The results of hardness testing are what will determine the success of the heat-treatment procedure and the materials’ response to that heat treatment. The results of a hardness test can tell us:
  • If the heat treatment was successful
  • An indication as to the cause of a failure
  • An indication of other mechanical properties of the metal
  • If the metal is in a condition that will ensure its functionality
  • A comparison to other hardness values
It is therefore 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 is conducting the test itself. The surface preparation of steel is what we will be focusing on in this blog.

Steel Surface
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 condition of:
  • Oxide surface
  • Surface decarburization
  • No surface decarburization
  • Carburized
  • Nitrided
  • Other surface treatments
Oxide and Decarburized Surface
If the steel is decarburized, it means the surface of the steel has lost surface carbon. If this has occurred, the steel will have:
  • An attack of oxygen at the surface
  • A loss of surface carbon
  • A change in 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
In order to achieve a good and true hardness result, it is necessary to test the substrate material by grinding underneath the oxide and decarburized layer. This applies to through-hardening steels only. This can involve a trial-and-error system of grinding off, say, 0.005 inches and hardness testing. Keep grinding off the surface layer until you reach a constant hardness result, which will be the true hardness of the steel.

Hardness Testing Machine
It is recommended that the hardness testing system that you are using is checked at the start of each working shift and compared to the previous shifts’ recorded readings. If you are heat treating a forging or casting, the most likely hardness-testing machine would be Brinell, which uses a 10-mm hardened steel ball and up to a 3,000-kg load application.

If you are checking a machined surface of through-hardened steel, such as an alloy steel or a tool steel, then one would check with a Rockwell system using 150-kg load. Surface preparation is just as important with Rockwell using a 150-kg load.

If you are checking a carburized case, the most accurate method to choose would be a Vickers or Knoop microhardness test system. This will involve selecting the area of the sample from the component, followed by:
  • Rough grind using 180-grit SiC pre-grind paper
  • Intermediate grind using 400-grit SiC pre-grind paper
  • Finish grind using 600-grit 600 SiC pre-grind paper
  • Rough polish using aluminum water-based slurry of 1 micron size
  • Final polish using aluminum water-based slurry of 0.5 micron size
  • Wash under cold running water
  • Flush the polished surface with alcohol
  • Blow dry using a warm air
Now you are ready to conduct your test for case-depth measurement and case-hardness profile. If you are testing the surface of a nitrided case or an FNC-formed case, then one would check the immediate surface hardness using Rockwell 15Kg “N” scale.