Rockwell Hardness

This is a property of material which describes its ability to be indented. The Rockwell method for measuring
hardness forces a steel point into the material and then measures the penetration of the point. The different letters in
the Rockwell reading describe the shape of the point and the load applied during the test. Units - Rockwell units
with the appropriate suffix letter. The letter and number cannot be separated. Higher numbers with the same letter
indicate harder materials. Harder materials have more resistance to penetration by another substance.

Rockwell hardness testing is a general method for measuring the bulk hardness of metallic and polymer materials. Although hardness testing does not give a direct measurement of any performance properties, hardness correlates with strength, wear resistance, and other properties. Hardness testing is widely used for material evaluation due to its simplicity and low cost relative to direct measurement of many properties. Rockwell hardness testing is an indentation testing method. An indenter is impressed into the test sample at a prescribed load to measure the material's resistance to deformation. Various indenter shapes and sizes combined with a range of test loads form a matrix of Rockwell hardness scales that are applicable to a wide variety of materials.

Regular Rockwell hardness testing-
Measures the bulk hardness of the material. There are separate scales for ferrous metals, nonferrous metals, and plastics.

Superficial Rockwell hardness testing-A more surface sensitive measurement of hardness than for the regular Rockwell scales. This technique is useful for samples with hardness gradients at the surface, to test small areas, and for thin samples. Superficial Rockwell hardness scales are N and T for metals and W, X, and Y for nonmetallic materials and coatings.

http://www.mee-inc.com/rockhar.html

In Rockwell testing, hardness is determined by measuring the comparative depth of two carefully controlled
indentations, one superimposed over the other.

First a Minor Load (one of a lesser force) is applied to a steel ball or sphero-conical diamond penetrator. Then,
while the Minor Load is still being applied, a Major Load (one of greater force) is applied at a precisely
controlled rate. The Major Load is removed and the hardness determination is displayed.

The hardness determination obtained represents the additional depth to which the Major Load has indented the
test specimen beyond the initial indentation of the application of the Minor Load. The displayed indication is
proportionally inverse, e.g. the higher the displayed determination of hardness, the shallower the indentations,
therefore the harder the material.

In the regular (R) Rockwell scale, the Minor Load is is 10 Kgf and the Major Load may be 60, 100, or 150 Kgf. In
the superficial scale (S), the Minor Load is 3 Kgf and the Major Load may be 15, 30, or 45 Kgf.

To ensure accuracy and consistency, the Major Load is under precise control and the penetrators are
manufactured to specific tolerances. Each point of hardness on the regular (R) scale is 0.00008 inch and 0.00004
inch on the superficial (S) scale, making the need for precise control of the applied forces and precision
manufacture of the penetrators readily apparent.

Charts, graphs, scales, values - http://www.ccsi-inc.com/p-rockwell1.htm

Rockwell Hardness Test

Hardness of metals is usually determined by measuring the resistance to penetration of a ball, cone, or pyramid.

The Rockwell method (ASTM 18-84 Standard Test Methods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials)
employs either a ball or a diamond cone in a precision-testing instrument that is designed to measure depth of penetration accurately. Two superimposed impressions are made, one with a load of 10 kg and the second with a load of 100 kg. The depth to which the major load drives theball or cone below that depth to which the minor load has previously driven it is taken as a measure of the hardness. The size of the ball generally used is 1/16 inch in diameter, but for hardened steels greater accuracy is obtained by use of a diamond cone (120 degrees with slightly rounded tip) applied under a major load of 150 kg. The method employing the ball is designated as the Rockwell B test, and that using the cone the Rockwell C test. The Rockwell C test can be utilized to measure the hardness of thin pieces since the depth of penetration of the cone is about 0.005 inches.

Concerning hardness conversions, Rockwell C values using the diamond cone are about one-tenth of the Brinell hardness for metals within the range of 300 to 600 Brinell.

http://www.prep.mcneese.edu/engr/engr316/rockwell/rockwell.htm

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