The Rockwell Hardness Scale: HRC, HRB, and Conversion Charts
Representative Infinita Engineering Visual explaining the four-step workflow for Rockwell Hardness Scale.What Is the Rockwell Hardness Scale?
The Rockwell hardness scale, governed by ASTM E18 (metallic materials) and ISO 6508, defines 30 distinct scales — designated by letters such as A, B, C, D, E, F, G, H, K, L, M, P, R, S, V — each using a specific indenter type and load combination suited to a particular hardness range and material class. Selecting the correct scale for the material being tested is essential; using the wrong scale produces invalid, non-repeatable results.
Most Common Scales
- HRC: 120-degree diamond spheroconical (Brale) indenter, 150 kgf major load — the standard scale for hardened and heat-treated steels, tool steels, and case-hardened components, covering roughly 20-70 HRC
- HRB: 1/16-inch steel or tungsten-carbide ball indenter, 100 kgf major load — used for softer metals: annealed steel, copper alloys, aluminium alloys, and malleable iron, covering roughly 0-100 HRB
- HRA: diamond indenter at a lower 60 kgf load — used for very hard, thin materials or thin case-hardened surfaces
Superficial Rockwell Scales
Superficial scales (e.g., HR15N, HR30T) use reduced preliminary (3 kgf) and total forces (15, 30, or 45 kgf), making them more sensitive to surface condition but suitable for thin materials, case-hardened surfaces, and small parts where a standard Rockwell indentation would be too large relative to the part.
Scale Selection Guidance
Ball indenters are generally used for softer materials; diamond indenters for harder materials, since a diamond indenter would simply destroy a soft material and a ball indenter would be too easily driven into a hard one to produce a meaningful reading. As a rule of thumb, specimen thickness should exceed roughly ten times the depth of indentation, with no deformation appearing on the opposite face.
Also Read – Polymer Hardness Testing: Common Uses, Methods & Scale Selection
Industry Specifications Referencing Rockwell Scales
- ASTM E18: standard for metallic materials, defining all 30 Rockwell and superficial Rockwell scales
- ASTM D785: Rockwell hardness of plastics (a related but distinct scale set)
- ASTM E140: hardness conversion tables between Rockwell, Brinell, Vickers, and approximate tensile strength
Conclusion
A Rockwell hardness number is meaningless without its scale designation — “64 HRC” and “64 HRB” describe wildly different material hardness, which is why the scale letter is never optional when reporting or specifying a Rockwell result.
What materials are tested using the HRC scale? The HRC scale is commonly used for hardened steels, tool steels, heat-treated alloys, and other relatively hard metals. It uses a diamond cone indenter.
What materials are tested using the HRB scale? The HRB scale is generally used for softer metals such as copper alloys, aluminium alloys, mild steel, and some annealed materials. It typically uses a ball indenter.
How is a Rockwell hardness test performed? A minor load is first applied to establish a reference position. A larger major load is then applied and removed, and the permanent indentation depth is converted into a Rockwell hardness value.
Can HRC and HRB values be directly compared? No. HRC and HRB use different indenters and test forces, so the values are not directly equivalent. A hardness conversion chart may provide an approximate comparison.
What factors can affect Rockwell hardness results? Surface finish, specimen thickness, curvature, vibration, spacing between indentations, incorrect scale selection, and instrument calibration can influence test accuracy.
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