The hardness of steel ranges from approximately 120 HV (Vickers) for soft annealed low-carbon steel to over 900 HV for hardened high-speed tool steel, and the specific value depends entirely on the grade and heat treatment condition. The Rockwell C scale (HRC), Brinell scale (HBW), and Vickers scale (HV) are the three measurement systems you will encounter for steel in engineering specifications. The Mohs scratch scale — the one with diamond at 10 — tells you almost nothing useful about steel, because steel’s entire hardness range compresses into a narrow band between Mohs 4 and 7.
This article provides the complete hardness reference for steel across all scales, explains what each scale measures and when to use each, and covers what the hardness values mean for the most common steel processing operations — machining, grinding, laser cutting, and wear.
Hardness of Steel: Reference Values by Grade Family
The complete hardness reference for steel, organized by grade and condition, across the three engineering hardness scales:
Structural and carbon steel
| Steel grade | Condition | HRC | HBW | HV | Mohs (approx.) |
|---|---|---|---|---|---|
| A36 structural | As-rolled | — | 119–159 | 125–170 | ~4 |
| 1018 mild steel | Cold-drawn | — | 126 | 131 | ~4 |
| 1045 | Normalized | — | 170 | 179 | ~4.5 |
| 1045 | Q&T (34 HRC) | 34 | 311 | 330 | ~5.5 |
| 4140 | Annealed | — | 197 | 207 | ~4.5 |
| 4140 | Q&T (28 HRC) | 28 | 270 | 284 | ~5 |
| 4140 | Q&T (54 HRC) | 54 | 513 | 546 | ~5.5 |
| 4340 | Q&T (50 HRC) | 50 | 475 | 505 | ~5.5 |
Stainless steel
| Grade | Condition | HRC | HBW | HV | Notes |
|---|---|---|---|---|---|
| 304 austenitic | Annealed | — | 201 | 210 | Max per ASTM A240 |
| 316 austenitic | Annealed | — | 217 | 228 | Max per ASTM A240 |
| 304 | Cold-worked | 20–30 | 220–280 | 235–295 | Work hardening raises hardness |
| 430 ferritic | Annealed | — | 185 | 195 | Lower than austenitic |
| 410 martensitic | Annealed | — | 207 | 218 | |
| 410 martensitic | Hardened + tempered | 38–44 | 363–420 | 385–445 | Standard knife-blade range |
| 440C martensitic | Hardened | 57–60 | — | 620–700 | High-carbon, high-Cr cutlery |
| 17-4 PH | H900 | 41 | 388 | 410 | Precipitation hardened |
Source: ASM Handbook, Volume 1; ASTM A240/A240M-22 (maximum hardness limits for stainless sheet/plate).
Tool steel
| Grade | Condition | HRC | HBW | HV |
|---|---|---|---|---|
| D2 (high-Cr, high-C) | Hardened | 61–63 | — | 740–780 |
| H13 (hot-work) | Hardened | 50–54 | — | 513–575 |
| M2 (Mo high-speed) | Hardened | 64–66 | — | 820–870 |
| T15 (W high-speed, Co) | Hardened | 67–69 | — | 870–940 |
| O1 (oil-hardening) | Hardened | 60–62 | — | 700–760 |
Wear-resistant plate (Hardox and equivalents)
| Grade | Condition | HRC | HBW | HV |
|---|---|---|---|---|
| Hardox 400 | As-delivered | 38–43 | 370–430 | ~400 |
| Hardox 450 | As-delivered | 43–47 | 425–475 | ~450 |
| Hardox 500 | As-delivered | 48–52 | 470–530 | ~500 |
| Hardox 600 | As-delivered | 57–62 | — | ~600 |
Source: ASM Handbook Vol. 1; SSAB Hardox product data sheets.
The Three Engineering Hardness Scales for Steel
Rockwell C (HRC): the shop floor standard for hardened steel
The Rockwell C scale uses a diamond Brale indenter at a 150 kgf major load. It is the standard hardness scale for:
- Hardened and tempered steel (heat treatment verification)
- Tool steel (after hardening)
- Wear-resistant plate (Hardox, AR400, AR500)
- Fastener standards (ISO and ASTM bolt and nut hardness limits)
Range: Meaningful from approximately 20–70 HRC. Below 20 HRC, the Rockwell B scale (using a 1/16″ ball indenter) is more appropriate.
What HRC measures: The depth of penetration of the Brale indenter under 150 kgf, minus the depth under a preliminary 10 kgf load. The scale is inverted — harder materials produce less depth, giving a higher HRC number.
Critical HRC values for steel processing:
- 20–30 HRC: readily machinable with HSS or carbide tooling at production feeds
- 35–45 HRC: hard turning requires carbide; grinding is preferred for precision work
- 50–55 HRC: conventional machining impractical; grinding and EDM are primary processes
- 60+ HRC: grinding, EDM, or hard turning with CBN (cubic boron nitride) tooling only
Brinell (HBW): the standard for structural and annealed steel
Brinell hardness uses a 10 mm tungsten carbide ball at 3,000 kgf (steel) or 500 kgf (softer materials). It is the standard for:
- Structural steel receiving inspection (A36, ASTM A572, HSLA plate)
- Carbon and alloy steel bar stock in the annealed or normalized condition
- Cast iron (where the indent diameter is large enough to average over the coarse microstructure)
Range: Meaningful from approximately 50–650 HBW. Above 650 HBW, the ball indenter deforms, and the results are unreliable.
