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Ammoniumparawolframat

Ammonium paratungstate is a white, crystalline tungsten salt. Its chemical formula is (NH4)10(H2W12O42)·4H2O.

APT is an intermediate from the processing of ores containing tungsten for the generation of tungsten or tungsten compounds, and contains about 85% WO3.

APT is the worldwide trading format for raw materials containing tungsten and is traded in MTU (metric ton units). One MTU contains 10 kg of WO3.

Carbide

This material has been and probably still is popularly known under the name WIDIA steel. WIDIA was the brand name introduced by Krupp in 1927. WIDIA is a contraction from the German for hard “like a diamond". The addition of "steel", however, is misleading. Carbide is not a steel alloy, but a composite material.

The main components of carbide are on the one hand the hard material tungsten carbide (WC), and on the other the binder metal cobalt (Co).  Both components are supplied as carbide raw materials in powder form. Tungsten carbide and cobalt are two very different materials. Their density and hardness are differ greatly. The different temperatures at which the two materials melt or boil are the key to selecting the right manufacturing process. Tungsten carbide will only begin to melt at a temperature at which the evaporating temperature of cobalt is almost reached. That means a manufacture based on melt metallurgy will fail due to the high melting temperature of the tungsten carbide, simply because there is no crucible material that could be used. Carbide is therefore manufactured by way of powder metallurgy, i.e. the sintering of a close mixture of WC and Co powders.

 Tungsten carbide (WC) - hard material

  • Density 15,63 g/cm³
  • Microhardness > 2.000
  • Melting temp. 2.870 °C

Cobalt (Co) - Metallic Binder

  • Density 8,90 g/ cm³
  • Microhardness 100 - 200
  • Melting temp. 1.495 °C
  • Boiling temp. 2.927 °C

Cobalt

The metallic binding phase of most carbides consists of cobalt. Cobalt is a ferromagnetic transition metal and has the atomic number 27. It has a density o 8.90 g/cm³, a melting temperature of 1,495°C, and a boiling temperature of 2,927°C.

Cobalt is the metal best suited as a binder metal for carbides based on tungsten carbide. It has the capability to increasingly dissolve tungsten carbide even in its solid state at increasing temperatures. The molten eutectic consisting of tungsten carbide and cobalt is ideally suited for the wetting of tungsten carbide grains. During cooling, the tungsten carbide and cobalt eutectic does not solidify; instead, a large part of the dissolved tungsten carbide is deposited on existing tungsten carbide grains. The ductility of the binder phase consisting of cobalt will therefore remain for the most part intact.

Composite materials

Composites form a very variegated group of modern materials. Their common characteristic is that two or more solids are closely combined to create a material that synergetically benefits from the positive features of the individual components.

Cutting materials

Cutting materials are those that are used for producing the cutting part of a machining tool with a geometrically specified blade or shearing tool.

Cutting materials must have the following properties: 

  • Hardness and compressive strength
  • Flexural strength and toughness
  • Edge stability
  • Thermal stability
  • Oxidation stability
  • Low adhesion tendency
  • Abrasion resistance.

The following groups of cutting materials are differentiated:

  • Tool steels
  • Carbides
  • Cutting ceramics
  • Cutting materials with high hardness made of boron nitride or diamond.

Cutting materials all have distinctive differences. As toughness and hardness are opposing material properties, the relevant cutting materials will also have very different uses. Increasing wear resistance and thermal stability will allow increasing cutting speeds. Increasing toughness and flexural strength will allow an increased feed. An ideal cutting material must cater for both.

Cutting materials – Background

The development of current cutting materials began in the middle of the 19th century. Carbon steels were used first, which worked at cutting speeds of approx. 5 m/min. The high-speed steels (HSS) introduced at the Paris World Exhibition brought about a doubling of cutting speeds around 1900. From that time onwards, the development of new high-speed steel types picked up momentum. High-speed steels were developed shortly before WWI, which allowed cutting speeds up to 30 m/min. Cast hard alloys (stellites) followed in 1914, which reached cutting speeds of up to 40 m/min. The next big step was the development of carbides. The company Osram received a patent for a powder metallurgy-based compound of WC and Co in 1923. This patent was purchased by Krupp in 1925, which resulted in the first tools equipped with carbide entering the market in 1927. All of a sudden, cutting speeds of up to 200 m/min were achievable.

