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In this video series we have often referenced the hardness of the metal. Specifications for the alloys of steel, stainless steel, nickel and cobalt alloys all typically have a hardness requirement.
So what is the hardness and how is it measured?
Let’s use the simple definition of hardness as the resistance a metal makes to a permanent indentation. To measure and compare the resistance; imagine that we have a standard size and shape ” indenter ” and a fixed amount of force to be applied. Seems like all we need to do is measure the resulting impression it makes in the metal. In “soft” material the impression is larger and deeper than in “hard” material. Results are expressed by measurement of the size of the impression or the depth of the impression made by the test procedure.
Here is where it gets a bit more complicated. Depending on the expected hardness and uniformity of the metal under test, the details of the indenter or the force applied or both change in order to give reliable results.
There are two common hardness tests in the alloys and steels we have been talking about in our video series. There are several other types of hardness tests for specialized applications that we won’t address today.
Brinell Hardness testing uses a 10-millimeter diameter ball shaped indenter and an applied force of 500 kilograms for soft metals and 3000 kilograms for steels and nickel alloys. For hard materials, the indenter is made of tungsten carbide
The indentation of the ball is approximately circular at the metal surface and its diameter is the measured result. The Brinell Hardness Number, ( “BHN” for short) represents the calculated load divided by the surface area of the partial spherical impression left in the metal. The Math is already done to make the calculation from the “Brinell Diameter” to the “BHN” in readily available tables.
Results are most commonly expressed as the BHN value but can as easily be the diameter.
For example a typical Brinell Hardness test on a type 304 stainless steel would leave a 4.25 millimeter diameter impression equating to a 201 BHN.
Rockwell Hardness testing is a bit more versatile in our applications. The impressions are made by much smaller indenters making it more convenient for testing even small samples or parts.
It uses either a diamond in a conical shape, known as the “Brale” indenter for hard metals or a 1/16″ dia steel ball shaped indenter for soft metals and applied loads of 60, 100, or 150 kilograms. Rockwell scales are designated by a letter or number and letter that define the combination of type of indenter and the applied load.
In the Rockwell test, a preload of 10 kilograms is applied and then the major load appropriate for the specific scale. The depth of the impression made by the major load is measured.
The Rockwell B scale hardness uses the 1/16″ ball indenter and a 100 kilogram major load.
The Rockwell C scale hardness uses the diamond “Brale” indenter and 150 kilogram major load for harder metals
In our 304 stainless steel example the typical Rockwell hardness would use the B scale and would be 94 on the RB scale..
Hardenable alloys generally require use of the Rockwell C scale
We hope this has given you the basics of these two types of hardness tests. Thank you for watching and this wraps up this week’s discussion on hardness testing.
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