Why are Rockwell C (HRC) readings invalid below 20 HRC?

Below 20 HRC (~226 HBW), the 150 kgf major load pushes the sharp 120° diamond cone too deeply into the soft metal substrate, exceeding the calibrated conical tip zone and causing nonlinear plastic flow. For soft steels, brass, and aluminum below 20 HRC (100 HRB), switch to the Rockwell B scale (HRB), which uses a 1.588 mm (1/16") tungsten carbide ball and a 100 kgf load.

Why are Vickers (HV) and Brinell (HBW) numbers nearly identical up to 350 HBW but diverge above 450 HBW?

The 136° diamond pyramid angle of the Vickers indenter was intentionally chosen by Smith and Sandland in 1924 to match the average tangent contact angle of a Brinell ball impression at d = 0.375 × D. However, above 450 HBW (~47 HRC), even a tungsten carbide Brinell ball flattens elastically under the 3,000 kgf load, yielding a lower HBW number than the rigid diamond Vickers pyramid.

What is the minimum workpiece thickness required for an accurate Rockwell C (HRC) test?

Under ASTM E18, the workpiece thickness must be at least 10 times the permanent indentation depth (10 × h). At 60 HRC (h = 0.080 mm), the minimum thickness is 0.80 mm; at 30 HRC (h = 0.140 mm), the minimum thickness is 1.40 mm. If the part or carburized case is thinner, an 'anvil effect' occurs and you must use Rockwell Superficial (HR30N / HR15N) or Micro-Vickers (HV0.5).

Why must I apply a cylindrical correction factor when testing HRC on round shafts?

When pressing a Rockwell diamond cone into a convex cylindrical shaft (especially diameters under Ø25 mm / 1.0"), the metal has less lateral support around the curvature than on a flat block, allowing the indenter to sink deeper and read 0.5 to 3.0 HRC points artificially low. Always add the ASTM E18 cylindrical correction factor based on shaft diameter.

Hardness Conversion Chart & Calculator: Rockwell HRC, Brinell HBW & Vickers HV

Under ASTM E140 and ISO 18265, metallic hardness scales are converted empirically across indenter geometries: Rockwell C (HRC) uses a 120° diamond Brale cone under a 150 kgf major load (valid from 20 to 70 HRC), Brinell (HBW) uses a 10 mm tungsten carbide ball at 3,000 kgf, and Vickers (HV) uses a 136° diamond pyramid. For carbon and alloy steels, Ultimate Tensile Strength is approximately UTS (MPa) ≈ 3.45 × HBW (UTS (psi) ≈ 500 × HBW).

Authored & Verified by ToolSpecVault Engineering Data Team

Mathematical Verification: Automated Geometric Verification (ISO / ASME / NIST Public Formulas) | Published October 26, 2026 | Verified October 2026

✓ First-Hand Computational Verification: Derived via formulas.py (ISO / ASME / NIST) & Audited Against Mitutoyo Micrometer & Gage Block Tolerance Bands (October 2026)

ASTM E140 Metal Hardness Converter: Rockwell (HRC), Brinell (HBW) & Vickers (HV)

Convert between Rockwell C (HRC), Vickers (HV), Brinell 10mm Carbide Ball (HBW), Rockwell A (HRA), and Ultimate Tensile Strength (MPa / ksi).

ASTM E140 / ISO 18265

Input Hardness Scale

Measured Hardness Value

Equivalent Hardness (HRC / HV / HBW / HRA)

45.0 HRC ≈ 446 HV ≈ 421 HBW

Rockwell A = 73.0 HRA | Shore Scleroscope ≈ 60 HS

Estimated Tensile Strength (UTS) & Tooling

1452 MPa (211 ksi)

Hard-Machining Regime: Use AlTiN / TiSiN Carbide at 60–90 m/min

How Do Rockwell C (HRC), Brinell (HBW), and Vickers (HV) Hardness Tests Differ Physically?

When a heat-treat blueprint or mill test report (MTR) specifies a material hardness in HRC, HRB, HBW, or HV, no single mathematical constant connects them because each test measures plastic deformation using a completely different indenter geometry and load stage, as standardized by ISO International Metrology Standard (ISO 18265 / ISO 6506 / ISO 6507 / ISO 6508) and NIST (National Institute of Standards and Technology) Standard Reference Material (SRM) hardness blocks.

The Rockwell C test (ASTM E18, HRC) applies a 10 kgf minor preload to seat a 120° diamond spheroconical indenter (0.200 mm tip radius) below surface scale, increases the load to a 150 kgf major load, and measures the permanent depth increase h in units of 0.002 mm: HRC = 100 - (h / 0.002 mm). By contrast, the Brinell test (ASTM E10, HBW) presses a Ø10 mm tungsten carbide ball into rough castings or forgings with 3,000 kgf and optically measures the wide indentation diameter d, while the Vickers test (ASTM E92, HV) presses a 136° square-based diamond pyramid under loads from 10 gf (micro-Vickers) to 100 kgf to inspect thin nitrided or case-hardened layers without breaking through the case.

