When to Use Hardness Testers？

Hardness is a material’s ability to resist deformation at its surface. A measurement is taken of the resulting indentation and converted to a hardness value relative to the hardness scale of the particular test. Many different standardized hardness tests exist, with each having an area of applicability to different types of materials, such as metals, ceramics, or rubbers. Measuring hardness has a number of benefits in comparing materials and providing quality control in manufacturing and hardening processes. This article will discuss hardness testing, its definition, how it works, its types, and benefits.

Read more

What Is Hardness Testing?

Hardness testing measures a material’s resistance to permanent deformation at its surface, by pressing a harder material into it. It is used in a number of industries for material comparison and selection, as well as quality control of a manufacturing or hardening process. A specifically manufactured indenter, with chosen dimensions, is used to press into the material being tested, with a prescribed force. The pressing time is also important in the hardness test. 

Hardness is not a fundamental physical property of a material, but rather a measured characteristic. It can however provide some valuable information about the strength and durability of a material, depending on the application it is intended for.

How Does Hardness Testing Work?

Hardness testing works by pressing a standardized, hard instrument into the material that is being tested. The instrument, called an indenter, is pressed into the sample for a standardized length of time (a number of seconds), and with a specific load (force) behind it. When the indenter is pressed into the material, it deforms the surface of the material by a measurable amount. This deformation is then measured, and the measurement is used to assign a hardness value to the material according to the test’s specific scale.

What Type of Materials Undergo Hardness Testing?

Hardness testing can be applied to various types of materials, some of which are listed below:

1. Metals and alloys.
2. Ceramics.
3. Elastomers.
4. Polymers and plastics.
5. Films.
6. Rocks and minerals.

What Is the Importance of Hardness Testing?

Hardness testing is important for a number of different reasons. The hardness of a material can be a critical parameter in its application. This can be the case for metal components used in cutting and grinding or on the opposite extreme, for rubbers used for shock absorption. It is important to be able to measure and compare the hardness of materials objectively. 

Having an objective, standardized hardness value for a material is important to allow quality control during manufacturing. Each unit would need to be within an acceptable range of the specified hardness. Furthermore, these objective hardness values allow for the comparison of different materials and are therefore important for material selection to match the needs of an application.

What Are the Benefits of Hardness Testing?

Hardness testing provides a valuable measure of a material characteristic with a number of benefits, including:

1. An automated hardness testing machine allows hardness to be measured relatively simply.
2. To provide meaningful hardness readings for a wide variety of materials, from soft to hard.
3. For some applications, the hardness of a material is the key characteristic that is valuable to know. This can be measured directly, rather than inferred by a physical property.

What Are the Challenges of Hardness Testing?

Hardness testing does have some limitations and challenges that can affect the accuracy and repeatability of hardness results, some of which are listed below:

1. It is necessary to have properly prepared the surface of the sample to be measured, usually by grinding and polishing. The surface condition has a significant influence on the hardness reading, so improper surface preparation can give inaccurate or spurious readings.
2. Most hardness tests are designed for flat surfaces, so curved surfaces do present a challenge. There are some correction factors that need to be applied to compensate for an indentation on a curved surface.
3. The size of the indentation can be difficult to ascertain objectively, particularly when manually reading the size of micro impressions through an eyepiece. This is mainly an issue with older machines. The latest models have cameras and software packages to automatically focus correctly, and display the edge of the impression clearly on a connected computer screen.
4. If the prescribed minimum distances between indentations are not followed according to the testing standards, the results of the hardness test will be affected. This is also true of an indentation too close to the edge of a sample.

What Are the Different Types of Hardness Test Methods?

