Processing of Tungsten Carbide
We present different procedures
The constant expansion of stamping and forming technology has also opened up new application possibilities for tungsten carbide on a large scale. In many cases, the performance of the tungsten carbide tools has had a decisive influence on the economic efficiency of the respective process.
Tools made of tungsten carbide are used today in all areas of stamping and forming technology, especially when the following requirements are met:
- high numbers of items or large numbers of production units (tool lifetime, short downtime, or high throughput);
- consistent quality of the products;
- economical production.
As a rule, tungsten carbide is used in large-scale production, where extreme resistance to abrasive, adhesive and/or surface-destroying stress is required with high mechanical strength at the same time. A typical example of the high-tech use of punching tools are chip holders made of a copper alloy.
In many cases, however, there are also other reasons that require the use of tungsten carbide tools, for example:
- so-called exotic, austenitic materials with a tendency to weld;
- forming in the limit range of strength properties (degree of deformation, tensile modulus);
- high dimensional accuracy of the products; in particular: the slightest springback or springback of the tools.
These requirements are usually only met by tungsten carbide or tools equipped with tungsten carbide, especially if they are optimally coordinated with the tool to be used and with the additional equipment, such that the whole works as an integrated production system.
Tungsten Carbide Compositions
In the last few years the range of tungsten carbide variants that can be used in stamping and forming technology has expanded continuously. Newly developed fine and very fine grain alloys have been added to the previously used types.
Because of its good toughness properties, tungsten carbide based on WC-Co is predominantly used in high-performance tools in stamping and forming technology.
Besides the cobalt content, WC-Co tungsten carbide is subdivided according to the WC grain size. The picture shows the structural differences of WC-Co alloys with different WC crystallite sizes:
Ultra-fine grain: mean grain size < 0.5 gm
Fine grain: mean grain size < 1.5 µm
Normal grain: mean grain size - 1.5-3 µm
Coarse grain: average grain size - 3 -20 µm.
In the punching and drawing area, there is a clear trend towards the use of fine-grain carbide. In contrast, in the last few years, especially for heavily stressed tools in massive forming, a clear trend towards hard metal with a coarser tungsten carbide grain and a medium cobalt content can be seen.
Prior to introduction of fine-grain tungsten carbide, fact was that the lower the crystallite size of a WC-Co alloy, the lower the flexural strength and the higher the hardness. In the case of fine-grain and ultra-fine-grain carbide, this principle is broken. In the case of these fine-grained WC-Co alloys, not only does the hardness increase, but, surprisingly, the flexural strength also increases with decreasing WC crystallite size. This fact is clear from the picture. In all three cases, the tungsten carbide is manufactured using the sinter-HIP process in order to avoid even the slightest porosity. Similar results are also obtained when sintered tungsten carbide is re-densified by hot isostatic means. By avoiding microporosity, a considerable increase in toughness is achieved, especially in tungsten carbide alloys with a low cobalt content.
Properties of the Different Types of Tungsten Carbide
The properties of finely structured tungsten carbide (ultra-fine grain and fine grain) differ significantly from the classic carbide types: With decreasing grain size, the hardness increases, so that the fine and ultra-fine grain carbide is noticeable due to its high hardness. The increase in hardness goes hand in hand with an increase in the coercive field strength.
With decreasing grain size, the hardness increases, so that the fine and ultra-fine grain carbide is noticeable due to its high hardness. The increase in hardness goes hand in hand with an increase in the coercive field strength.
The bending strength largely determines the use and application profile of the tungsten carbide type.
A smaller grain size of the carbide phase results in a reduction in the mean fine distance between the WC grains with the same Co content. An increase in the flexural strength can thus be achieved.
The fracture toughness of the tungsten carbide behaves in the same way as the flexural strength; it increases as the WC grain size falls. So there is a correlation between WC grain size, coercive field strength, flexural strength and fracture toughness. The increase in the fineness results in a significant increase in the fracture toughness.
Wear resistance: As the WC grain size decreases, the hardness and strength of fine and ultra-fine grain carbide increases and wear due to abrasion decreases. The harder tungsten carbide body offers greater resistance to abrasion. The same applies to corrosion wear due to the lower binding metal intermediate layer between the WC crystallites.
The thermal conductivity is also an important variable when using hard metal. As the grain size of the carbide phases decreases, the conductivity for heat also decreases, and consequently such tools and tool systems made of tungsten carbide are no longer exposed to such high temperatures.
The finer structure leads to improvements in all application-relevant properties such as hardness, toughness, strength and rigidity. However, the general conclusion cannot be drawn that selecting finely structured tungsten carbide always leads to an improvement in the application results.
Fine-structure tungsten carbide are widely used in various fields of application.
- drilling of high strength and hardened steel
- drilling of high-Si-containing Al alloys
- drilling of fiber-reinforced plastic
- HSC milling
- skiving and shaping in gear manufacturing
- hard milling with TiAlN-coated FK-HM (200-350m / min; 0.1-0.2mm / rev; 0.1mm)
- solid tungsten carbide hob
- broaching tools
- paper cutter
- print drill and micro milling cutter