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Sintered carbides, also known as cemented carbides, are a group of composites which are essentially aggregates of particles of refractory metallic carbides added or bonded with an iron-group metal using liquid phase sintering, forming a body with outstanding properties of high hardness and wear resistance. The most well-known sintered carbides are WC-Co composites. From a technical and commercial point of view, sintered carbides are one of the oldest and most successful powder metallurgy (P/M) products. They are a typical example of the benefits of manufacturing composite materials from disparate phases.
Cemented carbides (or sintered carbides) are common hard materials which have outstanding mechanical properties that make them commercially useful in machining, mining, metal cutting, metal forming, construction, wear parts, and other applications [1-3]. Since the early 20th century, the cemented carbides have been widely used in many manufacturing processes that benefit from their combination of high hardness, fracture toughness, strength, and wear resistance.
Tungsten monocarbide (WC, usually referred to as tungsten carbide) was discovered by Henri Moissan in 1893 during his search for a method to make synthetic diamonds . He found that the hardness of WC is comparable to that of diamond. This material, however, proved to be so brittle that its commercial use was seriously limited. Subsequently, research was focused on improving its toughness, and significant contributions to the development of cemented carbides were made in the 1920's by Karl Schröter . Employing cobalt (Co) as a binding material, Schröter developed a compacting and sintering process for cemented tungsten carbides (WC-Co) that is still widely used to manufacture WC-Co composites. Most of the further developments were modifications of the Schröter's process, involving replacement of part or all of the WC with other carbides, such as titanium carbide (TiC), tantalum carbide (TaC), and/or niobium carbide (NbC).
The over all advances made on sintered carbides are mainly due to three important events occurred chronologically.
It was the discovery of multicarbides around 1930's, particularly those of TiC, TaC, NbC, and Mo2C, added to WC-Co to form solid solutions which improved the performance of WC-Co in high speed machining of steels.
The introduction of the indexable insert tips in metal cutting and mining resulted in a revolution in the application of sintered carbides, particularly in the 1960's expanding automotive industry.
Thirdly the emergence of the coated tools in the late 1960's was the third important progress in sintered carbides technology which brought about profound benefits in increasing metal removal rates with enhanced tool life.
Based on the elements used in the composition, cemented carbides can be classified into two broader groups:
1) Straight grades
Straight grades, sometimes referred to as unalloyed grades, are nominally pure WC-Co composites. 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. 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:
a) Corrosion Resistant Grades
This group contains Cemented Carbide grades in which the binder phase has been specifically designed to raise corrosion resistance to a level exceeding that of the grades that contain Co alone as the binder phase. This is achieved by alloying Co with elements such as Nickel (Ni) and Chromium (Cr), or completely replacing it with a more corrosion-resistant alloy.
The susceptibility of the binder phase of Cemented Carbides to wet corrosion can result in wear problems. Corrosion mechanisms give rise to surface depletion of the binder phase, permitting the carbide grains to become detached relatively easily by the wear process. Awareness of this situation is important to the selection of the correct Cemented Carbide for a particular application, like carbide cutting tools.
Cobalt is unsuitable as a binder phase in wet corrosion conditions. Sandvik has developed a series of highly corrosion resistant grades for these applications (carbide cutting tools i. e.). As illustrated, straight WC-Co grades are corrosion resistant down to pH 7. This is also valid for WC-Co grades containing g- phase (i.e. TiC, TaC and NbC). The highest corrosion resistance is obtained for the TiC-Ni grades, which are resistant down to pH1. However, compared with the straight WC-Co grades, they have low strength and inferior thermal conductivity. In addition, they are difficult to grind and have poor brazeability, and thus they are used only when corrosion resistance requirements are high, combined with low demands in terms of mechanical strength and thermal shock resistance.
In most corrosion-wear situations, an optimum choice is the WC-Ni grades, which are resistant down to pH 2-3. These grades retain WC as the hard phase, and substitute Co for Ni; thus they exhibit mechanical and thermal properties similar to the WC-Co grades.
b) Cubic and Cermet Grades
Cubic and Cermet grades are one of the latest developments for Sandvik Hard Materials. This group consists of grades containing a significant proportion of g-phase, (i.e. TiC, TaC, NbC etc.) together with WC and Co. The main features of the g-phase are good thermal stability, resistance to oxidation and high temperature wear. These grades are designed to provide a favorable balance of wear resistance and toughness in can tooling applications that generate high temperatures and entail close contact with ferrous materials. These conditions arise in metal cutting or high-pressure sliding contact situations involving the welding and galling of surfaces. Other common terms for these grades are the "can tooling", "metal-cutting" or "mixed-crystal" grades. In the extreme case, these grades are designed without any WC phase. Such hard metal grades are called Cermets and give a unique combination of high temperature hardness, chemical wear resistance and low density. Cermets are traditionally avoided for wear parts because of being more brittle than standard WC-Co grades. New developments have allowed toughness to be improved significantly and cermets are now applied in a number of demanding applications from advanced engineering components to high performance metal sawing blades.
c) Dual Property (DP) Grades
This group contains grades which have had the distribution of their binder phase modified in such a way as to create a material with different properties in the surface zone compared with the bulk.
