Could you explain the most common problems when conducting Rockwell hardness testing?

Here are some of the most common Rockwell hardness testing problems, along with some tips on what to do if you encounter these types of problems (in no particular order).

1. Cleanliness of the part and tester is paramount.

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 as much as several Rockwell points.

2. Non-flat surfaces can alter readings.

a. Extremely rough or textured surfaces may give inconsistent readings. Remove any scale, debris, dirt and oil before testing. Light sanding of both the bottom and top surfaces is necessary before hardness testing.
b. Take into account the curvature of the surface.
c. A correction factor must be added to the hardness reading of small-diameter shapes for Rockwell C, A and D and varies with the apparent hardness and part diameter. (The correction factor to be added is shown in ASTM E18 Tables 2 and 3.)

3. Surfaces not perpendicular to the indenter will give false readings.
a. Surfaces should be flat within 2 degrees. 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.

4. Readings taken too close to the sample edge will both damage the indenter and give false readings.
a. Indentations should be no closer than 2½ times the indenter diameter from the edge. If the metal buckles outward, the indenter is too close to the edge and the reading is invalid.

Next week we will look at the remaining four problems and tips to solve them.