Hardness Conversion Reference

Compiled 2026-04-04 · 40 chunks, 12 posts · hardness · conversion · material-properties · heat-treatment · tooling-selection

Summary

Hardness conversion is critical for machinists to determine appropriate [[toolholder-selection]], cutting parameters, and [[insert-selection-guide]] choices. Understanding the relationship between Rockwell C (HRC), Rockwell B (HRB), Brinell (BHN), and Vickers (HV) scales allows proper material identification and machining strategy selection. Hardness directly affects machinability — too soft creates [[chip-control]] issues and built-up edge, while excessive hardness requires specialized tooling and reduced cutting speeds to prevent catastrophic [[tool-wear-diagnosis]] failures.

Hardness Scale Conversions

Primary Conversion Chart

HRC HRB BHN (3000kg) Vickers (HV) Approx. Tensile (ksi) Machinability Notes
68 - 739 940 - CBN/Ceramic only
65 - 688 832 - Hard turning territory
60 - 595 697 - Carbide insert limit
55 - 495 595 - Hardened steels
50 - 415 513 - Heat treated threshold
45 - 352 446 229 Quenched & tempered
40 - 302 392 196 Medium carbon steels
35 - 262 345 170 Normalized steels
30 - 229 302 150 Mild steels
25 100 202 266 134 Annealed condition
20 97 179 238 120 Soft steels

Material-Specific Hardness Ranges

[[4140-steel]]:

  • Annealed: 15-20 HRC (180-220 BHN)
  • Normalized: 25-30 HRC (240-280 BHN)
  • Heat treated: 35-45 HRC (320-430 BHN)

[[d2-tool-steel]]:

  • Annealed: 20-25 HRC (210-250 BHN)
  • Hardened: 58-62 HRC (650-720 BHN)

[[304-stainless]]:

  • Annealed: 15-25 HRC (150-200 BHN)
  • Work hardened surface: 35-45 HRC

[[cast-iron]]:

  • Gray iron: 20-30 HRC (180-260 BHN)
  • Ductile iron: 25-35 HRC (200-300 BHN)

Machining Implications by Hardness

Soft Materials (Under 25 HRC)

Cutting speeds: Higher SFM possible but chip formation problematic

  • Issues: Built-up edge, poor [[surface-finish-problems]], work hardening
  • Solutions: Sharp tools, positive rake angles, consistent feed rates
  • Tooling: HSS often superior to carbide for chip breaking

Medium Hardness (25-40 HRC)

Optimal machining range for most operations

  • Cutting speeds: Standard manufacturer recommendations apply
  • Tooling: Standard carbide grades perform well
  • Operations: All conventional machining methods viable

Hard Materials (40-55 HRC)

Requires specialized approach

  • Cutting speeds: Reduce SFM by 30-50% from annealed recommendations
  • Feeds: Maintain chip load to prevent [[work-hardening]]
  • Tooling: Tougher carbide grades, consider ceramic for continuous cuts

Very Hard Materials (55+ HRC)

Hard turning/milling territory

  • Cutting speeds: 50-200 SFM typical range
  • Tooling: CBN, ceramic, or specialized carbide only
  • Depth of cut: Must exceed work-hardened zone (0.006" minimum)

Hardness Testing Methods

Rockwell Testing

Most common in machine shops

  • HRC Scale: Diamond cone, 150kg load, for hardened steels
  • HRB Scale: 1/16" steel ball, 100kg load, for softer materials
  • Portable testers: ±2 HRC accuracy typical

Brinell Testing

Best for castings and forgings

  • Method: 10mm steel ball, 3000kg load
  • Advantage: Large impression averages material variations
  • Range: 100-650 BHN covers most machined materials

Vickers Testing

Most accurate but slowest

  • Method: Diamond pyramid indenter
  • Advantage: Single scale covers all materials
  • Use: Laboratory verification of critical parts

Shop Floor Conversion Rules

Quick Conversions (±10% accuracy)

  • HRC to BHN: BHN ≈ (HRC × 10) + 150 (for HRC 20-50)
  • BHN to HRC: HRC ≈ (BHN - 150) ÷ 10 (approximate)
  • Tensile strength: ksi ≈ BHN ÷ 3.3 (rough estimate)

Field Testing Methods

File test (traditional but inaccurate):

  • New file cuts: Under 60 HRC
  • File skates: Over 62 HRC
  • File bites slightly: 60-62 HRC range

Spark test (steel only):

  • More sparks = higher carbon content (not hardness)
  • Useful for material identification, not hardness measurement

Common Problems

Inconsistent Hardness Readings

Causes:

  • Surface decarburization after heat treatment
  • Work-hardened surface from previous machining
  • Improper test surface preparation
  • Scale thickness variations on hot-rolled materials

Solutions:

  • Test on freshly machined surface
  • Multiple readings across part
  • Consider core hardness vs. surface hardness

Heat Treatment Verification

Problem: Parts not meeting hardness specifications after heat treatment

  • Check: Uniformity across part (especially thick sections)
  • Verify: Proper quench medium and cooling rate
  • Test: Multiple locations, especially stress concentration areas

Machining Parameter Selection

Problem: Generic hardness-to-speed charts giving poor results

  • Reality: [[inconel-718]] at 35 HRC machines differently than [[4140-steel]] at 35 HRC
  • Solution: Use material-specific data, not just hardness number
  • Consider: Work hardening tendency, thermal conductivity, chemical composition

Advanced Applications

Hard Turning (50+ HRC)

Typical parameters:

  • Speeds: 200-400 SFM with CBN tools
  • Feeds: 0.004-0.012 IPR depending on nose radius
  • Depth: 0.020-0.100" per pass
  • Coolant: Flood or high-pressure preferred

[[thread-milling]] Hard Materials (60+ HRC)

Example from field: M1 tool steel at 64 HRC

  • Tool: TiAlN coated carbide thread mill
  • Speed: 150-200 SFM maximum
  • Feed: 0.0005-0.001 IPT
  • Method: Multiple spring passes, flood coolant critical
  • [[insert-selection-guide]] — Carbide grade selection by material hardness
  • [[work-hardening]] — How cutting parameters affect surface hardness
  • [[tool-wear-diagnosis]] — Recognizing hardness-related tool failures
  • [[4140-steel]] — Common heat treatable steel hardness ranges
  • [[d2-tool-steel]] — Tool steel hardness and machining considerations
  • [[surface-finish-grades]] — Hardness effects on achievable surface finish