"Why the cast iron's hardness is tested in rockwell hardness machine"?

Answer 1

The Rockwell Hardness test is used in quality control when fabricating parts or shapes in a variety of metals. The problem with making things with metals is that you're depending on the material to perform a certain task, based on its expected characteristics of hardness, brittleness, ductility, conductivity to electricity or heat, corrosion resistance, etc. Even with cast iron, small amounts of impurities may be present in the finished metal, which can have a big impact on the finished product. The Rockwell Hardness test is designed to give the engineer a basic idea of the qualities of the metal he has just cast or forged, and is relatively non-destructive -- it just leaves a little indentation on the metal surface (other tests like the Charpy V-notch, actually do break the metal being tested, and is only good for use on a test sample of a batch of metal, not on an actual metal product that you've put time and money into fabricating) If there is an unacceptable level of impurity in the metal, the Rockwell Hardness test can help tell you that, because the metal may be harder or softer than desired.

Answer 2

For light sections, such as piston rings and other light castings having a small graphite size, the Rockwell hardness test is often satisfactory.whereas,The Brinell hardness test is the one most frequently used for gray iron.

The Rockwell Hardness test also uses a machine to apply a specific load and then measure the depth of the resulting impression. The indenter may either be a steel ball of some specified diameter or a spherical diamond-tipped cone of 120° angle and 0.2 mm tip radius, called a brale. A minor load of 10 kg is first applied, which causes a small initial penetration to seat the indenter and remove the effects of any surface irregularities. Then, the dial is set to zero and the major load is applied. Upon removal of the major load, the depth reading is taken while the minor load is still on. The hardness number may then be read directly from the scale. The indenter and the test load used determine the hardness scale that is used (A, B, C, etc).

For soft materials such as copper alloys, soft steel, and aluminum alloys a 1/16" diameter steel ball is used with a 100-kilogram load and the hardness is read on the "B" scale. In testing harder materials, hard cast iron and many steel alloys, a 120 degrees diamond cone is used with up to a 150 kilogram load and the hardness is read on the "C" scale. There are several Rockwell scales other than the "B" & "C" scales, (which are called the common scales). A properly reported Rockwell value will have the hardness number followed by "HR" (Hardness Rockwell) and the scale letter.
Since in most cases the brinell hardness test can be considered a nondestructive test,brinell hardness is used as an indication of machinability, resistance to wear, and tensile strength.

the below link will give you a clear picture of assessing hardness of cast iron.