Hardness Test
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.
Indenter
WHY USE A HARDNESS TEST?
- Easy to perform
- Quick (1-30 seconds)
- Relatively inexpensive
- Non-destructive
- 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.
HARDNESS SCALES
There are five major hardness scales:
- Brinell - HB
- Knoop - HK
- Rockwell - HR
- Vickers - HV
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%
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