A Primer on Rockwell Hardness Testing

October 16, 2020 12:17 pm

The Rockwell hardness test is the industry standard measuring system used to determine how resistant a material is to another object. Hardness is defined as a material’s resistance to permanent indentation. This test was created to determine the hardness and strength of materials. After executing the Rockwell hardness test, you will be able to understand exactly how your material will perform and react to machining processes.

There are myriads of hardness tests out there, however the Rockwell hardness test is the most common hardness test used in the metalworking industry today, due to its speed and accuracy. The test must be carried out with a specific type of scale or tester, called a Rockwell hardness scale.

How It Works

The hardness test is done through a series of pressure tests.  An indenting tool, typically a diamond or ball indenter, presses against the material’s surface.  After a specific hold time, the load is removed, and the indent is measured. This is repeated with a heavier load, called the major load, and then repeated for a third time once again with the preliminary load. The difference between the initial and final indentation is measured, then converted to a hardness number, and that is how the hardness of the material is calculated.

The applied forces usually range between 15 kgf and 3,000 kgf but varies depending on the test and the type of material.

Here are a couple of visuals that should help you understand the concept.

Image Credit: https://www.unitedtesting.com/en-us/rockwell-hardness-testing

 

Image Credit: https://matmatch.com/learn/process/hardness-comparison

 

Check out the video below, it offers a practical and digestible introduction to working with a Rockwell hardness scale.

 

If you are interested in obtaining a complete understanding and holistic background of the Rockwell hardness test, we recommend diving deeper into the ASTM E18-20 Standard Test Methods for Rockwell Hardness and the Rockwell Superficial Hardness of Metallic Materials.

Best Practices

The accuracy this test can achieve is not to be taken lightly as there is a high level of human technique and care needed to maintain the integrity of the test. The operator of the Rockwell hardness scale must follow proper methods and practices. This can be achieved by paying close attention the manual and properly educating yourself (which can be done by watching tutorials like the video noted above).

A major factor to pay attention to is the support of the material. It is critical that the material be secure in place during the test process, as any movement will result in error. To get an idea of how delicate the process is, know that one point on the Rockwell measuring scale is 0.002 mm or 0.00008”. A movement of just 0.001” could result in an error of over 10 Rockwell points.

Factors like material type and thickness will require different scales. There are over 30 different Rockwell scales with their own distinguished test forces and indicator types. Make sure to select the scale right for your application needs.

Rockwell Hardness Scales (Testers)

The most common Rockwell hardness scales are B and C, however, there is a multitude of alternative scales. Check out this graph to get a better idea of which Rockwell hardness scale will meet your needs.

Image Credit: https://classes.mst.edu/civeng120/lessons/hardness/equipment/rockwell/index.html

 

A Rockwell hardness scale is an investment, so if you’re interested in checking out some models, here are some recommendations.

Starrett 3814 Analog Bench Hardness Tester

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This scale has the ability to handle Rockwell scales A through H and K and conforms to ASTM E18 standards. Featuring a high-quality casting and ergonomically designed for easy operation, this option is suitable for any lab, toolroom, workshop or inspection department.

Phase Ii Plus, Analog Rockwell Hardness Tester

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Also conforming to the ASTM E18 hardness standards, this scale is perfect for any environment, including heat treatment facilities, toolrooms, workshops, etc. With a direct analog reading, this scale has an advanced design and is easy to operate.

GUOCAO PHR-100 Portable Magnetic-type Rockwell Hardness Tester

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With high accuracy and complying with ISO6508 and ASTM E18, this scale has good repeatability and features a similar reading system to a micrometer, making it easy to read. Test the hardness of steel plates, pipes, axes, molds, huge forgings, and huge and medium-size heat treatment parts.

NEWTRY Portable Rockwell Hardness Tester

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This tester is compact, light and weighs only 1.2 kg. It is as accurate as desk testers and complies with the basic designing principles of Rockwell hardness testing. It is used to test thin and small parts, such as thin plates, thin tubes, thin axles, small balls, small stamping parts, and small supporting surfaces.

PTC 316 Steel Hardness Tester: Portable C Scale

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Not technically a Rockwell tester, but this tester measures the hardness of metals in the range of 20 to 65 HRC on the equivalent Rockwell C Scale.  It is exceptionally useful where the test piece is too large or heavy to test on a bench-type tester. This high quality, affordable, portable tester is a must for large workpieces, small machine shops, heat treaters, quality control personnel, and engineers.

Bench Top Rockwell Hardness Tester HR 150 A Testing Table

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This Rockwell tester can handle loads of  60kg, 100kg, 150kg with a max test height: 6.7″ and depth of 5.3.” A storage case with accessories is included and the weight of the unit is approximately 150 lbs.

 

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