Table 1: ASTM E140 / ISO 18265 Master Metal Hardness Conversion Chart — Rockwell C (HRC), Vickers (HV), Brinell 10mm Carbide Ball (HBW), Rockwell A (HRA), and Tensile Strength (UTS) — Verified October 2026.
Rockwell C (HRC — 150 kgf Cone)	Vickers (HV — 136° Pyramid)	Brinell (HBW — 3000 kgf, 10mm WC)	Rockwell A (HRA — 60 kgf)	Shore Scleroscope (HS)	Approx. Tensile Strength (MPa / ksi)	Typical Steel Condition & Machinability
20 HRC	238 HV	226 HBW	60.2 HRA	34 HS	`772 MPa (112 ksi)`	Annealed / Normalized 4140 — Easy CNC machining
25 HRC	266 HV	253 HBW	62.9 HRA	38 HS	`869 MPa (126 ksi)`	Pre-hardened P20 / 4140 — Standard carbide tooling
30 HRC	302 HV	286 HBW	65.3 HRA	42 HS	`986 MPa (143 ksi)`	4140HT / Grade 8.8 Bolts — Ideal strength-to-machinability
35 HRC	345 HV	327 HBW	67.9 HRA	48 HS	`1,131 MPa (164 ksi)`	Grade 10.9 Bolts / 4340HT — Reduce SFM by 25%
40 HRC	392 HV	371 HBW	70.4 HRA	54 HS	`1,282 MPa (186 ksi)`	Class 12.9 SHCS / H13 Hot-Work Die — TiAlN coated carbide
45 HRC	446 HV	421 HBW	73.0 HRA	60 HS	`1,462 MPa (212 ksi)`	Hardened Tool & Die Steels — Entering Hard-Milling regime
50 HRC	513 HV	481 HBW	75.9 HRA	67 HS	`1,696 MPa (246 ksi)`	Hardened S7 / 420SS Mold — Use AlTiN / SiN4 or CBN
55 HRC	595 HV	560 HBW	78.5 HRA	75 HS	`2,005 MPa (291 ksi)`	Case-Hardened Gears / Dowel Pins — PCBN turning / Hard mill
60 HRC	697 HV	654 HBW (WC Ball)	81.2 HRA	81 HS	`2,320 MPa (336 ksi)`	D2 / A2 / M2 Cutting Tools — PCBN, Ceramic, or Grinding
65 HRC	832 HV	739 HBW (Max)	84.2 HRA	91 HS	`2,550+ MPa (370+ ksi)`	HSS Taps & Drills — Diamond / CBN grinding or EDM

How to Estimate Steel Ultimate Tensile Strength (MPa & ksi) from Hardness

For quenched-and-tempered carbon and low-alloy steels (such as 1045, 4140, 4340, and 8620) between 200 HBW and 550 HBW (15 HRC to 55 HRC), indentation resistance is directly proportional to the alloy's Ultimate Tensile Strength (UTS or R_m). As documented in Machinery's Handbook (Industrial Press) and NASA Fastener & Mechanical Design Manual (NASA RP-1228) fastener metallurgy specifications, multiplying the Brinell hardness number (HBW) by 3.45 gives the approximate tensile strength in Megapascals (MPa), and multiplying HBW by 500 (or 0.500) gives the tensile strength in psi (ksi):

UTS (MPa) ≈ 3.45 × HBW | UTS (ksi) ≈ 0.500 × HBW

For example, pre-hardened 4140HT bar stock delivered at 30 HRC (286 HBW) has an ultimate tensile strength of approximately 286 × 3.45 = 987 MPa (143 ksi)—matching the exact strength class of an ISO Class 10.9 or SAE Grade 8 high-strength structural bolt!

Table 2: ASTM E140 Soft Metal Hardness Conversion Chart — Rockwell B (HRB, 100 kgf, 1/16" Ball) to Brinell (HBW) and Vickers (HV) for Mild Steel, Brass, and Aluminum — Verified October 2026.
Rockwell B (HRB — 100 kgf Ball)	Brinell Hardness (HBW)	Vickers Hardness (HV)	Approx. Tensile Strength (MPa / ksi)	Representative Unhardened Alloy
55 HRB	100 HBW	100 HV	`345 MPa (50 ksi)`	6061-T6 Aluminum / C11000 Copper
65 HRB	116 HBW	116 HV	`400 MPa (58 ksi)`	1018 Hot-Rolled Steel / Half-Hard C260 Brass
75 HRB	137 HBW	137 HV	`470 MPa (68 ksi)`	1018 Cold-Drawn Steel / 7075-T6 Aluminum
85 HRB	165 HBW	165 HV	`565 MPa (82 ksi)`	Annealed 304/316 Stainless / 1045 HR Steel
95 HRB (≈ 15 HRC)	210 HBW	210 HV	`715 MPa (104 ksi)`	Annealed 4140 / 8620 Alloy Steel
100 HRB (≈ 23 HRC)	240 HBW	240 HV	`825 MPa (120 ksi)`	Upper limit of HRB scale (Switch to HRC above 100 HRB)

CNC Cutting Speed & Tooling Adjustment Across 20 HRC, 45 HRC, and 60 HRC Steels

From a CNC machinist's perspective, workpiece hardness dictates both carbide grade selection and surface speed (V_c): below 32 HRC (300 HBW), standard PVD/CVD TiCN-Al2O3 coated carbide inserts cut freely at 150–240 m/min. Between 45 HRC and 52 HRC (the Hard-Machining threshold), switch to nano-layered AlTiN or TiSiN fine-grain carbide tools with negative land chamfers at 50–90 m/min. Above 55 HRC to 65 HRC (case-hardened gears, bearing races, and D2/M2 dies), deploy Polycrystalline Cubic Boron Nitride (PCBN) or Whisker-Reinforced Ceramic inserts in dry air blast! After checking your alloy hardness, calculate your lathe CSS speeds on our CNC Lathe G96 & G50 CSS Calculator, adjust high-efficiency milling feeds on our Radial Chip Thinning Calculator, compute sheet punching shear loads on our Sheet Metal Punch Tonnage Calculator, and verify fastener preload on our Bolt Torque & Preload Chart.