In everyday language, the words "HARDNESS" and "TOUGHNESS" are used interchangeably; however when used as metallurgical terms to describe material properties, the meanings are different and in some cases rnay be opposite.


TOUGHNESS may be thought of as the resistance to failure under sudden or impact loading, while HARDNESS is evident as resistance to wear under abrasive conditions or resistance to deformation under tensile, compressive or bending stresses.



Tool steels sacrifice TOUGHNESS in favor of HARDNESS, while machinery type steels (medium carbon or low alloy) aim for a balance of properties.



HARDNESS is determined by applying pressure to the steel surface using a standard indenter, and the most widely used hardness tests are the Rockwell and the Brinell.



Several combinations of indenter geometry and loading give a versatility to the Rockwell test which makes it suitable for testing a wide variety of metals. The most commonly used scale, the "c" scale, uses a 120^o angle diamond indenter with a load of 150 kilograms, and the depth of penetration is measured on a dial gauge, or with newer machines a digital read out. Most Rockwell testers are bench mounted and their use is restricted to relatively small parts perhaps up to 50lbs. The indentation is small, generally .003"-.008”.


In the most common Brinell test a hardened steel ball of 10mm. diameter is used with a force of 3,OOOlbs wt. The indentation might be as deep as 1.5mm. The Brinell test is used on larger samples, particularly castings where hardness may vary within a small area.


The Rockwell test may be used to check the surface hardness on a case hardened component, however the scale must be chosen with care. The commonly used "c" scale will not be suitable for case depths of less than .020" because a thin, brittle case will deform into the underlying softer core.(in much the same way as ice over mud)