There are a number of different hardness test methods, each suited to a certain scale of measurement, or type of material. Therefore, different methods should be selected according to the substance being tested. The most common hardness tests are explained below:

1. Rockwell: Developed to give a quick reading, mainly for metallic samples. The depth of the deformation made by the indenter is measured, and a deeper indentation is assigned a lower number, indicating a softer material. Two broad types of tests are used: the Rockwell superficial hardness and the Rockwell regular hardness tests.
2. Brinell: A spherical indenter is used, and the diameter of the resulting indentation is measured to give the hardness value. It is generally used for testing larger samples, or samples that are not perfectly homogenous since it causes a larger indentation that is less sensitive to local variations in material hardness.
3. Vickers: Uses a diamond pyramid to make a square indentation on the surface of the material being tested. The diagonal of the indentation is measured optically and is converted to the hardness value. The Vickers hardness test can be applied for micro-testing.
4. Knoop: Specifically used to test thin or brittle materials, as it gives a meaningful reading with a shallow indentation. It uses an elongated pyramid to make the impression, with the measurement of the long diagonal used to calculate the Knoop hardness. It is used for micro-hardness testing.
5. Mohs: A scale from 1 to 10 that provides an indication of the relative hardness of minerals based on how easily they can be scratched. A mineral’s hardness is measured by attempting to scratch it with materials of known Mohs hardness and ranking it accordingly.
6. Shore: Is used for softer materials such as elastomers and plastics. A spring-loaded indenter (called a durometer) is pressed into the material sample, and the depth of penetration is converted to a hardness value.

How Is a Hardness Test Different From a Ductility Test?

A hardness test uses a limited compressive force to apply a small deformation to the surface of a material, in order to assign a relative value for the material’s hardness. However, a ductility test uses a destructive tensile force to measure the amount of plastic deformation that a material can withstand before breaking.

Both types of tests apply a load to cause plastic deformation in a material. However, the direction of the load is different (compressive vs. tensile) and the extent of the load is different (limited to surface indentation vs. complete yield).

What Does Indentation Mean in Hardness Testing?

Indentation in hardness testing means a permanent deformation in the surface of the sample material. This indentation will have a measurable geometry depending on the shape of the instrument used to make the indentation. The measurable aspects of an indentation, which are converted into a hardness value, can be the depth of the indentation, or a variation of the diagonal of the size of the indentation of the surface.

How is Hardness Testing Measured?

Hardness testing is measured according to different scales, which depend on the exact hardness testing method being used. For all common scales, the hardness of a material is proportional to the hardness value—a harder material will have a higher hardness value. For some hardness tests, the scale selected is fairly arbitrary. For example, the Rockwell hardness test has a range of hardness values from 0 to 100 depending on the depth of the indentation. However, each hardness test defines its scale differently.

A hardness value does not have a unit but is simply a value on the scale associated with a particular test. However, hardness values are quoted with a designation of which scale and test were used to generate them. For instance, HR is Rockwell hardness, HB is Brinell hardness, HV is Vickers hardness, and so on.

What Are the Hardness Testing Standards?

There are various standards available for the different hardness testing methods. Generally, the internationally accepted standards for hardness testing are those issued by ASTM (American Society for Testing Materials) and ISO (International Organization for Standardization). The two standards are very similar for each corresponding type of hardness test but have small differences between them for details such as the minimum thickness of the sample. The common ASTM and ISO standards for the four most popular hardness tests in metallic materials are listed below:

1. ASTM E10: This standard covers Brinell hardness testing in metallic samples.
2. ASTM E18: This standard includes the various ways that Rockwell hardness can be measured by standard methods, specifically for metallic materials.
3. ASTM E384: This standard is for hardness testing on a micro-scale, and therefore includes the Vickers and Knoop hardness tests.
4. ISO : This standard mirrors ASTM E10, as it contains the standard method for measuring the Brinell hardness of metals.
5. ISO : This standard contains the details for the Vickers hardness test in metallic materials.
6. ISO : This standard is very similar to ASTM E18, as it deals with the Rockwell hardness tests, both superficial and regular, for metallic samples.
7. ISO : This standard covers Knoop hardness testing for metals.

What Is the Static Hardness Testing Method?

