What is Material Hardness?
Material hardness is a measure of how much a material resists changes in shape. The ability of a material to resist wear, tear, scratching, and abrasion is called hardness. Some examples of hard materials are diamond, boron carbide, quartz, tempered steel, ice, granite, and concrete.
Hardness testing enables you to evaluate a material’s properties, such as strength, ductility, and wear resistance, and so helps you determine whether a material or material treatment is suitable for the purpose you require. Hardness is just one mechanical measurement and properties such as toughness and strength need to be considered, as hard materials tend to have low toughness and can easily fracture.
The Methods of Hardness Testing
There are three primary procedures for determining a material’s hardness: scratch hardness testing, the Leeb rebound hardness test, and indentation hardness testing.
Scratch hardness testing
The Mohs scale of hardness can be used to determine the hardness of a material by comparing it to a list of standard materials with known hardness. The material’s hardness is measured on a scale between the material it barely scratches and the material it cannot scratch.
Leeb rebound hardness test
The Leeb Rebound Hardness Test (LRHT), invented by Swiss company Proceq SA, is one of the four most commonly used methods for testing metal hardness. This portable method is mainly used for testing sufficiently large workpieces (mainly above 1 kg). It is a type of non-destructive testing used to inspect large-sized workpieces weighing more than 1 kg. It also measures the coefficient of restitution.
Indentation hardness testing.
There are a variety of indentation-based methods for determining the hardness of a material. The Brinell test, the Vicker’s Diamond test, the Knoop hardness test, and the Rockwell test. All four techniques include material indentation. The hardness is determined by measuring the applied force and comparing it to a geometrical property of the indentation, such as its surface area or depth.
How to Process Material Hardness Testing?
The Rockwell hardness test
Rockwell is a fast hardness test method developed for production control, with a direct readout, mainly used for metallic materials. The Rockwell hardness (HR) is calculated by measuring the depth of an indent after an indenter has been forced into the specimen material at a given load.
- Generally, it is used for larger sample geometries.
- A ‘quick test’ is mainly used for metallic materials.
- can be used for advanced tests such as the Jominy (end-quench) test (HRC).
The Rockwell hardness test is based on an inverse relationship with the measurement of the additional depth to which an indenter is forced by a heavy total (major) load beyond the depth resulting from a previously applied preliminary (minor) load. Initially, a minor load is applied, and a zero data position is established. The major load is then applied for a set period of time before being removed, leaving the minor load in place. The resulting Rockwell number represents the depth difference from the zero datum position caused by the application of the major load.
The entire procedure can take from a few seconds to 15 minutes for plastics. Rockwell test results are obtained quickly and directly, without the need for a secondary, dimensional measurement requirement. The most common indenter type is a diamond cone ground at 120 degrees for testing hardened steels and carbides. Softer materials are typically tested with tungsten carbide balls ranging in diameter from 1/16′′ to 1/2′′. The Rockwell scale is a combination of indenter and test force. These combinations make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and then the respective scale. A recorded hardness number of HRC 63 denotes a hardness of 63 on the Rockwell C scale. Higher values indicate harder materials, such as hardened steel or tungsten carbide. These can have HRC values in excess of 70 HRC. Rockwell test forces can be applied using either a closed-loop load cell or a traditional deadweight system.
The Brinell hardness test
The Brinell hardness test is used for hardness testing larger samples of materials with a coarse or inhomogeneous grain structure. The Brinell hardness test (HBW) indentation leaves a relatively large impression when using a tungsten carbide ball. The size of the indent is read optically.
- used for materials with a coarse or inhomogeneous grain structure.
- used for larger samples.
- suitable for forgings and castings where the structural elements are large.
The Brinell test, another common hardness test type, involves applying a constant load or force, usually between 500 and 3000 Kgf, for a specified time (from 10 to 30 seconds) using a 5 or 10 mm diameter tungsten carbide ball. The load time period is required to ensure that the plastic flow of the metal has stopped. In some cases, lower forces and smaller-diameter balls are used. The Brinell test, like the Knoop and Vickers tests, uses only one test force. Following the removal of the load, the recovered round impression is measured in millimeters using a low-power microscope or an automatic measuring device.
Brinell testing is commonly used in the testing of aluminum and copper alloys (at lower forces) and steels and cast iron (at higher forces). Highly hardened steel or other materials are usually not tested using the Brinell method, but the Brinell test is particularly useful in certain material finishes because the indenter size and heavy applied force are more tolerant of surface conditions. Brinell testers are frequently designed to accommodate large parts such as engine castings and large diameter piping.
