Degree of hardness

1. In flake graphite cast iorn (GJL), the ratio between the actually achieved hardness, measured on the 30mm standard test rod, and the hardness calculated from the degree of saturation is referred to as the degree of hardness and calculated as follows:

Standard hardness     
= 530 – 344 Sc (if HB < 186)
= 930 – 744 Sc (if HB > 186)

However, this does not take the influence of the phosphor into account, and these calculations are thus only valid for cast iron with a low phosphor content. As a general rule, 0.1%P will increase the hardness by approximately 2.44 HB.

2. Degree of hardness of an abrasive wheel according to DIN 69100 with the following designation:

E, F, G     very soft
H, I, J, K     soft
L, M, N O     medium
P,Q,R,S     hard
T, U, V, W     very hard
X, Y, Z     extremely hard

The hardness of an abrasive wheel depends on the strength and quantity of the binder. It is determined by the resistance of the bond encountered by the grain when escaping from its bond under the influence of the abrasive force and depends on the hardess of the abrasive wheel.

3. Unit for the hardness of water: in the past, indicated as degree of hardness in English degrees (°e). Today, according to the valid SI measuring system, the concentration of the cations of the alkaline-earth metals, i.e. the total hardness, is indicated in mol per liter or, given the low concentrations, in millimol per liter (mmol/l), 1°dH = 0.1783mmol/l.

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