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Before knowing the function of the Rockwell hardness test, let us first understand the meaning of the term ‘Rockwell hardness’
Rockwell hardness defines the level of resistance of a metal object like a knife blade towards penetration or permanent deformation by another substance.
The engineers use the non-destructive metallurgical test named Rockwell Hardness Testing to determine how hard and strong a material is.
To achieve a broader application area for the best Rockwell process, several Rockwell hardness testing methods have been developed including the Rockwell and the Super Rockwell techniques.
The individual Rockwell hardness testing method can be distinguished from each other by:
This hardness tester uses digital automatic intelligence to test Superficial Rockwell hardness as well as ordinary Rockwell hardness. Its automatic abilities also include lifting, load-changing, loading and unloading, conversion, and testing which are all done automatically.Add to cart
This digital hardness tester features mostly automated operations including automatic conversion of hardness scales, automated calculations of hardness ranges, and average hardness. It has an improved accuracy using its 0.1HR resolution and an inbuilt microprocessor.Add to cart
ARH06 analog gauge resolution is 0.5HR, it is useful to improve the reading accuracy than traditional 1HR resolution.Add to cart
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To figure out the hardness, Rockwell hardness testers employ a differential-depth method.
At first, a primary test force (otherwise called as minor load or preload) is applied to a sample with the help of a ball or diamond indenter. This preload breaks through the surface to lower the consequences of surface finishing. After keeping the preliminary test force for a definite dwelling time, the baseline depth of indentation can be measured.
After this preload, an extra load, otherwise called as the major load, is applied to reach the total required test load. This force is held for a specific amount of time (dwell time) to allow elastic recovery. This major load is released followed by returning to the primary load. After holding the preliminary test force for a definite dwelling time, the final depth of indentation can be measured.
The difference in depth between the minor and major loads determines the hardness.
The Rockwell hardness value is thus derived from the difference between the baseline and the final depth measurements. This distance is converted to a hardness number. The preliminary test force is eliminated, followed by the elimination of indenter from the test specimen.
A dial or display shows the depth of penetration, and a higher number shows the level of hardness.
The primary test loads (preloads) can range from 3kgf (used in the ‘Superficial’ Rockwell scale) to 10kgf (used in the ‘Regular’ Rockwell scale). The complete test forces range from 15kgf to 150kgf (regular and superficial) to 500 to 3000kgf (macro-hardness).
The varied applications of Rockwell hardness meter include:
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Rockwell hardness tester, the world’s first tester designed on the principle of the Rockwell scale, was co-invented by Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) in 1914. The patent for this instrument was granted in the year 1919. At first, this instrument used only the single side contact to inspect the hardness of metals. There was no need for sample support, and the test accuracy was in line with the standardized ISO6508, not below desktop Rockwell hardness tester. Later, the co-inventors improved its design in the years 1919 and 1921 by setting the foundation of the modern Rockwell hardness tester.
In the year 1930, C.H Wilson had modernized the design to make it more complete. In recent times, the Rockwell Hardness Tester was produced with touch screen control, numerical display, and curve display, as well as automatic printing test data in China. This simple operational testing method takes a rapid and direct measurement from a display screen, dial indicator, or optical projection screen. Like the Vickers hardness test and Brinell hardness test, this Rockwell type has become the most common method but comparatively considered as easier to perform.
There are varied types of Rockwell Hardness Testers, according to the varied factors listed below:
There are four common types of Rockwell hardness tester with varied scales as listed below:
The Rockwell hardness is detected by the use of a primary test force (minor load or preload), followed by an extra load (major load) to reach the aggregate necessary test load, ultimately returning to the same primary test force (preload or minor load). The first minor load shows the zero or reference position. When the major load is applied and held for a fixed amount of time (dwell time), it allows elastic recovery. Next, the major load is eliminated, while still supporting the minor load to show the shift from zero or reference position in establishing a Rockwell hardness value.
The Rockwell scale can be compared with the Mohs Test and the Brinell scale.
The Brinell scale was developed by the Swedish engineer named Johann A. Brinell in the year 1910, and this scale operates as follows:
A load is applied to a strengthened steel ball that stays on a flat metal surface expected to be evaluated. Next, the diameter of the dent that creates is also assessed.
According to Sizes.com, the hardness level can be measured with this formula: “The Brinell number which shows the metal’s hardness is determined by the load on the ball in kilograms divided by the spherical surface area of the dent in square millimeters.”
Test Result Illustration:
A = Depth reached by the indenter after applying preload (minor load)
B = Position of indenter during Total load, Minor, and Major loads
C = Final position reached by the indenter after elastic recovery of sample material
D = Distance measurement showing the difference between preload and major load position. This distance helps in calculating the Rockwell Hardness Number.
Varied indenters such as conical diamond with a round tip for harder metals as well as the ball indenters ranging with a diameter from 1/16” to ½” for softer materials can be used.
While selecting a Rockwell scale, one should select the scale that indicates the largest load as well as the largest indenter that can be possible without surpassing the already defined operation and accounting conditions that may affect the test result. These conditions involve test specimens below the least thickness for the depth of indentation, a test impression that comes too close to the edge of the specimen or another impression, or evaluation on cylindrical specimens.
Moreover, the test axis must remain within 2-degrees of perpendicular to confirm precise loading. There should be no deflection of the tester or test sample during the loading step due to conditions like dirt below the test specimen or on the elevating screw. It is essential to keep the surface finish clean and remove the decarburization from heat treatment.
Sheet metal can be too thin as well as soft for Rockwell scale testing without exceeding the least thickness requirements as well as potentially indenting the test anvil. Here, a diamond anvil consistently influences the result.
There is one more special case in evaluating the cold-rolled sheet metal. The hardening can generate a gradient of hardness through the test sample, so any test can measure the average of the hardness over the depth of the indentation effect. But in this case, any Rockwell testing method proves to be doubtful. There is often a history of evaluation using a specific scale on a specific material that can be used and interpreted functionally by the operators.
A Rockwell hardness value includes three components:
The Rockwell hardness testing method has the following advantages:
The Rockwell hardness testing method has the following disadvantages:
The below listed five steps should be followed mandatorily before using the Rockwell hardness tester:
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