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Cemented Carbide is one of the most successful composite engineering materials ever produced. Cemented Carbide's unique combination of strength, hardness and toughness satisfies the most demanding applications.
A key feature of the Cemented Carbide is the potential to vary its composition so that the resulting physical and chemical properties ensure maximum resistance to wear, deformation, fracture, corrosion, and oxidation. In addition, the wide variety of shapes and sizes that can be produced using modern powder metallurgical processing offers tremendous scope to design cost-effective solutions to many of the problems of component wear and failure encountered in both the engineering and domestic environment.
The Cemented Carbides are a range of composite materials, which consist of hard carbide particles bonded together by a metallic binder.
The proportion of carbide phase is generally between 70-97% of the total weight of the composite and its grain size averages between 0.4 and 10 μm.
Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic Cemented Carbide structure from which other types of Cemented Carbide have been developed. In addition to the straight tungsten carbide – cobalt compositions – Cemented Carbide may contain varying proportions of titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). These carbides are mutually soluble and can also dissolve a high proportion of tungsten carbide. Also, Cemented Carbides are produced which have the cobalt binder phase alloyed with, or completely replaced by, other metals such as iron (Fe), chromium (Cr), nickel (Ni), molybdenum (Mo), or alloys of these elements.
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There are three individual phases which make up Cemented Carbide. In metallurgical terms, the tungsten carbide phase (WC) is referred to as the a-phase (alpha), the binder phase (i.e. Co, Ni etc.) as the b-phase (beta), and any other single or combination of carbide phases (TiC, Ta/NbC etc) as the g-phase (gamma). Other than for metal cutting applications, there is no internationally accepted classification of Cemented Carbides. |
This group of Cemented Carbides contains WC and Co only (i.e. two phases) and a few trace elements. These grades are classified according to their cobalt content and WC grain size.
The grades with binder content in the range 10-20% by weight and WC-grain sizes between 1 and 5 μm have high strength and toughness, combined with good wear resistance.
The grades with binder contents in the range 3-15% and grain sizes below 1 μm have high hardness and compressive strength, combined with exceptionally high wear resistance.
The Sandvik grade program also includes WC-Co grades which utilize a range of ultra-fine WC grain sizes (< 0.5 μm). With such fine, uniform grain sizes, a unique combination of hardness, wear resistance and toughness can be achieved.
The grades of Cemented Carbides in this group contain WC and Co as the main elements, although small additions or trace levels of other elements are often added to optimize properties. These grades are classified according to their Cobalt content and WC grain size and are often called the "straight grades". They have the widest range of strength and toughness of all the Cemented Carbide types and this is in combination with excellent wear resistance. This range of Cemented Carbides can be subdivided into its major application areas as follows:
HOW ARE CEMENTED CARBIDES MADE?
The manufacturing process begins with the composition of a specific tungsten carbide powder mixture - tailored for the application.
The tungsten carbide powder is compacted into a form.
In a high-temperature sintering furnace, the tungsten carbide structure of the blank is shaped at precise temperatures for strictly defined periods. During this heat treatment, the tungsten carbide blank undergoes shrinkage of some 50% in volume.
The sintered Cemented Carbide component gains its final finish by additional grinding, lapping and/or polishing processes.
The main use of tungsten (in the form of tungsten carbide) is in the manufacture of cemented carbides. After Scheele’s discovery of "Tungsten" in 1781, it took an additional 150 years before his successors’ efforts led to the application of tungsten carbide in the industry.
Cemented carbides, or hardmetals as they are often called, are materials made by "cementing" very hard tungsten monocarbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering.
MICROSTRUCTURE OF A WC-CO CEMENTED CARBIDE
The combination of WC and metallic cobalt as a binder is a well-adjusted system not only with regard to its properties, but also to its sintering behaviour.
The high solubility of WC in cobalt at high temperatures and a very good wetting of WC by the liquid cobalt binder result in an excellent densification during liquid phase sintering and in a pore-free structure. As a result of this, a material is obtained which combines high strength, toughness and high hardness.
The beginning of tungsten carbide production may be traced to the early 1920’s, when the German electrical bulb company, Osram, looked for alternatives to the expensive diamond drawing dies used in the production of tungsten wire.
These attempts led to the invention of cemented carbide, which was soon produced and marketed by several companies for various applications where its high wear resistance was particularly important. The first tungsten carbide-cobalt grades were soon successfully applied in the turning and milling of cast iron and, in the early 1930’s, the pioneering cemented carbide companies launched the first steel-milling grades which, in addition to tungsten carbide and cobalt, also contained carbides of titanium and tantalum.
By the addition of titanium carbide and tantalum carbide, the high temperature wear resistance, the hot hardness and the oxidation stability of hardmetals have been considerably improved, and the WC-TiC-(Ta,Nb)C-Co hardmetals are excellent cutting tools for the machining of steel. Compared to high speed steel, the cutting speed increased from 25 to 50 m/min to 250 m/min for turning and milling of steel, which revolutionized productivity in many industries.
Shortly afterwards, the revolution in mining tools began. The first mining tools with cemented carbide tips increased the lifetime of rock drills by a factor of at least ten compared to a steel-based drilling tool.
