Read the complete hardness expression
A report should identify material/sample, method or scale, test force, location, number of tests, individual and average results, and standard edition. “28 hardness” alone does not identify HRC, HRB, HV or another method.
ASTM E18-25 says Rockwell hardness at one location may not represent a whole part. ASTM E92-23 similarly notes that one Vickers or Knoop value may not represent bulk hardness.
Why one shaft can show different values
Q&T material may vary by surface, core or section; induction-hardened shafts have a depth gradient; chromium coating and steel substrate are different targets. Curvature, texture, decarburization, support and force also affect applicability.
State whether the test is on substrate, hardened case or coating, plus its relationship to surface and end face.
Conversion does not replace testing
ISO 18265:2013 says converted values apply directly only to the material actually tested; for other materials they are indicators and do not replace results from the correct standard method.
If an order accepts by HRC, use the agreed HRC method where practical. When conversion is necessary, retain the original value, basis and limitation.
What the buyer should agree
Record material/condition, range, method/scale, force where relevant, location/count, surface preparation, batch sampling and decision rule. Use separate inspection items for Q&T ground bar, induction-hardened case and hard chrome rather than one number for every layer.