The following scenario and question has been provided by one of our readers as a subject that dovetails into our recently completed 12-part series on Hardness Testing. We will set up the scenario here and address the Q&A next week.


My question revolves around the number of indentations required for a reported test value and operator options for hardness re-testing. I present the following scenario.

I have a contract to heat treat several hundred 440C actuator pins. The engineering drawing specifies a heat treatment per AMS 2759/5 with a hardness requirement of 58-62 HRC. The hardness indentations will affect the form/fit/function of the pins, so three hardness coupons are machined from the parent stock and heat treated with the parts.

The heat treatment is performed per the specification, and the parts are moved to quality control for Rockwell hardness testing. The hardness tester meets the requirements of ASTM E18 (latest revision) in that it is operating freely and passed the most recent indirect verification and the daily verification tests. The machine uncertainty based on the reference block, repeatability, non-uniformity and resolution is calculated as 0.23 HRC for a 95% confidence interval (k=2).

A single hardness measurement is performed on each coupon, where 58.2 HRC, 58.3 HRC and 57.9 HRC are measured. The operator verifies the following test parameters:

  • The test sample is supported rigidly, clean, flat, smooth and free from foreign material.
  • The anvil is seated, clean and free of raised edges or burrs.
  • The indenter is seated and clean.

A referee test where five hardness measurements distributed uniformly over the surface of the sample coupon are made, which result in 57.9 HRC, 58.2 HRC, 58.2 HRC, 58.0 HRC and 58.1 HRC, for an average of 58.0 HRC.

Check back next week for the reader’s questions and The Doctor’s answers.