Steel Hardness Conversion: HBW, HRC, HV and Tensile Strength

Hardness is the single most-quoted property on a steel mill test certificate. It correlates with tensile strength, costs almost nothing to measure, and is non-destructive — which is why heat-treat shops, gear makers and fastener houses live by it. The catch is that there are three scales in everyday use — Brinell (HBW), Rockwell (HRC/HRA/HRB) and Vickers (HV) — and they do not measure exactly the same thing. Below is how to convert between them, where the conversions come from, and how to estimate tensile strength from a hardness number without embarrassing yourself.

Three Scales, Three Operating Ranges

Each scale has a different indenter, a different load, and a different sweet spot. Using them outside their valid range is the most common reason a conversion looks wrong.

Brinell — HBW (ASTM E10 / ISO 6506)

Brinell uses a tungsten carbide ball, typically 10 mm diameter, pressed into the surface at a load of 3000 kgf (29.4 kN) for 10–15 seconds. The diameter of the resulting impression is measured optically, and HBW is calculated as load divided by the curved surface area of the impression. The "W" in HBW means tungsten carbide (Wolframcarbid in German); the older HBS designation for steel-ball indenters is obsolete.

HBW's valid range for steel is roughly 96 to 650 HBW. Above 650 HBW the ball deforms and the impression becomes unreliable. For very hard tool steels and carburised surfaces, you switch to HRC or HV. HBW is the standard scale for plate, bar, forgings and castings — basically anything where a 10 mm ball and 3000 kgf load leaves a measurable dent on a flat surface ≥ 4 mm thick.

Rockwell — HRC, HRA, HRB (ASTM E18 / ISO 6508)

Rockwell measures the depth of penetration of a diamond cone (HRC, HRA) or a hardened steel ball (HRB) under a defined minor (10 kgf) and major load. HRC uses a 120° diamond cone (Brale indenter) at 150 kgf major load; HRA uses the same cone at 60 kgf; HRB uses a 1/16-inch ball at 100 kgf. The depth is converted directly to a dimensionless number.

The valid ranges are:

• HRC: 20–70 HRC. Below 20 HRC the diamond cone produces a shallow, low-sensitivity reading — switch to HRB.

• HRA: 20–88 HRA. Used for carbide, thin hard coatings, cemented carbide and very high-hardness tool steels where HRC tops out.

• HRB: 0–100 HRB. Used for soft steels, annealed carbon bar, low-carbon sheet.

HRC is the dominant scale for hardened and tempered parts — shafts, gears, dies, blades. The advantage over Brinell is speed: a Rockwell reading takes 5–10 seconds and leaves a small, shallow indent that often does not require re-machining.

Vickers — HV (ASTM E92 / ISO 6507)

Vickers uses a square-based pyramid diamond with a 136° face angle, applied at loads from 1 kgf (HV1) up to 100 kgf (HV100) and beyond. HV measures the diagonal of the impression optically and computes hardness as load divided by surface area.

Vickers' strength is its wide range — the same indenter covers everything from soft annealed steel (~80 HV) to nitrided case layers (~1100 HV) without a scale break. Its weakness is that it requires an optical reading, which is slow and operator-dependent. HV is the standard scale for:

• Microhardness traverses across case-hardened parts (HV0.3, HV1)

• Welding procedure qualification

• Thin coatings and nitrided layers

• Reference calibration block certification

ASTM E140 / ISO 18265 Conversion Table

Both ASTM E140 ("Standard Hardness Conversion Tables for Metals") and ISO 18265 give the conversion values between HBW, HRC and HV for carbon, alloy and tool steels. The values are not theoretical — they are regressed from thousands of test points on quenched-and-tempered martensitic and bainitic steel. The conversion is accurate to roughly ±1 HRC for hardened steel and ±2–3 HBW for annealed steel. Beyond that, the indenter geometries differ enough that the conversion is only approximate.

Selected values from ASTM E140 for steel:

A note on the lower bound of HRC. ASTM E140 gives a small data block for the 19–22 HRC range but flags it as "approximate". The reliable part of the HRC scale starts at 20 HRC. Below that, switch to HRB or HBW.

Tensile Strength Estimation from Hardness

There is a useful empirical relationship between Brinell hardness and ultimate tensile strength for low- and medium-carbon steel:

σUTS (MPa) ≈ 3.45 × HBW for HBW ≤ 500 (low- and medium-carbon steel, quenched-and-tempered)

For higher-hardness alloyed steel (HBW 500–650), the multiplier drifts down to about 3.0–3.2 because the indenter measures a combination of strength and elastic recovery, and martensite does not convert to tensile strength linearly. The 3.45 multiplier also under-reads for low-strength annealed steel below ~150 HBW; there the multiplier is closer to 3.6.

