DIN 51917: Rockwell hardness test HR5/150 on carbon materials and graphite
The standard DIN 51917 specifies the Methods for hardness testing of carbon materials using Rockwell HR5 / 150 fixed. It defines the test conditions under which the hardness of technical carbon, graphite and related materials The hardness of these materials can be reliably determined. Since these materials are brittle, porous, and only slightly plastically deformable, the method provides reliable hardness values and thus forms the basis for quality assurance, production monitoring, and materials development.
Key facts at a glance
The Rockwell hardness test HR5/150 is considered Standard method for determining the hardness of graphite and technical carbon. It works quickly, is highly reproducible, and causes minimal damage to the sample. This makes it suitable for both individual testing in research and development and automated batch testing in production. Typical applications include testing electrographite, specialty graphites, EDM electrodes, current collectors, and furnace and high-temperature components.
The hardness test according to DIN 51917 provides important information about:
- Material quality
- degree of compaction
- microstructure
- porosity
- wear behavior
- process stability
What does DIN 51917 regulate?
DIN 51917 standardizes the hardness testing of carbon materials using the Rockwell HR5/150. It describes the entire testing procedure, from the indenter and test forces to the loading times and evaluation, and also specifies the requirements for the testing machine. It is the authoritative reference standard for many German manufacturers of graphite and carbon materials.
Purpose and scope
The aim of the standard is to determine the hardness of carbon materials in a reproducible and comparable manner. DIN 51917 uses a [method/method/method - context needed] for this purpose. Rockwell method modified for carbon materials using a ball, which is based on the general Rockwell test according to ISO 6508-1. Like all Rockwell methods, it works as a depth difference method: The permanent indentation depth is measured, not the indentation diameter.
The application range includes technical-grade carbon, graphite, and related carbon alloys. Metal-graphite alloys, such as carbon brushes in motors and generators, can also be tested using this method. The proprietary HR5/150 scale is necessary because carbon alloys differ significantly from metals in their mechanical behavior and would not be reliably measured using standard Rockwell scales.
The history of the Rockwell hardness test
The Rockwell hardness test was developed by American engineers at the beginning of the 20th century. Hugh M. Rockwell and Stanley P. Rockwell Developed and patented in 1919, the method quickly gained acceptance because hardness no longer had to be determined optically via an indentation diameter, but could be measured directly via the permanent indentation depth.
Today, Rockwell is one of the world's most important companies. Hardness testing method and is enshrined in both international ISO standards and numerous ASTM standards. While the general Rockwell methods apply primarily to metallic materials, separate standards have been developed for specific material groups: DIN 51917 for carbon materials and ASTM C748 for graphite materials.
Why carbon materials require special testing procedures
Carbon-based materials differ fundamentally from metals. Typical examples are:
- brittle fracture behavior
- low plastic deformation
- porosity
- anisotropic microstructures
- local density fluctuations
- sensitive surfaces
These materials cannot be reliably characterized using unsuitable testing methods: surface defects and microcracks can occur, and the measured values vary considerably. This results in poor repeatability and results that cannot be compared. DIN 51917 was specifically developed to minimize these influences and enable reproducible test results.
What does Rockwell HR5/150 mean?
The designation HR5/150 describes the most important parameters of the test procedure:
| designation | Significance |
|---|---|
| HR | Rockwell hardness, ball test |
| 5 | Ball diameter 5 mm (carbide ball) |
| 150 | Total test force 150 kgf (approx. 1.471 N) |
The test is performed using a carbide ball as the indenter. The total test force is applied in two stages: First, a pre-force is applied to define the reference plane for the measurement. Then, the force is increased to the total test force of 150 kgf and, after a defined holding time, reduced back to the pre-force. The hardness value is derived from the remaining indentation depth, i.e., the depth difference relative to the reference plane. This depth difference method enables a fast and highly reproducible hardness determination.
Related standards to DIN 51917
DIN 51917 is part of a series of Rockwell standards. For international comparison, the following reference standards are particularly relevant:
- DIN EN ISO 6508: general Rockwell hardness test, basis on which the ball test method of DIN 51917 is based
- ASTM E18: American equivalent of ISO 6508 for the general Rockwell test
- ASTM C748: American standard for graphite, uses the Rockwell-L scale (6,35 mm steel ball, 10/60 kgf)
Requirements for modern hardness testing machines
A standard-compliant test according to DIN 51917 places high demands on the testing technology. Four points are crucial:
- Precise test force control: Carbon materials are sensitive to impact loads. The force must be applied in a controlled and reproducible manner.
- High-resolution depth measurement: Rockwell hardness is determined by the permanent penetration depth. Therefore, highly accurate measurement technology is essential.
- Standard-compliant loading times: The prescribed times for pre-force, load application, holding time and unloading must be adhered to exactly.
- High repeatability: Especially with porous materials, many individual measurements must deliver stable results.
Factors influencing the measurement results
Several factors should be taken into account to obtain reliable and comparable results according to DIN 51917:
- Surface quality: The test surface should be flat, clean and as parallel as possible.
- Material homogeneity: Porosity and density variations affect the measured values.
- Test intervals: Multiple impressions must not influence each other.
- Environmental conditions: Temperature and humidity can play a role depending on the material.
- Calibration: Regular inspection and calibration of the testing machine ensures reliable results in the long term.
FAQ: Frequently asked questions about DIN 51917
What does DIN 51917 regulate?
DIN 51917 specifies the procedure for Rockwell hardness testing of carbon materials using a ball indenter (scale HR5/150). It defines the indenter, test forces, loading times, evaluation, and the requirements for the testing machine. In Germany, it is the authoritative reference standard for graphite and technical carbon.
What does HR5/150 mean?
The designation describes the test parameters: HR stands for the Rockwell hardness using the ball method, the 5 for the ball diameter of 5 mm, and the 150 for the total test force of 150 kgf (approximately 1.471 N). The test is performed with a hard metal ball, and the hardness value is determined by the permanent indentation depth.
Which materials are tested according to DIN 51917?
Carbon materials such as electrographite, specialty graphites, technical carbon, and carbon composites are tested. Metal-graphite materials, such as carbon brushes in motors and generators, can also be tested using this method. Typical applications include quality assurance, production monitoring, and research and development.
What are the differences between DIN 51917 and ASTM C748?
Both are based on the Rockwell principle of indentation depth measurement, but use different parameters. DIN 51917 tests with HR5/150 (5 mm carbide ball, 150 kgf), while the American standard ASTM C748 uses the Rockwell L scale (6,35 mm steel ball, 10 kgf preload, 60 kgf main load). Therefore, hardness specifications should always be given with both the standard and the Rockwell scale.
Why do carbon materials need their own testing procedure?
Carbon materials are brittle, porous, and only slightly plastically deformable. Inappropriate testing methods can cause spalling or microcracks and lead to highly variable, poorly comparable results. DIN 51917 is tailored to these properties and delivers reproducible results.
How often does a hardness tester for DIN 51917 need to be calibrated?
Testing machines and measuring systems should be regularly checked and calibrated. The exact interval depends on usage frequency and quality requirements; twelve months is typical. Only a valid calibration guarantees standards-compliant and traceable results.
