Hardness Conversion Calculator — Rockwell B/C, Brinell HB, Vickers HV, Knoop HK & Shore Scales
Convert hardness values between Rockwell (HRB, HRC), Brinell (HB), Vickers (HV), Knoop (HK), and Shore (HSD) scales. Includes tensile strength estimation from hardness.
Quick Answer
HRC 35 ≈ HB 327 ≈ HV 345. Tensile strength for steel: σ_uts ≈ 3.3 × HB = 1079 MPa. Hardness tests are cheap, fast, and nearly non-destructive — that’s why every shop floor uses them instead of tensile testing.
Why Hardness Conversion Matters
Every hardness scale measures material resistance to indentation — but each uses a different indenter, load, and measurement method. Converting between them is empirical, not theoretical.
1. Rockwell (HRB, HRC)
Measures depth of indentation. HRC: 120° diamond cone, 150kg load — for hardened steels (20-70 HRC). HRB: 1/16″ ball, 100kg — for softer metals (20-100 HRB). Rule: below HRC 20, use HRB. Above HRB 100, use HRC. The overlap region is roughly HB 230-250.
2. Brinell (HB)
10mm carbide ball, 3000kg load for steel. Measure indentation diameter with microscope. Broad range (HB 80-650), good for heterogeneous materials (cast iron). HB = 2P / (πD(D − √(D²−d²))). Slower than Rockwell but more representative of bulk hardness.
3. Vickers (HV)
Diamond pyramid indenter — micro and macro scales with same indenter shape. HV = 1.854 × P/d². Used for thin cases, coatings, and microhardness traverses. HV is geometrically similar across all loads — unique among hardness tests.
Common Mistakes
- Using conversion tables for work-hardened materials — Conversion tables (ASTM E140) are based on as-heat-treated materials. Cold-worked materials deviate significantly — work hardening changes the relationship between different hardness scales. Measure directly when possible.
- Converting between scales outside the valid range — ASTM E140 conversions are only valid within specified ranges. Converting HRC 15 to HB is not valid (below HRC 20). Converting HB 700 to HRC is not valid (above HRC 68). Extrapolation gives nonsense.
- Using hardness to estimate strength for non-ferrous metals — The σ ≈ 3.3×HB rule works for carbon and low-alloy steels only. Aluminum, copper, titanium, and stainless steels do NOT follow this ratio. Use tensile test data for non-ferrous — hardness-strength correlations are material-specific.
- Not accounting for case depth in Rockwell testing — The Rockwell indent penetrates ~0.1-0.15mm (HRC). For case depths <0.3mm, the core material influences the reading — you're not measuring case hardness. Use superficial Rockwell (HR15N, HR30N) or micro-Vickers for thin cases.
- Testing on rough, dirty, or curved surfaces — Surface roughness >3.2µm Ra, scale/oil, or curvature affects readings 2-5 HRC. Standard requires flat, clean, polished surface (600 grit minimum for Vickers, ground for Rockwell). Curved surfaces need correction per ASTM E18.
Frequently Asked Questions
What hardness scale should I specify on drawings?
Steel >250 HB: HRC (or HB if available). Steel <250 HB: HRB (or HB). Cast iron: HB (Brinell is standard for castings — the large indenter averages graphite flakes). Thin parts/case hardened: HR15N or HV. Coatings (<50µm): micro-Vickers (HV 0.1-0.5). Aluminum: HB (500kg load) or HRB for softer alloys.
How does hardness relate to machinability?
Low hardness (HB <150): gummy, built-up edge on tools, poor surface finish. Optimal: HB 180-250 (best machinability for most steels). Above HB 300: tool wear accelerates, requires carbide or ceramic tools. Above HB 400: difficult to machine — consider grinding or hard turning with CBN.
What is the minimum thickness for accurate hardness testing?
Rockwell HRC: 10× indentation depth ≈ 1.5mm minimum. Rockwell HRB: ~2mm minimum. Brinell HB (3000kg): 10× depth ≈ 8mm minimum for steel. Vickers micro: 1.5× diagonal spacing from edges, 2.5× between indents. For thin materials, use superficial Rockwell (HR15N, HR30N) or micro-Vickers.
Why do my hardness readings vary so much on the same part?
Acceptable variation: ±2 HRC for homogeneous steel, ±5 HB for cast iron (graphite causes scatter). Excessive scatter causes: (1) Surface condition (dirty, rough, decarburized layer), (2) Poor support (part moved during test), (3) Indenter damage (chipped diamond), (4) Segregation in material. Take 3-5 readings and average.
Can I convert hardness to yield strength?
Approximate for steel: σ_ys ≈ σ_uts − 70 MPa (for ductile steels), where σ_uts ≈ 3.3 × HB. So σ_ys ≈ 3.3×HB − 70 MPa. This is a rough estimate (±15%). For critical designs, tensile test is required per code. Our Shaft Stress Calculator needs material strength inputs.
What is microhardness and when is it used?
Microhardness (HV 0.01-1.0) uses loads 10g-1000g — indents visible only under microscope. Uses: (1) Case depth profiles (hardness vs depth traverses), (2) Individual phase hardness (martensite vs ferrite in dual-phase steel), (3) Weld HAZ mapping, (4) Thin foil/coating hardness. Standard Vickers (HV 1-50) is macrohardness.
How do I prepare a surface for hardness testing?
Rockwell: machine-ground surface, no oxide scale, ≤3.2µm Ra. Brinell: same as Rockwell. Vickers macro (HV 10-50): ground + polished to 1µm (mirror finish) for accurate diagonal measurement. Vickers micro: polished to 0.05µm (colloidal silica finish) for indent visibility. Roughness affects reading by 2-10% depending on scale and load.
What is the difference between hardness and hardenability?
Hardness = measured resistance to indentation on the final part. Hardenability = how deep the steel hardens during quenching (Jominy test). High hardenability doesn’t guarantee high hardness — it means hardness penetrates deeper. 1040 has low hardenability (shallow hardening) but can reach HRC 55 at surface. 4340 has high hardenability (deep hardening) and similar peak hardness.