The 1950ies saw the introduction of carbides with a high titanium-carbide content, and the first ceramics (up to 500 m/min). Around this time, the first synthetic diamonds were introduced. In the 1960ies, super-hard cutting materials based on boron carbide joined the fray. At the same time, the first coating systems for carbides entered the market. As an improvement on the very brittle cutting ceramics, silicon nitride was introduced in the late 1970ies. The last great development step to date was the development of finest-grain carbides, which are characterised in particular by their high toughness and equally high hardness.

Density

Brittle materials must be completely free from defects. Such defects could occur on the surface or in the interior of the solid. One type of defect is. The higher the percentage of, the lower the material density. The density of the material will therefore already hint whether or not the material was sintered to its theoretical density.

Density furthermore indicates the approximate content of binder metal.

Extrusion

In an extrusion process (extrusion moulding), a plastic mass is pressed out via a nozzle (mouthpiece).

The presses can be piston or screw presses. The moulded body will have the diameter of the nozzle after extrusion. If the extrusion head contains nozzles equipped with integrated tools, then moulded bodies with holes (channels) can be produced along the entire length of the pressed out strand (like perforated bricks).

Extrusion is a continuous shaping method. A virtually endless strand can be produced.

Flexural strength

The flexural strength test (ISO 3327) is the commonly used method for determining the strength of brittle materials. The sample is deposited on two contact points. At one of the points, stress is applied until breaking. Previously, flexural strength was generally tested on samples with a rectangular cross section. Today, samples with a circular cross section (Ø 3.3 x 30 mm) are generally preferred, as they more realistically reflect the characteristics of rotating tools.

The flexural strength of brittle materials is limited by the number of defects beyond a specific size. A volume correlation does exist, as the probability of finding a defect increases with the size of the sample. It is therefore important to create a carbide that is virtually free of defects. The smallest surface fault could result in material failure.

Green body shaping

The objective of green body shaping is to produce a shape of a component part as close as possible to its final contour before sintering. The green body is processed with mechanical processes like drilling, milling, or grinding.

One benefit of a preshaped green body is that the final processing will be faster and less energy-intensive. Resulting waste material can be reintroduced to the manufacturing process after separation into individual material groups.

The processing will require a sufficient dimensional allowance to take into account the volume loss during sintering (shrinkage).

Hardness

Hardness is defined as the resistance a material displays against the intrusion of another, harder body. Hardness is the most important parameter for the user. The wear and cutting behaviour of tools is governed by their hardness.

Hardness is determined using the Vickers impression process in accordance with ISO 3878. The test is generally performed with a load of 30 kg. The measured values are expressed as HV30.

The lower the cobalt content of the carbide alloy and the lower the grain size of the tungsten carbide powder used, the higher the hardness value.

Machining groups

ISO 513: Due to the large variety of cutting materials, it is impossible to create standards for cutting material types according to their own characteristics. A classification of cutting materials therefore follows their application.

Every cutting material manufacturer assigns their own products to these main application groups or application groups.

A classification of this kind is quite problematic and extremely difficult to do for the carbide manufacturer. There are numerous influencing factors in its application: Carbide substrate, coating, blade geometry, edge rounding, cutting data, processing machine (stiffness), tool cooling, chip disposal options, the amount of material for machining. All these are usually unknown variables for the carbide manufacturer

Magnetic properties

The magnetic properties of carbide stem from the inherent magnetic properties of the ferromagnetic binder alloy, and are a result of the volume content of binder alloy and its own content of foreign elements.

Coercivity Hc [kA/m] (ISO 3326)

When a ferromagnetic material is introduced into a magnetic field, the magnetic flux density will be induced up to a saturation value in that area. When the magnetic field strength is reduced back to 0, a magnetic flux density, the so-called remanence, will remain. To reduce this remanence back to 0, an opposing magnetic field must be created.

The magnetic field strength required to return the remanence of a ferromagnetic material back to 0 (hysteresis loop) is called coercivity.The coercivity of a carbide indicates the state of the ferromagnetic binder phase. The more finely veined the binder phase and the higher its resulting force in the carbide, the higher the required coercivity to reverse the magnetisation. This is sometimes also referred to as magnetic hardness. The coercivity increases the finer the WC microstructure, and the lower the cobalt content.

Magnetic saturation 4ps [µTm³/kg]

Magnetic saturation is the max. value of magnetic induction, and is also referred to as magnetic flux density.

The magnetic properties are determined on the one hand by the magnetic characteristics of the binder metal, and on the other by the foreign elements contained therein.

