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Hardness conversion: HRC, HB, HV and tensile strength per ASTM E140 / ISO 18265
A drawing says 58–62 HRC, your tester reads Vickers, and the customer wants a tensile value. Conversion tables make this possible — but only within one material group, only approximately, and only if the report says it was converted.
Definition: Hardness conversion is the use of empirical tables such as ASTM E140 or ISO 18265 to estimate the value one hardness scale (for example Rockwell C) would have given from a measurement made in another (Brinell, Vickers, Rockwell A or superficial N), or to estimate tensile strength from hardness; the result is approximate, valid only for the material group the table was built from, and must be reported as a converted value.
1. Why conversion tables exist and why they are approximate
Rockwell, Brinell and Vickers do not measure the same thing. Rockwell C records the depth a diamond cone penetrates under 150 kgf (1471 N) after a 10 kgf preload. Brinell records the diameter of an impression left by a 10 mm tungsten carbide ball under 3000 kgf (29.42 kN). Vickers records the diagonals of a pyramid impression at anything from 1 to 120 kgf. Each scale samples a different volume of metal and work-hardens it to a different degree; there is no physical law linking them.
What exists instead is data. ASTM E140 and ISO 18265 were built by testing thousands of specimens of known composition and heat treatment on several scales and fitting curves through the results. That is why the tables are:
- Empirical — the curves are fitted to results, not derived from theory.
- Material-specific — E140 publishes separate tables for non-austenitic steels (Rockwell C range and Rockwell B range), nickel and high-nickel alloys, cartridge brass, austenitic stainless steel plate and sheet, copper, alloyed white cast irons and wrought aluminium products. A steel table used on brass gives a wrong answer with no warning.
- Approximate — a converted value carries an extra uncertainty on top of the measurement uncertainty. For steels this is of the order of ±2 HRC or a few per cent in HV/HB; for non-ferrous alloys it is larger.
Never convert across material groups. Austenitic stainless steel, in particular, work-hardens so heavily under the indenter that its HRC–HV–HB relationship differs visibly from carbon and alloy steel, which is why E140 gives it its own tables.
2. Conversion table for non-austenitic steel (ASTM E140 Table 1)
This is the table that covers carbon steels, alloy steels, tool steels, cast steel and martensitic stainless steel in the hardened, tempered, normalised or annealed conditions — the bulk of what an Indian heat-treatment shop or forging QC department tests.
| HRC | HV | HBW 10/3000 | HRA | HR15N | Approx. tensile Rm (MPa) |
|---|---|---|---|---|---|
| 68 | 940 | — | 85.6 | 93.2 | — |
| 66 | 865 | — | 84.5 | 92.5 | — |
| 64 | 800 | — | 83.4 | 91.8 | — |
| 62 | 746 | — | 82.3 | 91.1 | — |
| 60 | 697 | 654 | 81.2 | 90.2 | — |
| 58 | 653 | 615 | 80.1 | 89.3 | — |
| 56 | 613 | 577 | 79.0 | 88.3 | — |
| 55 | 595 | 560 | 78.5 | 87.9 | 2075 |
| 52 | 544 | 512 | 76.8 | 86.4 | 1880 |
| 50 | 513 | 481 | 75.9 | 85.5 | 1760 |
| 48 | 484 | 455 | 74.7 | 84.3 | 1635 |
| 45 | 446 | 421 | 73.1 | 83.0 | 1480 |
| 42 | 412 | 390 | 71.5 | 81.5 | 1340 |
| 40 | 392 | 371 | 70.4 | 80.4 | 1250 |
| 38 | 372 | 353 | 69.4 | 79.4 | 1180 |
| 35 | 345 | 327 | 67.9 | 77.7 | 1100 |
| 32 | 318 | 301 | 66.3 | 76.1 | 1005 |
| 30 | 302 | 286 | 65.3 | 75.0 | 965 |
| 28 | 286 | 271 | 64.3 | 73.9 | 900 |
| 25 | 266 | 253 | 62.8 | 72.2 | 850 |
| 22 | 248 | 237 | 61.5 | 70.5 | 785 |
| 20 | 238 | 226 | 60.5 | 69.4 | 760 |
Brinell values stop at about 650 HBW because above that the ball itself deforms and the test is no longer valid (IS 1500 / ASTM E10 limit the method to 650 HBW). Tensile values are rounded and differ by 1–3 % between editions and sources; treat them as an indication, not a property.
