ASTM E18-05 now requires the use of carbide ball indenters, which produce results 0.5-1.0 Rockwell point below that of the old steel indenters.
The Rockwell Hardness test is probably the most commonly performed hardness test. It is easy to perform and the hardness number is obtained directly from the testing machine. The test consists of applying a minor load of 10 kg to seat the indenter and then applying a major load (typically 100 kg) to create a permanent depression beyond that caused by the minor load. The hardness number, which is read directly from a dial or digital scale on the testing machine, is inversely related to the additional penetration caused by application of the major load.
There are numerous variations of the Rockwell Hardness test involving different penetrators and loads. A 120o diamond cone indenter with a slightly rounded point is used for testing very hard materials. Most materials are tested using hardened steel ball indenters. Steel indenters include 1/16, 1/8, 1/4, and 1/2 inch diameters. Major loads can be 60, 100 or 150 kg. (superficial hardness tests use lower load values).
The following contain images of Rockwell indentations taken with a light microscope and a scanning electron microscope.
Rockwell hardness numbers should be reported by specifying the type test and the scale on which the test was made. For example 64 HRC means the hardness number is 64 as determined on a Rockwell C scale. Most Rockwell hardness tests are performed on either the Rockwell B or C scales. The B scale utilizes a 1/16 inch diameter penetrator and a 100 kg major load while the C scale uses a conical diamond penetrator and a 150 kg major load.
Rockwell hardness number, HR, is a number derived from the net increase in the depth of impression from a fixed minor load to a major load and then returned to the minor load. Rockwell hardness numbers are always quoted with a scale symbol representing the penetrator, load, and dial used.
Scale Symbol Indenter Major Load, kgf Dial Figure
A Diamond 60 Black Cemented carbides, thin steel, and shallow case-hardened steel B 1/16" ball 100 Red Copper alloys, soft steels, aluminum alloys, malleable iron C Diamond 150 Black Steel, hard cast irons, pearlitic malleable iron, titanium, deep case-hardened steel, and other materials harder than HRB 100 D Diamond 100 Black Thin steel and medium case-hardened steel and pearlitic malleable iron E 1/8" ball 100 Red Cast iron, aluminum and magnesium alloys, bearing metals F 1/16" ball 60 Red Annealed copper alloys, thin soft sheet metals G 1/16" ball 150 Red Phosphor bronze, beryllium copper, malleable irons. Upper limit is HRG 92, to avoid possible flattening of ball. H 1/8" ball 60 Red Aluminum, zinc, lead K 1/8" ball 150 Red Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not give anvil effect. L 1/4" ball 60 Red M 1/4" ball 100 Red P 1/4" ball 150 Red R 1/2" ball 60 Red S 1/2" ball 100 Red V 1/2" ball 150 Red
The governing standard for hardness testing of plastics is ASTM D785. The procedure is as follows.
Set the dial gage within 10 seconds.
Apply the major load for 15 seconds. The application time of the major load must be controlled carefully if reproducible results are to be obtained.
Read the Rockwell hardness number 15 seconds after removal of the major load.
Corrections can be made for specimens with curved surfaces based on the radius of curvature.
ReferencesMetals Handbook, Ninth Edition, Vol. 8 "Mechanical Testing", American Society for Metals, 1985, Hardness Testing pp 71-113.
Handbook of Experimental Stress Analysis, by Hetenyi, John Wiley & Sons, Inc., 1950, pp 22-25.
Hardness Testing, 2nd Ed., ASM International, 1999, pp 29, 121.