A so-called h phase Co3W3C, a double carbide consisting of Co and W, is created if tungsten remains dissolved in the Co phase after sintering due to an insufficient carbon balance. Cobalt in this compound is no longer ferromagnetic, and will therefore not contribute towards any magnetic properties. When carburisation increases, tungsten is no longer dissolved in cobalt; it forms a mono-tungsten carbide WC instead, and crystallises as such. The cobalt content contributing to the magnetic properties will increase. From a certain degree of overcarburisation onwards, free carbon is separated from the microstructure. Knowledge of the magnetic properties of the binder metal will allow quite accurate statements regarding the carburisation state of the carbide without any knowledge of the actual microstructure.

The h phase and free carbon will weaken the microstructure of the carbide and are therefore undesirable. A measurement of this property is of critical importance, as the measured magnetic saturation will allow an inference regarding the degree of carburisation of the carbide alloy.

Main application groups

Due to the large variety of cutting materials, it is impossible to create standards for cutting material types according to their own characteristics. A classification of cutting materials was introduced in accordance with their application (ISO 513).

Every cutting material manufacturer assigns their own products to these main application groups or application groups.

A classification of this kind is quite problematic and extremely difficult to do for the carbide manufacturer. There are numerous influencing factors in its application: Carbide substrate, coating, blade geometry, edge rounding, cutting data, processing machine (stiffness), tool cooling, chip disposal options, the amount of material for machining. All these are usually unknown variables for the carbide manufacturer.

Microstructure

The microscopic microstructure of a material determines its macroscopic characteristics. There are countless different cemented carbide alloys (with differences in grain size of the original tungsten carbide, differences in the binder metal content, differences in the additives, e.g. Cr3C2, VC, TaC, TiC), which are also significantly different in terms of their application characteristics. For a cemented carbide to be judged as good quality, it must fulfil only two basic requirements: it should be largely free from pores, and should have a mostly homogenous microstructure.

A microstructural examination will allow the assessment of the homogeneity of the microstructure and its pore content.

The following characteristics can be observed and determined from a polished state cut (ISO 4505):

A-type porosity (pores up to 10 µm)
B-type porosity (pores between 10 and 25 µm)
C-type porosity (carbon porosity)

The following microstructural phases can be observed from an etched state cut (ISO 4499):

Tungsten carbide phase (α-phase)
Binder phase, generally cobalt (β-phase)
Other individual or combined carbide phases (γ-phase)
Double-carbide phase (η-phase) – insufficient

Mixing

The aim of mixing is the homogenisation of all mixture components weighed out in accordance with the recipe, and the removal of any agglomerates. Grinding, and therefore a reduction of the powder grains is undesirable. Overly intensive grinding could result in an energy input into the mixture that could have a negative impact on the microstructure developing in the sintering process.

So-called attritors can be used for mixing, as well as a ball mill or vibrating tube mill. The grinding occurs in conjunction with an organic grinding liquid, which prevents the oxidation of the powder components, and which has to be stripped from the mixture completely once the grinding process is completed.

A pressing additive must be added to the mixture depending on the intended shape of the product.

A pourable mixture must be produced for cold isostatic pressing (CIP). The consistency must be such that the compact will not fall apart after pressing.

For extrusion purposes, the addition of the pressing additive must result in a plasticisable mass similar to modelling clay.

Monostatic pressing

Uniaxial dry pressing: The powder is filled into a die and compacted, and the resulting compact is removed from the die.

This is a discontinuous procedure. Suitable only for simple geometries. Has the advantage of good reproducibility, dimensional accuracy, and limited drying characteristics.

Phase diagram

It may be helpful to imagine the powder metallurgy manufacturing process like the activities of a bakery. A mass is formed from liquid and powder raw materials, which is then transformed into a solid in a kiln. A so-called phase diagram will help explain what happens during the sintering process.

The occurrence of specific phases at specific temperatures is usually depicted in a so-called phase diagram. The visualisation of a tungsten-carbon-cobalt system would require a ternary system, which can only be depicted in 3D.

Tungsten monocarbide will separate from the molten mass only if a stoichiometric proportion of tungsten : carbon = 1 exists. It will therefore suffice to explore a stoichiometric carbon content of 50% (equal to 6.13 weight percent of carbon) in the tungsten carbide (WC). This can be done in the quasi binary system WC-Co (see phase diagram below).

