Hardness Converter

What Are Hardness Scales and How Do They Relate?

Hardness measures a material's resistance to localized plastic deformation. Different scales use different indenters and loads: Rockwell C (HRC) uses a diamond cone with 150 kgf, Brinell (HB) uses a 10 mm tungsten carbide ball with 3000 kgf, and Vickers (HV) uses a diamond pyramid with variable loads.

Conversions between scales are empirical, not mathematical — they are based on correlation tables from ASTM E140. The relationship is approximately linear only within limited ranges. HRC is most commonly used for hardened steels (20-68 HRC), while HB is preferred for softer materials and castings.

Typical hardness values: Mild steel ~120 HB, Medium carbon steel (quenched & tempered) ~30 HRC, Tool steel (hardened) ~60 HRC, Case-hardened surface ~700 HV. The Shore column here is the scleroscope rebound test (HS) that ASTM E140 converts — not the Shore D durometer used for rubber and plastics, which only shares the name and measures indentation on a different scale.

The relationship between hardness and tensile strength is approximately linear for carbon and alloy steels: UTS (MPa) ≈ 3.45 × HB. This provides a convenient non-destructive method to estimate tensile strength from a portable hardness tester in the field. However, this correlation does not apply to non-ferrous metals or heavily cold-worked steels.

Modern automated hardness testers can perform conversion automatically, but understanding the underlying relationships is essential for quality engineers who must validate test reports and resolve disputes between measurement scales used by different suppliers.

Formula: HRC ≈ HV/10.2 − 3 (rough approximation, 20-60 HRC range) HB ≈ HV × 0.95 (for HB < 350) Actual conversions use ASTM E140 lookup tables

Example Calculation

A steel part measures 45 HRC. From ASTM E140 tables: this corresponds to approximately 446 HV, 421 HB, and roughly 62 HS on the Shore scleroscope scale. The approximate tensile strength is about 1480 MPa.

When to Use This Calculator

• Receiving a material test certificate with hardness in HB but your drawing specifies HRC — conversion is needed for acceptance decision
• Estimating tensile strength from a portable hardness tester reading during incoming material inspection
• Comparing material specifications across international standards where different hardness scales are used (e.g., JIS uses HV, ASTM uses HRC)
• Verifying heat treatment results when the available tester uses a different scale than the specification requires

Common Mistakes to Avoid

• Treating ASTM E140 conversions as exact — they are empirical approximations with ±2 HRC typical uncertainty, and accuracy degrades outside the 20-60 HRC range
• Using conversions designed for carbon/alloy steels on stainless steel, aluminum, or non-ferrous metals — the correlation tables are material-specific
• Converting across multiple scales sequentially (e.g., HRC→HV→HB) — each conversion adds error; always convert directly from the measured scale
• Ignoring the test method limitations — HRC requires a minimum thickness of 10× indentation depth, and HB is unsuitable for hardened steels above 450 HB

How to Interpret Results

• HRC 20-30: Medium hardness — typical of quenched & tempered structural steels suitable for general machinery
• HRC 30-45: Hard — typical of high-strength bolts (Class 10.9), gears, and shafts requiring wear resistance
• HRC 45-58: Very hard — tool steels, bearing races, and case-hardened surfaces
• HRC 58-68: Extremely hard — cutting tools, gauges, and wear plates; material becomes brittle at this range
• HB below 200: Soft and machinable — suitable for general machining and forming operations

Related Standards & References

• ASTM E140 — Standard hardness conversion tables for metals (the basis for all conversions)
• ASTM E18 — Standard test methods for Rockwell hardness
• ASTM E10 — Standard test method for Brinell hardness
• ISO 6508 — Metallic materials — Rockwell hardness test
• ISO 6506 — Metallic materials — Brinell hardness test

ASTM E140 Hardness Conversion Table

The full conversion table this calculator interpolates from — HRC values from 20 to 68, and their corresponding HB, HV, and Shore scleroscope (HS) readings.

Frequently Asked Questions

Which hardness scale should I specify on engineering drawings?

Use HRC for hardened steels (heat-treated parts, tools, springs) above 20 HRC. Use HB for softer steels, castings, and non-ferrous metals below 350 HB. Use HV for thin sections, case depths, and when a wide range is needed on a single scale. Always specify the scale and test conditions.

Are hardness conversions exact?

No. ASTM E140 conversions are empirical approximations valid only for carbon and alloy steels. Accuracy degrades for stainless steels, non-ferrous metals, and at extreme ranges. The tables have ±2 HRC typical uncertainty. For critical specifications, test directly in the required scale rather than converting.

Can I estimate tensile strength from a hardness measurement?

Yes, for carbon and alloy steels. The approximate relationship is UTS (MPa) ≈ 3.45 × HB (Brinell). For example, 200 HB ≈ 690 MPa tensile strength. This is a field estimation method — not a substitute for actual tensile testing — but is widely used for incoming inspection and material verification when destructive testing is not practical.