Hardness Test Correction Factors for Round Bar
A hardness reading taken on a round bar is not the same value as a reading taken on flat stock of the same material. This page carries the published correction factors for cylindrical workpieces on the Rockwell scales, with the diameter limits that decide when a correction is needed.
Why a curved surface reads low
When an indenter is forced into a convex surface there is less lateral support for the indenting force, so the indenter sinks deeper than it would into a flat surface of the same hardness and the reading comes out low. On a concave surface the extra support produces the opposite effect, and the reading comes out high.
Correction values are added to the observed reading on a convex surface and subtracted from it on a concave surface. The published values are averages to the nearest half Rockwell number, taken from measurements by several laboratories and from analysis of the same problem.
Above 25 mm (1 in.) diameter the difference is negligible on the Rockwell scales. Below that diameter, and particularly on softer material where the indentation is larger, the curvature has to be corrected before the value is compared with a flat surface or with another diameter.
Regular Rockwell scales may be used on cylinders down to 6.35 mm (0.25 in.) diameter, which is why the 3.175 mm column is blank for the C, D and A scales. The superficial correction values run down to 3.175 mm (0.125 in.), and below that diameter the part belongs to a microindentation method.
A result taken on a curved surface should not be reported without the radius of curvature. Accuracy on cylindrical workpieces also depends on the alignment of the elevating screw, the V-anvil and the indenter, and on the surface finish and straightness of the bar. For Brinell testing the radius of curvature has to be greater than 2.5 times the diameter of the indenter ball.
Correction factors for cylindrical workpieces, standard Rockwell
Add the value on the outer surface of a cylinder and subtract it on an inner surface. The columns are the workpiece diameter.
Diamond indenter, Rockwell C, D and A scales
| Observed reading | 3.175 0.125 in. | 6.350 0.250 in. | 9.525 0.375 in. | 12.700 0.500 in. | 15.875 0.625 in. | 19.050 0.750 in. | 22.225 0.875 in. | 25.400 1.000 in. |
|---|---|---|---|---|---|---|---|---|
| 80 | … | 0.5 | 0.5 | 0.5 | … | … | … | … |
| 70 | … | 1.0 | 1.0 | 0.5 | 0.5 | 0.5 | … | … |
| 60 | … | 1.5 | 1.0 | 1.0 | 0.5 | 0.5 | 0.5 | 0.5 |
| 50 | … | 2.5 | 2.0 | 1.5 | 1.0 | 1.0 | 0.5 | 0.5 |
| 40 | … | 3.5 | 2.5 | 2.0 | 1.5 | 1.0 | 1.0 | 1.0 |
| 30 | … | 5.0 | 3.5 | 2.5 | 2.0 | 1.5 | 1.5 | 1.0 |
| 20 | … | 6.0 | 4.5 | 3.5 | 2.5 | 2.0 | 1.5 | 1.5 |
1.588 mm (1/16 in.) ball indenter, Rockwell B, F and G scales
| Observed reading | 3.175 0.125 in. | 6.350 0.250 in. | 9.525 0.375 in. | 12.700 0.500 in. | 15.875 0.625 in. | 19.050 0.750 in. | 22.225 0.875 in. | 25.400 1.000 in. |
|---|---|---|---|---|---|---|---|---|
| 100 | … | 3.5 | 2.5 | 1.5 | 1.5 | 1.0 | 1.0 | 0.5 |
| 90 | … | 4.0 | 3.0 | 2.0 | 1.5 | 1.5 | 1.5 | 1.0 |
| 80 | … | 5.0 | 3.5 | 2.5 | 2.0 | 1.5 | 1.5 | 1.5 |
| 70 | … | 6.0 | 4.0 | 3.0 | 2.5 | 2.0 | 2.0 | 1.5 |
| 60 | … | 7.0 | 5.0 | 3.5 | 3.0 | 2.5 | 2.0 | 2.0 |
| 50 | … | 8.0 | 5.5 | 4.0 | 3.5 | 3.0 | 2.5 | 2.0 |
| 40 | … | 9.0 | 6.0 | 4.5 | 4.0 | 3.0 | 2.5 | 2.5 |
| 30 | … | 10.0 | 6.5 | 5.0 | 4.5 | 3.5 | 3.0 | 2.5 |
| 20 | … | 11.0 | 7.5 | 5.5 | 4.5 | 4.0 | 3.5 | 3.0 |
| 10 | … | 12.0 | 8.0 | 6.0 | 5.0 | 4.0 | 3.5 | 3.0 |
| 0 | … | 12.5 | 8.5 | 6.5 | 5.5 | 4.5 | 3.5 | 3.0 |
Source: ASM Handbook, Vol. 8, correction factors for cylindrical workpieces tested with standard Rockwell hardness testers.
Correction factors for cylindrical workpieces, superficial Rockwell
The superficial scales are used on thin sections and shallow case depths, where the regular scales are too heavy for the part.