Prerequisite for the creation of a moulded body is the joining of the individual powder particles. The merging process commences at around 800°C in solid-state sintering. A material exchange of the individual particles occurs on the basis of diffusion-controlled transposition processes. At this point, tungsten carbide will begin to dissolve in the cobalt. Conversely, cobalt will never dissolve in tungsten carbide. The consistency of the cobalt binder phase will follow the demarcation line of the g range. At these temperatures, around 80% of the required densification is already reached alongside the inevitable shrinkage of the compact. The first molten mass will appear at approx. 1,300°C, significantly below the temperature at which pure cobalt will begin to melt. Tungsten carbide will immediately begin to separate in the molten mass until the eutectic melt is reached (54% Co, 46% WC). A continuing rise in temperature will dissolve additional tungsten carbide along the demarcation line of the tungsten carbide and molten mass (S) range on the right.

The sintering temperature for a carbide with 6% cobalt content will have a liquid phase of 11.8 weight percent, which equals 15.6 volume percent. The liquid phase will wet the tungsten carbide particles completely, and will penetrate between the agglomerated hard material particles. For this process to occur, it is important that the binder metal can sufficiently wet the hard material particles. Surface tension will cause the tungsten carbide particles to flow together, which results in a further shrinkage of the compact.

Reprecipitation processes of the hard material phase occur during the sintering process. Tungsten carbide continuously dissolves, while tungsten carbide simultaneously deposits onto existing hard material interfaces.

Once the sintering temperatures begin to decrease, the solvency of the tungsten carbide in the cobalt declines continuously, and the dissolved tungsten carbide will once again deposit onto existing hard material particles. As the aim is to maintain a state of the lowest possible energy at all times, a minimisation of the interface energy (i.e. a minimised specific surface area) is desirable, which will encourage grain growth. During the processes described above, small crystallites will dissolve in favour of the growth of large crystals. This undesirable grain growth can be counteracted by adding small amounts of foreign atoms, the so-called dope carbides.

Due to the differing expansion coefficients of the cobalt and tungsten carbide phases, the cobalt phase will be subject to tensile stress and the tungsten carbide phase subject to compressive stress after cooling. This will delay a breaking of the brittle carbide phase under mechanical stress.

Porosity

In contrast with ductile materials, the pores in carbide and other brittle materials cannot be closed via plastic deformation. Carbides undergo only a minor plastic deformation before breaking. Due to an internal notch effect, pores can result in material failure far below the theoretical strength characteristic for a specific carbide type.

The following characteristics can be observed and determined from a polished state cut (ISO 4505):

A-type porosity (pores up to 10 µm)
B-type porosity (pores between 10 and 25 µm)
C-type porosity (carbon porosity)

Powder metallurgy

Powder metallurgy is a sub-field of metallurgy, which deals with the manufacture of metal powders and the production of components from these powders (ISO 3252).

The manufacturing process encompasses three main manufacturing steps:

  • Powder production
  • Compacting and densification 
  • Consolidation by way of sintering

Powder metallurgy is part of the primary shaping processes.

The material group traditionally manufactured by way of powder metallurgy is the ceramics group. It is of no consequence whether the end products are traditional ceramics like bricks or porcelain, or technical ceramics, e.g. aluminium oxide or silicon nitride. For that reason, early developers called powder metallurgical products made from metallic powders "metal ceramics".

Powder metallurgy, as opposed to hot-melt metallurgy, offers the great advantage that composites can now be created from source materials that could not be hot-melted into a compound.

In cemented carbides, completely different materials like tungsten carbide (melting temperature 2,870°C) and cobalt (melting temperature 1,495°C, boiling temperature 2,927°C) can now be combined to form a composite material.

Primary shaping

Primary shaping is the forming of an amorphous material into a solid moulded body. Four options are available for its creation:

  • Starting from a liquid state material, the liquid is poured into a die and left to solidify. This is referred to as the casting process
  • In the deposition of solids from a vapour phase, a solid can be coated. This process is known as evaporation
  • Ionised materials from an electrolyte can be deposited onto surfaces. This is referred to as galvanisation.
  • Bulk materials can be formed into a particular shape, and can then be solidified in a heat treatment process. This process is called sintering.

Sintering

Sintering is the heat treatment process with which the loosely pressed powder material is densified to reach its final consistency.

The compact contains a large amount of free energy. The sintering process transforms the compact into a more stable solid. All outer and inner surfaces of the compact are minimised (external open pores, enclosed pores, grain boundaries).

Temperature and time-induced

The outer shape of the solid changes during material transport and due to the resulting rearrangement of individual particles. It shrinks. Individual particles join together, while the pore volume decreases.

