Knoop

How to read and represent a Knoop hardness value

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A Knoop hardness value consists of the following components:

1. The numeric hardness value;

2. The two letters "HK", standing for "Hardness according to Knoop";

3. The applied test load in kgf;

4. According to ISO 4545: The dwell time of the test load, but only if this is not between 10 and 15 seconds (uncommon in practice)

Examples:

870 HK 1
870 …hardness value
HK …as per Knoop
1 …with test load of 1 kgf (or test force of 9.807 N )

870 HK 1/30
870 …hardness value
HK …as per Knoop
1 …with test load of 1 kgf
30 …dwell time to test force outside recommended duration, in this case: 30 seconds