Rockwell hardness test
The Rockwell hardness test is a standardized method in which hardness is determined by measuring the permanent indentation depth of a test indenter. Since no optical measurement of the indentation is required, hardness determination is particularly fast. Rockwell is therefore ideally suited for high-volume production and serial testing. Furthermore, Rockwell hardness testers are usually more cost-effective compared to optical hardness testing machines. In practice, people often simply refer to "Rockwell hardness," usually meaning the HRC scale. However, the Rockwell method actually comprises a total of 30 standardized hardness scales for different materials and applications.
the essentials in brief
- The Rockwell hardness test The hardness is determined by the permanent penetration depth.
- The process works with Preload and main load.
- Depending on the Rockwell scale, a Diamond cones or hard metal balls for use.
- Rockwell is particularly suitable for Fast and recurring tests.
- Clean contact surfaces and stable sample positioning are crucial for reliable results.
- The Hardness testing method Rockwell is in the standard series DIN EN ISO 6508 .
What is the Rockwell hardness test?
The Rockwell hardness test is an indentation test. A test specimen is pressed into the surface of the material with a defined force. Unlike... Brinell, Vickers or Button The hardness value is not determined by optical measurement of a test indentation, but by the permanent indentation depth.
The HRC scale is particularly well-known for hard and hardened steels. Its practical application range extends up to approximately 70 HRC. For higher hardness levels, the Vickers, Knoop, or Rockwell HRA scale methods are typically used. HRC is the most widely used Rockwell scale; however, depending on the material and testing requirements, numerous other scales with different test specimens and test forces are available.
Rockwell is the fastest and cheapest hardness testing method and is therefore the most widely used.
For which materials is the Rockwell hardness test suitable?
Depending on the scale, the Rockwell hardness test is suitable for hard, medium-hard, and softer metallic materials. Crucially, the scale, test specimen, test force, and material must be compatible.
Typical applications are:
- hardened steels
- Tool steels
- Spring steels
- hard to very hard metallic materials
- Softer metals with matching Rockwell scales and a ball bearing
- Recurring inspections in goods receipt, production and quality assurance
This method should not be used across the board for every testing task. For small test areas, thin layers, or precisely positioned individual measurements, other methods, especially Vickers or Knoop, are often more suitable.
How does the Rockwell hardness testing method work?
The Rockwell hardness testing method uses a preload and a main load. The preload reduces surface influences and play in the testing system before the actual measurement takes place.
The process can be summarized in a few steps:
- The test piece is positioned cleanly and stably on the test table.
- A preload is applied to bring the system into a defined starting position.
- The measuring system is set to zero path at this reference position.
- The main load is applied and held for a specified holding time.
- The main load is relieved again and transferred to the preload.
- The Rockwell hardness value is calculated from the measured, permanent penetration depth (under preload).
The advantage lies in the rapid determination of results. Since no optical indentation measurement is necessary, Rockwell is particularly suitable for repetitive tests with clearly defined test conditions.
Measurement principle: Why Rockwell measures via penetration depth
Rockwell is a depth difference method. It measures how deeply the test specimen has penetrated the material permanently after the main load has been applied and removed. This depth difference forms the basis for the hardness value.
The HRC scale uses a diamond cone. The calculation principle follows the following approach:
HRC = 100 – h / 0,002
Here, h represents the permanent depth difference in millimeters. Even very small changes in penetration depth have a significant effect on the Rockwell hardness value. An additional depth change of 0,002 mm corresponds to one Rockwell unit.
Therefore, clean contact surfaces, stable sample positioning, and play-free contact between the test piece and the support are particularly important. Contamination, rust particles, burrs, or an uneven underside can distort the result.
What do HRC, HRB and other Rockwell scales mean?
Rockwell is not a single hardness value, but encompasses various scales. They differ in terms of test specimen, test force, and application.
| Scale | test specimens | Typical application |
|---|---|---|
| HRC | diamond cone | Hard steels and hardened components |
| HRB | carbide ball | Softer metals, copper alloys and aluminum alloys, depending on the application |
| HRA | diamond cone | Thinner hard layers or very hard materials, depending on the standard and test task |
| Super-Rockwell | Depending on the scale | Thinner samples or tests with smaller test loads |
The appropriate scale should always be selected based on the material, component geometry, expected hardness and standard requirements.
Requirements for reliable Rockwell measurements
Rockwell measures very small depth differences. Therefore, the results are sensitive to errors in the setup, sample positioning, and surface condition.
The following are particularly important for reliable measurements:
- The test piece must rest stably and without play.
- The contact surface must be clean and free of particles.
- The underside of the sample should be flat, smooth and free of rust, burrs or dirt.
