Hardness is usually tested by pressing a hard indenter into the part and measuring the dent. Brinell is the standard choice for castings and many forgings, while Rockwell and Vickers are used for harder surfaces and smaller areas.
Hardness is quick, cheap and almost non-destructive, so it is the most common check on every batch. Results usually appear on the material test certificate. It is also easy to get wrong, so you should know which method is used and where.
The main methods
- Brinell presses a hard ball, often 10 mm in diameter, into the surface and measures the indent diameter. It suits cast iron and coarse-grained metals because the large indent averages the structure. It is covered by ISO 6506 and ASTM E10.
- Rockwell measures the depth of an indent from a diamond cone or a ball. It is fast and suits hardened steel and finished parts. It is covered by ISO 6508 and ASTM E18.
- Vickers uses a diamond pyramid and measures the indent diagonals. It suits thin sections, case depth checks and small areas. It is covered by ISO 6507 and ASTM E92.
- Portable rebound testers, often called Leeb testers, measure how a small impact body bounces back. They are covered by ISO 16859 and ASTM A956.
Where the test is taken
Location matters as much as method. The as-cast skin of a casting and the decarburised surface of a forging are often softer than the metal beneath. A test must be taken on a prepared, ground spot, deep enough to reach sound metal.
Hardness also varies through a section. Thick parts cool more slowly and can be softer in the middle. Your drawing should mark where hardness is measured, and for heavy forgings, whether a test on a cut sample is needed.
Portable testing with care
Portable testers are useful on parts too heavy for a bench machine and for checking many parts quickly. But results depend on the part's mass, thickness, surface finish and the conversion table used. A thin or light part can give a false reading.
Treat portable results as a screen. Check the tester against a reference block before use, and confirm any doubtful parts on a bench Brinell or Rockwell machine. Converting between scales is approximate, so specify the scale you want and test in it.
What to agree with the supplier
- The method and scale, for example HBW for Brinell with a tungsten carbide ball.
- The hardness range, not just a minimum.
- The test location and surface preparation.
- How many parts per batch or heat are tested.
- Calibration records for the testing machine and reference blocks.
Next step
Hardness depends heavily on heat treatment for forgings and on the casting grade. We check testing set-ups during audits and can have hardness verified on your lots before loading. To plan inspection, book a free sourcing review.