Pay attention to the stable support
â—† The test point should be close to the support point and parallel to the support force
â—† When the pipe wall is thin, put a proper core in the pipe
In the heat treatment process, sometimes the metal material will change (for example, 20Cr steel is carburized and quenched from alloy structural steel to low alloy tool steel). In this case, care should be taken to select an appropriate metal material.
The discreteness of the hardness of the workpiece itself also causes the error of the test value. The hardness distribution should be analyzed based on experience to reasonably explain the error of the test value. Incorrect operation methods, sample preparation, and probe configuration can also cause errors.

In short: all hardness machines are not omnipotent, not all problems can be solved, perfect! The portable Leeb hardness tester has simple measurement values, small traces, extensive hardness value measurement, convenient portability, and is not limited by space and orientation. It is a useful supplement and extension for desktop hardness machines! In the mold, roll, container manufacturing, forging and other industries, there is a trend of spreading, and its popular role cannot be ignored!

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