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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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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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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