Material hardness affects machinability, wear resistance, and part function. Different industries and applications use different hardness scales, making conversions essential. This guide covers the major hardness scales, their relationships, and when to use each.
Major Hardness Scales
Rockwell Hardness (HR)
The most common industrial scale. Measures depth of penetration under load. Different scales for different materials:
- HRC (Rockwell C): Diamond cone indenter, 150 kg load. Used for hardened steel (20-70 HRC range)
- HRB (Rockwell B): 1/16" ball, 100 kg load. Used for softer steel and non-ferrous metals (0-100 HRB)
- HRA: Diamond cone, 60 kg load. Thin materials and case-hardened surfaces
Brinell Hardness (HB)
Uses a 10mm steel or carbide ball with loads from 500-3000 kg. Measures diameter of the impression. Standard load is 3000 kg for steel (HBW 10/3000). Good for rough or heterogeneous surfaces due to larger indentation.
Vickers Hardness (HV)
Uses a diamond pyramid indenter. Measures diagonal of the impression. Works across the entire hardness range with one scale. Common in metallurgical labs and for micro-hardness testing.
Convert Between Hardness Scales
Instantly convert Rockwell, Brinell, Vickers, and tensile strength values for any steel.
Quick Conversion Reference
| HRC | HRB | HB (3000kg) | HV | Tensile (ksi) |
|---|---|---|---|---|
| 20 | - | 226 | 238 | 110 |
| 25 | - | 253 | 266 | 122 |
| 30 | - | 286 | 302 | 138 |
| 35 | - | 327 | 345 | 158 |
| 40 | - | 371 | 392 | 180 |
| 45 | - | 421 | 446 | 205 |
| 50 | - | 481 | 513 | 234 |
| 55 | - | 545 | 595 | 266 |
| 60 | - | 613 | 697 | 302 |
When to Use Each Scale
Use HRC When:
- Testing hardened steel (heat-treated parts)
- Quick shop-floor testing is needed
- Hardness is above ~20 HRC
Use HRB When:
- Testing annealed or normalized steel
- Testing brass, bronze, or aluminum alloys
- Material is too soft for HRC
Use Brinell When:
- Testing castings or forgings with rough surfaces
- Averaging hardness across heterogeneous microstructures
- Specifications call for Brinell values
Use Vickers When:
- Laboratory or metallurgical work
- Microhardness testing (case depth, thin coatings)
- Need one scale across very soft to very hard materials
Common Material Hardness Ranges
- Annealed mild steel: 70-85 HRB (120-150 HB)
- Cold-rolled steel: 85-95 HRB (180-220 HB)
- Quench and tempered 4140: 28-32 HRC (depending on temper)
- Through-hardened tool steel: 58-62 HRC
- Case-hardened surfaces: 58-65 HRC
- Carbide: 90+ HRA (off the HRC scale)
Conversion Limitations
Hardness conversions are approximate because:
- Each test measures something slightly different
- Conversions are empirical, based on carbon/alloy steels
- Different materials may not follow standard conversion curves
- Surface condition affects test accuracy
For critical applications, specify the exact scale and test method required rather than relying on conversions.
Testing Tips
- Surface must be clean, smooth, and perpendicular to the indenter
- Minimum thickness: 10× indentation depth for Rockwell
- Space indentations at least 3× diameter apart
- Take multiple readings and average
- Calibrate testers regularly with certified blocks
Understanding hardness scales helps you communicate specifications accurately, verify heat treatment results, and troubleshoot material performance issues. When in doubt, test in the scale specified by your drawing or customer.
