Hardness converter: HV, HB, HRC and Rm
The mill certificate quotes HB, the joint test report quotes HV10, and the shop instrument reads HRC. The converter brings them onto one scale using the ISO 18265 table for unalloyed and low-alloy steels.
Three scales, one metal
Vickers (HV) — a diamond pyramid, loads from tens of grams
up to 100 kg. The impression is small, which is why the HAZ is measured
this way: nothing else fits in the narrow band beside the weld. Written
HV10, it means a 10 kgf load.
Brinell (HBW) — a carbide ball. The impression is large, so the reading averages the structure: fine for parent metal, poor for the HAZ. Above 650 HBW the method is not used — the ball itself deforms.
Rockwell C (HRC) — a diamond cone read straight off the dial, no microscope. Fast, which is why the shop likes it. Below 20 HRC the scale is not specified.
A few rows of the table
| HV | HBW | HRC | Rm. MPa |
|---|---|---|---|
| 150 | 143 | — | 480 |
| 200 | 190 | — | 640 |
| 250 | 238 | 22.2 | 800 |
| 300 | 285 | 29.8 | 965 |
| 350 | 333 | 35.5 | 1125 |
| 380 | 361 | 38.8 | 1220 |
| 400 | 380 | 40.8 | 1290 |
| 450 | 428 | 45.3 | 1455 |
| 500 | 475 | 49.1 | 1630 |
| 600 | 570 | 55.2 | 1995 |
| 700 | 665 | 60.1 | — |
The full table steps in 10 HV; the calculator interpolates between rows exactly as you would by hand on paper.
The relation Rm ≈ 3.2 · HV holds for steels in the annealed, normalised and quenched-and-tempered condition and drifts at high hardness. For austenitic steels, work-hardened material and cast iron, ISO 18265 gives separate tables, and the figures on this page do not apply to them.
Where a welder needs it
- Qualifying a procedure. Hardness testing is part of the test set in EN ISO 15614-1 and appears in the WPQR next to the mechanical results.
- Chasing the cause of a crack. A hard HAZ points to cooling that was too fast; from there you go to the t8/5 cooling time and to preheat.
- Reading a certificate. The mill certificate gives HB while the requirement is written in HV — without a conversion the two cannot be compared.
Frequently asked questions
Does a conversion replace a measurement?
No. ISO 18265 says plainly that the tables are for orientation, not a substitute for testing in the scale required. If the procedure asks for 380 HV10, HV10 is what has to be measured: a converted HRC is not evidence. The scatter of the conversion alone reaches several HRC points.
Why is HAZ hardness measured at all?
It is the cheapest way to catch a quenched structure. A hard band beside the weld means the joint cooled too fast, and together with hydrogen that gives cold cracks. The measurement is taken on a macro section, along a line crossing weld metal, HAZ and parent metal.
Where does the 380 HV10 limit come from?
From EN ISO 15614-1: for steels of groups 1 and 2 without post-weld heat treatment the maximum is 380 HV10, and 320 after treatment. Other groups have their own values, and the full table belongs in the standard rather than in anyone’s memory.
Updated: