Hardness testers report on different scales, but a single material has one hardness. This tool uses the ASTM E140 correlation for steel to convert a value between Rockwell C, Rockwell B, Brinell, and Vickers, and estimates the corresponding tensile strength.
When you need hardness conversion
The typical scenario: your workshop has a Rockwell C tester, but the material specification in your drawing calls out a Vickers range, or the supplier’s certificate reports Brinell. You need to confirm compliance without re-testing. Another common case is estimating tensile strength from a hardness test on a finished part, where cutting a tensile specimen would destroy the part.
What each scale actually measures
Each scale presses a different indenter under a different load and reads the residual impression:
| Scale | Indenter | Load | Best for |
|---|---|---|---|
| Rockwell C (HRC) | Diamond cone (Brale) | 150 kgf | Hardened steel, range ~20–70 HRC |
| Rockwell B (HRB) | 1/16” steel ball | 100 kgf | Soft steel, aluminium, brass, ~0–100 HRB |
| Brinell (HB / HBW) | 10 mm carbide ball | 3,000 kgf | Soft to medium steel, castings |
| Vickers (HV) | Diamond pyramid | Variable | Wide range; thin sections and case-depth work |
Because each presses a different geometry, there is no exact equation linking the scales. The numbers only correlate because the same specimens were measured on every machine and the results tabulated — that is the ASTM E140 standard.
How it works
This tool stores a slice of the ASTM E140 table for carbon and alloy steel and interpolates linearly between the two nearest rows for whichever value you enter.
Tensile strength for steel is then estimated from the Brinell value with the long-standing rule of thumb:
UTS (MPa) ≈ 3.45 × HBSo a part at 200 HB has an approximate tensile strength of 690 MPa. The factor 3.45 comes from empirical measurements on a broad range of steels and is an approximation, not a specification.
Worked example
For example, a spring steel component is measured at 55 HRC on the workshop tester. The material specification says the part must fall between 550 and 650 HV. Converting 55 HRC using the ASTM E140 table gives approximately 595 HV, which sits comfortably within that window. The same conversion also yields an approximate Brinell of 540 HB and an estimated tensile strength of around 1,863 MPa.
Scale ranges and edge cases
Each scale has a working range; outside it the indenter or the load produces unreliable results:
- HRC below about 20 is not meaningful — use HRB or HB instead for soft steel.
- HRB above 100 saturates; hardened steel above this threshold needs HRC or HV.
- Brinell becomes unreliable above about 600 HB because the carbide ball deforms.
- Vickers is the only scale that reliably spans the whole range from very soft to very hard, and is the choice for case-hardening depth measurements where the zone is too thin for other indenters.
Tips and notes
- Conversions at the extremes of the table are less accurate; if the result lands near an edge, consider re-testing on a scale better suited to that range.
- The tensile estimate is a planning figure only — it carries uncertainty that grows with alloy content and processing. Specify a real tensile test for structural design values.
- Vickers and Brinell agree closely below roughly 300 because they share the same kgf/mm² basis; above that they diverge because the Brinell ball begins to deform.
- These correlations apply to steel only. Aluminium, copper, titanium, and polymers need their own conversion data.