Measuring the Hardness

Hardness is indicated in a variety of ways, as indicated by the names of the tests that follow:

Static indentation tests:
A ball, cone, or pyramid is forced into the surface of the metal being tested. The  relationship of load to the area or depth of indentation is the measure of  hardness, such as in Brinell, Knoop, Rockwell and Vickers hardness tests.

Rebound tests:
An object of standard mass and dimensions is bounced from the surface of the workpiece being tested, and the height of rebound is the measure of hardness.  The Scleroscope and Leeb Rebound Tests are examples.

Scratch file tests:
The idea is that one material is capable of scratching another. The Mohs and file hardness tests are examples of this type.

Plowing tests:
A blunt element (usually diamond) is moved across the surface of the workpiece being tested under controlled conditions of load and shape. The width of the  groove is the measure of hardness. The Bierbaum test is an example.

Damping tests:
Hardness is determined by the change in amplitude of a pendulum having a hard pivot, which rests on the surface of the workpiece being tested. The Herbert Pendulum test is an example.
Cutting tests:
A sharp tool of given shape is caused to remove a chip of standard dimensions from the surface of the workpiece being tested.

Abrasion  tests: A workpiece is loaded against a rotating disk and the rate of wear is the measure of hardness.

Erosion tests: Sand or other granular abrasive is impinged on the surface of the workpiece being tested under standard conditions, and loss of material in a given time is the measure of hardness. Hardness of grinding wheels is measured by this testing method.

Electromagnetic  testing: Hardness is measured as a variable against standards of known flux density.

Ultrasonic testing:
A type of indentation test

Within each of these classes of measurement there are individual measurement scales. For practical reasons conversion tables are used to convert between one scale and another.

Comparison of hardness scales

TEST TEST METHOD TEST FORCE RANGE INDENTER TYPE ASTM TEST METHOD MEASURE METHOD
Rockwell Regular 60, 100, 150 kgs Conical Diamond & Small Ball E 18 Depth
Superficial 15, 30, 45 kgs Conical Diaomond & Small Ball E 18 Depth
Light Load 3, 5, 7 kgs Truncated Cone Diamond N/A Depth
Micro 500, 100 grams Small Truncated Cone Diamond N/A Depth
Macro 500, 3000 kgs 5, 10 mm Ball E 103 Depth
Micro-Hardness Vickers 5 to 2000 grams 136 Pyramid Diamond E 384 Area
Knoop 5 to 2000 grams 1300 x 1720 Diamond E 384 Area
Rockwell Type 500, 3000 grams Truncated Cone Diamond N/A Depth
Dynamic .01 to 200 grams Triunghiular Diamond N/A Depth
Brinell Optical 500 to 3000 kgs 5mm, 10mm Ball E 10 Area
Depth 500 to 3000 kgs 5mm, 10mm Ball E 103 Depth
Shore Regular 822 (A), 4550 (D) grams 35 Cone (A) 30 Cone (D) D 2240 Depth
Micro 257 (A), 1135 (D) grams 35 Cone (A) 30 Cone (D) N/A Depth
IRHD Regular 597 grams 2.5 mm Ball D 1415 Depth
Micro 15.7 grams .395 mm Ball D 1415 Depth

There is, in general, no simple relationship between the results of different hardness tests. Though there are practical conversion tables for hard steels, for example, some materials show qualitatively different behaviors under the various measurement methods. The Vickers and Brinell hardness scales correlate well over a wide range, however, with Brinell only producing overestimated values at high loads.



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