In all these applications, there has been a continuous expansion in the consumption of cemented carbide from an annual world total of 10 tons in 1930; to 100 tons around 1935; 1,000 tons in the early 1940’s; through 10,000 tons in the early 1960’s and up to nearly 30,000 tons at present.
The development of metal cutting tools has been very rapid over the last four decades, having been greatly stimulated by much improved design and manufacturing techniques, e.g. the introduction of indexable inserts in the 1950’s and the invention of coated grades around 1970.
The first coating was with a thin layer (~5 µm thick) of titanium carbide made by a Chemical Vapour Deposition (CVD) process. It improved the lifetime of tools by a factor of 2 to 5.
This technique has since been improved by multilayer coatings, where layers of alumina, titanium nitride, titanium aluminium nitride and other materials have been added which have further improved the lifetimes by 5 to 10 times.
However, coating and improved design are only one side of the coin. Continuous improvement of intermediates and manufacturing techniques led to improved performance of hardmetals and opened new areas of applications. The introduction of solvent extraction in tungsten chemistry, new techniques in hydrogen reduction and carburization improved the purity and uniformity of tungsten and tungsten carbide powder.
In parallel, new powder milling, spray drying and sintering techniques resulted in improved hardmetal properties and performance. Notably, the continuous improvement of vacuum sintering technology and, starting from the late 1980’s, hot isostatic pressure sintering (SinterHIP) led to new standards in hardmetal quality.
The history of tungsten powder metallurgy, and especially that of the hardmetal industry, is characterized by a steadily widening range of available grain sizes for processing in the industry; while, at the same time, the grain size distribution for each grade of WC powder became narrower and narrower.
The most important reason for this widening of the spectrum of available WC grades is that, besides those variations achieved by cobalt contents and some carbide additives, the properties of WC-Co hardmetals such as hardness, toughness, strength, modulus of elasticity, abrasion resistance and thermal conductivity can be widely varied by means of the WC grain size. While the spectrum of available WC grain sizes ranged from 2.0 to 5.0 µm in the early days of the hardmetal industry in the mid 1920’s, the grain sizes of WC powders now used in hardmetals range from 0.15 µm to 50 µm, or even 150 µm for some very special applications.
Based on the wide range of grain sizes now available, not only very hard and abrasion resistant, but also very tough, hardmetals can be produced for widespread applications in high tech tools, wear parts and mining tools as well as for many sectors of the engineering industry.
There has been a rapid development in mining and stone cutting tools, with improved performance which has led to the increasing substitution of steel tools by cemented carbide tools, in particular in the oil industry. Notably, the use of very coarse grained hardmetals is growing in this application area.
A large portion of the tungsten volume in cemented carbide is today used in wear part applications, where there is a wide range of products from the very small (such as balls for ball-point pens) to large and heavy products, such as punches, dies or hot rolls for rolling mills in the steel industry.
Most of these wear parts and the mining tools are made of straight WC-Co hardmetals without any addition of other carbides.
Fine and ultrafine grained WC hardmetals have become more and more important today in the field of wear parts, tools for chipless forming and cutting tools for cast iron, non ferrous alloys and wood.
Application range of straight grade cemented carbides
The first submicron hardmetals were launched on the market in the late 1970s and, since this time, the microstructures of such hardmetals have become finer and finer. The main interest in hardmetals with such finer grain sizes derives from the understanding that hardness and wear resistance increase with decreasing WC grain size.
A special application for these fine or ultrafine WC hardmetals, involving large quantities of cemented carbide, is in drills for the drilling of the very fine holes in printed circuit boards for the computer and electronic industries. For this purpose, new cemented carbide compositions, based on extremely fine-grained carbide, have been introduced.
Basic data for different WC-Co and WC-(W,Ti,Ta,Nb)C-Co hardmetal grades |
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|
Grade (wt%) |
Hardness HV30 |
Compressive strength (N Ч mm -2) |
Transverse rupture strength (N Ч mm -2) |
Young’s modulus (kN Ч mm -2) |
Fracture toughness (MPa Ч m -1/2) |
Mean thermal expansion coefficient (10 -6 Ч K -1) |
|
WC-4Co |
2000 |
7100 |
2000 |
665 |
8.5 |
5.0 |
|
WC-6Co/S* |
1800 |
6000 |
3000 |
630 |
10.8 |
6.2 |
|
WC-6Co/M** |
1580 |
5400 |
2000 |
630 |
9.6 |
5.5 |
|
WC-6Co/C*** |
1400 |
5000 |
2500 |
620 |
12.8 |
5.5 |
|
WC-25Co/M |
780 |
3100 |
2900 |
470 |
14.5 |
7.5 |
|
WC-6Co-9.5 (Ti,Ta,Nb)C |
1700 |
5950 |
1750 |
580 |
9.0 |
6.0 |
|
WC-9Co-31 (Ti,Ta,Nb)C |
1560 |
4500 |
1700 |
520 |
8.1 |
7.2 |
S* = submicron; M** = fine/medium; C*** = coarse