Static hardness testing uses an indenter that is pressed and held against the surface of the material being tested. The indenter is therefore static for most of the test and causes a permanent deformation of the surface of the material. In static hardness testing, therefore, the indenter pushes through the elastic deformation of the material and forces a plastic deformation. This is the most common approach, and the widely used Rockwell, Brinell, Vickers, and Knoop hardness tests are all static hardness tests.

What Is the Dynamic Hardness Testing Method?

Dynamic hardness testing measures the rebound velocity of a moving indenter as it hits the material sample and rebounds. The hardness value obtained is determined from the rebound distance or the rebound velocity. It is typically applied in cases in which a static hardness test (completed on a testing machine) may not be practical, such as out in the field, or when large specimens are involved. Dynamic hardness testing is usually not as precise as static hardness testing.

Can Hardness Testing Be Conducted on Any Types of Materials?

No, hardness testing cannot be performed on any type of material. Broadly speaking, hardness testing can be carried out on almost every type of material, but there are some exceptions for which it cannot be applied. For instance, the material must be solid, or at least gel-like. Even some solid materials cannot be tested for hardness, such as if they have a highly irregular surface, or are non-homogeneous.

Summary

Xometry provides a wide range of manufacturing capabilities including CNC machining, 3D printing, injection molding, laser cutting, and sheet metal fabrication. Get your instant quote today.

Capital contains other products and information you need, so please check it out.

Disclaimer

The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.

Understanding the different hardness testing types and systems can be useful to determine an optimal solution. Hardness testing functionality has evolved and now users can dial into world-class caliber instrumentation which more closely aligns with their applications at hand.

If using an older Rockwell hardness systems, the operator may have to manually control most or all of the test procedure steps. The majority of today's newer machines automatically perform the entire test. Also, when leaving a mark or indentation is not an option, non-destructive ultrasonic technology can be used.

Rockwell Hardness     

When testing the hardness of carbon steel, alloy steel, cast iron, non-ferrous metals and engineering plastics, Digital Rockwell Benchtop Hardness Testers can be used to directly measure in the most popular regular Rockwell hardness scales and can quickly convert that hardness value into HB, HV, HK and many other scales. Desirable hardness tester features include the ability to obtain ultra precise results, a wide measuring range, and scale/ selectable test force capabilities. Also, automatic main test force loading/unloading, a high resolution digital display and USB data storage are all very advantageous.

A user-friendly touchscreen interface can speed operations and the ability to use USB output to a flash drive is excellent for data mobility. There are options to apply the weight load such as on digital systems that use weights to apply the load or use a closed loop load cell to apply the weight load. The latter affords greater precision and repeatability. With a weight loaded system, the level of the machine is of great importance, so that the weights drop correctly. This is a less critical matter when using a load cell system.

Conforming to ASTM E-18 Superficial Rockwell Hardness standards, hardness testers in this category offer excellent repeatability in all Superficial Rockwell Hardness scales. Superficial Rockwell hardness testing is designed for very thin and soft workpieces. The systems are ideally suited for a wide range of environments including inspection labs, heat treat facilities, tool rooms, workshops and laboratories. For more versatility, twin hardness testers are capable of testing in all of the regular Rockwell and Superficial Rockwell hardness scales.

Dolphin nose systems allow for the hardness testing of inner, as well as exterior, diameters. The systems are generally larger in size than other Bench Rockwell systems, offering greater testing heights and depths. Dolphin nose models offer a manual handle that activates the preload system, or an advanced Auto Z-axis preload system. Using the Auto Z-axis preload system, after placing the workpiece in testing position, the operator only needs to press the start button for the machine to complete the testing process.

Brinell Hardness

Brinell hardness testing is commonly used for very large, porous testing of less hard metals, such as castings. Benchtop systems available today can handle the most popular Brinell hardness applications, and incorporate the latest innovative closed-loop technology. A test load is applied via a closed-loop control unit with a load cell to apply weight loads up to 3,000kg, a DC motor and an electronic measurement and control unit. The result is highly accurate Brinell hardness measurements at all test loads up to 0.5%. A common load overshoot or undershoot, also known as traditional dead weight or open-loop, system is eliminated. The absence of mechanical weights not only eliminates friction problems but also makes the equipment less sensitive to misalignments caused by vibrations. The systems are ideal for laboratories, workshops, tool rooms and inspection labs.

