The outcome of each test should have a label identifying the method used, as it is dependent on it. There is no intrinsic significance of hardness value or number, hence it cannot be used directly like tensile test value. The value is only useful for comparing different treatments or materials. Testing is widely employed for process control and inspection, and the outcome is used in estimating mechanical properties like tensile strength. It is usually done using testing machines fitted with an indenter that is enforced into test matter over a period of time.
Each machine also makes use of different load or force application system, while recording a hardness value in kg-force as per the individual scales. During this test, an accurately controlled force is maintained when an indenter, generally a carbide ball, is forced into the test model for a specific period of time. Upon removal, it leaves an encircling indentation, the measurement of which is taken to calculate material hardness as per the formula.
In this article, we want to look into the four different methods of indentation hardness testing methods.
First developed over one hundred years ago, the Rockwell hardness test is a rapid hardness testing method primarily used for quality assurance and production control. It uses a diamond cone or steel ball to first apply a minor load of around 3kgf before an additional major load is applied; typically, exceeding 10kg.
Measuring the permanent change in depth from the minor baseline enables engineers to calculate the Rockwell hardness number. Like the Rockwell scale, Brinell hardness testing typically concerns larger loads approaching a nominal maximum force of 3,kgf which requires a larger, harder indenter tip. Brinell indenters have spherical tungsten carbide tips that leave a relatively wide, deep impression on surfaces.
As a result, Brinell hardness testing is typically used for larger samples. The Brinell method is the most commonly used hardness measurement technique in industry. To mark the surface of the sample, this test method uses a steel ball indenter with a diameter that varies according to the sample to be measured.
A microscope or measuring magnifier complete the device. The Rockwell method is the most universal because it uses both a penetrating diamond cone and a penetrating steel ball to measure the hardness of a material. The Vickers method is a microhardness testing technique. It is similar to the Brinell method. It is suitable for all solid and metallic materials and for all hardness ranges from 10 gf to kgf.
The difference between the Vickers method and the Brinell method is that the latter uses a diamond penetrating pyramid with a square base and a fixed load. An optical instrument measures the impression. This technique allows light test loads as well as macro loads up to 30 kg. The Knoop method is another microhardness testing technique, an alternative to the Vickers technique, that uses a narrow diamond shaped indenter. This technique allows for light test loads of just a few grams.
It is mainly used to assess the risk of cracking in brittle materials such as ceramic and to test the hardness of thin layers of a sample, including coatings. Cylindrical Samples A correction to a test result is needed when testing on cylinder shapes with small diameters due to a difference between axial and radial material flow.
Roundness correction factors are added to your testing result based on the diameter of convex cylinder surfaces. Sample Thickness Your sample should have a minimal thickness that is at least 10x ten times the indentation depth that is expected to be attained. There are minimum, allowable thickness recommendations for regular and superficial Rockwell methods Scales Sometimes it is necessary to test in one scale and report in another scale.
Conversions have been established that have some validity, but it is important to note that unless an actual correlation has been completed by testing in different scales, established conversions may or may not provide reliable information.
Refer to ASTM scale conversion charts for non-austenitic metals in the high hardness range and low hardness range.
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