Material Thickness When Choosing a Rockwell Scale

This article discusses the use of Rockwell scales for thin materials.

Mimum Thickness HR 15N HR 30N HR 45N HRC
mm in Hardness
0.15 0.006 92 - - -
0.21 0.008 90 - - -
0.25 0.010 88 - - -
0.30 0.012 83 82 77 -
0.36 0.014 76 78.5 74 -
0.41 0.016 68 74 72 -
0.46 0.018 - 66 68 -
0.51 0.020 - 57 63 -
0.56 0.022 - 47 58 69
0.61 0.024 - - 51 67
0.66 0.026 - - 37 65
0.71 0.028 - - 20 62
0.76 0.030 - - - 57
0.81 0.032 - - - 52
- - - 45

Material thickness is of primary importance during Rockwell scale selection. The 30 Rockwell scales are classified by the total test force and the indenter type, so a force or load that is excessive for a particular material thickness is influenced by the support anvil when the material is completely penetrated. Interruptions in flow of material can lead to erroneous readings and serious misinterpretation of the actual material hardness.

ASTM Reference Tables

To help the decision-making process, ASTM provides scale thickness requirements in tabular and graphical forms. Buehler recommends that these can be used as a reference guide to decide the suitable scale according to material thickness. Generally, the material must be a minimum of 10 times the depth of the indentation when a diamond-type indenter is used, and at least 15 times the depth if a ball-type indenter is used.

If needed, the actual depth of an indentation can be determined to confirm whether this requirement is being satisfied. Usually, depth calculation is not needed as the ASTM reference graphs and tables provide enough information to make an educated decision. A rule of thumb that is followed to ensure accurate test results is that material deformation should not be evident on the supporting (underside) of the material surface.

When the material thickness is reduced, it becomes essential to decrease the applied force in order to reduce “breakthrough” of the indenter. This can be achieved by selecting a Rockwell scale that uses lower test forces and has the suitable conversion values.

As an example of referencing the tables to determine suitable hardness scale, suppose there is a requirement to perform a hardness test on a steel sheet of 0.51 mm. (0.20 in.) thick with an approximate hardness value of 57 HRC. ASTM E18 shows that the material needs to have a thickness of 0.76 mm (0.030 in) to obtain valid results, removing the possibility to conduct a standard HRC scale test on this particular sample.

A review of ASTM E 140 conversion tables shows that 57 HRC can be approximately converted to 15N 88.9, 30N 74.8, or 45N 63.2. According to the ASTM minimum thickness chart, for a material of 0.51 mm thick there are two scales to choose from: HR30N and HR45N.

According to the chart, the 0.51 mm thick material should be at least HRN30 57 or HRN45 63, concluding that either of the converted scales is appropriate (the converted value is HR45N 63.2 and HR30N 74.8). In this case, HR45N can be selected as it is the higher of the two test force scales.

The material can be tested on the Vickers or Knoop microhardness scales when suitable Rockwell scale is not possible due to not meeting minimum thickness requirements.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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