Rockwell (unit)
Rockwell is an internationally used unit of measurement for the hardness of technical materials. The abbreviation consists of HR ( Hardness Rockwell , hardness according to Rockwell) to identify the test method, followed by another letter, which indicates the scale and thus the test forces and bodies.
The test procedure is based on the penetration depth of a test body in the material. This results in increasing penetration depths with softer materials, which require thicker specimens. To accommodate this, eleven scales with different test forces and bodies are standardized according to EN ISO 6508-1. These scales bear the letters A – H, K, N and T. The standardized representation consists of the hardness value, the test method and the scale, for example 65 HRC or 90 HRH.
Scales A, C and D
For scales A, C and D, a diamond cone with a 120 ° tip angle and a test pre-load of 98.0665 N is used, the additional test force for scale A is 490.3325 N, for scale C 1372.931 N and for scale D 882.5985 N. it is usually used the method according to the scale C (C stands for engl. cone , dt. cone). HRA is also common, as thinner workpieces can be tested here.
It is the permanent penetration depth of the specimen in millimeters.
Scales B, E – H and K
These scales are suitable for softer workpieces such as medium-hard steels or brass . According to the latest standard, a sintered carbide ball (W) is used as a test body after 2006, before steel balls (S) were also permitted. For this reason, the hardness scale has also been expanded by 30 degrees. For scales B, F and G the ball has a diameter of 1.5875 mm ( 1 ⁄ 16 inch), for scales E, H and K it is 3.175 mm ( 1 ⁄ 8 inch). The preliminary test force always corresponds to 98.0665 N, but the additional test force is broken down as follows: scales B and E 882.5985 N, scales F and H 490.3325 N and scales G and K 1372.931 N. The most common is the scale B (B stands for English ball , German ball or sphere ).
Scales N and T
A diamond cone with a 120 ° point angle is used for scale N, and a steel or hard metal ball with a diameter of 1.5875 mm is used for scale T. What both methods have in common is the lower test preload of 29.41995 N and that they are standardized in three different test force variations. This is indicated by a number that is synonymous with the N and T scales. 15 stands for 117.6798 N, 30 for 264.77955 N and 45 for 411.8793 N. The full abbreviation is HR15N or HR45T, for example.
Examples
A shaft for a gearbox , for example, a hardness of 48 HRC, have a stainless steel - knife blade " Nirosta " the hardness of 39 HRC, Swiss Army knife, the hardness 55 HRC, a normal kitchen knife blade from rostträgem Steel 52-56 HRC, a high quality chef's knife from rostträgem steel 59 -61 HRC, and powder metallurgical blades reach 66 HRC. A knife blade made of Japanese Ao-Gami steel (blue paper steel) has a hardness of up to 67 HRC, one made of Shiro-Gami steel (white paper steel) has a hardness of up to 65 HRC. Tungsten carbide blades have a hardness of up to 71 HRC.
Above approx. 64 HRC, the machining options ( turning , drilling , milling ) usually end . Machining in the hardness range from 60HRc to 64HRc is usually carried out with coated carbide tools during drilling and milling. Turning contours are machined with ceramics up to the upper limit mentioned. Alternatively, hardened materials can be ground or eroded . While hardness plays no role in eroding, corundum or diamond-coated discs are used for grinding, depending on the hardness.
The most common procedure is to prepare production parts down to a few tenths of a millimeter in the "soft" state. After the heat treatment, the workpieces can then be finished by grinding, eroding or hard milling. In classic tool and machine construction, tolerances of +/- 5 µm are achieved here.
Individual evidence
- ↑ Stephan Kallee: Stanley Pickett Rockwell - One of the two inventors of the Rockwell hardness testing machine. Published on November 21, 2018. Retrieved November 21, 2018.
- ↑ List of the toughest knives in the world