The result of hardness testing is what will determine the success of the heat-treatment procedure and the material’s response to that heat treatment. The results of a hardness test can:
- Indicate if the heat treatment was successful
- Give a possible indication as to the cause of a failure
- Give an indication of other mechanical properties of the metal
- Indicate if the metal is in a condition that will ensure its functionality
- Provide a comparison to other hardness values
Therefore, it is most important that the test is conducted in the appropriate and accurate manner in order to achieve the “real results.” The preparation of the metal surface for hardness testing is of as much importance as the test itself. The surface preparation of the steel to be tested is what we will be focusing on in Part 1.
Steel Surface Preparation
The heat-treatment process, or pre-condition of the steel, will determine the surface condition to be tested at that point. The steel can be in a variety of conditions.
- Surface oxide formation (scale)
- Surface decarburization (loss of surface carbon due to unstable processing conditions)
- No surface decarburization
- Other surface treatments
Surface Oxide and Decarburized Surface Preparation
If the steel is decarburized, it means the surface of the steel has lost surface carbon. If decarburized, the following may be true:
- It has the potential for an attack at the steel sample surface. Oxygen will most probably be the culprit to cause 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
- Change in the surface metallurgy because of carbon-content variations
If the steel has been through-hardened, it is necessary to test the substrate material by grinding beneath the oxide and decarburized layer in order to achieve a good and true hardness. The use of a fine grinding wheel with only a light surface pressure should be used to remove the decarburized/oxide layer. Excessive pressure must NOT be applied to the surface grinding procedure because this will create friction (generating heat) that will begin to adversely 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 surface 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.
We will conclude this discussion in Part 2.