# Rockwell

BACKGROUND INFORMATION
ROCKWELL HARDNESS TESTING (DIN EN ISO 6508)

In 1907, Ludwik published the proposal to use the depth of penetration of a diamond cone to assess the hardness of a material. Furthermore, the influence of the sample surface should be eliminated by applying a preliminary test force. It was not until 1922 that the American Rockwell, building on Ludwik’s idea, succeeded in developing a useful hardness testing method. Because of the simplicity of the method, Rockwell hardness testing quickly found its way into industry.

INDENTER FOR ROCKWELL HARDNESS TESTING

A diamond cone with a point angle of 120° or two hardened steel balls (diameter 1/16 inch = 1.5875 mm or 1/8 inch = 3.175 mm) are used as indenters in the Rockwell test. The steel balls may only be used if this is explicitly required in the product specification or is agreed separately. If required or agreed, carbide balls of 6.356 or 12.70 mm may also be used.

 120°DIAMOND CONE HRA, HRC, HRD, HR 15 N, HR 30 N, HR 45 N Ø 3.175MMHARDENED STEEL BALL HRE, HRH, HRK Ø 1.5875MMHARDENED STEEL BALL HRB, HRF, HRG, HR 15 T, HR 30 T, HR 45 T, HR Bm, HR Fm

THE 3 STEPS OF ROCKWELL HARDNESS TESTING

1. The indenter is first pressed into the surface of the specimen with a specific preload F0. This is to exclude influences of the surface on the hardness value, such as scale layers, hardenings, etc. Under this preload, the indenter initially penetrates the specimen by the distance t0. To generate the preliminary test force, the specimen is usually pressed against the indenter until the required test force is applied.

2. In a second step, the additional test force F1 is applied. The indenter now penetrates the specimen by a further amount Dt. The additional test force must be applied (starting from the preload) without shock or vibration and without overshooting within a period of 2-8 s and must be kept constant for 4±2s.

3. In the third step, the load is relieved again up to the preload F0. The indenter moves back upwards by the elastic portion of the deformation tel so that the remaining indentation depth is tbl. The hardness value is either displayed directly on the hardness tester or can be calculated from the measured indentation depth.

t0 = indentation depth through test preload F0
tel = elastic back deformation by relief to the test preload F0
tbl = permanent indentation depth after relief to the test preload F0

RESULTS OF A ROCKWELL HARDNESS TEST

The result of a Rockwell hardness test is determined as follows:

 In the HRC method, Z is 100 and the scale division Skt is 0.002 mm. If a permanent indentation depth of 0.12 mm is measured, the Rockwell C hardness is 40 HRC.

According to DIN EN IS 6508-1 the result of a Rockwell hardness test is depicted as follows:

60 HRC W   ⇒   Rockwell hardness value
60 HRC W   ⇒   Generic marker for "Hardness Rockwell"
60 HRC W   ⇒   Label for the hardness scale
60 HRC W   ⇒   Label for the material of the indenter in case a ball is used (not shown for diamond cone)

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