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