This is a diagram of a Knoop diamond.  Knoop diamonds are used in conjunction with a micro hardness tester, which makes impressions in material using loads from 10 to 1000 grams.  Unlike the depth of penetration Rockwell test, the micro hardness test is measured optically.  You measure the impression from tip to tip to get your Knoop value. Some micro hardness systems can be connected to a computer which can automate the process of making and reading the impression.  Since you measure the impression optically, the surface finish needs to be suitably smooth. Your reading will first be in microns, which are then plugged into a formula which gives the Knoop hardness.  Newer micro hardness tester and those that use a computer for automation will do this for you.  ASTM E-384 is the specification that governs micro hardness testing.

This is an explanation of the Brinell test method.  Brinell test is similar to a micro test in that a load is applied and the resulting impression is measured optically.  That is where the similarity ends though.  Developed in the 1920’s, the Brinell test uses loads from 187.5 to 3,000 kilograms.  With a Brinell tester, surface finish is not as critical as on a micro hardness tester.  You measure the impression using a Brinell scope, which will give you a reading in millimeters.  Once you have this value, there is a formula to get the Brinell hardness value.  While Brinell testing is not too popular here in the Northeast, it is used extensively throughout the Midwest, particularly in the Detroit area.  Brinell tests are usually performed on large cast or forged parts.  ASTM E-10 is the specification that governs Brinell hardness testing.

This is a drawing of a Vickers diamond.  The Vickers diamond is used with either a micro hardness tester (loads from 10 to 1,000 grams) or a macro hardness tester (loads 1 to 10 kilograms).  Again the impression is measured optically.  With the Vickers diamond, you actually read all four tips to get two readings, referred to as d1 and d2.  You then average the two together to get your reading in micros.  Similarly to the Knoop test, you use the micron reading in a formula to get your Vickers value.  Also like the Knoop, these systems can be fully automated with the help of a computer.  There are micro hardness testing systems that are available with both Knoop and Vickers diamonds; those are known as dual indenter machines.  ASTM E-384 is the specification that governs micro hardness testing.

HRC = -1.25501E+02 + 2.76747E+00(HRA) - 5.94178E-03(HRA)²

The above formula is a conversion from HRA (A scale) to HRC (C scale).  Where the formula calls for HRA, you would insert your A scale reading.  There are 33 Rockwell scales total, and there are conversion formulas for a great number of them.  For any applicable scale that uses a diamond to make the impression, there is a conversion to HRC.  For those scales that use a ball, the conversion is to HRB.  There is no conversion from a ball to a diamond scale, or vise versa.  These formulas are found in the appendices of ASTM E-140.

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Should you have any question about the above explanations or about hardness testing in general, please do not hesitate to contact our office.  Thanks for stopping by our booth.