Why hardness conversion is approximate
Hardness tests use different indenters, loads, measurement methods, and material responses. Brinell measures an indentation diameter from a ball, Rockwell measures depth under load, and Vickers measures diagonal length from a diamond pyramid. Because the physics are not identical, conversion is empirical.
ASTM E140 and similar references provide conversion tables for specific materials and ranges. The selected material family matters. A conversion developed for carbon steel may not be valid for aluminum, copper alloys, tool steel, cast iron, or hardened case layers.
- Manufacturing engineers use conversions for drawing interpretation.
- Quality teams use them for incoming material screening.
- Buyers use them to compare supplier specifications written in different scales.
How to calculate a hardness conversion
There is no universal formula that accurately converts all hardness scales. The correct method is table lookup for the material and scale range, with interpolation between adjacent table entries when needed. In simple form: converted value = table value at the measured hardness, or an interpolated value between two table rows.
Example: if a steel conversion table lists HRC 30 as approximately HB 286 and HV 302, then a measured HRC 30 part may be communicated as about 286 HB or 302 HV for comparison. The word about matters because the converted number inherits the table's uncertainty and material limits.
Hardness and tensile strength
Some tables also estimate tensile strength from hardness, especially for steels. This is useful for rough screening, but it is not a replacement for tensile testing. Heat treatment, microstructure, cold work, section size, and material chemistry all affect the relationship.
If a contract, code, or drawing requires a specific hardness scale, report the actual test result on that scale whenever possible. Use conversion only when the governing document allows it or when the value is clearly labeled as approximate.
- Use the required test scale for acceptance decisions.
- Use conversions for preliminary comparison and communication.
- Document the conversion table or standard used.
Common hardness conversion mistakes
A common mistake is converting outside the valid range of a table. Rockwell B, Rockwell C, Brinell, and Vickers ranges overlap only partly, and very soft or very hard materials may not convert reliably.
Another mistake is ignoring surface condition and test setup. Case-hardened parts, coatings, thin sections, curved surfaces, decarburization, and rough finishes can produce hardness readings that are not representative of the bulk material.
Frequently asked questions
Is there an exact formula from HRC to HB?
No. HRC to HB conversion is empirical and material-dependent. Use an appropriate conversion table and treat the result as approximate.
Can converted hardness be used for acceptance?
Only if the governing specification allows conversion. Otherwise, test on the required scale.
Why do different charts give different values?
They may be based on different materials, standards, rounding, interpolation methods, or hardness ranges.
Does hardness prove tensile strength?
No. Hardness can estimate tensile strength for some materials, but tensile testing is required when strength certification is needed.