Why Brinell is preferred for structural steel: The 10 mm ball produces a large indentation that averages over multiple grains and any microstructural variations — important for coarse-grained as-rolled steel, where a small indenter would give variable results depending on where it landed.
The HBW–UTS relationship for steel: For carbon and low-alloy steels, there is an empirical correlation:
- UTS (MPa) ≈ HBW × 3.45 (approximately)
- For A36 at HBW 130: estimated UTS ≈ 130 × 3.45 ≈ 449 MPa (ASTM A36 specifies 400–550 MPa UTS — within range)
This relationship is approximate (±10–15%) but useful for quick property estimation when only hardness data is available.
Vickers (HV): the universal and research scale
Vickers hardness uses a square pyramidal diamond indenter at loads from 1 gf to 120 kgf. The same indenter geometry works from the softest metal to the hardest coating, making HV the only scale that spans the complete hardness range without scale changes.
Where HV is used for steel:
- Microhardness measurements (case depth profiling in carburized or nitrided steel)
- Hardness mapping across welds (HAZ hardness after welding)
- Thin coatings and surface layers
- Research and international reporting (ISO standards tend to use HV more than HRC)
- Comparing metals with very different hardness ranges (from soft copper at HV 40 to tool steel at HV 900)
The case depth of a carburized steel gear, for example, is defined as the depth at which HV drops below a specified threshold (often HV 550 for medium-duty gearing). This measurement requires the Vickers scale because the depth resolution needed (measuring hardness every 0.1 mm through the case) is impractical with Rockwell or Brinell.
Mohs Hardness and Steel: Why It Tells You Almost Nothing
The Mohs scratch hardness scale ranks materials by which can scratch which, from Mohs 1 (talc) to Mohs 10 (diamond). It was developed by Friedrich Mohs in 1812 for mineralogy, specifically for the field identification of rocks and minerals.
For engineering steel, the Mohs scale is nearly useless:
- Mild steel (A36, 1018): approximately Mohs 4–4.5
- Hardened tool steel (D2, M2): approximately Mohs 6–6.5
- The Mohs scale cannot distinguish between A36 at HRC 17 and M2 at HRC 65 — both fall between 4 and 7
The Mohs scale’s coarse 10-step resolution compresses all engineering steels into a band that spans only about 2.5 steps (4 to 6.5). This is why fabricators, metallurgists, and machine tool engineers use Rockwell, Brinell, or Vickers — all three provide 100–200× finer resolution within the steel hardness range.
The one legitimate use of Mohs for steel: Mohs gives useful relative comparisons when asked “Can X scratch Y?” For practical shop floor use: a steel file (approximately Mohs 6.5) will scratch soft aluminum (Mohs 2.5–3) easily, will scratch copper (Mohs 3) easily, and will leave a mark on glass (Mohs 5.5). It will barely mark or not mark ceramic tile (Mohs 6.5–7). This level of scratch testing — using a file or a hardened steel scriber — is the correct application of Mohs-type thinking in fabrication.
Hardness Conversion: Rockwell C, Brinell, and Vickers for Steel
ASTM E140 provides the standard hardness conversion tables for steel. The conversions are empirical (not calculated from first principles) and have accuracy of approximately ±5% for normal carbon and alloy steels. The conversions are not valid for stainless steel, cast iron, or non-ferrous metals — those materials require their own conversion tables.
| HRC | HBW | HV | Approximate UTS (MPa) | Steel condition example |
|---|---|---|---|---|
| 20 | 226 | 238 | 780 | Through-hardened 4140, medium temper |
| 25 | 253 | 266 | 870 | |
| 30 | 286 | 301 | 985 | 4140 Q&T, 28 HRC target |
| 35 | 327 | 345 | 1,130 | |
| 40 | 375 | 396 | 1,295 | Hardox 400 |
| 45 | 432 | 456 | 1,495 | |
| 50 | 513 | 544 | 1,775 | High-strength tool steel |
| 55 | 595 | 631 | 2,060 | |
| 60 | 697 | 746 | 2,410 | D2 tool steel range |
| 65 | — | 882 | — | M2 HSS range |
| 70 | — | 1,037 | — | Upper HSS + specialty steels |
Source: ASTM E140, Standard Hardness Conversion Tables for Metals.
Note on UTS estimation: The HBW × 3.45 ≈ UTS (MPa) approximation degrades above 40 HRC because it was empirically derived for steels in the 100–400 HBW range (annealed to moderately hardened). Above 50 HRC, use actual tensile test data from the specification rather than the hardness-UTS correlation.
