Hardness testing is one of the fastest and most widely used quality control methods in manufacturing and maintenance. A hardness test takes seconds, requires minimal surface preparation, and produces a number that correlates directly to tensile strength, wear resistance, and machinability. In plant environments, portable hardness testers verify heat treatment results, assess material condition after service, and confirm incoming material meets specifications.
There is no single universal hardness scale. Different test methods use different indenters, loads, and measurement techniques, each suited to particular materials and applications. The three dominant methods in industrial practice are Rockwell, Brinell, and Vickers. Understanding when to use each method and how to convert between scales is essential knowledge for machinists, inspectors, and plant engineers.
This guide covers the principles behind each major test method, the applicable standards, conversion between scales, and practical guidance for selecting the right test for common industrial materials.
Rockwell Hardness Testing
The Rockwell test measures the depth of indentation under a specific load, which makes it fast and easy to read — the result appears directly on the dial or digital display without optical measurement. The test applies a minor load (10 kgf) to seat the indenter, then a major load (60, 100, or 150 kgf depending on scale), then releases back to the minor load. The difference in penetration depth between the minor load before and after the major load determines the hardness number.
The most common Rockwell scales for steel are HRC (C scale) and HRB (B scale). HRC uses a diamond cone (Brale) indenter with a 150 kgf major load and is used for hardened steels, typically reading 20 to 68 HRC. Below 20 HRC, the test becomes insensitive; above 68 HRC, the diamond indenter risks damage. HRB uses a 1/16-inch steel ball indenter with a 100 kgf major load and covers softer steels, brass, and aluminum in the range of 0 to 100 HRB.
Other Rockwell scales include HRA (tungsten carbide and thin case-hardened surfaces), HR15N and HR30N (superficial scales for thin materials and coatings), and HRF (soft metals with a 1/16-inch ball and 60 kgf load). The scale letter must always be reported with the number — a reading of "58" is meaningless without specifying "58 HRC" or "58 HRB."
The Rockwell test standard is ASTM E18. Key requirements include minimum specimen thickness (at least 10 times the indentation depth), flat surface perpendicular to the indenter, and minimum spacing between indentations (at least 3 indentation diameters apart). Testing on curved surfaces requires correction factors or a fixture that supports the specimen rigidly.
Brinell Hardness Testing
The Brinell test presses a hardened steel or tungsten carbide ball (typically 10 mm diameter) into the test surface under a heavy load (500 to 3000 kgf) for a specified dwell time (10 to 30 seconds). The resulting impression is measured optically with a low-power microscope, and the Brinell hardness number is calculated as: BHN = 2P / (π x D x (D - sqrt(D² - d²))), where P is the applied load in kgf, D is the ball diameter in mm, and d is the measured impression diameter in mm.
The standard load for steel is 3000 kgf with a 10 mm ball (designated HBW 10/3000). For softer metals like aluminum and copper alloys, the load is reduced to 500 kgf (HBW 10/500) to prevent an excessively large impression. The load-to-diameter-squared ratio (P/D²) should be held constant when comparing results across different ball sizes.
Brinell testing is particularly well-suited for castings, forgings, and rough-surfaced materials because the large ball indenter averages out local microstructural variations. A Rockwell test on a gray iron casting may give erratic readings due to graphite flakes near the surface, while the Brinell impression spans enough material to produce consistent, representative results.
The primary disadvantage of Brinell testing is that it requires optical measurement of the impression diameter, which introduces operator subjectivity. Two operators measuring the same impression may disagree by 0.05 to 0.10 mm, which can change the BHN result by 10 to 20 points on hard materials. Proper lighting, calibrated comparison microscopes, and averaging two perpendicular diameter measurements improve repeatability.
