September 7, 2021
These carbides were not exactly as hard as diamond, but harder than most hard materials known at the time. It was thus logical to produce tools and extremely durable components from tungsten carbide, which in terms of tool life even outpaced steel parts by far. The invention, however, did not experience immediate success as the tungsten carbide products were much too brittle, the molten carbides generally containing cavities and graphite precipitates. This is why Henri Moissan received the Nobel Prize in Chemistry not for having discovered carbides but for analysing and isolating the element fluorine and for inventing the electric furnace named after him.
Was that already to be the end of the brief history of carbides? Far from it! In fact, Hugo Lohmann and Otto Voigtländer managed to produce wear-resistant workpieces in 1914 by crushing the molten tungsten carbide into an extremely fine powder, after which the material was subjected to pressing and sintering. But here once again the problem was that the material was too brittle! In order to improve the toughness of the material, ferrous metals like chrome and titanium were added to create an alloy.
The decisive step towards success, however, was taken in 1923, when Karl Schröter of the Osram Research Association succeeded in developing sintered carbide. He managed this by first mixing the tungsten monocarbide powder (chemical formula WC) with 5 to 10% cobalt powder and then heating the pressed parts resulting from this mixture to a temperature close to the melting point of cobalt. What happened? A eutectic melt was created because tungsten carbide dissolves in cobalt. This melt wets the tungsten carbide crystals in the form of a hard solder and causes them to contract into the smallest possible space, thus creating a very dense body. Hitherto unrivalled strength values were achieved and cemented carbide eventually became a material to be reckoned with on the market – which was underlined by the issuing of the first patent on 30 March 1923.
Over these last almost 100 years a lot has happened in the field of development – has the focus shifted from the pioneering achievement to refining the process? Well, this is only partly true as ‘cemented carbide’ actually does not exist. Cemented carbides represent a group of materials which differ from other hard materials such as ceramics, corundum or diamond in view of their high hardness and metallic properties. Cemented carbide is therefore a powder-metallurgical two-phase material consisting of a hard material phase and a metal binder phase.
Microscopic image
In the case of sintered cemented carbide, the hard and brittle tungsten carbides are combined with relatively soft but tough metals, such as cobalt, nickel or iron, to form a type of composite material. In the sintering process, the carbide powder, with grain sizes ranging from 0.1 to 20 µm, is sintered together with the tough binder metals at a temperature between 1,300° and 1,500°C and sometimes also at high pressure, up to 100 bar, to yield a solid and dense structure. The original volume is thus reduced by up to 50 per cent.
In contrast to a pure melt, not all materials here have been melted, but rather ‘baked together’. The metal binder fills the gaps creating a brazing effect between the carbide grains. Basically this structure can be compared to concrete, where single hard particles (such as gravel) are firmly bonded with cement.
Cemented carbides have evolved into a kind of multi-talent: whenever tools and components are subject to extreme stress, that's when cemented carbide comes into its own. High hardness, wear resistance and toughness combined with other mostly adaptable high-performance characteristics, make cemented carbide the ideal material for numerous applications. At present over a hundred different carbide grades with varying compositions for specific applications are available, for instance, for steel machining, hot rolling or injection moulding applications, to name just a few.
General advantages of cemented carbide at a glance:
Cemented carbides cannot be limited to a specific range of applications; they are just too versatile. As a matter of fact, where other materials fail, cemented carbide components prevail in terms of wear protection, tool life, hardness and reliability. Thanks to their wide range of hardness, wear resistance and toughness, they have risen to become superstars in the cutting tools sector: diamond may be the hardest existing material, but due to its low fracture toughness, it cannot be used economically or at all in many applications. Cemented carbide owes its enormous flexibility to its composition which – depending on the application – provides either the required toughness or optimal wear resistance.
The graph shows the discrepancy between the two parameters wear resistance and toughness, based on physical laws: the ideal cutting material (which for now exists only theoretically) would be positioned at the top right of the graph, showing both maximum wear resistance and maximum toughness, but such a cutting material does not exist.
When a material is exposed to external static or dynamic stress, this inevitably leads to mechanical tensions. During application, particularly with impact loads, both the strength and the ductility of the material have to be taken into account. These two properties represent the basis for the concept of toughness, which is defined as ‘the capacity to resist fracture or crack propagation’. The higher the binder metal content and the larger the grains, the higher the toughness.
