The hardness of a material may be defined in the following ways:
(i) the ability to scratch other materials
(ii) the ability to resist scratching
(iii) the ability to resist plastic spreading under indentation
(iv) the ability to resist elastic deformation under indentation
(v) the ability to resist deformation by rolling
Hardness tests are based on pressing a hard substance, such as a diamond or a steel sphere having known dimensions, into the material under test. The hardness can be determined from the size of indentation made for a known load. The three principal hardness tests are:
(a) the Brinell test,
(b) the Vickers test, and
(c) the Rockwell test.
The Brinell Test
In a standard Brinell test, a hardened steel ball having a diameter of 10 mm is squeezed into the material under a load of 3000 kg. The diameter of the indentation produced is measured under a microscope. The Brinell hardness number, HB , is given by:
Variations on the standard test include smaller loads used for soft materials, balls of different diameters (usually restricted to 1, 2 and 5 mm) and the steel ball being replaced by one made of tungsten carbide for use with very hard materials. Values of Brinell hardness number vary from about 900 for very hard materials having an equivalent tensile strength for steel of 3000 MPa,
down to about 100 for materials having an equivalent tensile strength for steel of about 350 MPa.
The following precautions are required when carrying out a Brinell test.
(i) The material must be sufficiently wide and thick. The impression must have its centre not less than two and a half times its diameter from any edge. The thickness must be at least ten times the depth of the impression, this depth being given by: Brinell hardness number.
(ii) The surface of the material should, if possible, be ground flat and polished.
(iii) The load should be held for 5 s.
(iv) Two diameters of the impression at right angles should be read and their mean used in the calculation.
(v) When stating the result of the test the ball number and load should be stated; for example, H10/3000 D 410
Special machines are made but a Brinell test can be carried out on most universal testing machines.
For materials of the same quality and for families of materials an approx- imate direct proportional relationship seems to exist between tensile strength and Brinell hardness number. For example, a nickel-chrome steel which is hardened and then tempered to various temperatures has tensile strengths
varying from 1900 MPa to 1070 MPa as the Brinell hardness number varies
from 530 to 300. A constant of proportionality k for: tensile strength D (k ð hardness) in this case is 3.57 for all tempering temperatures. Similarly, for a family of carbon steels, the tensile strength varies from 380 MPa to 790 MPa as the carbon content increases. The Brinell hardness number varies from 115 to 230 over the same range of carbon values and the constant of proportionality in this case is 3.35. Because of the general approximate relationship between tensile strength and hardness, tables exist relating these quantities, the tables usually based on a constant of proportionality of about 3.35.
In a Vickers diamond pyramid hardness test, a square-based diamond pyramid is pressed into the material under test. The angle between opposite faces of the diamond is 136° and the load applied is one of the values 5, 10, 30, 50 or 120 kg, depending on the hardness of the material. The Vickers diamond hardness number, HV , is given by:
where F is the load in kilograms and d is the length of the diagonal of the square of indentation in millimetres.
The Rockwell Test
The Rockwell hardness test is mainly used for rapid routine testing of finished material, the hardness number being indicated directly on a dial. The value of hardness is based directly on the depth of indentation of either a steel ball or a cone shaped diamond with a spherically rounded tip, called a ‘brale’. Whether the steel ball or brale is selected for use depends on the hardness of the material under test, the steel ball being used for materials having a hardness up to that of medium carbon steels.
Several different scales are shown on the dial, and can include Rockwell A to H scales together with Rockwell K, N and T scales. Examples of the scale used are:
Scale A: using a brale and a 60 kg load Scale B: using a brale and a 150 kg load
Scale C: using a 1/16th inch steel ball and 100 kg load, and so on.
The big advantage of the Rockwell test over Brinell and Vickers tests is the speed with which it can be made. As it is also independent of surface condition it is well suited to production line testing. British Standards, however, require hardness numbers to be based on the surface area of any indentation.
Other Hardness Tests
The Brinell, Vickers and Rockwell tests are examples of static hardness tests. Another example is the Firth Hardometer test, which is very similar to the Vickers test.
Examples of dynamic hardness tests are those using the Herbert pendulum Hardness Tester and the Shore Scleroscope. The former uses an arched rocker resting on a steel or diamond pivot; hardness can be indicated by the time taken for ten swings or by the difference between an initial angular displacement and the first swing. The Scleroscope is a portable apparatus in which a diamond-tipped hammer falls on to the material under test. The height of the rebound gives the hardness number.
Other Non-destructive Tests
Flaws inside a casting can be revealed by X-ray methods. Surface flaws can be revealed by electro-magnetic methods and by those using ultra-violet light. The former are applicable only to ferrous metals but the latter can be used for other metals and for other materials, such as plastics and ceramics.
To give an indication of the toughness of a material, that is, the energy needed to fracture it, impact tests are carried out. Two such tests are the Izod test,
principally used in Great Britain, and the Charpy test that is widely used in other parts of Europe.
In an Izod test, a square test piece of side 10 mm and having a vee-notch of angle 45° machined along one side, is clamped firmly in a vice in the base of the Izod test machine. A heavy pendulum swings down to strike the specimen and fractures it. The difference between the release angle of the pendulum measured to the vertical and the overswing angle after fracturing the specimen is proportional to the energy expended in fracturing the specimen, and can be read from a scale on the testing machine. An Izod test is basically an acceptance test, that is, the value of impact energy absorbed is either acceptable or is not acceptable. The results of an Izod test cannot be used to determine impact strength under other conditions.
A Charpy test is similar to an Izod test, the only difference being the method of mounting the test specimen and a capability of varying the mass of the pendulum. In the Izod test, the specimen is gripped at one end and is supported as a cantilever, compared with the specimen being supported at each end as a beam in the Charpy test. One other difference is that the notch is at the centre of the supported beam and faces away from the striker.