Different families of steel have different multipliers:

The 3.45 multiplier is built into most old procurement specs and material strength charts. The ISO 18265 Table A.1 gives a more elaborate breakdown by steel grade family, and is the document to cite when arguing the case in writing.

A worked example: an AISI 4140 bar tested at 280 HBW should be expected to show a tensile strength around 970 MPa. If a Q&T 4140 test bar at 280 HBW shows only 850 MPa UTS in the lab, something is wrong — either the bar is under-tempered (insufficient tempering time at temperature), the test sample is axial vs transverse (anisotropy can drop UTS by 5–7%), or the hardness was measured on a decarburised surface. This is why hardness and tensile testing are done on the same heat-lot bar.

Different Steels Convert Differently

The conversion numbers in ASTM E140 are regressed from quenched and tempered carbon and alloy steels — martensitic or bainitic, fairly homogeneous. They are less reliable for:

• Austenitic stainless steel (304, 316): the material work-hardens under the indenter, so HBW/HRC tend to read higher than the underlying tensile strength would suggest. ISO 18265 lists separate conversion data for austenitic stainless.

• Grey and ductile cast iron: the graphite phase crushes under the indenter. HBW conversions for cast iron are entirely different from those for steel — multiply by ~1.0–1.5 for grey iron.

• Case-hardened parts: a surface reading at 60 HRC over a soft core will not match the bulk tensile. Use the hardness traverse (HV0.3 across the case) and compute effective case depth per ASTM E384.

• Cold-drawn bar: cold work raises HBW without raising UTS proportionally. A cold-drawn 1045 bar at 200 HBW has a UTS closer to 700 MPa, not the 690 MPa that the annealed multiplier predicts.

This is why a procurement spec should never say "hardness and tensile are equivalent". They are not — they correlate, with a multiplier that depends on the grade and the metallurgical condition.

Specifying Hardness on a Purchase Order

A common mistake is to write "hardness: 28–32 HRC" on a PO without specifying the test location, surface condition or scale. The supplier picks the cheapest interpretation, and you get a soft core under a thin hard case. The clause to write is:

Hardness: 28–32 HRC per ASTM E18, measured on a finished or semi-finished surface free of decarburisation, scale and oxide. Three readings minimum, spaced ≥ 3 mm apart, all to fall within range. Surface preparation per ASTM E18 §7. For bars >50 mm diameter, hardness to be verified at mid-radius, not on the as-rolled surface.

Additional clauses to consider:

• For through-hardened parts: "Through-thickness hardness traverse at one end of the bar per ASTM E384, variation ≤ 2 HRC across the section."

• For case-hardened parts: "Effective case depth 0.8–1.2 mm at 550 HV (HV1) per ASTM E384."

• For forged bar: "HBW measured on a machined flat at mid-radius; the as-forged surface is not acceptable for hardness verification."

FAQ

Q: Why does my 4140 bar read 30 HRC but the mill cert shows 285 HBW?
30 HRC ≈ 285 HBW per ASTM E140 — they are equivalent. The mill may report in HBW because Brinell is the standard production-floor test for Q&T bar; the customer may specify HRC because it is the design-scale number. Both are correct.

Q: Why is the 3.45 multiplier giving me a wrong UTS?
Because the multiplier is for medium-carbon Q&T steel between 150 and 500 HBW. If the steel is austenitic stainless, cast iron, case-hardened, or above 500 HBW, use the correct multiplier from ISO 18265 Table A.1.

Q: Can I use a portable Rockwell tester on as-rolled bar?
Only for a screening reading. As-rolled scale and decarburisation can drop the apparent HRC by 2–5 points. For acceptance, machine a small flat first or test on the milled end face.

Q: Is Vickers more accurate than Rockwell?
More sensitive, but not necessarily more accurate. Vickers is the reference scale used to certify Rockwell test blocks. For production-floor work, Rockwell is faster and good enough. For laboratory or case-depth work, Vickers is the only choice.

Q: What is HBW vs HB?
HB is the old, non-specific designation — it could mean a steel ball (HBS) or a tungsten carbide ball (HBW). Since ISO 6506:2005 and ASTM E10-14, only the tungsten carbide ball is standard, so HBW is the only designation. If a mill cert says "HB", assume HBW.

Talk to Qilu Metal

Qilu Metal supplies Q&T bar, forged bar and plate in AISI 4140, AISI 4340, AISI 4130 and AISI D2 tool steel with hardness verified per ASTM E18 / E10 / E92 on machined surfaces, full mill test reports and traceability. If you need a specific hardness window for a hardened-and-tempered part, send the drawing and the spec — our metallurgy team will confirm the heat-treat recipe, the test location and the acceptance band, and quote against it. Email enquiry@qilumetal.com.