3. Softer steel: HRB, HB and HV
Below roughly 20 HRC the C scale loses resolution and E140 switches to the Rockwell B range (1/16 in ball, 100 kgf). This is the table for annealed and normalised carbon steel, mild steel plate, bright bars and most forgings before heat treatment. Note that in this range Vickers and Brinell numbers are practically identical.
| HRB | HBW 10/3000 | HV | Approx. tensile Rm (MPa) |
|---|---|---|---|
| 100 | 240 | 240 | 800 |
| 95 | 210 | 210 | 705 |
| 90 | 185 | 185 | 620 |
| 85 | 165 | 165 | 565 |
| 80 | 150 | 150 | 515 |
| 75 | 137 | 137 | — |
| 70 | 125 | 125 | — |
| 65 | 114 | 114 | — |
| 60 | 105 | 105 | — |
Two practical warnings. First, HRB 100 and HRC 20 overlap in hardness (about 240 HB against 226 HB) but are separate tables — choose the scale by the material's expected hardness before testing, not after. Second, a Brinell test on soft steel to IS 1500 should use a force-diameter ratio of 30, i.e. 10/3000 or 2.5/187.5; both give the same HBW number, and the 2.5/187.5 combination is what a bench Rockwell-cum-Brinell tester uses.
4. Hardness to tensile strength: the 3.45 rule and where it fails
For carbon and low-alloy steels the Brinell number tracks the ultimate tensile strength closely enough for a rule of thumb:
Rm (MPa) ≈ 3.45 × HBW (≈ 3.3–3.5 depending on grade) → Rm (kgf/mm²) ≈ 0.35 × HBW
Check it against the table: 286 HB × 3.45 = 987 MPa against a listed 965 MPa; 421 HB × 3.45 = 1452 against 1480. The rule is good to about ±5 % over 150–450 HB for annealed, normalised, quenched and tempered steel. It fails in these cases:
- Cold-worked steel — drawn wire, cold-rolled strip and cold-headed fasteners. Work hardening raises hardness more than it raises tensile strength; the rule over-predicts Rm.
- Austenitic stainless steel — the ratio is lower and varies with the degree of cold work; use the E140 austenitic tables for hardness only and do not infer tensile strength.
- Non-ferrous metals — aluminium, copper and brass have their own ratios, which vary with temper and alloy. Use the material-specific tables and, if tensile strength matters, test it.
- Cast iron — graphite flakes give grey iron a tensile strength far below what its matrix hardness suggests.
- Very high hardness — above about 50 HRC, tensile strength is limited by fracture, not by flow, and published tables stop giving values above about 55 HRC for good reason.
5. Measure in the right scale, and report conversions honestly
The cleanest way to avoid conversion error is not to convert. Choose the scale the specification calls for and a machine that can test it directly:
- Hardened and tempered steel, case-hardened parts, tools: Rockwell C directly.
- Castings, forgings, plate, bar stock: Brinell. The large 10 mm ball averages over coarse grains, porosity and segregation that a Rockwell point reading would miss, and most IS product standards for castings and forgings specify HB.
- Thin sheet, tubes, small parts, case depths under 0.4 mm: Rockwell superficial (HR15N, HR30N, HR15T) or Vickers at a load chosen so the indentation depth is less than one-tenth of the thickness. Do not test a 0.8 mm sheet in HRC and convert; the anvil effect corrupts the reading before the table does.
- Reference and dispute work, welds, coatings, micro-hardness: Vickers. It is the only scale continuous from very soft metals to the hardest steels and carbides, and it is the reference method that ISO 18265 tables are built around.
When a conversion is unavoidable — the drawing is in HRC and only a Vickers machine is available, or a customer wants a tensile indication from a hardness survey — IS 1586, IS 1500 and IS 1501 (which adopt ISO 6508, 6506 and 6507) and the ISO standards themselves are clear: there is no general conversion, conversions should be avoided unless agreed, and a converted value must be identified as such. Write it like this:
Measured: 697 HV 30 → Report: "≈ 60 HRC (converted from HV 30 per ASTM E140 Table 1)"
Always show the measured scale, the load, the table used and, if the customer requires it, the conversion uncertainty. A report that shows only "60 HRC" when the machine never touched a diamond cone is a non-conformance in any audit that looks at raw data.
6. ISO 18265 vs ASTM E140
Both standards do the same job and their steel tables agree within the stated uncertainties, but they are organised differently and an Indian lab will meet both.
- ASTM E140 is organised by material group and by Rockwell range, with tables for non-austenitic steel (C range and B range), nickel alloys, cartridge brass, austenitic stainless plate and sheet, copper, white cast iron and wrought aluminium. It gives conversion equations for some groups so software can interpolate, and its appendices describe the uncertainty of the conversions. It uses the tungsten carbide ball (HBW) for Brinell and quotes American practice such as HRC, HRA, HRB and the superficial N and T scales in full.
- ISO 18265 (which superseded DIN 50150) is organised into annexes A to F by material — unalloyed and low-alloy steels and cast steel in Annex A, steels in specific heat-treatment conditions (hardened, tempered, cold-work and high-speed tool steels), hard metals, and non-ferrous groups such as copper and aluminium alloys. Its tables are built with Vickers as the reference hardness and express tensile strength as an indicative value only. Each table states the conditions under which it was derived so the user can judge whether their material fits.
