We continue our discussion of hardness testing by focusing on tips for Rockwell and Rockwell Superficial testing.

Here are some of the most common errors that often occur in Rockwell hardness testing:

1. Cleanliness of the part and tester

a. Remove and clean the indenter and anvil prior to operation and at shift change. 
b. A small amount of debris can alter the reading by several points.

2. Curvature of the surface

a. 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.
b. The correction factor to be added is shown in Tables 4 and 5 of ASTM E18.

3. Non-flat surfaces

a. Extremely rough or textured surfaces may give inconsistent readings.
b. Remove any scale, debris, dirt and oil before testing.

4. Surfaces not perpendicular to the indenter

a. Surfaces should be flat within 2 degrees.
b. Be careful when taking readings on mounted samples. They must be flat, thick and not flex under load. A microhardness test may be more appropriate.

5. Readings taken too close to the sample edge

a. Indentations should be no closer than 2.5 times the indenter diameter from the edge.
b. If the metal buckles outward, the indenter is too close to the edge and the reading is invalid.

6. Readings taken too close together

a.  Indentations should be three diameters apart.

7. Sample is too thin

a. The material should have a thickness at least 10 times the depth of the indentation.
b. Minimum acceptable thicknesses can be found in Tables 6 and 7 of ASTM E18.

8. Parts that are not supported

a. Large and irregularly shaped parts need to be supported.
b. Parts that move, even slightly during the test, produce a false reading. Change the anvil to one that keeps the part stationary using the variety that should be available with your tester. Additional outside support (such as a Steady-Rest) may also be required.

9. Diamond is damaged or ball is flattened

a. Periodically remove the indenter from the hardness tester and inspect the tip using a low power (10-50x) stereomicroscope or loop. Look for a chipped or cracked diamond or a flattened ball.

Next Time

We'll continue our discussion by focusing on microhardness testing.

References

1. Stone, Alan and Daniel H. Herring, “Practical Considerations for Successful Hardness Testing,” Industrial Heating, April 2006.
2. Lysaght, Vincent E., and DeBellis, Anthony, Hardness Testing Handbook, American Chain and Cable Company, 1969.
3. Wilson Instruments Division, Instron Corporation, Norwood, MA (www.wilsoninstruments.com)
4. Herring, Daniel H., “Common Pitfalls in Hardness Testing,” Wire Forming Technology International/Spring 2012.
5. ASTM Specification Nos. E3, E10, E18, E103, E140 and E384 (www.astm.org).