Rockwell B to Rockwell C

How Do You Convert Rockwell B to Rockwell C?

Rockwell B (HRB) and Rockwell C (HRC) are two scales on the same testing machine that use different indenters and loads. HRB uses a 1/16-inch steel ball at 100 kgf, measuring softer materials (annealed steels, brass, aluminum). HRC uses a diamond cone at 150 kgf, measuring hardened steels. The scales overlap in a narrow band around 20-25 HRC / 95-100 HRB, but direct conversion across the full range is not meaningful because each scale is designed for a different hardness regime. Where conversion is needed — typically in the overlap zone when a part is borderline between the two scales — the relationship is approximately HRC = (HRB - 104) / 1.76, valid only for HRB values above about 90. Below 90 HRB, the material is too soft for HRC, and the conversion produces meaningless numbers. This conversion arises most often in heat treatment, where incoming material might test at 95 HRB (soft) and the same material after hardening tests at 55 HRC. It is mainly a transition-zone decision rather than a full-range conversion tool.

How to Convert Rockwell B to Rockwell C

1. Determine if your HRB value falls in the valid overlap range (approximately 90-100 HRB).
2. For HRB in the 90-100 range, convert to an approximate HV first: HV = 1.76 x HRB - 21.
3. Then convert HV to HRC using the HV-to-HRC formula or ASTM E140 table.
4. For example, 95 HRB: HV = 1.76 x 95 - 21 = 146.2 HV. This is below the HRC range (HRC requires about 226+ HV).
5. If HRB is below 90, do not attempt to convert to HRC — the material is too soft for the HRC scale.

Real-World Examples

History of Rockwell B and Rockwell C

Stanley Rockwell designed multiple scales for his hardness tester, each optimized for a different hardness range. The B scale (ball indenter, 100 kgf) was intended for soft metals, and the C scale (cone indenter, 150 kgf) for hard metals. There are actually 30 different Rockwell scales (A through V, then superficial scales), but B and C dominate industrial use. The B-to-C overlap zone was an inevitable consequence of designing discrete scales for a continuous hardness spectrum. ASTM E18 (the Rockwell test standard) recommends against using HRB above 100 or HRC below 20, precisely because both scales lose accuracy in those extremes.

Common Mistakes to Avoid

• Converting low HRB values (below 80) to HRC. This is invalid — the material is far too soft for the Rockwell C scale, and any calculated HRC value would be meaningless.
• Assuming HRB and HRC can be compared by subtracting a fixed offset. The scales are not linearly related — they use different indenters (ball vs. cone) and different loads (100 vs. 150 kgf).
• Testing in the overlap zone (95-100 HRB or 20-25 HRC) and expecting precise results. Both scales lose accuracy at their extremes. If a part consistently tests near these limits, consider using the Vickers or Brinell scale for a more reliable measurement.
• Forgetting that the indenter type changes between scales. Ball indentation on soft material and diamond penetration on harder material do not respond the same way, which is why the conversion only works in a narrow overlap band.

Frequently Asked Questions