Hardness Testing Methods Compared

## What Hardness Measures Hardness is the resistance of a material to permanent indentation. All hardness tests follow the same basic principle: press an indenter of defined geometry into the material surface under a controlled load and measure the size or depth of the residual indentation. Harder materials produce smaller or shallower indentations. Hardness correlates roughly with yield strength and tensile strength, which is why hardness is used as a production acceptance test and field inspection method where a full tensile test is impractical. For steels in the range 150–400 HB, the approximate relationship is: **UTS (MPa) ≈ 3.45 × HB** This is an approximation valid for many steels but does not apply reliably to aluminum alloys, stainless steels, or cold-worked metals. ## Brinell Hardness Test (HB or HBW) The Brinell test presses a 10 mm tungsten carbide ball into the surface under a standard load (typically 3000 kgf for steel, 500 kgf for aluminum) for 10–15 seconds, then measures the diameter of the indentation with a calibrated optical measuring device. **HBW = 2F / (πD(D - √(D² - d²)))** where F = force in kgf, D = ball diameter in mm, d = indentation diameter in mm. Strengths of the Brinell test: - Large indentation area averages over microstructural heterogeneities (inclusions, graphite flakes in cast iron, etc.) - Applicable to coarse-grained materials and castings - Can be used on rough surfaces that would give unreliable Rockwell readings Limitations: - Minimum test piece thickness: approximately 10× indentation depth; generally not suitable for thin sections or surface layers - Soft materials: the ball may sink excessively at high loads (the valid range is roughly 16–650 HBW with a 10 mm ball and 3000 kgf load) - Not suitable for case-depth measurement or small parts ## Rockwell Hardness Test (HR) The Rockwell test measures indenter penetration depth under a preliminary minor load (10 kgf), then under a major load, then back to the minor load. The Rockwell hardness number is directly read from the machine, making it the fastest hardness test for production use. The choice of indenter and major load defines the Rockwell scale: | Scale | Indenter | Major Load | Typical Application | |-------|----------|------------|---------------------| | HRC | 120° diamond brale | 150 kgf | Hardened steel (20–67 HRC) | | HRB | 1/16" ball | 100 kgf | Annealed steel, aluminum, brass (0–100 HRB) | | HRA | 120° diamond brale | 60 kgf | Cemented carbides, thin hard layers | | HR30N | Diamond | 30 kgf | Superficial hardness of thin case layers | HRC is by far the most commonly used scale for hardened steel, tool steels, and the specification of heat treatment results. A hardened AISI D2 die at 60 HRC and a soft annealed 1045 at 85 HRB would both be measured with the Rockwell machine but on different scales. Limitations: - Results are affected by surface roughness and flatness; curved surfaces require correction factors - Not reliable on inhomogeneous microstructures or coatings (the indentation may span different phases) - Minimum specimen thickness: approximately 10× indentation depth (a rough guide is t ≥ 0.5 mm for HRC) ## Vickers Hardness Test (HV) The Vickers test uses a square-based diamond pyramid indenter with a 136° face angle. The diagonal length of the square indentation is measured with an optical microscope and converted to hardness: **HV = 1.854 × F / d²** where F is in kgf and d is the mean diagonal in mm. The Vickers scale is continuous and consistent across the full hardness range of engineering materials (5 to 3000 HV), unlike Rockwell which requires scale changes. It is the preferred scale for: - Research and metallographic work at low loads (microhardness, 0.01–1 kgf) - Measuring hardness of individual phases, weld microstructures, and heat-affected zones - Thin coatings and case-hardened layers - Highly alloyed and exotic materials - International specifications where a universal scale is preferred Microhardness testing (Vickers at 0.01–1 kgf load, HV 0.01 to HV 1) is used to map case depth in carburized or nitrided parts by traversing from the surface through the case to the core and plotting the hardness profile. ## Hardness Conversion Tables Hardness scales cannot be converted by an exact formula because different tests measure different volumes and different aspects of resistance to indentation. Conversion tables (ASTM E140) provide approximate equivalents based on empirical data from thousands of tests on normalized steel: | HRC | HBW | HV | Approx. UTS (MPa) | |-----|-----|----|-----------------------| | 20 | 226 | 238 | 760 | | 30 | 286 | 302 | 985 | | 40 | 371 | 392 | 1275 | | 50 | 481 | 513 | 1655 | | 60 | 628 | 682 | — (too brittle) | These conversions apply to steel; they should not be used for aluminum, copper, titanium, or other metals.