Vickers and Knoop Microhardness Testing
The Vickers test uses a square-based diamond pyramid indenter with a 136-degree included angle. The load can range from 1 gf to 120 kgf, making it applicable from microhardness testing of thin coatings to macro testing of structural components. The Vickers hardness number (HV) is calculated from the applied load divided by the surface area of the impression: HV = 1.854 x P / d², where P is load in kgf and d is the mean diagonal length of the impression in mm.
The Vickers scale is continuous and approximately linear, unlike Rockwell which has multiple overlapping scales. This makes Vickers ideal for research and for materials that span a wide hardness range. A single Vickers scale covers everything from annealed aluminum (20-30 HV) through hardened tool steel (700-900 HV) to cemented carbide (1200-2000 HV).
Knoop testing is a variant that uses an elongated diamond indenter (length-to-width ratio of 7:1) at very low loads (typically 10 to 1000 gf). The elongated impression is shallower than a Vickers impression at the same load, making Knoop ideal for testing thin layers, case depths, brittle materials, and individual microstructural phases.
Both Vickers and Knoop require polished specimen surfaces and optical measurement of the impression diagonals, which makes them slower and more labor-intensive than Rockwell testing. They are standard methods in metallurgical laboratories and for quality control of heat-treated surfaces, coatings, and welds.
Converting Between Hardness Scales
Hardness conversions between scales are empirical — they are based on experimental correlation, not mathematical derivation. The most widely used conversion tables are in ASTM E140, which provides conversions for steel between Rockwell C, Rockwell B, Brinell, Vickers, Knoop, and approximate tensile strength. These conversions are reliable for carbon and alloy steels but should not be used for non-ferrous metals, stainless steels, or hardmetals without verifying against material-specific data.
Key reference points for carbon and alloy steel: 60 HRC = 654 HV = 613 BHN (approx) = 324 ksi tensile. 40 HRC = 392 HV = 371 BHN = 180 ksi tensile. 20 HRC = 228 HV = 226 BHN = 100 ksi tensile. These are approximate values; the actual ASTM E140 tables should be used for specification compliance.
The correlation between Brinell hardness and tensile strength for medium carbon and alloy steels follows the approximate relationship: Tensile Strength (PSI) = BHN x 500. This rule of thumb is accurate within about 10 percent for steels in the 120-400 BHN range and is one of the most useful quick checks in plant maintenance. If a shaft reads 300 BHN, you can estimate its tensile strength at approximately 150,000 PSI.
Be cautious with conversions outside the tested range. Converting a Rockwell B reading to HRC, or a Brinell number above 630 to anything else, stretches the correlation data and may produce inaccurate results. When in doubt, test directly on the scale required by the specification rather than converting from a different scale.
Selecting the Right Test Method
Choose the test method based on the material, specimen geometry, required accuracy, and the specification requirements. For production testing of hardened steel parts (gears, shafts, bearings), Rockwell C is the standard — it is fast, requires no optical measurement, and is specified in most heat treatment standards. For incoming material verification on plate, bar stock, and castings, Brinell testing is preferred because the larger impression gives more representative results on heterogeneous materials.
For thin materials (less than 1/4 inch), case-hardened surfaces, coatings, and weld heat-affected zones, Vickers or Knoop microhardness testing is appropriate. The low loads produce small impressions that can map hardness gradients across a weld or through a case depth profile. A typical case depth study uses Knoop indentations at 0.001 to 0.002 inch intervals from the surface inward, measuring the hardness gradient from case to core.
Portable hardness testers (Leeb rebound type) are invaluable for field testing of installed equipment — checking hardness of pressure vessel walls, large castings, weldments, and structural members that cannot be brought to a laboratory. Leeb testers measure the rebound velocity of an impact body and convert to HLD (Leeb D scale), which can then be converted to HRC, HRB, HB, or HV. Accuracy depends on specimen mass, surface condition, and orientation.
When a specification calls for a specific test method, use that method. Converting results from a different test introduces uncertainty and may not be accepted by the customer or inspector. If the spec says "58-62 HRC," test on the Rockwell C scale, not on Vickers and convert.