The following questions and comments have been asked by readers of our recent series of blogs on Hardness Testing (Parts 1-8). Special thanks to Debbie Aliya (Aliya Analytical, www.itothen.com), George Vander Voort (Vander Voort consulting, www.georgevandervoort.com) and Alan Stone (Aston Metallurgical Services Company, www.aston-met.com) for their contributions to this subject.

Test specimen thickness: Test specimen should be 10 times thicker than the depth of hardness indentation.

Question 1: Is there an easy or convenient way to figure out if you have met this criterion?

Response: ASTM E10 talks about the specimen (Table 4) and that the minimum thickness is based on 10 times the depth of the Brinell indent. This is very conservative for a BCC metal and more realistic for an FCC or HCP metal.

You can calculate the depth of a Brinell indent and multiply by 10 or just use Table 4. The equation that was given to calculate the indent depth is: Depth = L / (pi)DHB

where L is the load in kgf (usually 3,000 kgf for steels), D is the ball diameter (10 mm usually for steels) and HB is the Brinell hardnes number.

So, if a steel is tested with a 10-mm-diameter ball and a load of 3,000 kgf and the indent diameter is 3.2 mm, the depth would be 0.26307 mm. Ten (10) times this would be rounded off to 2.6 mm.

In ASTM E10, they now have a different formula to calculate the depth: h = D – (D2-d2)1/2 / 2

where h is the indent depth, D is the ball diameter and d is the indent diameter.

For the same example as above, I calculate a depth of 0.2629. Ten (10) times this depth (when rounded off) would again be 2.6 mm.

Question 2:  You state “should be no more than 2.5 times the diameter of the indenter ball.” This should read “at least 2.5 times the diameter of the indenter ball” rather than “no more than.”

Response:Paragraph 7.6.4 of ASTM E10 states that “the minimum radius of curvature of the surface shall be two and a half times the diameter of the ball.” I do not see any correction factors for testing on a curved surface in E10.

More Q&As next time.