Heat Treating of Tool Steels – Hardness

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Hardness is the resistance of the metal to plastic deformation usually by indentation. Indentation is usually measured by various hardness tests, such as Brinell, Rockwell, Knoop, and Vickers. The most common tests are Brinell & Rockwell scales.


The term hardness may also refer to resistance to wear, scratching and cutting.

Common misconceptions of hardness are the following:

The higher the hardness, the better the tool/part; not true at all since tooling steels have operational “sweet spot hardness” that enable the best performance.
Testing of hardness on Rockwell “c” scale should not be used below 22 RC as the results will not be accurate, use the Brinell scale. Testing of hardness above 68rc should be done on the Vickers or Knoop scale to be most accurate.
Just because you achieved the Rockwell “C” you wanted, does not mean your tool/part is good, unless you followed proper procedures during the heat treat process.

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