If decarburization (a loss of surface carbon in the steel/surface) exists, then the steel will have been subjected to:

  • Oxygen at the surface
  • A loss of surface carbon (indicated by low hardness)
  • A change in the surface chemistry
  • A change in the mechanical properties of the surface in relation to the material’s core/substrate

For through-hardened steels, in order to achieve a good and true hardness result, it is necessary to test the substrate material by grinding beneath the oxide and decarburized layer.

The use of a fine grinding wheel with only a light surface pressure should be used. This will create surface friction (thus generating heat) that can begin to adversely affect the accuracy of the hardness result obtained. If the heat-treated surface is a previously ground/polished finished surface, then one cannot grind that surface.

It is very likely that the surface may not be decarburized. If that is the case, then one should select an area where no indenter impression will affect the finish-ground surface. This could be a sidewall face, the reverse side of a die or a representative test coupon that has been treated with the product.

One cannot always grind into the finished surface. If this occurs, then it will most likely mean that the treated surface will begin to be depleted in carbon, nitrogen or boron.

Care must now be given to the load, particularly if the formed case is thin. A heavy load will penetrate the resultant case and will result in a false/incorrect reading. A simple rule of thumb is “The shallower the case, the lighter the hardness-testing load.”

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Fig. 2. Microscopic example of steel decarburization (Gunnerson, 1963/B. Edenhofer)