Hardness testing measures how strongly a material resists permanent surface indentation. Engineers use it because hardness is quick to measure, requires only a small mark, and often gives a useful estimate of wear resistance and strength. In metals, a harder surface usually means greater resistance to plastic deformation, which matters for gears, tools, bearings, fasteners, and structural parts.
Key Facts
- Hardness is resistance to permanent indentation, scratching, or localized plastic deformation.
- Rockwell hardness uses depth of penetration under a minor load and a major load, then reports values such as HRC or HRB.
- Brinell hardness number is HB = 2F / (pi D(D - sqrt(D^2 - d^2))), where F is load, D is ball diameter, and d is indentation diameter.
- Vickers hardness number is HV = 1.854 F / d^2, where F is load and d is the average diagonal length of the square indentation.
- For many steels, ultimate tensile strength can be estimated by UTS in MPa ≈ 3.45 HB, but the correlation depends on material and heat treatment.
- A smaller indentation under the same load usually indicates a harder material.
Vocabulary
- Indentation
- An indentation is the permanent mark left when a hard indenter is pressed into a material surface.
- Rockwell hardness
- Rockwell hardness is a hardness value based mainly on the depth of penetration made by an indenter under specified loads.
- Brinell hardness
- Brinell hardness is a hardness value calculated from the size of a circular impression made by a hard ball under a known load.
- Vickers hardness
- Vickers hardness is a hardness value calculated from the diagonal length of a square impression made by a diamond pyramid indenter.
- Plastic deformation
- Plastic deformation is a permanent change in shape that remains after the applied force is removed.
Common Mistakes to Avoid
- Using the wrong hardness scale, such as comparing HRC directly to HB without conversion, is wrong because each method uses different indenters, loads, and measurement principles.
- Placing indents too close together is wrong because the deformed zone around one indent can harden or weaken the nearby material and distort the next reading.
- Testing on a rough, curved, or dirty surface is wrong because the indenter may not contact the material evenly, causing inaccurate depth or size measurements.
- Treating hardness as exactly equal to strength is wrong because hardness and tensile strength are correlated only approximately and depend on alloy, microstructure, and heat treatment.
Practice Questions
- 1 A Vickers test uses a 30 kgf load and produces an average diagonal length of 0.50 mm. Use HV = 1.854 F / d^2 to calculate the Vickers hardness.
- 2 A steel sample has a Brinell hardness of 220 HB. Estimate its ultimate tensile strength using UTS in MPa ≈ 3.45 HB.
- 3 A technician must test a thin hardened coating on a softer steel base. Explain why a shallow indentation method such as micro Vickers may be better than a large Brinell indentation.