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Rockwell Hardness Testing

The Rockwell hardness test method, as properly defined in ASTM E-18 standards, is the most commonly used hardness test method among all the other methods. The customer should get a copy of these standards and understand them well before attempting a Rockwell test.

 

The Rockwell test is comparatively easier to perform and more precise than other hardness testing methods. The Rockwell test method can be used on all metals, except in the condition where the test sample structure or surface conditions would bring too many variations; where the indentations can be too large; or where the sample size or shape may prohibit its use. 

This Rockwell hardness testing method measures the permanent depth of indentation created by a force/load on an indenter. At first, a preliminary test force (preload or minor load) is applied upon the sample with the help of a diamond or ball indenter. This preload breaks through the surface to decrease the effects of surface finish. After holding this preliminary test force for a specific period of dwell time, it is possible to measure the baseline depth of indentation.

After completing the preload step, an additional load (referred to as the major load) is added to attain the total required test load. This intense force is held for a predetermined amount of time, referred to as the dwell time, which permits the elastic recovery. This load is then released by returning to the original preliminary load. Again, after holding the preliminary test force for a specific dwell time, the final depth of indentation is also measured. The Rockwell hardness value can be finally calculated from the difference in the baseline and final depth measurements. This distance is then converted to a hardness number. The preliminary test force is then removed followed by the removal of the indenter from the test specimen.

In the Rockwell hardness test, preliminary test loads (preloads) can range from 3 kgf (‘Superficial’ Rockwell scale) to 10 kgf (‘Regular’ Rockwell scale). The complete test forces also range from 15kgf to 150 kgf (superficial and regular) with a maximum level of 500 to 3000 kgf (macrohardness). 

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.

Rockwell-Hardness-test

  

Varied indenters such as conical diamond having a round tip for harder metals along with the ball indenters ranging with a diameter from 1/16” to ½” for softer materials can be used in this method. While selecting a Rockwell scale, it is mandatory to select the scale that indicates the largest possible load as well as the largest possible indenter without surpassing the already defined operation and accounting conditions which may possibly affect the test result. These conditions involve test specimens under the least thickness for the depth of indentation, a test impression that appears too close to the edge of the specimen or another impression, or evaluation on cylindrical specimens.

Furthermore, the test axis must remain within 2-degrees perpendicular to confirm the accurate loading. There should not be any deflection of the tester or test sample during this loading step due to varied conditions like dirt below the test specimen or upon the elevating screw. It is mandatory to keep the surface clean and eliminate the decarburization from heat treatment. Sheet metal can remain too thin as well as soft for Rockwell scale testing without crossing the least thickness requirements as well as potentially indenting the test anvil. In this testing procedure, a diamond anvil consistently influences the result.

There is one more special case in evaluating the cold-rolled sheet metal. The hardening step causes a gradient of hardness through the test sample, so any test can determine the average of the hardness over the depth of the indentation effect. But in this case, any Rockwell testing method proves to be totally doubtful. There is often a history of evaluation using a specific scale on a specific material. This can also be used and interpreted functionally by the operators. For more information about Rockwell hardness tester, contact us.

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