David Pye's series on hardness testing continues.

Rockwell Hardness Testing Scales and Applications

Superficial Rockwell hardness values are expressed by the desired result number suffixed by a number and a letter that show the load and indenter combination. However, the increasing use of materials other than steel and brass as well as thin materials necessitates a basic knowledge of the factors that must be considered in choosing the correct scale to ensure an accurate Rockwell test.

The choice is not only between the regular hardness test and superficial hardness test, with three different major loads for each, but also between the diamond indenter and the ball (1.588, 3.175, 6.35 and 12.7 mm diameter) indenters. An example of the expression is as follows: 80 HR30N indicates a reading of 80 on the superficial Rockwell scale using a diamond indenter and a major load of 30 kg.

The majority of applications for testing steel, brass, and other materials are covered by the Rockwell C and B scales.

Selection of Scale Based on Material Type

Standard Rockwell scales and typical materials for which these scales are applicable are selected for the hardness test. If, for example, a hard material such as steel or tungsten carbide is to be tested, a diamond indenter would be used. This will limit the choice of scale to one of six: Rockwell Scale A, B, C, 15N, 30N or 45N.

The next step is to determine which scale will provide the best accuracy, sensitivity and repeatability. Typically, as the thickness of the sample decreases, the major load must also decrease.

Test-Block Calibration

It is strongly recommended that you test the unit for calibration and accuracy at the commencement of each shift. It is further recommended that you maintain a shift log book of test calibrations, signed and dated with the result of the calibration test for “documentary evidence” of practice.

Testers not used regularly should be checked before use. This check uses standardized test blocks to determine whether the tester and its indenter are in calibration. Before commencing the calibration test (as with all hardness testing), make sure that the unit is clean and free from dust. As a point of interest, ensure that the machine is clean and ensure that the Rockwell C scale test block is clean to demonstrate the sensitivity of the Rockwell C scale test.

Conduct three to four indentation tests to show the unit’s accuracy, and note the result from the test block. Then take a hair (one single human hair) and place it between the hardness test table and the test block. Take a reading and note the result. You will find that the result is approximately 5 Rockwell C scale points low! This why it is absolutely necessary to ensure the machine's cleanliness for accurate results.

If a test unit is used continuously on a given hardness scale, the recommended practice is to check it at the high, middle and low ranges of the scale. An example of this practice is to check the complete Rockwell C scale, with the full test range using the test blocks values at 63, 45 and 25 HRC. If only one or two ranges are used, the test blocks should be selected that fall within 5 hardness numbers of the testing range on any scale using a diamond indenter and within 10 numbers on any scale using a ball indenter.

A minimum of five tests should be made on the standardized surface of the block. The test unit is in calibration if the average of these tests falls within the tolerances indicated on the side of the test block. Should the average result of five readings fall outside of the Rockwell test block limits, the ball indenter should be inspected visually; or in the case of a diamond indenter, the diamond point should be examined using at least a 10x magnifier. If there is any indication of damage, the damaged component must be replaced.

Test Block Calibration (NIST)

The National Institute of Standards and Technology (NIST) has developed a new traceable U.S. Rockwell hardness test block. ASTM Subcommittee E-28.06 began working on revisions to ASTM E 18 to mandate the use of NIST traceable test blocks in the calibration of the hardness test blocks and testers, particularly for aerospace HT hardness testing. NIST initially released the Rockwell C scale, but they will eventually maintain standards for most of the commonly used scales, such as HRB, HRA, HR30N, HR30T, HR15N and HR15T.