Simply stated, hardness is the resistance of a material to
permanent indentation. It is important to recognize that hardness
is an empirical test and therefore hardness is not a material
property. This is because there are several different
hardness tests that will each determine a different hardness
value for the same piece of material. Therefore, hardness is
test method dependent and every test result has to have a
label identifying the test method used.
Hardness is, however, used extensively to characterize materials
and to determine if they are suitable for their intended use.
All of the hardness tests described in this section involve the use
of a specifically shaped indenter, significantly harder than the
test sample, that is pressed into the surface of the sample using
a specific force. Either the depth or size of the indent is
measured to determine a hardness value.
Why Use a Hardness Test?
- Easy to perform
- Quick - 1 to 30 seconds
- Relatively inexpensive
- Finished parts can be tested - but not ruined
- Virtually any size and shape can be tested
- Practical QC device - incoming, outgoing
The most common uses for hardness tests is to verify the heat
treatment of a part and to determine if a material has the properties
necessary for its intended use. Establishing a correlation between
the hardness result and the desired material property allows this,
making hardness tests very useful in industrial and R&D applications.
There are five major hardness scales:
Each of these scales involve the use of a specifically shaped
diamond, carbide or hardened steel indenter pressed into the
material with a known force using a defined test procedure.
The hardness values are determined by measuring either the
depth of indenter penetration or the size of the resultant
indent. All of the scales are arranged so that the hardness
values determined increase as the material gets harder.
The hardness values are reported using the proper symbol,
HR, HV, HK, etc. indicating the test scale performed.
Five Determining Factors
The following five factors can be used to determine the correct hardness
test for your application.
- Material - grain size, metal, rubber, etc.
- Approximate Hardness - hardened steel, rubber, etc.
- Shape - thickness, size, etc.
- Heat Treatment – through or casehardened, annealed, etc.
- Production Requirements - sample or 100%