There are big differences between the toughness of different cemented carbides, which can be explained by their microstructure: cemented carbides with low binder content and small grain size tend toward highly critical crack formation, thus leading to spontaneous component failure (due to cracks).
In cemented carbide with high binder content and coarse grain, cracks in the binder matrix are deflected or completely prevented from spreading. This is called ‘subcritical crack formation’ and prevents or at least significantly delays spontaneous failure of the components.
For such cases there are carbide grades which are designed for maximum fracture toughness and are used, for example, for forging hammers. They work the workpiece with high force and must be able to withstand consistent impact stress without failing or, in other words, breaking. In terms of the composition of the cemented carbide, this means a cobalt content of up to 30% with a grain size of 10 µm.
Fracture toughness diagram: comparison of carbide grades with different grain sizes
Transverse rupture strength is normally used to find out the mechanical strength of a particular material. Fractures are mostly due to defects in the structure and on the surface of the component. It is therefore absolutely essential that the cemented carbide has a homogeneous structure and no surface defects.
When high transverse rupture strength is required, carbide grades with medium cobalt content and small or medium grain size are used, for example, in micro-drills for circuit boards. They are sometimes thinner than a human hair and exposed to high bending forces during drilling. In order to withstand these forces, carbide grades with a cobalt content of around 8.5% and a WC grain size of <0.5 µm may be used for such drills.
Transverse rupture strength diagram: comparison of carbide grades with different grain sizes
Corrosion: every production hall's worst nightmare. If a metal material reacts with its environment, this inevitably results in a measurable modification of the material. In most cases this change impairs the functionality of the metal component or even leads to total failure and fatal consequences for the entire process.
Cemented carbides are also subject to corrosion. In cemented carbides, corrosion in acid water-borne solutions (pH less than 7) causes a reduction of the binder phase surface, such that in the worst case only a carbide 'skeleton’ remains. Cobalt leaching (‘Co leaching’) weakens the bond between adjacent carbide grains while increasing the rate of degradation. The lower the pH value, the greater the tendency for corrosion.
When the metal binder content is low, the carbide 'skeleton' is more pronounced. Consequently, this type of carbide grade shows higher wear resistance and corrosion resistance than do cemented carbides with a higher metal binder content. In practice, however, this is not sufficient to increase the lifetime to any significant degree. Due to their limited corrosion resistance, pure WC-Co carbides are therefore often not suitable for application fields with difficult corrosion conditions. The only solution is to use materials like Co/Cr, Co/Ni or Ni/Cr rather than pure cobalt binders.
In extremely alkaline media (pH 11 and higher), the carbide phase or tungsten carbide (WC) is evenly abraded by corrosion. This leads to a slightly increased rate of degradation, which (subsequently) can be noticed on the component as increased wear.
Comparison of standard grades and corrosion-resistant grades (microscopic images)
Most applications today require an individual approach as they are not standard applications. And in the case of cemented carbides? It is the same for them, as specific properties are more important than others depending on the application. Thus the process always begins by choosing the right carbide: thanks to such a broad selection, there is no problem finding the optimal grade for every application.
The main thing that matters is high carbide quality: porosity, structural peculiarities and defects in the microstructure have an adverse impact on the mechanical properties. In order to achieve the best possible result, it pays to count on the experience and specific know-how of producers of premium cemented carbide. This is the only way to systematically improve specific properties, for example, through alloy additives, which increase the temperature resistance or corrosion resistance of the cemented carbide.
As for all cemented carbides, the hard WC phase provides hardness, heat resistance and wear resistance while the metal binder ensures good toughness of the material. The extremely high modulus of elasticity ensures that cemented carbide deforms only slightly when exposed to pressure. This combination of characteristics make tungsten carbides the ideal material for numerous applications. The advantages are further boosted by the fact that the properties can be varied widely, which is why cemented carbide is used in many different application fields, either in applications with high impact stress or high bending stress or high stress in terms of wear.
The cemented carbides used by far most frequently are based on WC or Co. They are used not only in metal cutting (ISO application group K), but also for products in the wood and stone machining sector and for numerous wear parts. In addition to the simple WC-Co carbides, there are also mixed carbides which contain titanium, tantalum or niobium carbides. They are used for steel cutting (ISO application group P) and for metal sawing.
