Introduction to Portable Leeb Hardness Tester

The portable leeb hardness tester, commonly referred to as the rebound method, determines hardness from voltages generated by the electromagnetic induction principle. Pressing the button on top of the impact device releases a spring that propels an impact body through a guide tube toward the part’s surface, as shown in Fig. 1.

The impact body passes through a coil surrounding the guide tube. The motion of the impact body’s magnet through the coil creates a voltage across the coil windings. When the impact body’s indenter strikes the part, a portion of its energy is lost in the material. The softer the material, the greater the energy loss and the slower the rebound velocity.

The magnet returning through the coil creates a second voltage whose amplitude is relative to the velocity of the impact body. Figure 2 illustrates the voltage generated by the impact body traveling through one coil.

Fig. 1 — Illustration showing how the impact body travels through the guide tube of a rebound method portable hardness testing machine.
 

A variety of impact devices are available, with the D device being the most common model. The D device has a 3-mm-diameter tungsten carbide ball indenter and a spring force to create an indentation size suitable for homogeneous materials. The E device provides the same spring force as the D device but uses a 3-mm-diameter diamond indenter for testing parts in excess of 50 HRC. The G device uses a larger 5-mm-diameter tungsten carbide ball indenter and a heavier spring to create a larger indentation, which is necessary for testing coarse-grained forgings and castings. Some manufacturers provide variants of the D device for use when access is limited.
 

Rebound Applications

The rebound method requires large, massive parts. Due to its large indentation size, it is the best solution for coarse-grained castings and forgings. Examples of applications include the following:

• Steel mill rolls
• Engine blocks
• Shafts or axles
• Extrusion dies
• Crane wheel pulleys.

Factors Influencing the Rebound Method

• The rebound method creates a large force of short duration during impact. Thin and lightweight parts may yield, causing erroneous and random readings. For example, the D device requires a minimum weight of 5 kg and a minimum wall thickness of 20 mm. Smaller parts can be tested but require the aid of an additional support fixture.
• Gravity affects the velocity of the impact body; therefore, as the test direction changes, a correction factor is required. Most testers require the operator to manually input any changes to the direction prior to performing a test. However, more advanced instruments sense the direction and compensate automatically.
• The measured HL value is not frequently specified in test procedures. To display the common Brinell and Rockwell scales, the operator must choose the appropriate material group to select a conversion table created for a specific Young’s modulus. Since conversion tables are based on empirical relationships, they must be considered only approximations.
• The tungsten carbide ball indenter of the D and G devices may become flattened, causing high readings, or, for all models, the guide tube may require cleaning to prevent low readings. Periodic checks on a test block will detect these conditions.

Above sharing just for your reference. When you’re interested in investing a hardness tester to improve your quality process, select a supplier who offer a quality service and meet your specific needs.

If you have any enquiry with hardness tester, welcome to contact us by email: enquiry@mspmetrology.com