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Leeb hardness Tester

 Traditional hardness measurement equipment including Rockwell, Brinell, and Vickers is stationary that require fixed workstations in segregated testing areas or labs. Mostly, these hardness measurement methods are selective, which involves destructive tests on samples. With the help of individual results, it is possible to draw statistical conclusions for the entire batches. The portability of these Leeb hardness testers helps in achieving higher inspection rates without damaging the samples, which in turn streamlines the processes at the reduced cost.

The traditional methods of hardness testing are based upon properly defined physical indentation hardness tests. The hardest indenters of well-defined sizes and geometries are constantly pressed into the material with a particular force. Several deformation parameters like the indentation depth in the Rockwell method, are documented to give the measurement of hardness.

 

According to the Leeb principle, the hardness value is taken from the energy loss of a well-defined body by affecting the sample metal, similar to the Shore scleroscope. The Leeb quotient (vr/vi) is considered as the amount of the energy loss by plastic deformation, where the impact body recovers quickly from harder test samples compared to the softer ones. This results in delivering a greater value of 1000×vr/vi. A magnetic impact body allows the deduction of velocity from the voltage induced by the body while moving through the measuring coil. The quotient 1000×vr/vi is defined as the Leeb rebound hardness unit HL.

 

In the traditional static tests, apply the test force uniformly with increasing magnitude, where dynamic testing methods use an instantaneous load. This test takes merely 2 seconds using the standard probe D, which leaves an indentation of only ~0.5 mm in diameter on steel or steel casting having a Leeb hardness of 600 HLD. Comparatively, a Brinell indentation on the same material will be ~3 mm equivalent to the hardness value of ~400 HBW 10/3000, having a standard-compliant measuring time of approximately 15 seconds in addition to the time taken for measuring the indentation.

 

What are the applications of the Leeb Hardness Tester?

  • Leeb testers can very well inspect the hardness of ferrous and nonferrous materials.
  • The resultant measurements are taken in the Leeb Scale here. But depending on the instrument, the units can be converted to Brinell, Vickers, Rockwell B & C, and Shore.
  • The testing materials include alloy tool steel, cast steel, stainless steel, copper, cast irons, aluminum, bronze, etc.
  • Leeb Rebound testers are not appropriate to inspect the hardness of thinner parts like thin-walled, sheet metal, or very lightweight parts.
  • With the help of an ultrasonic hardness tester, parts below 2mm (0.11″) well supported/combined with a heavier part or up to any thickness can be inspected.

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About Leeb Hardness Tester You Should Know

  1. What are the testing standards for Leeb hardness testers?

There are varied testing standards for Leeb testers, which are listed below:

  • American standards:

o ASTM A956 “Standard Test Method for Leeb Hardness Testing of Steel Products”

o ASTM E140 – 12be1 “Standard Hardness Conversion Tables for Metals Relationship Among Vickers Hardness, Brinell Hardness, Rockwell Hardness, Knoop Hardness, Superficial Hardness, Leeb Hardness, and Scleroscope Hardness”

  • German standards and specifications:

o DIN 50156-1 “Metallic materials – Leeb hardness test – Part 1: Test Method”

o DIN 50156-2 “Metallic materials – Leeb hardness test – Part 2: Calibration and Verification of the testing devices”

o DIN 50156-3 “Metallic materials – Leeb hardness test – Part 3: Calibration of reference blocks”

o VDI/VDE Guideline 2616 Part 1 “Hardness testing of metallic materials”

o DGZfP Guideline “Mobile Härteprüfung”

  • Official European standard projects:

o PrEN ISO 16859-1 “Metallic materials – Leeb hardness test – Part 1: Test method”

o PrEN ISO 16859-2 “Metallic materials – Leeb hardness test – Part 2: Calibration and Verification of the testing devices”

o PrEN ISO 16859-3 “Metallic materials – Leeb hardness test – Part 3: Calibration of reference test blocks”

  • Official International standards projects:

o ISO/DIS 16859-1 “Metallic materials – Leeb hardness test – Part 1: Test method”

o ISO/DIS 16859-2 “Metallic materials – Leeb hardness test – Part 2: Calibration and Verification of the testing devices”

o ISO/DIS 16859-3 “Metallic materials – Leeb hardness test – Part 3: Calibration of reference test blocks”

 

 

  1. What are the characteristics of the Leeb test?

The Leeb hardness test is a dynamic method that has the following characteristics:

  • It follows the standardized procedures (ISO 16859 and ASTM A956).
  • The impact velocity stays between 1.4 and 3.0 m/s, as per the method.
  • It is a rebound test method, where the velocity of an impact body before and after the impact is calculated to detect the hardness of a test sample.
  • The ratio of the impact velocity to the rebound velocity is considered as a measure of the dynamic hardness of a test sample in Leeb units.
  • Shape and material type of the impact body: Ball-shaped indenter with different radii. Tungsten carbide cobalt, ceramic, or diamond type.

