David Pye's series on hardness testing continues.
ASTM E-18 Rockwell Hardness Testing
This particular aspect of E-18 focuses and defines the test method as the forcing of a conical diamond indenter (or a hardened steel ball) into the surface of a material. The test occurs under two loads: the first being the primary load and the second being the main load. Then the depth is measured. The diamond indenter has a point radius at the diamond tip of 0.20 mm and steel balls of several diameters.
Vickers Hardness Testing
The Vickers hardness test machine and method were developed in England in 1925 by R. Smith and G. Sandland. It was originally known as the 136-degree diamond-pyramid hardness (DPH) test due to the shape of the diamond indenter. The early testing machines were first manufactured by Vickers-Armstrong Limited of Crayford, Kent, U.K. The test method and the test machine gained popularity, and the name of Vickers became the recognized designation for the hardness test.
Vickers Diamond Indenter Impression
The simple principle of the method is based on “the ability to resist load application.” The load is applied by a system of variable weights, depending on the material thickness, its composition and the process applied to the steel (e.g., case hardening). The diamond-shape pyramidal indenter is forced under a predetermined load.
The applied load can range from 1-120 kg to be “pushed” into the material being tested. After the indenter has been pushed/pressed into the material by the applied load and reached equilibrium conditions without any further penetration, the force remains applied for a specific period of time, which is between 10-15 seconds for normal test times. The load application is then released.
The impression is now ready to be measured. The resulting indentation diagonals are measured and averaged to give a value in millimeters. The measurements of the diagonals must be the same dimension (otherwise a false result is obtained) and are then used to calculated the appropriate Vickers hardness number (HV).
The Vickers hardness number is a derived and measured number related to the applied force and the surface area of the measured unrecovered indentation produced by a diamond base pyramidal diamond indenter. The Vickers indenter has included face angles of 136 degrees.
The procedure is relatively slow due to manual operation, and the test procedure is not well-suited for routine testing. Typical test and measurement times are in the one-minute range as opposed to the new electronic load cell and automation, but the sample preparation is the same.
Measurement of diagonals is operator-dependent, with possible eyestrain and fatigue adding to test errors. Once the diamond indentation has been made and the impressed axis measured, the measurement is then related to a set of tables based on the load applied to make the indentation.
The hardness resulting tables are based on the selected load weight for the test. The dual measured axis is then read across the table for the appropriate load weight and acquired measurement.