CARBIDE.WHAT IS IT MADE OF?
Carbides are sintered composite materials made of hard metal materials and a binding agent. They are usually made with a combination of tungsten carbide and cobalt (WC+Co). In addition to Tungsten (WC), also Titanium (TiC), Tantalum (TaC), Chromium (CrC) or other carbides are used as hard materials as well. Cobalt (Co), Nickel (Ni), Iron (Fe) and Nickel-Chromium (NiCr) are most frequently used as binding agents.
When chemist Henri Moisson searched for a way to make synthetic diamonds in 1894, the expression “Wolframcarbide” (WC) was widely publicized. About 20 years later industrialists Hugo Lohmann and Otto Voigtländer finished their process to produce parts made of Wolframcarbide, which were sintered just below the melting point. In 1914 they patented their cast tungsten carbide, but failed to establish it in the marketplace, mainly due to its brittleness. That was not accomplished until nine years later, when Karl Schröter and Heinrich Baumhauer developed sintered tungsten carbide. Osram bought their patent in 1923. Industrial use beginning in 1926 got things going. Krupp Hartmetall successfully launched its material named “Widia” – as in diamond (German: “wie Diamant”) – in the marketplace. In the USSR tungsten carbide was developed under the name “Pobedit” starting in 1929 by a company bearing the same name.
TUNGSTEN CARBIDE.THAT’S HOW HARD IT IS!
The hard metal material most commonly used is Wolframcarbide (WC), which is a high-performance material composed of Wolfram and carbon. Its defining characteristic is its hardness, which is close to the hardness of a diamond. Wolframcarbide is obtained from tungsten ores “wolframite” and “scheelite”. These are mined primarily in China, Russia, Canada, Austria and Portugal. The manufacturing process is called carburization, which is a metal treatment process that adds carbon to a metal surface with low carbon content – thereby increasing the hardness of the metal. Varieties of Wolframcarbide-Cobalt carbides are the most important in today’s market.
THE BEST CARBIDE?A BINDING MATTER.
The main binder for carbides is usually cobalt, because it has a positive impact on the sintering process. If increased corrosion resistance is desired, a nickel-binder is the material of choice. The most common binding agents in today’s carbide production are Cobalt (Co), Nickel (Ni), Iron (Fe), and Nickel-Chromium (NiCr). In this context it is important to remember that the hardness of a carbide part increases with decreasing content of binding agent and decreasing grain size.
ONE MATERIAL,MANY ADVANTAGES.ONLY CARBIDECAN!
Carbide is extremely multi-faceted in its application. The material impresses with its enormous versatility and provides a large number of advantages and features that make it desirable for use across multiple industries. For most DURIT customers, the following three features are the most important ones:
In numerous industrial processes, there are abrasive effects between different materials, resulting in premature wear of one of the two materials. Tungsten carbide provides – due to its versatility – ideal options to significantly reduce premature wear.
The degree of hardness is determined by the Vickers method The force of a 30 kg (294 Newton) weight is used to produce a measurable surface indentation in reference to a defined diamond. The degree of hardness of the tungsten carbide component increases with decreasing content of binding agent and decreasing grainsize.
Compared to other materials, tungsten carbide distinguishes itself with an enormously high pressure resistance. Pressure resistance increases with decreasing binder content and decreasing grain size. Tungsten carbide grades with a low carbide grain size and a low binder content attain a pressure resistance of approximately 7,000 N/mm2.
Carbide production in a nutshell: Powder containing tungsten carbide and the chosen binder, is ground and mixed to achieve the desired composition. Subsequently it is dried. The resulting granulates are then pressed into shape. For this process, different methods of direct and indirect forming are available. The pellet, also called “green compact”, can then be processed mechanically. Finally, the part is sintered at about 1,300 to 1,500 °C.
THE SINTERING PROCESS.
Sintering is a thermal process under anaerobic conditions. In carbide production, this process firmly embeds the tungsten carbides into a binding matrix. For this purpose, the binder in the green compact is heated to its liquid phase. It then fills the interstitial spaces and surrounds the carbides. The HIP sintering process includes an injection of argon at high pressure after reaching the liquid phase. This provides the carbide with additional density and ensures a homogeneous, non-porous structure.
The term “blank” or “carbide blank” refers to a workpiece that requires further processing. In tungsten carbide production, the blank is the part that emerges from the sintering process. For the powder mixture to become an actual carbide blank, it must be pressed in a mold. Good to know: DURIT blanks feature a VERY low grinding excess – perfect for subsequent processing.
HOW LONG DOESTHE CARBIDE-PRODUCTIONPROCESS TAKE?
CARBIDE GRADES.AN OVERVIEW.
What carbide grades are used where? Remember: depending on composition and structure, carbide has very different properties. So far there has been no uniform systematization, typification or generally applicable technical term. At DURIT, we differentiate between the TWO TYPES GD and BD:
Carbides for forming technology, wear and corrosion protection.
Carbides for forming and mining technologies.
FROM NANO-FINE TOEXTRA COARSE.CARBIDE GRAIN SIZES.
The balance of hardness, wear resistance and toughness makes carbide so special. The exact properties are determined by the composition. One decisive factor is the selection of grain size. That means: The finer the grain, the higher the hardness and wear resistance.
» NANO-FINE GRAIN < 0.2 µm
» ULTRA-FINE GRAIN 0.2 – 0.5 µm
» MICRO GRAIN 0.5 – 0.8 µm
» FINE GRAIN 0.8 – 1.3 µm
» MEDIUM GRAIN 1.3 – 2.5 µm
» COARSE GRAIN 2.5 – 6.0 µm
» EXTRA COARSE GRAIN > 6.0 µm
CARBIDE.WHATFASTENING OPTIONSARE POSSIBLE?
How can carbide be combined with other materials or products? The choice of the most useful fastening option is fundamentally dependent on the application of the respective workpiece. Tungsten carbide ultimately offers numerous options: i.e. soldering, gluing, shrink-fitting, casting or mechanical fastening.
Before gluing tungsten carbide, the adhesion surface must be treated and mechanically roughened. The glues that we use feature a high thermal stability up to 200 °C. The material can be easily glued. In many cases, gluing is a good alternative to soldering. However, at higher temperatures, soldering is the only option.
HOW PRECISELYCAN CARBIDES BEMACHINED?
This is primarily determined by the geometry of the workpiece: Contoured parts can usually only be processed economically in green compact form. Due to shrinkage of the green compact in the sintering process, accuracy within one tenth can be accomplished. Round or standard geometric shapes can be processed with considerably higher precision of up to 3 µm.
CARBIDE.HOW HARD IT CAN BE!
The hardness of tungsten carbide is determined in Vickers throughout Europe. The Vickers process is a static hardness test. Founded in 1828, Vickers Limited was an important British machine manufacturer and defense contractor. In the United States the Rockwell test (HRA) is preferred. Tungsten carbide actually offers a broad range of available hardness. It ranges from “soft” grades with a hardness of 770 HV30 to highly wear-resistant grades with a hardness of up to 2,000 HV30.
CAN CARBIDESBE PLATED?
Carbides can be coated using PVD or CVD processes.
The PVD method with low coating temperatures around 450 °C is preferable to the CVD method with coating temperatures of 900-1,100 °C. Tungsten carbide is frequently used as a basis for coatings due to its high pressure resistance which prevents the egg shell effect when coating parts.