The numerous carbide grades required for the various applications can be distinguished according to three criteria: WC grain size (α phase), binder metal content (β phase) and the addition of other metal binders (γ phase). Using these three parameters, in particular the WC grain size and metal binder content, the material properties can be varied considerably.
The triumph of cemented carbide is largely due to the balance between hardness/wear resistance and toughness. The exact properties are determined by the composition of the cemented carbide. The all decisive factor is the choice of the grain size. The smaller the grains, the harder and thus more wear-resistant is the material.
Example of submicron grain: These grades are normally used for abrasive materials which tend to suffer from edge-build up and are thus prone to high wear. They achieve maximum edge stability and thus have a low tendency to adhesion. Normal grain grades are the ideal compromise between toughness and wear resistance, without being simply a gap filler. They are frequently used for non-ferrous metals or steel.
Comparison of grain sizes (microscopic image)
The metallic binder phase of most cemented carbides consists of cobalt. Cobalt accounts for 4% up to 30% of the total mass (and up to 12% for grades used for metal cutting) and notably improves the transverse rupture strength compared to pure tungsten carbide. In the history of cemented carbide research, various metals have been tested as binder phases, but cobalt has established itself in the long term. It forms the strongest bonds with tungsten carbide, wetting it very well as both solidify in a hexagonal (lattice) structure.
The WC grains have a diameter between 10 µm and 0.5 µm; small grains improve both hardness and toughness. They have a prismatic shape. Between them there is the cobalt matrix. Ideally, the matrix is formed solely of tungsten carbide and cobalt. If the material does not contain enough carbide, this results in the eta phase, a carbide consisting of Co3W3C, which reduces toughness. Too much carbon, on the other hand, leads to elemental carbon (graphite), which also reduces the strength. Part of the carbon and the tungsten have been dissolved in the cobalt.
Special applications require special treatments, which is also the case with cemented carbide. Special alloy additives sometimes lead to small but very valuable optimisations which provide the decisive kick for the respective applications. Small quantities of vanadium carbide (VC, up to 0.8%), chromium carbide (Cr3C2) or tantalum-niobium carbide (both up to 2%) are used, for example, as doping additives as they ensure a fine-grain structure.
Cemented carbide is a real multi-talent and the best-performing material in various industries when it comes to wear resistance: whether in metal forming, sawing, drilling and reaming, milling, abrasive water-jet cutting, profiling and planing, forming and casting, shredding or ultrasonic cutting– there’s no getting around application-optimised cemented carbide.
There is a wide range of extremely wear-resistant carbide products which include an equally wide range of different components for a variety of industries. In wood and stone working, cemented carbide is used, for instance, in the form of saw tips for circular saws and drill bits for drilling all metal alloys. The automotive industry uses wear-resistant carbide components for Common Rail Systems. In the oil and gas industry, carbide components minimise the downtime in pipelines and in exploration drilling operations.
Whether stamping, bending, blanking, metal forming, powder pressing or fine cutting – with our active parts made of carbide, you can achieve high output rates and quantities, enabling you to turn out mass-produced parts economically. This is ensured by the optimised edge stability of products for blanking and lamination tools as well as by their reliability and process stability and better tensile strength combined with reduced tendency to and speed of corrosion.
Scratchproof watch cases, components for metal forming and tool making, water-jet nozzles, sputter targets for the production of diamond-like carbon coatings (DLC), high-pressure tools for the production of synthetic diamonds or blanks for hob milling cutters or rotary cutter blanks are also manufactured today from cemented carbide. Special grades approved by the US Food and Drug Administration are even used for medical systems and in the food industry.
Cemented carbide in the cutting tools sector plays an extraordinary role as it clearly shows better wear resistance than high-speed steel (HSS), withstands higher working temperatures and can be optimised very well to the requirements of various processes. At the same time, it costs less than polycrystalline diamond (PCD) tools, for example.
Even taking into account that today things are no longer as they used to be in the pioneering phase of research: cemented carbide development and production still represent a broad field with lots of options to be explored. This is why innovative companies strive every day to offer the optimal cemented carbide for the respective application.
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