What is a hardness tester?

Using a Hardness Tester allows you to evaluate the properties of a material, such as strength, ductility, and abrasion resistance, and thus helps you determine whether the material is a material or materials for a purpose. Whether your requirements are appropriate or not.

Definition of hardness test “A test to determine the resistance of a material to permanent deformation by the penetration of other harder materials.” However, hardness is not an essential feature of a substance. Therefore, when concluding a toughness test, you should always evaluate a small amount in relation to the following:

• Load given on the indentation
• Specifications of specific loading time and specific loading time
• A special indentation geometry

How to test with a hardness tester?

A hardness test is usually performed by pressing a dimensional object (indentation) against the surface of the material you are testing. Hardness is determined by measuring the penetration depth of the indentation or measuring the size of the effect left by an indentation.

• Hardness tests that measure the depth of indentation penetration include Rockwell, instrumental indentation test, and ball indentation hardness.
• Hardness tests that measure the size of the affected indentation include Vickers, Hemp, and Brinell.

Types of hardness testers

1. Pen hardness tester
2. pendulum

 The best method of hardness test

How to choose the test method

The hardness test you choose should be based on the microstructure – for example homogeneity – of the test material, as well as the type of material, the size of the part and its condition.

In all hardness tests, the material under the recess must represent the entire microstructure (unless you try to identify the various components in the microstructure). Therefore, if a microstructure is very large and heterogeneous, you need a larger image of a homogeneous material.

There are four main difficulty tests, each with its own advantages and requirements. There are different standards for these tests that describe in detail the methods and applications of the hardness test.

Important considerations when choosing a hardness test method are:

• The type of material whose hardness is tested
• Is compliance with the standard required
• Approximate hardness of materials
• Homogeneity / heterogeneity of materials
• Piece size
• Whether installation is necessary
• Number of samples tested
• Accuracy required result

Four of the most common indentation hardness tests

Rockwell Hardness Test

Rockwell is a fast hardening method for production control that is used with direct readings, mainly for metallic materials. Rockwell hardness (HR) is calculated by measuring the depth of an indentation after an indentation enters the sample material at a given load.

• Commonly used for larger sample geometries
• “Fast test” is mainly used for metal materials
• Can be used for advanced experiments, such as Jominy (HRC)

Vickers hardness test

Vickers is a tough test for all solids, including metallic materials. Vickers hardness (HV) is calculated by measuring the oblique length of a depression in the remaining sample material by introducing a pyramidal diamond load with a certain load. Indentation diagonals are measured using a table or formula to determine stiffness.

• Used to test the hardness of all solids, including metals
• Suitable for a wide range of applications
• Includes a subset of welding hardness tests.

Hemp hardness test

Knoop (HK) is an alternative to the Vickers test in the microhardness test range. It is mainly used to overcome cracking in brittle materials as well as to facilitate hardness testing of thin films. The indentation is an asymmetric pyramidal diamond, and the distance is measured by long-diameter optical measurements.

• Used for hard and brittle materials such as ceramics
• Suitable for small stretched areas such as coatings

Brinell hardness test

The Brinell hardness test is used to test the hardness of larger specimens in coarse or inhomogeneous materials. Brinell hardness test (HBW) indentation using a tungsten carbide ball has a relatively large effect. The size of the indentation is called optical.

• Used for materials with coarse or heterogeneous grain structure
• Used for larger samples
• Suitable for forging and casting which are large structural elements

How to ensure accuracy and repeatability in hardness test

Proper application of hardness testing requires careful preparation and execution. However, once you have the basics, most hardship tests offer good accuracy and reproducibility.

Factors affecting hardness testing

Several factors affect the results of the hardness test. As a general rule, the less load you use on the hardness test, the more factors must be controlled to ensure an accurate hardness test result.

Here are some of the most important factors to consider to ensure an accurate conclusion of the difficulty test.

• External factors such as light, soil, vibration, temperature and humidity must be controlled
• The tester and stage must be firmly fixed on a horizontal table and the specimen must be secured or held in a holder or anvil.
• The indentation should be perpendicular to the test surface
• When using Vickers, Knoop or Brinell, the brightness settings should be constant during the test
• The tester must be re-calibrated / verified each time you change the recessed lens or object lens.

Surface preparation requirements for hardness tester tests

You must prepare the surface before testing the hardness of metal or other materials. The required surface conditions depend on the type of test and the load used. In general, the quality of the surface preparation has a direct effect on the hard test result, so before deciding on a low level preparation, you should consider the trade-off between the surface quality and the changes in the test results.

Macro-
surface hardness testing is usually sufficient and sometimes does not require preparation.

Microhardness test
Due to the lower loads used in the hardness test, the microhardness test requires a polished or electro-polished surface. It is important that the margins / corners of an optically examined image are clearly visible. This can be done mechanically, chemically or electrochemically. It is important that heating or cold work does not alter the surface characteristics of the sample.

Deformations

Cutting and grinding may cause deformation. Depending on the hardness test load, these surfaces must be removed by polishing up to 6.0, 3.0 or 1.0 μm.

For small loads (less than 300 gf1) the surface must be completely free of deformation and the samples need oxide or electrolyte to achieve a completely damage-free surface. You should also consider that soft and / or malleable materials (ie less than 120-150 for HV) are more sensitive when introducing preparation products.

Definition of test loads with hardness tester

Formally, hardness test loads are expressed in Newtons (N). Historically, however, it has often been expressed in kilograms of force (kgf), heat force (gf), or pond (p). The correlation between kgf, kp and N is: 1.0 kgf = 1000 gf = 1.0 kp = 9.81 N

• The term microhardness test is commonly used when indentation loads are less than 1 kg / kg.
• The term macro hardness test is used when the load is more than 1 kg / kg

If standards allow, use the highest possible load / force for the largest inflow to ensure the most accurate results.

The loads used in each of the four methods for testing the hardness of metal materials * comply with various ISO and ASTM standards.

Source : 

https://www.struers.com/en/Knowledge/Hardness-testing#hardnesstestinghowto