Oxidation
Carbide die alloys are resistant to oxidation in air up to 600
degrees Fahrenheit. Oxidation is relatively slow from 600-1000
degrees, but rapid above 1000 degrees.
Fortunately,
even in warm forming, the carbide component seldom reaches critical
oxidation temperature, so the superior high temperature properties
of the cemented tungsten carbide die alloys can be utilized.
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Hardness
The highest cobalt alloy, the softest grade, has a hardness greater
than that of the hardest steel alloys. The fine grained alloys
have hardness values greater than the normal grain size alloys.
Experience
has shown that the hardness of cemented tungsten carbide alloys
is related to abrasive wear. Relative abrasive resistance, measured
as a volume loss under specified abrasive conditions, decreases
with an increase in cobalt content of the alloy, but not linearly.
Fine grained alloys with higher hardness increase the abrasion
resistance and lower the volume loss considerably below that of
the normal grain size alloys.
Hardness decreases
with increasing temperature but not as rapidly as for hardened
steel alloys. Therefore, the WC alloys are applicable to warm
deformation tooling, providing other critical properties, such
as heat checking or oxidation, are not predominant factors.
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Density
All cemented tungsten carbide alloys approach their theoretical
density when sintered.
Density is
not critical to the problems encountered in metal deformation
tooling and is used mainly as a means of quality control by the
producer. A density considerably less than theoretical indicates
the presence of abnormal porosity or in adequate sintering. Microporosity
is not considered to be detrimental to the application except
at very high stress levels. However, macroporosity, if encountered
in a carbide component part, is a potential stress riser even
under low stress levels and should be avoided.
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Fatigue
Strength
The fatigue strength of the cemented tungsten carbide die alloys
under cyclic stressing is important to metal deformation applications.
Unfortunately, published qualitative fatigue data are very limited.
Qualitatively, from actual applications, the carbide die alloys
have proven resistant to fatigue and superior to the best steel
alloys.
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Thermal
Shock Resistance
Thermal shock resistance is a measure of a material to withstand
one or more drastic temperature changes without cracking. This
property must be taken into consideration for each set of carbide
tooling, especially if warm metal deformation is being done.
It has been
proven that heat sets up compressive forces on the surface of
a punch or die. On cooling, tensile forces become predominant.
Since the compressive strength for the cemented tungsten carbide
die alloys is greater than the tensile strength, the cooling cycle
presents the greater probability for failure. When the tensile
stresses developed on the surface during cooling exceed the tensile
strength, surface cracks of "heat checks" develop. This
condition is detrimental to the surface of the workpiece and may
initiate failure of the tooling.
In general,
tungsten carbide alloys are especially subject to "heat checking"
when used in warm deformation tooling. This tendency is eliminated
or minimized by using the higher cobalt grades or by decreasing
time of contact between the warm metal part and the carbide component.
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Transverse
Rupture Strength
Transverse rupture strength
resting is the most widely used procedure for evaluating the mechanical
strength of the cemented tungsten carbide alloys. This is due
to the relative simplicity of the procedure and test specimen
configuration compared to other methods. The test is performed
by applying a measured concentrated load on the center of a bar
specimen supported between two stationary bars spaced a fixed
distance apart.
Measured
fracture strength, or bending strength, is a function of the cross
sectional area of the test specimen, the width of the test span,
and the rate of applying the load. As the specimen size increases,
the fracture strength per unit cross sectional area decreases.
This tendency should be taken into consideration in designing
carbide tools.
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Compressive
Strength
Compressive strength of carbide die alloys is of major importance.
The forces encountered in punches and dies during actual loading
are usually complex, so uniaxial compressive strength data should
be used as a first approximation.
Carbide
alloys subjected to compressive stresses deform elastically but
not plastically, The alloy fractures when the elastic limit is
exceeded. For the higher cobalt alloys there appears to be a slight
tendency for plastic deformation (0.1-0.2 percent) but the magnitude
is too small to measure unless precise equipment is used. Compressive
strength decreases with increasing cobalt content.
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Impact
Strength
Impact strength is a measure of a material's ability to withstand
mechanical shock. Typical data for the cemented tungsten carbide
die alloys is obtained by fracturing Charpy unnotched test bars
in standard testing equipment.
The
impact strength increases linearly with cobalt content for the
normal grain size alloys; with an increase in average tungsten
carbide grain size and with increased temperature. For the fine
grained alloys, there doesn't appear to be any major change in
impact strength.
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