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 (VanderVoort consulting, www.georgevandervoort.com) and Alan Stone (Aston Metallurgical Services Company, www.aston-met.com) for their contributions to this subject.
Comments & Questions (Parts 4 and 5, Rockwell Hardness Testing Tips)
Rockwell hardness measurements: These are actually a conversion of the depth of the indentation left by the indenter.
Comment 1: Each Rockwell point represents 0.00008 inches of depth. For a reading of 60 HRC, subtract the major and minor loads, or 100-60 x 0.00008 = 0.0032 inches. So, the depth of a 60 HRC reading is 0.0032 inches.
Comment 2: Right. They talk about this in the ASM Hardness Testing book. The 100 and 60 are not really the major and minor loads, though. They are the dial readings.
Response: I do not see anything in E 18 today about indent depth. But the depth historically was 0.002 mm for each standard Rockwell hardness number difference and 0.001 mm for each superficial Rockwell hardness number difference. Equations 4 and 5 in 188.8.131.52 sort of imply this but not directly. This does not tell you the absolute depth of a Rockwell indent, as far as I can tell, because it does not include the depth from the minor load.
Lysaght and DeBellis and H.E. Boyer give depth information that is similar but incomplete. They say that one Rockwell number represents a penetration of 0.002 mm (0.00008 inch). “Therefore a reading of C60 indicates penetration from minor to major load of (100-60) x 0.002 mm = 0.080 mm or 0.0032 inch.” This does not include the penetration due to the 10-kg minor load.
Curvature of the surface
Comment 1: A correction factor must be added to the hardness reading of small-diameter shapes for Rockwell C, A and D, which varies with the apparent hardness and part diameter. The correction factor to be added is shown in Tables 4 and 5 of ASTM E18.
Comment 2: Good info.
Comment 3: You might note that technically these corrections only work for convex and not concave surfaces.
Response: In the current version of ASTM E 18, Tables A6.1 to A6.4 in Annex A6 cover correction factors for Rockwell C, A and D (Table A6.1), B, F and G (Tables A6.2), 15N, 30N and 45N (Table A6.3) and for 15T, 30T and 45T (Table A6.4) for tests “on convex cylindrical surfaces of various diameters.”
1. V. E. Lysaght and A. DeBellis, Hardness Testing Handbook, American Chain and cable Company, 1969.
2. H. E. Boyer (ed.), Hardness Testing, ASM International, 1987.