Hardness is a characteristic of a material. It is defined as indentation strength, which is determined by measuring the permanent depth of the indentation. More simply, when using a fixed force value (load) and a specific indenter, a smaller indentation represents a harder material. This indentation hardness value is obtained by measuring the depth or area of the indentation using one of 12 different test methods.
Factors to Consider in Hardness Testing
The following sample characteristics should be prioritized before selecting the hardness test method to use:
- Sample size
- Cylindrical samples
- Thickness of the sample
- Ruler
- Reproducibility and repeatability of equipment
Sample Size
The smaller the part size, the lower the load to form the desired indentation. It is extremely important for small sized parts to meet minimum thickness requirements and to keep the indentation away from internal or external boundaries. Larger parts need to be properly secured with appropriate clamps to ensure that they do not move or slip during testing. Those parts that protrude from the anvil or are not easily supported by the anvil should be clamped in place or properly supported.
Cylindrical Samples
Due to the difference between axial and radial material slip, the test results need to be corrected when testing small diameter cylindrical samples. According to the diameter of the cylindrical convex surface, the arc correction parameter is added to the test result. Also, it is important to maintain a minimum spacing equal to approximately 2 to 1/2 times the diameter of the indentation from the edge or another indentation.
Thickness of The sample
The minimum thickness of the sample should be at least ten times the depth of the indentation. Both ordinary and superficial Rockwell methods have minimum thickness requirements.
Scale
Sometimes it is necessary to measure on one scale and report on another. The conversion function is necessary, but it is important to note that it may not provide reliable information unless actual correlations have been done by testing at different scales.
Reproducibility and Repeatability of Hardness Testing Equipment
Equipment repeatability and reproducibility studies are used to calculate the ability of operators and instruments to perform tests within tolerances for a particular test sample. Hardness testing also has inherent variables, which are excluded with standard equipment R&R procedures and formulas for actual test samples. Variations in material and the inability to test in the same area with a depth gauge are two significant factors that affect GR&R results. To minimize these effects, it is best to study high-consistency standard blocks in order to minimize these built-in changes.
Methods of Hardness Testing
The Rockwell test method defined in ASTM E-18 is the most commonly used hardness test method. Rockwell testing is generally easier to perform and more accurate than other types of hardness testing methods. The Rockwell test method is used for all metals unless the structure or surface conditions of the metal being tested would cause too much variation; and those where the indentation is too large for the application; or the size or shape of the sample limits the test.
The Rockwell method measures the depth of permanent deformation of the indentation created by the force/load on the indenter. First, an initial loading force (often referred to as a preload) is applied to the sample using a diamond indenter. This load represents a zero or reference position that penetrates the surface to reduce the effect of surface finish.
After the preload is complete, an additional load called the main load is added to achieve the desired total test load. Taking into account elastic recovery, maintain this force for a predetermined time (hold time). The main load is then released, and the indentation depth deviation from the final position from the preload position, as well as the indentation depth deviation between the preload and main load, is measured. This distance is converted into a hardness value.
The Brinell test method is defined by the ASTM E10 standard. Most of the time it is used to measure materials with rough surfaces or structures such as castings and forgings. Brinell testing often uses a very heavy test load (3000 kgf) and a 10 mm wide indenter in order to equalize most of the surface and subsurface inconsistencies of the indentation.
The Brinell method loads a tungsten carbide ball of a fixed diameter (D) with a predetermined test load (F), holds it for a certain preload time and then unloads it. The indentation is then taken at least two diameter measurements - usually perpendicular to each other and then averaged (d). The measured average diameter was then converted to a Brinell hardness value using a conversion table. The force range is between 500 and 3000 kgf.
Generally, a Brinell hardness tester produces an indentation, and then a special Brinell microscope or optical system is used to measure the diameter of the indentation. The measurements are then converted to Brinell values using the Brinell formula or a formula-based conversion table.
Ball indenters can range in diameter from 10mm to 1mm. In general, lower loads and spherical diameters are for easier comparison with other instrument results, such as Rockwell units with small load ranges.
Vickers test method, Vickers test method also known as microhardness test method is mainly used for small parts, thin slices or hardened layer depth work. The Vickers method is based on an optical measurement system. Microhardness test procedure According to the requirements of ASTM E-384, a diamond indenter is loaded according to the specified low load range to form an indentation, and is measured and converted into a hardness value.
This test is useful for a wide variety of materials testing as long as the test samples are carefully prepared.
Inverted conical diamond with a square base is used for the Vickers scale test. While "macro" dimensional loads can range as high as over 30kg, conventional loads are light in the range of a few grams to one or a few kilograms. This microhardness method is used to test metals, ceramics, composites - virtually any type of material.
Since the indentation of the Vickers test is very small, it is suitable for different kinds of applications: inspecting very thin materials such as metal foils or measuring parts, surfaces of small parts or small areas, measuring individual microstructures or measuring the depth of hardened layers. This measurement describes a profile of hardness change by making a series of indentations on a partial profile.
The Knoop test method, also known as the microhardness test method, is primarily used for small parts, thin sheets, or deep work in hardened layers. The Vickers method is based on an optical measurement system. Microhardness Test Procedure Load indentation according to ASTM E-384 at a specified low load range and measure and convert to a hardness value.
As long as the Knoop test sample is carefully prepared, the test is useful for a wide variety of materials testing. The inverted conical diamond indenter is tested on the Knoop scale. This indenter is different from the inverted cone type indenter used in the Vickers test above. The shape of the Knoop indenter is relatively extended or rectangular.
The Knoop method is usually used when the indentations are very closely spaced or very close to the edge of the sample. The width of the Knoop indentation provides higher resolution for the measurement and the indentation is shallower. Therefore, it can be used for very thin materials.
In the Knoop test, a predetermined test load is applied with an inverted conical diamond indenter for a specified holding time.
The inverted conical indenter used in the Knoop test is more slender than the indenter in the Vickers test. Uninstall after the retention time. Unlike the Vickers test, which averages the longitudinal and transverse lengths of the indentation, the Knoop method only measures the transverse length of the indentation. Then convert to Knoop hardness value according to the conversion table.
Since the indentation of the Knoop test is very small, it is suitable for different kinds of applications including inspecting very thin materials such as metal foils or measuring the surface of parts, small parts or small areas, measuring individual microstructures or measuring the depth of hardening of hard layers. This measurement shows the change in hardness by making a series of indentations on a partial section.
The hard layer depth is the thickness of the hard layer on the sample. Hard layer hardening improves wear resistance and fatigue strength of components under dynamic and/or thermal stress. Hardened steel components are typically used in applications requiring high wear resistance and strong rotation. The properties of hard layer hardening are mainly determined by the surface hardness, effective hardening depth and depth profile of residual stress. Transmission and engine components are examples of hardening.
Hard coat depth testing is generally performed by creating a series of hardness indentations from the edge of the sample towards the center. The hardness evolution is then plotted and the distance from the surface to the hardness limit (HL) is calculated.
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