Hardness of Steel in Laser Cutting
For laser cutting applications, the hardness of the steel being cut affects three things:
1. Laser cutting process parameters. Hardness itself does not directly affect laser cutting — the process melts and vaporizes the metal, so hardness (resistance to plastic deformation) is irrelevant at cutting temperatures. What matters is carbon content (which affects the oxygen-assist combustion chemistry) and thermal conductivity (which affects heat extraction from the kerf). Wear-resistant steel (Hardox 400, AR plate) cuts on oxygen-assist fiber laser in the same way as A36, because the carbon content is similar (~0.15–0.30%) and the thermal properties are comparable.
2. HAZ microstructure. Laser cutting produces a very narrow heat-affected zone (HAZ) — typically 0.1–0.5 mm depending on power and speed. In hardened steel, this HAZ zone is quenched and tempered by the laser thermal cycle. Depending on the preheat temperature and prior condition of the steel, the HAZ in hardened steels may be softer (tempered back) or harder (re-hardened from the austenite temperature followed by air cooling) than the base material. For precision tool steel components, this edge condition must be evaluated and the thin edge zone may need grinding to remove.
3. Post-cut edge handling and processing. Laser-cut edges on Hardox 400–600 and similar wear plate retain hardness close to the bulk value — the thin, rapidly quenched HAZ may be slightly harder than the bulk, but it is narrow. Parts cut from wear-resistant plate should be handled with awareness that the edge is hard enough to damage softer tooling and that standard drilling and tapping tooling cannot penetrate the edge material of high-Hardox grades. Carbide tooling is required for any post-cut machining.
Learn more:
Defining Coefficient of Friction Steel on Steel for Engineering
Frequently Asked Questions About the Hardness of Steel
Q: What is the hardness of steel in Rockwell?
It varies by grade and condition. Mild steel (A36, 1018 as-rolled): not typically tested on HRC (too soft — use Brinell, ~120–160 HBW, or Rockwell B scale). Medium-carbon normalized (1045): HBW 170. Quenched and tempered 4140: 28–54 HRC depending on temper. Wear-resistant plate Hardox 400: ~40 HRC. Tool steel D2 hardened: 61–63 HRC. High-speed steel M2 hardened: 64–66 HRC.
Q: What is the Mohs hardness of steel?
Mild and structural steel: approximately 4–4.5 on the Mohs scale. Hardened tool steel: approximately 5.5–6.5. The Mohs scale is not a useful hardness scale for steel in engineering — its 10-step resolution cannot distinguish between mild steel and fully hardened tool steel, which differ by a factor of 5–6 on the Vickers scale. For steel hardness in engineering, use Rockwell C (HRC) for hardened steel or Brinell (HBW) for softer structural steel.
Q: What is the hardness of stainless steel compared to carbon steel?
Annealed austenitic stainless (304): HBW 201, HV ~210 — comparable to normalized medium-carbon steel. Annealed A36: HBW 119–159, HV ~125–170 — softer. Cold-worked 304 stainless can reach HRC 25–35 through work hardening, approaching medium-heat-treated carbon steel. Martensitic stainless steel (440C, hardened): HRC 57–60, HV 620–700 — in the range of quality tool steel.
Q: How does heat treatment change the hardness of steel?
Dramatically. 4140 steel in the annealed condition: HBW 197 (no meaningful HRC). After quench and temper to moderate hardness: 28 HRC (HBW 270). After quench and temper to high hardness: 54 HRC (HBW 513). The hardness increases because quenching produces martensite (a supersaturated, highly distorted BCC structure that resists dislocation motion) and tempering reduces brittleness while retaining much of the hardness. The final HRC value is controlled by the tempering temperature: higher tempering temperature = softer but tougher; lower tempering temperature = harder but more brittle.
Q: Can you tell the hardness of steel by its color after laser cutting?
No. The color of a laser-cut steel edge reflects the surface oxide layer and heat-affected zone temperature — not the bulk hardness of the material. Hardened tool steel and soft mild steel can produce similar edge appearance after oxygen-assist laser cutting. Hardness measurement requires an indentation test (Rockwell, Brinell, or Vickers on a prepared surface) or a dedicated hardness tester. Portable Rockwell and Brinell hardness testers are available for field verification.
References
- ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys, 10th Edition. ASM International, Materials Park, OH, 1990. (Hardness values for carbon, alloy, and tool steels by grade and temper; HRC-HBW-HV relationships; hardness and strength correlations; martensite hardness as a function of carbon content; stainless steel hardness ranges.)
- ASTM E18, Standard Test Methods for Rockwell Hardness of Metallic Materials, ASTM International, current edition. (Rockwell C and B scale methodology; indenter specifications; minor and major load requirements; hardness scale applicability ranges; basis for all HRC values cited in this article.)
- ASTM E10, Standard Test Method for Brinell Hardness of Metallic Materials, ASTM International, current edition. (10 mm tungsten carbide ball indenter; 3,000 kgf load for steel; indentation measurement procedure; hardness calculation from ball diameter and indentation diameter; basis for all HBW values cited.)
- ASTM E140, Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, Scleroscope Hardness, and Leeb Hardness, ASTM International, current edition. (HRC-HBW-HV conversion table for steel cited throughout this article; accuracy limitations at extreme hardness values; material-specific conversion tables for stainless steel, cast iron, and non-ferrous metals.)
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