Diamond indenter, Rockwell 15N, 30N and 45N scales
| Observed reading | 3.175 0.125 in. | 6.350 0.250 in. | 9.525 0.375 in. | 12.700 0.500 in. | 15.875 0.625 in. | 19.050 0.750 in. | 22.225 0.875 in. | 25.400 1.000 in. |
|---|---|---|---|---|---|---|---|---|
| 90 | 0.5 | 0.5 | … | … | … | … | … | … |
| 85 | 0.5 | 0.5 | 0.5 | … | … | … | … | … |
| 80 | 1.0 | 0.5 | 0.5 | 0.5 | … | … | … | … |
| 75 | 1.5 | 1.0 | 0.5 | 0.5 | 0.5 | 0.5 | … | … |
| 70 | 2.0 | 1.0 | 1.0 | 0.5 | 0.5 | 0.5 | … | 0.5 |
| 65 | 2.5 | 1.5 | 1.0 | 0.5 | 0.5 | 0.5 | … | 0.5 |
| 60 | 3.0 | 1.5 | 1.0 | 1.0 | 0.5 | 0.5 | … | 0.5 |
| 55 | 3.5 | 2.0 | 1.5 | 1.0 | 1.0 | 0.5 | … | 0.5 |
| 50 | 3.5 | 2.0 | 1.5 | 1.0 | 1.0 | 1.0 | … | 0.5 |
| 45 | 4.0 | 2.5 | 2.0 | 1.0 | 1.0 | 1.0 | … | 1.0 |
| 40 | 4.5 | 3.0 | 2.0 | 1.5 | 1.0 | 1.0 | … | 1.0 |
1.588 mm (1/16 in.) ball indenter, Rockwell 15T, 30T and 45T scales
| Observed reading | 3.175 0.125 in. | 6.350 0.250 in. | 9.525 0.375 in. | 12.700 0.500 in. | 15.875 0.625 in. | 19.050 0.750 in. | 22.225 0.875 in. | 25.400 1.000 in. |
|---|---|---|---|---|---|---|---|---|
| 90 | 1.5 | 1.0 | 1.0 | 0.5 | 0.5 | 0.5 | … | 0.5 |
| 80 | 3.0 | 2.0 | 1.5 | 1.5 | 1.0 | 1.0 | … | 0.5 |
| 70 | 5.0 | 3.5 | 2.5 | 2.0 | 1.5 | 1.0 | … | 1.0 |
| 60 | 6.5 | 4.5 | 3.0 | 2.5 | 2.0 | 1.5 | … | 1.5 |
| 50 | 8.5 | 5.5 | 4.0 | 3.0 | 2.5 | 2.0 | … | 1.5 |
| 40 | 10.0 | 6.5 | 4.5 | 3.5 | 3.0 | 2.5 | … | 2.0 |
| 30 | 11.5 | 7.5 | 5.0 | 4.0 | 3.5 | 2.5 | … | 2.0 |
| 20 | 13.0 | 9.0 | 6.0 | 4.5 | 3.5 | 3.0 | … | 2.0 |
Source: ASM Handbook, Vol. 8, correction factors for cylindrical workpieces tested with superficial Rockwell hardness testers. The 22.225 mm column is blank for the 15T, 30T and 45T scales in the source table.
What the correction is worth in practice
On a 6.35 mm bar, a 60 HRC reading carries a correction of 1.5 points and a 30 HRC reading carries 5.0 points. At the hardness tool steel is used at, the correction is small, but it is larger than the repeatability of the test, so it decides whether a bar sits inside a narrow acceptance band.
A certificate that gives 58.5 HRC on a round bar without naming the diameter cannot be compared with a flat specimen result. State the diameter, the scale and the load, and say whether the correction has been applied.
The same table covers tube, where the bore is a concave surface, and wire, where the reading falls below the range of the regular scales. Below 6.35 mm diameter the regular scales are out, and the superficial N scales carry the correction down to 3.175 mm.
Before quoting a hardness value
A hardness reading belongs to a scale, a load and a part geometry. Comparing a value taken on a curved surface with a flat specimen, or a superficial value with a C scale value, produces a disagreement that has nothing to do with the material.
The factors on this page are published reference data and are approximate by construction. Confirm the acceptance criteria against the standard that governs your own product, and state the scale, the load, the location and the part diameter on the certificate.
Related reference pages on this site: Minimum thickness for hardness testing, Hardness testing methods, Tool steel hardness conversion calculator, Tool steel quality verification, Tool steel supply.
Source: ASM Handbook, Volume 8, Mechanical Testing and Evaluation (ASM International), article Selection and Industrial Applications of Hardness Tests by Andrew Fee, Consultant, and article Macroindentation Hardness Testing by Edward L. Tobolski, Wilson Instruments Division, Instron Corporation and Andrew Fee, Consultant. Values are reproduced for reference only. Aobo Steel supplies tool steel in the annealed condition.