Table of Contents
What Is Rockwell Hardness?
Rockwell hardness is a measure of a material's resistance to indentation. The test uses a diamond cone or steel ball indenter pressed into the material under a specified load, and the depth of penetration determines the hardness number. The Rockwell C scale (HRC) uses a diamond cone with a 150 kgf major load and is the most common scale for testing hardened steels.
Different hardness scales exist because no single test covers all materials. Brinell testing uses a large ball indenter suited for castings and forgings. Vickers testing uses a diamond pyramid and works across all hardness ranges. Converting between scales requires empirical relationships since the tests measure different physical responses.
Conversion Formulas
These are polynomial approximations valid for carbon and alloy steels in the range of 20-65 HRC. Actual conversions depend on the specific material and should be verified against ASTM E140 tables for critical applications.
Hardness Conversion Table (Steel)
| HRC | HB | HV | Tensile (MPa) |
|---|---|---|---|
| 20 | 226 | 238 | 780 |
| 30 | 286 | 302 | 987 |
| 40 | 371 | 392 | 1280 |
| 50 | 481 | 513 | 1660 |
| 60 | 613 | 697 | 2116 |
Rockwell Scales
| Scale | Indenter | Major Load | Typical Use |
|---|---|---|---|
| HRA | Diamond cone | 60 kgf | Cemented carbides, thin steel |
| HRB | 1/16" steel ball | 100 kgf | Soft steels, copper, aluminum |
| HRC | Diamond cone | 150 kgf | Hardened steels, hard alloys |
Frequently Asked Questions
Are these conversions exact?
No. Hardness conversions are empirical approximations because each test measures a different aspect of material behavior. The relationships depend on the material type, heat treatment, and work hardening state. For critical applications, always refer to ASTM E140 or ISO 18265 conversion tables.
What HRC is considered hard steel?
Steel above 50 HRC is considered hard. Knife blades typically range from 55-62 HRC. Tool steels are usually 58-65 HRC. File-hard steel is about 60-65 HRC. Below 20 HRC, the Rockwell B scale is preferred.
Why do hardness values matter in engineering?
Hardness correlates with wear resistance, machinability, and tensile strength. It is a quick, non-destructive quality control test that can verify heat treatment results, weld integrity, and material conformance without cutting test specimens.