Rockwell C to Vickers

How Do You Convert HRC to Vickers?

The Rockwell C (HRC) and Vickers (HV) hardness scales are the two most widely used hardness tests in metallurgy and manufacturing. Converting between them requires an approximate relationship because the tests use fundamentally different methods: Rockwell C measures the depth of penetration of a diamond cone under a specific load, while Vickers measures the size of an indentation made by a diamond pyramid. The most commonly used approximation is HV = 0.171 times HRC squared plus 1.65 times HRC plus 103, which is accurate within about 3% for the typical HRC range of 20-65. ASTM E140 provides official conversion tables based on extensive experimental data, and these tables are the authoritative reference for critical applications. In practice, machinists, heat treaters, metallurgists, and quality control engineers convert between these scales daily — a part might be specified at 58-62 HRC on an engineering drawing, but tested on a Vickers machine that reads 653-746 HV. Understanding this conversion lets you verify that hardened steel parts meet specification regardless of which testing machine is available.

How to Convert Rockwell C to Vickers

1. Start with your Rockwell C (HRC) value.
2. Apply the polynomial approximation: HV = 0.171 x HRC² + 1.65 x HRC + 103.
3. For example, 50 HRC: HV = 0.171 x 2,500 + 1.65 x 50 + 103 = 427.5 + 82.5 + 103 = 613 HV.
4. For critical applications, refer to the ASTM E140 conversion table rather than a formula.
5. Common reference points: 20 HRC = about 238 HV, 40 HRC = about 392 HV, 60 HRC = about 697 HV.

Real-World Examples

History of Rockwell C and Vickers

The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd, a British engineering firm. They designed it as an improvement over the Brinell test, using a diamond pyramid indenter that could test any material regardless of hardness. The Rockwell test was invented in 1914 by Hugh and Stanley Rockwell in Connecticut, originally for testing ball bearings. The Rockwell test became dominant in American manufacturing because of its speed — it gives a direct reading without measuring an indentation under a microscope. The Vickers test requires optical measurement but provides more consistent results across a wide hardness range and can test very thin or small samples (microhardness). The need for conversion tables between these scales led ASTM to publish E140, first issued in 1951 and periodically updated. The conversions are empirical — derived from testing thousands of specimens on multiple machines — because there is no theoretical relationship between a depth-based measurement (Rockwell) and an area-based measurement (Vickers).

Common Mistakes to Avoid

• Treating HRC-to-HV conversion as a simple linear relationship. The relationship is nonlinear — the HV equivalent increases more steeply at higher HRC values. Using a linear approximation like "HV = HRC x 10" gives wildly wrong results.
• Applying the conversion outside its valid range. HRC is only meaningful from about 20 to 70. Below 20 HRC, the Rockwell B scale (HRB) is used instead. Extrapolating the HRC formula below 20 gives meaningless results.
• Assuming the conversion is exact. ASTM E140 conversions have an inherent uncertainty of 2-5% because different hardness tests measure fundamentally different material responses. Two identically hardened specimens may show slightly different HRC and HV readings.
• Using a steel conversion on carbide, coating, or non-ferrous hardness data. ASTM E140 tables are material-specific, so a conversion that is reasonable for hardened steel may be misleading for another alloy system.

Frequently Asked Questions