Software driven digital optical systems offer advantages over manual microscopes that are supplied with several hardness testing machines. Connected to a PC, laptop or tablet, the operator can push a single button to take automatic and instant measurements. All graphics can be saved, along with test results, in either word or excel formats.

Vickers/ Knoop Hardness

Micro Vickers/ Knoop Hardness Testers are cost-effective options in Vickers hardness applications, that are ideal for those who do not perform high volume testing each day. Testing is done on extremely thin/small workpieces, and often used for checking the hardness of layers, or platings and coatings on small parts in a laboratory environment. A high level of preparation is needed for such testing, including but not limited to, a high degree of polishing.

There are three types of turret control including a basic manual turret for changing from optics to indentation and back to optics for measuring. A second type incorporates an automatic turret, giving operators greater freedom to change the turret position by a button on the tester keypad. The most popular Vickers/Knoop Hardness Testers have a turret control option using software to control the entire test with a one-click process using calibrated auto edge detection. Precision video and measurement software also allows for clicking of the indent edges in software, then deriving a hardness reading on screen. 

Designed for the accurate hardness testing of small precision parts, thin materials, case-hardened layers and all sorts of steel components, Macro Vickers Hardness Testers utilize larger weight loads of up to 50kg. This type of test bridges the gap between the superficial Rockwell and micro Vickers machines. These systems have a manual turret.

Shore Testers (Durometers)

Shore Portable Hardness Testers are targeted for testing materials such as rubber, soft plastics and leather, and versions are also available for the testing of hard plastics such as bowling balls and hard hats. Electronic Durometers for measuring Shore A and Shore D values are designed to fit comfortably and firmly in a user's hand. A large LED display and simple 3 button control make this device easy to use.

 Portable Hardness Testers

There are two popular digital methods of portable hardness testing. The first is "Dynamic Impact", based on the Leeb principle of hardness, developed by DietMar Leeb in the 's. A spring loaded impact body is thrust to the test surface, affecting rebound. Initial thrust and rebound speed is measured in a non-contact mode, and is calculated as a Leeb hardness value and then automatically converted to Rockwell C, B, Brinell, Vickers and Shore Values. Also, the portable benefit means the tester can be brought to the workpiece, which is especially useful when testing large and/ or cumbersome parts. This method has resulted in efficient, fast and accurate portable hardness testing results.

However, when a mark or indentation on the workpiece must be avoided, ultrasonic testing is a great solution. Advanced Non-Destructive Portable Hardness Testers utilize ultrasonics with Ultrasonic Contact Impedance (UCI) technology, enabling a portable hardness tester to test special surfaces on small and thin workpieces without marking the surface. These units can test metals as thin as 2mm throughout all scales, hard or soft. UCI technology is available on both manual and motorized systems. The motorized probe systems are used for very thin testing of coatings and platings, or surfaces with a very high polish finish.

UCI is based on a 136 degree diamond at the end of a vibrating rod being depressed into the test surface at a fixed load. The difference in Ultrasonic vibration frequency is then calculated into a hardness value. The UCI test procedure is slower than the Dynamic Impact style, however the UCI method has the advantages of being non-destructive and able to test thin and small work parts.

Previously, destructive indentations made on such sample pieces meant the garbage heap for those tested. Using ultrasonics, this is no longer the outcome. These systems have an open architecture and can be calibrated to read any metal, in any hardness scale, with reference samples to perform initial calibration. Ultrasonic portable hardness testing is ideal for applications such as bearings, pistons and valves, among many others. Key industries for this type of testing include aerospace, automotive and medical parts as well as knife blade manufacturing, to name just a few.