This entirely new concept, developed by Sandvik, enables components to be produced which contain distinct microstructural zones, each with different binder content. Thus, each zone has different properties - hence the term "Dual Property".
d) Sandvik DP Carbide
For conventional Cemented Carbides, wear resistance and toughness are related in such a manner that an improvement in one property results in a deterioration in the other.
Sandvik has developed an entirely new type of WC-Co Cemented Carbide in which wear resistance and toughness can be improved independently of each other. By means of a controlled redistribution of the cobalt binder phase, Cemented Carbide components can now be made which contain three distinct microstructural zones, each of which has different properties. These gradients, together with the differences in thermal expansion, redistribute the internal stresses. For example, it is possible to create a very hard and wear-resistant surface layer which is simultaneously pre-loaded with compressive stresses to prevent the initiation and propagation of cracks.
Carbide having such a distribution of properties has high wear resistance at the surface combined with a tough core. These materials have therefore been given the designation DP - Dual Property. Carbide component's initial application area was in rock drilling. Other applications of carbide components, such as tools for tube and wire drawing and cold heading dies, have also confirmed improved performance.
e) Nano, Ultrafine and Submicron Grades
Grades with binder content in the range of 3-10 wt% and grain sizes below 1 Î¼m have the highest hardness and compressive strengths, combined with exceptionally high wear resistance and high reliability against breakage. These grades are used in a wide range of wear parts applications and in cutting tools and carbide drill bits designed for metallic and nonmetallic machining for which a combination of high strength, high wear resistance and sharp cutting edges are essential.
Ultrafine grades, an example: Today the trend is towards miniaturization: digital cameras, laptops and mobile phones are becoming even smaller and are expected to include more features. This has resulted in more complex printed circuit boards with a greater number of components per surface area. To meet this demand, PCB manufacturers are compelled to drill more and smaller holes with smallers carbide drill bits. This shift in drill size has increased the demands on tool material. The smallest drill-diameter in carbide drill bits today is only 10-20 Î¼m. To facilitate the use of tiny carbide drill bits and raise productivity, spindles with increasing rpm are being developed. It is now possible to purchase a standard PCB NC machine with a maximum speed of 300,000 rpm.
Sandvik Hard Materials has supplied high performance Cemented Carbide blanks and carbide drill bits to toolmakers in the printed circuit board (PCB) industry since 1983. During 1986-88 the ultra-fine grades (UF grades) were developed and introduced in the market. The ultrafine grade family boosted our customers' productivity and became the market-leading material for tools like carbide drill bits in the PCB industry.
f) Fine and Medium grades
The grades with binder contents between 6-30% and grain sizes of 1-3 Î¼m are used in wear parts and cutting tools and carbide rolls when an element of improved strength and shock resistance is required.
g) Medium Coarse, Coarse and Extra Coarse grades
Grades with binder contents between 6-15 wt% and grain sizes above 3Î¼m are used in Oil & Gas and mining applications where resistance to high impact stresses and abrasive wear are required.
Coarse grades: In today's competitive Oil & Gas drilling environment, the pursuit of faster, economical and superior wells has conjured a host of technological advances.
However, in the end, it all comes down to the drilling bit. Cemented Carbide is an ideal material for drilling inserts and carbide drill bits due to its high hardness, compressive strength and thermal conductivity. Research and Development within Sandvik Hard Materials has taken these properties and used innovative techniques to improve the toughness and impact resistance, while reducing the risk of thermal effects of the carbide drill bits during drilling.
The variety of material that Sandvik can supply provides coverage for a variety of application needs. In soft rock/heat generation formations, engineers typically select grades that are extra coarse with high binder content. These characteristics result in high fracture toughness and prolonged carbide drill bit life. Medium coarse grades with low binder content are generally used in drilling hard formations. This results in high hardness (better abrasion resistance), but low fracture toughness, ultimately having a higher penetration rate but increased likelihood for fracture.
Sandvik manufactures a large variety of inserts and carbide drill bits used in rotary & percussion rockbits for Oil & Gas and mining industries. Extreme drilling conditions, whether rotary, percussion or downhole, require unique solutions. Sandvik has the technology to supply the customers with inserts and carbide drill bits that perform every time.
ii) Alloyed grades
Alloyed grades are also referred to as steel cutting grades, or crater resistance grades, which have been developed to prohibit cratering during the machining of steel. The basic compositions of alloyed grades are 3-12 w/o Co, 2-8 w/o TiC, 2-8 w/o TaC, and 1-5 w/o NbC. The average carbide particle size of these grades is usually between 0.8 and 4 Î¼m.
These straight and alloyed grades pretty much cover most of the cemented carbides. However, these carbides can also be classified based on their applications or even features at times, which are more suitable from and application point of view.