European drawings and work under ISO 6508/6506/6507 will refer to ISO 18265; American drawings and anything under ASTM A370 will refer to E140. Where a purchase order names neither, agree the table in writing before testing.
7. How digital testers convert, and a worked example
A digital or touch-screen hardness tester converts in software. The controller measures what the machine physically does — depth for Rockwell, impression diameter for Brinell (from a camera or an entered reading), diagonals for Vickers — calculates the primary hardness number, and then looks up a stored E140 or ISO 18265 table, interpolating between rows, to show the secondary scales. The tester therefore has an opinion about the converted value, not a measurement of it. Three consequences follow:
- The operator must select the correct material group in the menu before testing; the default is almost always "steel", and a brass fitting tested on the steel table will show a wrong HRB-to-HB conversion.
- The test report and stored data must record the primary scale. A well-designed tester prints the measured value in large type and the converted values labelled as converted; if the report format hides which is which, change the format.
Worked example. A forging shop receives EN 19 (42CrMo4) shafts specified 28–34 HRC after quench and temper. The shop has a Brinell tester and measures 311 HBW 10/3000 (impression diameter 3.44 mm) at three points. From the non-austenitic steel table, 311 HBW falls between 301 HBW (32 HRC) and 319 HBW (34 HRC), interpolating to about 33 HRC; the tensile indication is about 1050 MPa (the tables give 1025–1070 MPa depending on edition, and 3.45 × 311 = 1073). The report should read: "311 HBW 10/3000 (mean of 3); ≈ 33 HRC converted per ASTM E140 Table 1; ≈ 1050 MPa indicative." With a conversion uncertainty of about ±2 HRC, the part is safely inside 28–34 HRC — but had it read 297 HBW (≈ 31.5 HRC, or 29.5–33.5 with uncertainty), the shop would be right to retest the part directly on a Rockwell machine before shipping.
Frequently asked questions
Is 60 HRC equal to how many HB?
For non-austenitic steel, 60 HRC corresponds to approximately 654 HBW (10 mm tungsten carbide ball, 3000 kgf) and 697 HV per ASTM E140 Table 1. This is close to the 650 HBW upper limit of the Brinell method itself, so in practice steel at 60 HRC should be tested in Rockwell C or Vickers, not Brinell, and the HB figure treated as nominal.
Can I convert aluminium HRB to HB with the steel table?
No. Aluminium alloys have their own table in ASTM E140 (wrought aluminium products) and in ISO 18265, and the relationship between HRB and HB differs from steel because aluminium is softer, has a lower modulus and work-hardens differently. Using the steel table on aluminium, brass or stainless steel is one of the most common causes of disputed hardness results.
Why do two conversion charts disagree?
Usually because they come from different standards (E140 vs ISO 18265 vs an old DIN 50150 or SAE J417 chart), different editions, or different material groups, and because tensile columns are rounded from ksi or kgf/mm². Differences of one to two HRC points or 2–3 % in HB are within the stated conversion uncertainty. Agree which table applies to the order and quote it on the report.
What HRC is 1000 MPa steel?
A quenched and tempered carbon or alloy steel with a tensile strength of about 1000 MPa sits at roughly 32 HRC, 300 HBW or 318 HV. Remember this only holds for non-austenitic steel in a heat-treated or annealed condition; for cold-worked, austenitic or non-ferrous material the hardness–tensile relationship is different and the specification should call for a tensile test.
Is Vickers to Rockwell conversion reliable above 65 HRC?
It becomes less reliable. Above about 65 HRC the Rockwell C scale is compressing very small depth differences (68 HRC is about 940 HV), the diamond indenter itself is under high stress, and the datasets behind the tables are sparse because few steels reach that hardness. For carbides, nitrided cases, hard coatings and ceramics use Vickers or Knoop directly and do not convert to HRC at all.
Do Indian standards accept converted values?
IS 1586, IS 1500 and IS 1501, which adopt ISO 6508, 6506 and 6507, state that there is no general process for converting between scales and that conversions should be avoided unless a reliable basis is agreed between the parties. A converted value is acceptable when the purchase order or product standard allows it and the report clearly shows the measured scale and the table used.
Which ETS machine
- RASN-TSFA and RSNE-TTouch-screen Rockwell testers that read HRC directly and display converted HV, HB and tensile values from built-in conversion tables, with the measured scale on the report.
- RASNB-TSFA and RSNE-250Rockwell and Brinell on one machine, so hardened parts and forgings can each be measured in their own scale instead of converted.
- B3000-PC3000 kgf Brinell with camera measurement of the impression, automatic HBW calculation and converted values shown alongside the measured hardness.
- ETS-BIMSPortable camera kit that measures 1–7 mm Brinell impressions on any existing tester and applies ASTM E140 conversions to the measured HBW.
- ETS VM 50-TSPC1–50 kgf Vickers tester for the reference method behind ISO 18265 tables — thin sections, case depths and dispute work.
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