The Knoop hardness test
The Knoop (HK) is an alternative to the Vickers test in the micro hardness testing range. It is mainly used to overcome cracking in brittle materials as well as to facilitate the hardness testing of thin layers. The indenter is an asymmetrical pyramidal diamond, and the indent is measured by optically measuring the long diagonal.
- It is used for hard and brittle materials, such as ceramics.
- Suitable for small elongated areas, such as coatings,
The Vickers hardness test
The Vickerstest is a hardness test for all solid materials, including metallic ones. The Vickers Hardness (HV) is calculated by measuring the diagonal length of an indent in the sample material left by introducing a diamond pyramid indenter with a given load. The diagonals of the indent are measured optically to determine the hardness, using a table or formula.
- used for hardness testing of all solid materials, including metallic materials.
- It is suitable for a wide range of applications.
- This includes a sub-group of hardness testing of welds.
Knoop, or Vickers testing, is also performed by pressing an indenter of specified geometry into the test surface. Unlike Rockwell testing, the Knoop or Vickers test uses only one test force. The resulting impression or un-recovered area is then measured using a high-powered microscope and filar measuring eyepieces or, more recently, automatically with image analyzing software. The Knoop diamond creates an elongated rhombic-based diamond-shaped indent with a long-to-short diagonal ratio of about 7 to 1. Knoop tests are commonly performed at test forces ranging from 10g to 1000g.
Knoop tests are also known as microhardness or micro indentation tests and are best used in small test areas or on brittle materials because minimal material deformation occurs on the short diagonal area. The Vickers diamond produces a square-based pyramidal shape with an indentation depth of about one-seventh of the diagonal length. To meet all testing requirements, the Vickers test has two distinct force ranges: micro (10g to 1000g) and macro (1kg to 100kg). Because the indenter is the same for both ranges, Vickers hardness values are continuous across the entire metal hardness range (typically HV100 to HV1000). Vickers tests, also known as macro-indentation tests, are used on a wide range of materials, including case-hardened and steel components. Vickers indents are also less sensitive to surface conditions than the Knoop test. In both test types, the measured area is used in a formula that includes applied force to determine a hardness value. Tables or automatic electronic or imaging measurements are a more common and convenient way to generate Knoop and Vickers hardness numbers.
The Leeb hardness test
The Leeb hardness test, otherwise called the Leeb Rebound Hardness Test (LRHT), is considered one of the four most 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 more than 1 kg. It also measures the coefficient of restitution. 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 the energy loss due to deformation. The impact body rebounds faster from harder samples than the softer ones, resulting in a larger value of 1000* vr / vi which is quoted as the Leeb rebound hardness unit HL. In a Leeb hardness tester, 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:
- Approach phase: The impact body is accelerated in the direction of the test surface in the approach phase.
- 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 a standstill. This elastic spring-back nature of the impact body, as well as the specimen, helps the impact body to rebound.
- Rebound phase: Here, the impact body is again accelerated out of the impact phase with the resultant energy.
Hardness Comparison and Conversion
There is a variety of hardness-testing methods available, including the Vickers, Brinell, Rockwell, Meyer, and Leeb tests. Although it is impossible in many cases to give an exact conversion, it is possible to give an approximate material-specific comparison table.
If you are interested in hardness comparison and conversion, you can do further research from ASTM International‘s standard hardness conversion tables for metals relationship among brinell hardness, vickers hardness, rockwell hardness, superficial hardness, knoop hardness, scleroscope hardness, and leeb hardness.
Many of the conversion values presented in the tables were obtained from computer-generated curves of actual test data. Most Rockwell hardness numbers are presented to the nearest 0.1 or 0.5 hardness number to permit accurate reproduction of these curves. Conversion of hardness values should be used only when it is impossible to test the material under the conditions specified, and when conversion is made it should be done with discretion and under controlled conditions. Each type of hardness test is subject to certain errors, but if precautions are carefully observed, the reliability of hardness readings made on instruments of the indentation type will be found comparable. Differences in sensitivity within the range of a given hardness scale (for example, Rockwell B) may be greater than between two different scales or types of instruments. The conversion values, whether from the tables or calculated from the equations, are only approximate and may be inaccurate for the specific applications.
Hardness testing plays an important role when designing devices and products, We need the data to verify the heat treatment, structural integrity, and quality of components, to determine if a material has the properties necessary for its intended applications.
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