Leeb Hardness Testing

The Leeb hardness testing, otherwise called as Leeb Rebound Hardness Test (LRHT), is considered as one of the four commonly used methods to test the hardness of the metal. It is a type of non-destructive testing used to inspect large sized workpieces weighing above 1 kg. It also measures the coefficient of restitution.  

 

Unlike other stationary traditional methods including Rockwell, Brinell, and Vickers which were implemented only in the laboratories or testing areas, the Leeb method proves to be portable by achieving high testing rates at a reduced cost.

How to measure hardness by Leeb hardness tester?

The hardness of metallic materials can be determined as per the ISO 16859 and ASTM A956 standards.

In the Leeb hardness testing method, the hardness value can be calculated from the energy loss of an impact body after impacting upon a metal. This Leeb quotient is equivalent to the measure of that energy loss due to deformation. The impact body rebounds faster from harder samples than the softer ones, resulting in a larger value 1000* vr / vi which is quoted as the Leeb rebound hardness unit HL.

When an impact device accelerates an impact body with the help of spring force, the velocity of the impact body gets segregated into three phases:

  1. Approach phase – Here, the impact body is accelerated in the direction of the test surface with the help of the spring force.
  2. Impact phase – In this phase, the impact body as well as the specimen remain in direct contact. The specimen gets plastically or elastically deformed and the impact body comes to the standstill. This elastic spring-back nature of the impact body, as well as the specimen, helps the impact body to rebound.
  3. Rebound phase – Here, the impact body is again accelerated out of the impact phase with the resultant energy.

These velocities are measured without any contact through an induced voltage, produced by a magnet moving in a defined coil of the impact device. This induced voltage is logged electronically where the peak values, the point of the impact phase as well as the rebound phase, are used to determine the Leeb hardness, as seen in the figure below. The ratio of the rebound velocity (vr) to the impact velocity (vi) multiplied by a factor of 1000, shows the Leeb hardness value (see the formula below).

 

                                             Rebound velocity                   vr

HL = ————————— * 1000 = —- * 1000

                                             Impact velocity                      vi

 

Legend:

A – Peak value of the approach phase

B – Peak value of rebound phase

t – Time

U – Voltage

 

Classification of the Leeb hardness test

There are seven hardness testing methods that vary from each other with respect to three factors namely:

  • Impact velocities
  • The shape of the indenter
  • Material type of the indenter of the impact unit

These differences between the various hardness testing methods are listed in the following table:

 

Test method

Impact velocity

[m/s]

Kinetic impact energy

[mJ]

Rebound velocity

[m/s]

Maximum distance between indenter ball and test surface

[mm]

Material of indenter

Mass of impact body

[g]

Spherical radius

[mm]

Application range

HLC

1.4

3.0

0.49 – 1.344

2.00

WC-Co

3.1

1.5

350 – 960 HLC

HLD

2.05

11.5

0.615 – 1.8245

2.00

WC-Co

5.45

1.5

300 – 890 HLD

HLDL

1.82

11.95

1.1092 – 1.729

2.00

WC-Co

7.25

1.39

560 – 950 HLDL

HLD+15

1.7

11.2

0.561 – 1.513

2.00

WC-Co

7.75

1.5

330 – 890 HLD+15

HLE

2.05

11.5

0.615 – 1.886

2.00

PCD

5.45

1.5

300 – 920 HLE

HLG

3.0

90.0

0.9 – 2.25

3.0

WC-Co

20.0

2.5

300 – 750 HLG

HLS

2.05

11.4

0.82 – 1.886

2.00

C

5.40

1.5

400 – 920 HLS

There are two types of Leeb hardness testers such as bench (stationary, classical) as well as portable (mobile, electronic).

  1. Bench hardness tester– This bench hardness tester is a traditional type used to determine the hardness of a sample employing the standard methods of Rockwell, Vickers, Brinell, or Shor. Hence, they can be called ‘bench Rockwell hardness tester’ or ‘bench Brinell hardness tester’. There are many other bench hardness testers used to measure the hardness of the sample not only using just one scale but varied hardness scales. For example, there is a bench hardness tester used to measure hardness using Brinell, Rockwell, and Vickers scales. All these bench hardness testers are very large and heavy, weighing more than 50 kg. Therefore, these testers are installed either on a table or a floor. Its advantages include the standard implementation along with the easier and direct method of hardness determination.
  2. Portable hardness testers– Unlike the bench testers, portable hardness testers are smaller in size with lightweight. These testers are compact in nature, which determines the hardness using various hardness scales. Moreover, the modern portable hardness testers can detect the hardness by employing all hardness scales. Its major advantages include user convenience as well as portability. There is no need to carry samples into the testing units or workshops. The modern portable hardness tester can electronically save the test results in the internal storage, followed by transferring them to a computer to generate a report. These reports employ a specialized program named ARM, which is provided along with the portable testers. However, at times, there might be a requirement for the bench hardness testers, though it is too rare. Anyway, 99 cases out of 100 users always prefer portable devices.

 

Application of Leeb hardness testing

There are various applications of Leeb hardness testers which are listed below:

  • Inspect the hardness of both ferrous as well as nonferrous materials
  • Though the measurement results are considered in the Leeb scale, the units can be converted to Rockwell B & C, Vickers, Brinell, and Shore according to the instrument. The reason is they have a wider measuring range.
  • Leeb testers can also test materials such as stainless steel, cast steel, cast irons, alloy tool steel, copper, bronze, aluminum, etc.
  • Leeb testers are inappropriate to determine the hardness of thinner parts such as very lightweight parts or the thin-walled sheet metal
  • Using an ultrasonic hardness tester, the Leeb Rebound tester can inspect the parts below 2mm (0.11″) supported or combined with a heavier part or up to any thickness
  • Measure the hardness of all metals with the EL background light, and large screen LCD that shows all parameters and functions.
  • Automatically detect the type of impact devices by testing at any angle, even in upside down position.
  • Measure the die cavity of molds, bearings, heavy work pieces, as well as other parts
  • Analyzed the installed machineries as well as the permanently assembled parts
  • Failure analysis of steam generator, pressure vessel, and other equipment.
  • Identify the material in the warehouse of metallic samples
  • Detecting the surface of a small hollow space
  • A quick inspection in large range as well as multiple measuring areas for large scale workpiece.

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