Article updated on 8 December 2020
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Rockwell hardness testing is a common analytical technique for evaluating the hardness of metallics and synthetic materials.
Rockwell Hardness Test
While hardness testing does not provide a direct assessment of product or raw material performance qualities, hardness correlates directly to qualities like strength and wear resistance. Hardness testing is commonly used for material evaluation due to its simplicity and low cost, relative to other assessments of these qualities. Rockwell hardness does have a relationship to tensile strength in some structural alloys, such as steel and aluminum.
The Rockwell hardness test involves making an indentation on the test material. The indenter is either a conical diamond or a steel ball. Indenter ball diameters range from 1/16 to 1/2 inches and are chosen based on the test parameters.
Advantages of Rockwell Hardness Test
Typically, the Rockwell test is easy to execute and more precise than other kinds of hardness evaluations. The Rockwell test technique is used on all metals, except when the test would introduce too many surface variations; when indentations would be too large; or where the sample otherwise prohibits its use.
To begin the test, an initial test force, frequently called preload or minor load, is put to a sample. The purpose of this preload is to break through the exterior and overcome the effects of surface finish. After the preliminary force is held for a specified length of time, the baseline level of indentation is measured.
Next, a secondary load, called the major load, is added to attain the overall test load. This force is sustained for a pre-specified length of time to allow for elastic recovery. The major load is then released, and the preliminary load is returned. The final degree of indentation is then assessed.
Rockwell Hardness Number
The Rockwell hardness number used to evaluate a sample is calculated based on the depth of permanent deformation into the sample, or the difference in indenter position before and after the major load is applied.
The various indenter varieties and range of test loads form a matrix of Rockwell hardness scales, which can be applied to a wide range of materials. Every Rockwell hardness scale is identified by a letter signifying the indenter type and the two loads used for the test. A Rockwell hardness number is a combination of the numerical hardness value and the letter for the scale preceded by the letters, HR. For instance, a hardness value of 80 on the Rockwell A scale is denoted as 80 HRA.
When choosing a Rockwell scale, a general rule of thumb is to pick the scale that identifies the biggest load and the biggest indenter possible without going beyond identified operation conditions and considering conditions that may have an impact on the test outcome. These conditions include test samples below the minimum thickness for the depth of indentation; a test impression too near the edge of the sample or another indentation; or testing on cylindrical samples.
Furthermore, the test axis ought to be within 2 degrees of perpendicular to make for precise loading; there ought not to be any deflection of the test sample or tester during the evaluation due to factors like debris. It is critical to keep the exterior finish clean and decarburization from heat treatment ought to be taken off.
Unique Cases
While Rockwell hardness testing can be used on a wide range of samples, there are certain cases with unique considerations.
Sheet metal may be too thin and soft for evaluating due to minimum thickness requirements and possible damage the test anvil.
Cold-rolled sheet metal can have a gradient of hardness across its surface. One popular solution is to use a specified scale on a material that operators are used to and can functionally interpret.
Sources and Further Reading
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