If the steel is decarburized, it means the surface of the steel has lost carbon. If either of the conditions described in part 1 exists, the steel will have been subjected to:
- The potential for an attack at the steel sample surface will most probably be oxygen to cause a decarburization at the steel surface
- A loss of surface carbon (indicated by low hardness)
- A change in the surface of the steel chemistry
- A change in the mechanical properties of the steel surface in relation to the core/substrate steel
- A change in the surface metallurgy because of carbon content variations
If the steel has been through-hardened 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. Use of a fine grinding wheel with only a light surface pressure to remove the decarburized/oxide layer can be used (with extreme care so as not to overheat the part by the grinding or cutting action). Excessive surface pressure when grinding will create friction that will begin to affect the accuracy of the hardness result obtained.
The result of good surface grinding techniques will ensure that there will be a reasonably smooth surface without ridges. If surface ridges are present, this can cause the indenter to slip off the peak of the ridge and give a false reading.
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 finished ground surface. This could be a sidewall face or the reverse side of a die or a representative test coupon that has been treated with the product.