 

  1. What is the procedure of the Leeb Hardness Tester?

The Leeb hardness test works as per the guidelines of ISO 16859. In this procedure, an impact device speeds up the impact body with the help of a spring force. This velocity of the impact body is classified into three phases:

  1. Approach phase – Here, the impact body is accelerated towards the test surface with the help of the spring force.
  2. Impact phase – Here, the impact body and the sample are in direct contact. The sample is thereby plastically and elastically deformed where the impact body is brought to a standstill completely. This elastic type spring-back of the impact body, as well as specimen, causes the impact body to rebound.
  3. Rebound phase – Here, the impact body is once again accelerated out of the impact phase with the resultant energy.

The velocities are measured without any contact with the help of an induced voltage, produced by a moving magnet in a well-defined coil of the impact device. The induced voltage signal is noted electronically. The peak values, the point of the impact phase, as well as the point of the rebound phase, help in calculating the Leeb hardness. Consider the figure given below. The ratio of rebound velocity vr to impact velocity vi, multiplied by a factor of 1000, gives the measurement of Leeb hardness. The below mentioned formula is used for calculation.

         

HL = (Rebound velocity/Impact velocity) * 1000 = (vr/vi) * 1000

     

Legend:

A – Peak value of the approach phase

B – Peak value of the rebound phase

t –   Time

U – Voltage

 

  1. How are the individual hardness testing methods distinguished from each other?

The individual hardness testing methods distinguish from each other with respect to:

  • The impact velocities
  • Shape and the material type of the indenter of the impact unit

 

The differences between the individual hardness testing methods can be seen 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

 

  1. How are the values of the Leeb hardness tester read and interpreted?

The Leeb test hardness value consists of three components:

  1. Numerical hardness values
  2. The two capital letters ‘HL’ which stands for ‘Hardness according to Leeb’
  3. The designation of the Leeb scale that describes the impact device and the associated parameters.

 

  1. What are the characteristics of a portable Leeb hardness tester?
  • Highly precise
  • Easy-to-use and convenient
  • This equipment is versatile which tests the varied range of materials and sizes
  • Portable, compact, and lightweight
  • The readings can be converted to Brinell (HB), Vickers (HV), Shore (HSD), and Rockwell (HRB and HRC) hardness scales

The major new features of Leeb Portable Rebound Hardness Tester includes the integration of QualiTip, a portable hardness tester with statistic data display, huge storage capacity, backlight display to use in dark areas, USB interface for both data transfer to your PC or printer as well as recharging, and much more.

 

  1. How does the Leeb hardness tester work?

A spring present in the probe projects the tungsten carbide ball-tipped indenter at the test piece. The impact velocity is measured at once prior to impact followed by on rebound. The quotient is calculated and shown as the Leeb Hardness Value. On harder materials, the rebound velocity proves to be higher than softer materials, which will absorb more impact energy.

 

For the proper repetitive measurements, certain conditions listed below must be met:

  • The material should be smooth and free from rust, paint, oxide, etc.
  • The grain size of the test part should remain small in terms of the indentation size.
  • The material cannot be moved or deflected by the impact of the indenter.
  • The correction factor should be applied for the material’s elastic modulus.
  • Even the correction for the effect of gravity should also be made.

 

  1. What are the two major types of Leeb hardness testers available in the market?

There are two types of Leeb hardness testers namely, bench (stationary, classical) and portable (mobile, electronic).

 

  1. Bench hardness tester – This bench hardness tester is a classic type that measures the hardness using the standard methods of Rockwell, Brinell, Vickery, or Shor. Generally, the name of this hardness tester includes the name of the hardness scale like ‘bench Brinell hardness tester’ or ‘bench Rockwell hardness tester’. There are several bench hardness testers that permit users to determine the hardness not only using one scale but several hardness scales. For instance, there is a bench hardness tester which allows the users to calculate hardness by Brinell, Rockwell, and Vickers scales. All these Bench hardness testers are too big and heavy (more than 50 kg). Hence, they should be installed either on a floor or a table. The main benefits of such devices include the classic implementation as well as the easy direct method of hardness measurement.

 

  1. Portable hardness testers – Contrary to the bench hardness testers, portable ones are in small size with light weight. As they are compact in nature, they can measure the hardness using several hardness scales. Generally, the modern portable hardness testers can assess the hardness of all hardness scales. The major benefit of portable hardness testers is the convenience for the users to carry the device anywhere especially next to the measuring samples. You need not carry samples into the workshops. A portable hardness tester is an electronic device that has a set of useful features which include saving the test results in the internal storage. Next, these results can be transferred to a computer to generate a report of control in a specialized program named ARM, which is supplied along with the portable hardness testers. Though portable hardness testers have a few advantages, there are some cases that require only the bench hardness testers. Anyway, such cases are too rare. Hence 99 cases out of 100 users always consider portable devices.

 

Since portable devices are very useful and handy, they are considered as the best solution.

 

 

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