ASTM E18 Revises Test Method Replacing Steel Ball Indenters With Carbide Balls

ASTM subcommittee E28.06 for indentation hardness has passed a revision to the E18 test method for Rockwell testing that virtually eliminates the use of ball indenters made from steel. Effective immediately the material used to make indenter balls is carbide. The old version of the standard, E18-03e1, allowed the use of carbide or hardened steel balls, however, the new version, E18-05, requires carbide balls for all scales that use ball indenters.

This reason for this change is to improve the performance of the Rockwell tests that use ball indenters and to bring the standards in line with the corresponding ISO requirements. It is not uncommon when testing in the high end of the most popular ball scale– HRB, to damage the ball. Testing materials in the hard range of the scale can flatten a steel ball within a few tests. Accidental tests on most anvils will immediately render a steel ball useless. As a result, thousands of bad tests have been performed because of damaged steel balls. Carbide balls are nearly indestructible when used properly. The result will be the elimination of bad test results due to a damaged ball. The increased life of the harder carbide balls will outweigh their higher cost.

There will be small shift in the readings as result of this change. Since the carbide ball is much stiffer and harder than a steel ball, the depth of penetration during a Rockwell test is greater. The hardness value will therefore be lower. Several studies have been performed to determine the differences and they show a 0.5 to 1.0 point shift downward. In most situations, the tolerances associated with the hardness of soft materials are quite high and 1 point is not significant. If your tolerances are very tight, this shift could be important.

This shift will be more apparent when using test blocks to verify the performance of you Rockwell tester. The higher HRB scale tolerances are +/- 1.0 so the shift could easily cause a tester to fall out of calibration if you are using a carbide ball indenter but you are not using test blocks that have been calibrated using a carbide ball indenter. It is critical that the test blocks you use are calibrated with the same type ball you have in your tester.

There will be a period of adjustment, however, once everything is converted to carbide the result will be a greatly improved Rockwell testing process. The new E18.05 will allow the use of hardened steel balls under special circumstances. This provision is intended for very specific applications only.

View actual test data showing the Rockwell hardness performance differences between the use of tungsten carbide and steel ball indenters. Read helpful hints on how to change from steel to carbide balls.