The Features of Hard Metal

Technical features

Hard metal has some truly unique characteristics. On these pages we will examine this topic from a purely technical point of view.

Hardness

The “hardness” of the Widia must surely be stated as a primary feature. In fact it is considered as the most important physical property for practical applications although it is not the only reason that has determined its commercial success. Its resistance to abrasion is extraordinary. The hardness is calculated using a drill notching sample with a penetration diamond for ASTM standard B-294. The values ​​of hardness of the Widia are expressed in terms of Rockwell “A” or Vickers values. In nature, the only hardest material of this type of metal is diamond: only the diamond is able to scratch the carbide carbonate. Silver and gold, in comparison, are much softer metals.

Density

Another distinctive feature is its density. This property is calculated with ASTM B311 standard. The density of cemented carbide varies according to its composition. Being a composite alloy, its constituent gradients have specific variable densities. By combining these materials in different proportions it is possible to create a variation in the density of the resulting material. A density of 14.5 g / cc is typical for a 10% cobalt mix. This value presents twice the density of wrought iron 1040: an element to keep in mind especially when weight is an important factor in a practical application.

Resistance to Transverse Breakage

The mechanical strength of cemented carbide is generally determined by the method of transverse fracture strength rather than by a tensile test as is commonly done for steel. This methodology is used because friable materials are very sensitive to non-alignment of tensile tests and surface defects, which could cause stress concentration and lead to incorrect test results. The transverse breaking strength is determined by placing a standard sample (for ASTM B-406, ISO 3327) between two supports and loading it up to the breaking point. The value obtained is called the transverse breaking strength or cohesive strength and is measured in relation to the weight that caused the breakage. This test detects the load on a single area of ​​the unit and is expressed in psi or N / mm2. Since the cemented carbide has a range of fracture values ​​differentiated by the existence of micro-voids, characteristic of all friable materials, this test is performed by carrying different trials: the resulting reference value is evaluated on the average of all the tests.

The values ​​for the breaking strength shown in the properties graphs provided by the manufacturers reflect the mechanical force operated only for a specific area. Erroneously, many engineers – even those working in the metallurgical industry – consider this value as a force value of the model. This data is used to evaluate the degree to which the alloy should work in a particular application, expecting a direct correspondence with this value. In reality, these results decrease as the size of the targeted area decreases: the value of the strength of the model should be calculated in relation to its actual size.

Another factor influencing the mechanical properties of cemented carbide – in particular the transverse breaking strength – is its granular size. The more the granule size increases, the more the breaking strength and the wear resistance decreases.

Compressive strength

This is another of the most important attributes of cemented carbide. Ductile materials under compression simply swell or expand without fractures, but a friable material does not hold this type of test for the occurrence of cutting fractures more than for true compression. Cemented carbide exhibits a high level of compressive strength when compared to most other materials and the value increases with decreasing mixture content and granule size. Regarding the size of the granulate and the content of the mixture, the values ​​between 400K-900K psi (7kN / mm2) are typical for cemented carbide.

Impact strength

The cemented carbide reveals a striking impact force, especially at high temperatures,

containing 25% cobalt binder with a coarse granular structure. Transverse breakage is often erroneously used as a measure of impact resistance when, in truth, fracture toughness is a better indicator of the cemented carbide’s ability to withstand any mechanical shock or impact. Fracture strength varies according to the size of the granulate and the contained binder.

Resistance to fatigue

When a material is subject to repeated cycles of fluctuation, several damages may occur. These problems can happen even if the material undergoes a stress lower than what might have been caused if the load stress had been constant. The fatigue properties are evaluated by subjecting some samples to a stress cycle and calculating the number of cycles that take place until the damage occur. Several large companies have conducted this type of cemented carbide tests and have written their reports on the matter. For example, the Swedish company Sandivik has verified that the fatigue strength of cemented carbide in a load compression can result in between 65% and 85% of the compressive strength at 2 x 106 cycles. The resistance to fatigue increases with the decrease in the size of the tungsten carbide granulate and with the decrease in the binding content.

Resistance to Corrosion

Tungsten carbide particles are resistant to the most corrosive substances. It is a binding material that is subject to leaching in the presence of a strong acid or an alkaline solution. The binding material will leach from the surface of the hard metal, leaving a skeletal structure, with no support. The carbide particles will be scraped rather quickly, exposing a new area of ​​the surface that can be attacked. When the binder is low, the carbide skeleton is denser. A low binder gradation shows a slightly higher combination of resistance to wear and corrosion than those with a gradation with a higher binding content. These particles are also hard to crumble or weld and are used in specific applications where corrosion and wear resistance are an indispensable necessity while resistance to mechanical strength and thermal shock are so important.

Thermal properties

The carbide shows a very low Linear Expansion Coefficient. About half compared to steel. A carbide grade with 8% cobalt indicatively has a linear expansion coefficient of 5 * 10-6 / ° C in a temperature range of 20 to 400 ° C. The Thermal Conductivity is approximately twice that of an unalloyed steel and a third compared to copper. The specific heat capacity of a generic grade of hard metal is about 150-350 J / (Kg * ° C), ie about halfway to an unalloyed steel.

Electric and magnetic properties

Hard Metal has a low electrical resistivity and a typical one is 20 μOcm. As a consequence of the low resistivity, the carbide is a good conductor, having a conductivity value that is about 10% less than copper. Due to the cobalt or nickel content, the carbide also presents ferromagnetic properties at room temperature. Therefore the Curie Temperature is included in the range between 950 and 1050 ° C, depending on the composition of the degree. Magnetic permeability is very low and is a function of cobalt content. It increases with the cobalt content. A typical value is in the range 2 to 12 when the vacuum value is equal to 1.