Why are hardness scales converted at all?
The same part gets measured on different machines. The heat treater's report says HRC, the customer drawing asks for HB, the lab certificate gives HV. All three describe hardness, but each uses a different indenter, a different load and a different measuring principle. That is why there is no fixed factor such as "1 HRC equals so many HB".
Standard conversion tables are an experimental comparison of the same material measured by different methods. A conversion is therefore an approximate comparison, not a measurement. The tool below does that conversion, and also works out hardness directly from the indentation you read on a Brinell or Vickers machine.
Hardness Conversion and Measurement Calculator
HRC to HB to HV conversion
Brinell (HBW) calculation
A ball of a given diameter is pressed into the surface under a given load. The diameter of the remaining indentation is measured, and hardness is worked out from those three values.
Show the formula used
F is the load in kgf, D the ball diameter in mm, d the mean indentation diameter in mm. A load entered in newtons is divided by 9.80665 first.
Vickers (HV) calculation
A diamond pyramid with a 136° included angle is used. Both diagonals of the indentation are measured, averaged, and hardness is calculated from that average.
Show the formula used
F is the load in kgf, d the mean diagonal in mm. A load in newtons is divided by 9.80665; the SI form HV = 0.1891 × F(N) / d² gives the same answer.
Which test should I use?
| Method | How it measures | Where it is used | Standard |
|---|---|---|---|
| HRC Rockwell C | From the depth a diamond cone reaches under load. The machine shows the result directly, no calculation needed. | Hardened steel, tool steel, hard castings. The quickest method on the shop floor. | ISO 6508 |
| HB / HBW Brinell | From the diameter of the mark left by a carbide ball, read by eye or optically. | Castings, forgings, structural steel, softer and medium-hard parts. The wide indentation averages out an uneven structure. | ISO 6506 |
| HV Vickers | From the two diagonals of the mark left by a diamond pyramid. | A very wide hardness range. Thin case-hardened or nitrided layers, weld zones, small parts and microhardness work. | ISO 6507 |
Practical rules of thumb
- Thin or small parts: avoid Brinell. The wide indentation crushes the part and can show through the back. Vickers or superficial Rockwell suits better.
- Measuring a case-hardened or nitrided layer: HRC misleads, because the indenter punches through the hard skin into the softer core. Section the part and take a microhardness profile in Vickers.
- Castings and coarse-grained material: a small indentation may land on a single grain. Brinell gives a more representative result thanks to its larger mark.
- Rough or scaled surfaces: clean them first. A reading taken on an oxidised, blackened or unground surface does not show the real hardness.
How tempering temperature changes hardness
The most common shop-floor situation is this: the part comes out of quenching at 56 HRC, you temper it, measure again and read 52 HRC. That is not a fault. Tempering is not done to lower hardness, it is done to take the brittleness out. As-quenched steel is at its hardest but also at its most brittle; used without tempering it cracks.
But not every steel behaves the same way, and this is where many people get caught out.
Cold-work tool steels: hardness falls as temperature rises
In unalloyed and low-alloy cold-work tool steels, hardness drops steadily as the tempering temperature goes up. That is why these steels should be kept below about 200 °C in continuous service. If the part heats past its tempering temperature in use, it loses hardness permanently.
Hot-work and high-speed steels: hardness rises around 500 °C
Hot-work tool steels and HSS behave differently. Hardness dips slightly first, then climbs again at roughly 500-550 °C. This is called secondary hardening, and it comes from alloy carbides precipitating out of the structure. It is the reason these steels keep their hardness while working hot.
| Steel | Hardening | 400 °C | 500 °C | 550 °C | 600 °C |
|---|---|---|---|---|---|
| 1.2343 / H11 hot work | 1000-1040 °C oil, 52-56 HRC | 53 HRC | 54 HRC | 52 HRC | 48 HRC |
| 1.2344 / H13 hot work | 1020-1080 °C oil, 52-56 HRC | 54 HRC | 55 HRC | 54 HRC | 50 HRC |
| 1.2365 / H10 hot work | 1010-1050 °C oil, 52-56 HRC | 50 HRC | 51 HRC | 52 HRC | 50 HRC |
| 1.2367 hot work | 1030-1080 °C oil, 52-56 HRC | 52 HRC | 54 HRC | 53 HRC | 50 HRC |
| 1.2714 / L6 forging dies | 830-900 °C oil, 52-58 HRC | 50-48 HRC | 46-44 HRC | 43-41 HRC | 40-38 HRC |
These are typical tempering-curve figures. The actual result depends on the austenitising temperature, the soaking time, section thickness and furnace control. Always work from the material producer's own data sheet for exact values.
What the table shows in practice: a low-alloy steel such as 1.2714 gives about 50 HRC after tempering at 400 °C, while a hot-work steel such as 1.2344 gives 55 HRC after tempering at 500 °C - a higher hardness at a higher temperature. The rule "raise the tempering temperature and hardness drops" does not hold for every steel.
The whole sequence - annealing, stress relieving, hardening and tempering - is covered in order in Steel Heat Treatment.
Frequently asked questions
What is 45 HRC in HB and HV?
For non-austenitic steels the standard conversion table puts 45 HRC at roughly 421 HB and 446 HV. It is not one exact number though; the table comes from experimental comparison and can be a few points out depending on the material.
Why is there no conversion for aluminium or stainless?
Because the same table does not apply. Austenitic stainless work-hardens at the surface during the test, and aluminium and copper alloys behave quite differently. Applying the carbon-steel table to them would produce a number, but the wrong one. We would rather give no figure than a wrong figure.
Why no Brinell equivalent above 60 HRC?
Because the Brinell test is not performed at that hardness. Above roughly 650 HBW the carbide ball itself starts to deform and the indentation no longer reflects true hardness. The table can produce a number, but it cannot be measured in practice. HRC or HV is used in that range.
Why does the indentation-to-ball ratio matter?
If the Brinell indentation is too small, measuring accuracy suffers; if it is too large, material piles up around the ball and the result is distorted. ISO 6506-1 requires the load to be chosen so that the indentation diameter stays between 0.24 and 0.60 of the ball diameter. The calculator shows this ratio and warns you if it falls outside.
Which measurement should I ask for after hardening?
If the part is solid and hardness is wanted through the body, Rockwell C is the most practical. If the part was case hardened, nitrided or induction hardened, case depth is what matters: section the part and run a microhardness profile in Vickers. For large castings, Brinell is preferred.
The same part gives two different hardness readings - which one is right?
First compare the test conditions: surface cleanliness, whether the part was firmly supported, and whether the measuring point was far enough from an edge and from the previous indentation. If all of that is sound, the part may genuinely differ - cooling rate changes with section thickness, and a thicker section stays softer. That is the limit of hardenability.