- The test surface must be suitable for the chosen method.
- The sample must be sufficiently thick.
- Edge distances and distances between test impressions must be maintained.
- The indenter and test force must match the selected Rockwell scale.
Since even the smallest additional movements can change the measured penetration path, sample preparation is particularly important for Rockwell.
Minimum distances and edge distances at Rockwell
Rockwell hardness indentations must not be placed too close together or too near the edge. During the hardness test, material is locally displaced and hardened. An additional indentation placed too close can influence this process and produce a false hardness reading.
The distance to the edge of the test piece is also important. If the distance is too small, the material can deflect laterally. The test specimen then penetrates deeper, which can lead to a falsely low hardness reading.
The specific minimum distances and edge distances must be checked and adhered to according to the applicable standard.
Advantages and limitations of the Rockwell hardness test
| Benefits | boundaries |
|---|---|
| Very fast results determination | High demands on stable support and clean sample underside |
| No optical impression measurement required | Not ideal for very small test areas |
| Well suited for serial and recurring tests. | Incorrect scale leads to unreliable results. |
| Established and standardized procedure | Contamination, burrs, or unevenness can greatly affect the result. |
Standards for hardness testing according to Rockwell
The Rockwell hardness test is part of the standard series. DIN EN ISO 6508 regulated. For practical application, the following areas are particularly relevant:
- DIN EN ISO 6508-1 describes the testing procedure.
- DIN EN ISO 6508-2 It covers the verification and calibration of testing machines and indenters.
- DIN EN ISO 6508-3 This concerns the calibration of hardness comparison plates.
- ASTM E18 is relevant for Rockwell tests according to American standards.
The standard specifies, among other things, the test specimen, test force, test procedure, evaluation, and reporting of results. For comparable results, the required standard should be clearly defined before the test.
Common mistakes in Rockwell hardness testing
Errors in Rockwell hardness testing often arise from an unsuitable scale, an inappropriate specimen, or an insufficiently stable support. Typical sources of error include:
- incorrect Rockwell scale
- unsuitable test piece (material)
- dirty or uneven surface
- The sample is not lying stably on the surface.
- The sample is too thin.
- Test impressions are too close to the edge or too close together
- damaged or worn penetrator
- missing or unrated calibration
- Direct comparability of different Rockwell scales or other hardness testing methods is assumed.
Careful preparation and a suitable selection of methods are therefore crucial for reliable measurement results.
What Rockwell hardness testers are available?
Various instrument concepts are available for Rockwell hardness testing. The selection depends on the testing task, test volume, scale range, degree of automation, and documentation requirements.
Possible device types include:
- Simple Rockwell hardness testers with analog pointer instrument (only cheap, low-quality products)
- digital Rockwell hardness testers
- Universal hardness tester with Rockwell function
- Automatic Rockwell systems for recurring tests or test series (Jominy)
- Devices with force measuring cell and digital evaluation (now the standard)
For regular testing volumes, digital or automated systems are advantageous because they can standardize testing procedures and document results. For varying methods or multiple scales, a universal hardness tester may be a suitable solution.
When is Rockwell the right method?
| Application | Suitable for Rockwell? |
|---|---|
| Rapid serial testing | Ja |
| Hardened steels and HRC applications | Ja |
| Recurring incoming goods inspections | Ja |
| Very small components or thin layers | Probably not, depending on the scale and standard. |
| Precise positioning of individual test points | More likely Vickers |
| Coarse or inhomogeneous cast structures | Rather Brinell |
| Visual documentation of the impression is required. | Brinell or Vickers? |
Frequently asked questions about the Rockwell hardness test
What is the Rockwell hardness test?
The Rockwell hardness test is a standardized hardness testing procedure in which hardness is determined by the permanent indentation depth of a test specimen.
How does the Rockwell hardness test work?
First, a preload is applied. The measuring system is then zeroed. Next, the main load is applied and held. After the load is released back to the preload, the remaining difference in penetration depth is determined, and the Rockwell hardness value is calculated from this.
What does HRC mean?
HRC is a Rockwell scale that uses a diamond cone. It is commonly used for hard and hardened steels.
For which materials is Rockwell suitable?
Depending on the scale, Rockwell is suitable for hard, medium-hard, and softer metallic materials. The test specimen used, the test force, and the applicable standard are crucial factors. Special Rockwell scales are also used for plastics, graphite, carbon alloys, hard metals, and sintered materials. The measured quantity is always the permanent indentation depth of a test specimen.
Why is the contact area so important for Rockwell products?
Rockwell measures very small depth differences. Contamination, rust, burrs, or play between the test piece and the support can alter the measured penetration depth and distort the result.
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