Application Of The Leeb Hardness Tester in The Field Of Heat Treatment

1.Detection Principle

Leeb Hardness Test is a dynamic test method, using the principle of elastic impact to get the hardness. However, due to the majority of operators on its dynamic test method of the test conditions of insufficient understanding, there are not strict or improper operation and other issues, resulting in the process of Leeb Hardness Tester application to produce detection errors. The concept of Leeb Hardness is proposed by Dr. Leeb, which is a prescribed mass of impact body under the action of elasticity, a certain speed impact on the surface of the specimen, with the ball head at 1mm from the surface of the specimen rebound velocity and impact velocity ratio multiplied by 1000, defined as the Leeb Hardness Calculation formula:

HL is the Leeb hardness; V_R is the impactor rebound velocity, m·s^-1; and V_A is the impactor impact velocity, m·s^-1.

2.Precautions

Leeb hardness testers are usually equipped with a material option function, before use must be adjusted to the corresponding material option of the test object, the material option is divided according to the modulus of elasticity E, the parameters of the option will have a direct impact on the accuracy of the test results.

Due to the alloying elements in the metal and a variety of compounds in the metallurgical and heat treatment process will form a multi-phase, diffuse, high modulus of elasticity of the microstructure, so that changes in the dynamic mechanical properties of the material will make the modulus of elasticity is very sensitive to changes in the Riehl's hardness change, and Riehl's hardness is different from the base material Leeb's hardness. Different from the base material's Leeb Hardness, it is very sensitive to changes in elastic modulus, but also different from the base material's Leeb Hardness. As a rule, materials with a higher alloy content have a higher modulus of elasticity, so steels with the same modulus of elasticity can be categorized into 2-3 choices from low to high alloy content.

3.The Selection of Impact Devices

In order to apply to a variety of field environment testing, Richter hardness tester impact device models, different models have different purposes, all models are included: D type, DC type, S type, E type, D + 15 type, DL type, C type, G type. Among them, the D-type impact device belongs to the general-purpose, usually as a standard configuration, applicable to general metal specimen hardness testing; DC-type impact device is shorter, the use of a special force ring, other parameters and the D-type impact device is basically the same, applicable to the space of the smaller hardness test, such as holes, cylindrical cylinders, etc.; G-type impact device with a larger diameter of the ball head, the impact energy is higher, the surface quality of the object to be measured requirements Wider range, suitable for rough surface parts and components of the G-type impact device. Apply to the surface of the rougher parts and large castings and forgings of the hardness test; C-type impact device is shorter, the impact energy is about 1/4 of the D-type device, suitable for surface hardening layer, thin layer and the quality of the smaller test object detection.

Impact body and the measured object in the collision process, without special stabilization of small mass specimens will be under the action of the impact force to the impact direction of the instantaneous acceleration of the movement, when the collision is over, the impact of the body's rebound kinetic energy and speed will be reduced, ultimately leading to hardness values smaller than the standard hardness value. So the quality of the test object can not meet the minimum requirements of the impact test detection conditions need to be coupled, such as in the specimen surface and heavy support between the application of viscous film.
Leeb hardness tester on the thickness of the test object requirements not only can not be pierced, but also to prevent the test object in the impact force occurs when the phenomenon of elastic or plastic deformation, so the thickness of the impact device must meet the relevant conditions of detection.
Leeb hardness tester on the surface roughness of the test object has certain requirements, the surface roughness of the specimen will lead to the detection of low values, high hardness of the specimen is more obvious. General-purpose (D-type) impact device on the test specimen surface roughness Ra requirements for less than 2.0 μm. With a variety of other hardness test methods on the indentation spacing, indentation margins have certain requirements, the Leeb's hardness test on the spacing of the indentation also has the corresponding requirements. Due to a hardness indentation and its surrounding a certain region of the material will occur cold hardening, the new indentation and its stress zone should leave the former region, otherwise the hardness will be high. Due to the indentation stress influence area, when the center of the indentation is too close to the edge of the measured object, the hardness value will be low.

Leeb hardness tester in the test force should try to ensure that perpendicular to the surface of the test object, such as can not be perpendicular to the impact body and specimen collision after the direction of the rebound force will deviate from the direction of the impact body rebound, and its rebound velocity will be less than the speed should be, the hardness will be low. The worse the verticality, the lower the bias is more obvious.
It is also necessary to pay attention to the radius of curvature of the specimen surface of the concave-convex cylindrical and spherical surfaces on the test results, as well as the detection of hardness of the surface hardening layer should pay attention to the effective thickness of the hardened layer.

The Richter hardness test shall be performed at least three times and the arithmetic mean calculated. If the difference in hardness exceeds 20 HL, the number of tests shall be increased and the arithmetic mean calculated.