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The Truth About Rockwell Hardness Testers

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Introduction

Rockwell Hardness Testers, like other hardness testers, measure the hardness of materials by measuring the materials resistance to penetration under a specific force. Unlike other testers, however, the Rockwell testers measure the net increase in the depth of indentation (rather than the hemispherical area) as and when a load is applied. The hardness measure is displayed directly on the scale (dial gauge) attached to the Rockwell tester.

Rockwell Hardness Testers make use of different indenters and scales. Thus, the indenter can be either a steel ball of some specified diameter or a spherical diamond-tipped (tip radius 0.2 mm) cone angled at 120 degrees. Steel balls of varying diameters are used for soft materials and the diamond-tipped cone is used for testing harder materials.

As far as Rockwell scales are concerned, B and C are the most commonly used scales; but there are many other scales like E, K, L, M and R scales. The type of indenter being used and the test load together go on to decide the hardness scale.

Rockwell Hardness is recorded complete with the hardness number followed by 'HR' (Hardness Rockwell), which is again followed by a letter representing the Rockwell scale. Thus, 125 HRC indicates that the sample material tested reads 125 on the C Hardness Rockwell scale.

Rockwell Hardness Testing Procedure

The Rockwell Hardness Testers make use of two different loads as part of the testing method. Initially, a minor load of 10 kg is applied on the material surface to make a denting. This not only holds the indenter in place but also does away with surface irregularities. Thereafter, the dial is once again set to zero and the major load applied. The difference in the depth of the dent caused by the two different forces (the minor and major loads) provides the Rockwell hardness number, which may be read directly from the scale.

Applications

Rockwell Hardness Testers are used to determine the hardness of ceramic substrates. These testers are ideal for measuring the relative resistance to indentation of various plastics, especially the harder plastics such as Acetal, Nylon and Polycarbonates.

Superficial Rockwell Hardness Tester

Beside the standard Rockwell Hardness Testers, there is this Superficial Tester used to test small parts and thin sheets. Like the regular testers, this tester uses a minor load (in this case it is just 3 kg weight) and a major load (which again is much less, between 15 and 45 kg depending on the indenter used). If the indenter used is the steel ball, the Superficial Rockwell Hardness Number is accompanied by T. In case the steel ball is replaced by diamond cone, the T in the Superficial Rockwell Hardness Number is replaced with an N.

Author: Robert Allen

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