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The Rockwell Hardness Calculator determines the Rockwell hardness number from the permanent depth increment left after applying and removing a major load on a material surface. Rockwell hardness testing is one of the most widely used methods for measuring material resistance to indentation, governed by standards ASTM E18 and ISO 6508.
The test operates in three stages: apply a preliminary (minor) load to seat the indenter, apply the major (total) load that drives the indenter deeper, then remove the major load while maintaining the minor load. The permanent increase in depth of penetration $$d$$ (in mm) resulting from the major load is measured and converted to a hardness number. For the HRC scale (diamond cone indenter, 150 kgf total load): $$\text{HRC} = 100 - \frac{d}{0.002}$$ For the HRB scale (1/16" steel ball indenter, 100 kgf total load): $$\text{HRB} = 130 - \frac{d}{0.002}$$ where $$d$$ is measured in millimeters and each unit of $$d/0.002$$ represents 0.002 mm of penetration depth. The Rockwell method's advantage is its speed, simplicity, and direct reading — no optical measurement of indent size is needed.
HRC is used for hardened steels, tool steels, and hard alloys (typical range 20–70 HRC), while HRB covers softer metals like brass, soft steel, and aluminum alloys (typical range 0–100 HRB). Understanding the correct scale selection and depth measurement is essential for quality control in manufacturing, heat treatment verification, and material acceptance testing.
The Rockwell hardness test measures the net increase in indentation depth under specific loading conditions. The procedure follows these steps:
The hardness number is then calculated from the depth increment. For scales using a diamond cone (Brale) indenter: $$HR = 100 - \frac{d}{0.002}$$ For scales using a ball indenter: $$HR = 130 - \frac{d}{0.002}$$ The constant 0.002 mm represents one unit of Rockwell hardness depth. A smaller $$d$$ (less penetration) yields a higher hardness number, reflecting greater resistance to deformation.
Key considerations for accurate testing include: minimum specimen thickness (at least 10 times the indentation depth), minimum spacing between indentations (at least 3 times the indent diameter), and a flat, smooth test surface. The test piece must be rigidly supported on an anvil appropriate for the specimen geometry.
A higher Rockwell number indicates a harder material with greater resistance to permanent deformation. For HRC: values of 20–30 represent medium-hard steels, 55–65 represent hardened tool steels, and values above 65 indicate extremely hard materials. For HRB: values of 60–80 cover annealed low-carbon steels, while values above 100 suggest the material should be tested on a harder scale (HRC). If the depth increment is very small (high hardness), ensure the correct scale is selected — testing a very hard material on the HRB scale can damage the ball indenter.
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A depth increment of 0.08 mm on the HRC scale gives h = 0.08/0.002 = 40, so HRC = 100 − 40 = 60. This is typical for hardened tool steel after heat treatment.
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A depth increment of 0.14 mm on the HRB scale gives h = 70, so HRB = 130 − 70 = 60. This is consistent with annealed brass or soft copper alloys.
The Rockwell hardness test measures a material's resistance to indentation by pressing an indenter into the surface under a specified load and measuring the permanent depth of penetration. The result is expressed as a dimensionless number on a specific scale (e.g., HRC, HRB). It is standardized by ASTM E18 and ISO 6508.
Use HRC (diamond cone, 150 kgf) for hard materials: hardened steels, tool steels, case-hardened surfaces, and hard alloys (typically 20–70 HRC). Use HRB (1/16" ball, 100 kgf) for softer materials: annealed steel, brass, copper, aluminum alloys (typically 0–100 HRB). If HRB exceeds 100, switch to HRC.
The depth increment $$d$$ is the net increase in indentation depth after the major load is applied and removed, measured in millimeters from the initial datum set by the preliminary load. It represents permanent plastic deformation only, since elastic recovery occurs when the major load is removed.
The preliminary load (typically 10 kgf) seats the indenter on the specimen surface, pushing through any surface roughness, scale, or oxide layer. This establishes a consistent reference datum from which the permanent depth increment is measured, improving test repeatability.
The specimen thickness should be at least 10 times the permanent depth of indentation. For HRC at a typical depth of 0.1 mm, minimum thickness is about 1 mm. For HRB with deeper penetration, thicker specimens are needed. No deformation should be visible on the back surface of the specimen.
For carbon and alloy steels, approximate conversions exist between HRC and ultimate tensile strength (UTS). For example, 25 HRC ≈ 840 MPa, 40 HRC ≈ 1250 MPa, 60 HRC ≈ 2070 MPa. These are empirical correlations (ASTM E140) and apply only to specific material groups — they should not be used for non-ferrous metals.
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