Other than in the traditional ceramics industry, which uses pusher-type kilns, carbide is sintered discontinuously in autoclaves, as only then the required framework conditions can be created.

Sintering shrinkage

The moulded body will decrease in volume during the sintering process, as it will be reduced to almost its theoretical density during that process. Any pore volume in the green body will be removed during the sintering process. Volume shrinkage in cemented carbide amounts to around 38-48%, which would be equal to a linear shrinkage of about 15-20%.

This shrinkage factor must be taken into consideration for component drawings; all drawing dimensions for the green body must be increased by a specific percentage (allowance). This allowance will amount to approximately 17-24%.

Toughness

Toughness is the ability to avoid breaking.

Palmqvist method: critical stress intensity factor KIc (stress occurring at the crack end).

Carbide is a brittle material, as virtually no plastic deformation occurs before breaking. The stress intensity factor allows us to calculate crack propagation in the material. Individual types of carbides will display significant differences in behaviour depending on their composition. The Palmqvist method cannot be used for continuous production control as it is very elaborate. As a first step, a hardness impression must be generated. Any cracks occurring on the corners will then be measured. The KIc value will then be calculated using MNm-3/2.

It is worth mentioning here that any KIc values provided in documentation should be considered with caution, as results can differ significantly depending on measuring conditions and sample preparation.

Tungsten

Tungsten is a chemical element with the element symbol W and the atomic number 74. It is rated as a transition metal, and is listed in the periodic table in the 6th subgroup or chromium group. Tungsten is a gleaming white metal, which is brittle in its pure form, and has a density of 19.3 g/cm³. It has the highest melting point (3,422°C) of all pure metals, and the second highest boiling point (5,930°C). Its most popular use is therefore as the filament in light bulbs.

Tungsten is a so-called refractory metal, characterised by its very high melting point and extreme hardness. The other refractory metals are Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo. The metals Ti, V, Nb, Ta, and Cr are in their carbide form popular additives for carbide alloys.

Tungsten carbide

Tungsten carbide is a chemical compound of tungsten and carbon. Tungsten carbide has a distinctive metallic character although it is not a metal, which explains its good electrical and heat conductivity, as well as its good wettability with the metal cobalt.

Tungsten ore

The Earth's crust contains about 0.0001 g/t of tungsten. This metal has to date not been documented in its native (pure) form in nature.

Tungsten ore contains tungsten mostly in the form of the minerals scheelite – CaWO4, ferberite – FeWO4 and also hubnerite – MnWO4. Its content of tungsten oxide is a maximum of approximately 1.5%, but generally only 0.5%. These ores will therefore have to be enriched into a concentrate in close proximity of its mining site by separating it from its vein material. The concentrate will then contain approximately 65-70% tungsten oxide.

China is the world leader in mining production with 75-90%, followed by a wide margin by the Russian Federation, Canada, Vietnam, and more. The most important known source of tungsten in Europe is in Febertal Valley in the Hohen Tauern Mountain Range (federal state Salzburg, Austria).

Wear resistance

The key characteristic of cemented carbides is their wear resistance.

Distortion, separation, and friction processes occur in the cutting area during machining. The cutting material is subject to a variety of stresses during that process, including high compressive stresses, high cutting speeds, and high temperatures. Increasing wear on the machining and free surface of the cutting material will signal its end of life.

Wear resistance is a surface property. Friction contact will result in both surfaces losing material. The term wear denotes the loss of minute particles, while abrasion is the result of larger particles breaking away from the surface. Both of these effects are further exacerbated by environmental media, as well as corrosive and oxidation processes. In terms of a high resistance of the cutting edge against plastic deformation, cemented carbides should have a high hot hardness and compressive strength. They will also need high flexural strength and sufficient toughness.

Wear is a very complex process, and therefore very difficult to assess. There are some tests that can be carried out under specific laboratory conditions. The result will, however, only be valid for the examined material pairings and relevant test conditions, and cannot be applied to other conditions.

Factors that are easier to measure and assess offer indications regarding wear behaviour. These are toughness and flexural strength.

Wettability

A liquid that is unable to wet a surface will draw together to form a drop. The cohesive forces in the liquid will prevail.

The full wettability of a surface will cause the liquid to spread. The adhesive forces between the liquid and the surface will prevail.

Full wettability exists between tungsten carbide and liquid, allowing the liquid cobalt to encase the tungsten carbide particles, and the surface