The physical properties of these composites depend on microstructural features, such as grain sizes, size distributions, grain shapes, orientations, misorientations, and the volume fraction of the carbide phase. The hardness, toughness, and fracture strength of WC-Co composites range from 850 to 2000 kg/mm2 (Vickers hardness, HV), from 11 to 25 MPa (critical stress intensity factor, plane strain fracture toughness, KIC), and from 1.5 to 4 GPa (transverse rupture strength, TRS), respectively. Also, it is known that the wear resistance of these materials is five to ten times higher than that of a typical tool steel. The details of the physical properties of WC-Co composites will be described in Chapter 2. It should be noted at the outset that while the relationships between the mechanical properties and the mean grain size, carbide volume fraction are known, the influence of grain shape, size distribution, and interface character distribution are not yet clear. Furthermore, it is not clear how changing these microstructural characteristics beyond normally observed ranges alters the properties of the composites.
The applications of sintered carbides are in a sense a mirror to show the features of this group of composites. Their utilisations surprisingly cover almost every industry. These applications may be classified into the following five basic categories
1. Metal Cutting
Metal cutting tools must be able to withstand high temperatures and temperature gradients, severe thermal shocks, fatigue, abrasion, attrition, and diffusion wear, because of the intimate contact between the work piece and the tool materials during chip removal. Tool temperature and contact time between the newly-formed chips and the tool are often around 1000oC and a millisecond, respectively. Contact stress may reach up to 200-500 MPa. Sintered carbides usually have a high modulus of elasticity, but exhibit little ability to undergo plastic deformation. Therefore the sintered carbides tips have been used as indexable inserts supported by the tool body, usually plain carbon steel with medium carbon content, of adequate section size in order to withstand the localised contact
stresses induced during heavy cuts. The majority of carbides consumed in industry are for metal cutting applications. A total of 90-95% of the cutting tool market is covered by steels and sintered carbides, and almost 95% of the available sintered carbides are WC based. Generally, the hardest grades of sintered carbides are selected for light continuous finishing cuts, while the tougher grades are used for roughing and heavy cuts or for intermittent cutting involving vibrational or impact forces.
2. Metal Forming
Both hot and cold metal forming operations are carried out using sintered carbides tools and dies. Cold drawing of rods, wires, and tubes employs sintered carbides dies and mandrels, while in the cold rolling of strips and foils with good surface finish, carbide rolls are advantageous. Hot working tools, including extrusion dies and drop-stamping dies, have been made of sintered carbides, although they suffer from lack of toughness and thermal shock resistance in comparison with nickel-base high-temperature alloys which are dominant materials in this field.
3. Earth Drilling
In mining industry, carbide tools are widely used for picks, rotary drills, pucussion drills, and other tools subjected to severe wear by the minerals involved. It is estimated that almost 90% of aIl pneumatic drilling of hard rocks is done with carbide insert tips. Carbides insert tips are almost mandatory for drilling rocks harder than limestone, and their use has all but made the conventional hard faced steel teeth obsolete, both on the basis of performance and economics.
4. Wear Protection
Owing to their resistance to abrasive wear, sintered carbides also find extensive utilisation in applications where abrasive wear is of prime concern. Typical examples using sintered carbides for wear protection include nozzles and valves in plastic processing, guides and cones for wire drawing, brick mould liners, facing for hammers in hammer mills, jaw crushers, ball milling linings, sand blast nozzles and wear pads in machining. Obviously, the severity of wear damage in these applications depends on the nature of the abrading materials and surrounding medium, the temperature and the relative speed of constituent components of the wear system.
5. High-Rigidity Structural Components
The high modulus of sintered carbides, about three times that of steels, enables them to be used in applications where high rigidity is the prime requirement e g. boring bars. The diversified applications of sintered carbides may suggest a need for a wide range of grades. The classification of these carbides is somewhat confusing and controversial, since it is based on their applications rather than compositions or properties. The simplest classification of sintered carbides recognises two broad categories:
1. The "straight tungsten carbides", used primarily for machining cast iron, austenitic steel,
nonferrous and non-metallic materials;
2. The grades containing considerable percentages of titanium and tantalum carbides, used primarily for machining ferritic steels.'
As for the research activities surrounding sintered carbides, any other P/M products have never caught more attention. The earlier efforts were engaged in explaining the empirical relationships between production conditions and properties. By the early 1950's, most of the basic steps essential for understanding the production processes and properties measured for quality control, ie. hardness and transverse rupture strength, had already been well known. With the aid of more advanced scientific methods and facilities, many of the classic mysteries of sintered carbides, from basic physical metallurgy, physical and chemical properties, wear and other operating mechanisms in numerical practical applications, to the design of new applications were solved or more clearly understood during the last three decades. The occurrence of coated sintered carbides further expanded the spectrum of research and development work. Consequently, publications are in plenty.
Some Recent Developments in the field of Cemented Carbides
Over the past two decades, substantial research efforts have focused on the synthesis and sintering of nanosized tungsten carbide powders in an attemot to manufacture cemented carbide materials with nanocrystalline grain structure. It has the potential to significantly improve the mechanical properties of these materials. It would be quite advantageous to explore these improved properties to increase the lifetime of tungsten carbide tools. Due to industrial significance, efforts are being made to produce tungsten carbide based materials with nanoscale grain sizes. As far as synthesis of nanosized powders is concerned, many different processes have been used to produce nanostructured tungsten carbide and tungsten carbide-cobalt composite powders. These technologies range from improvements to the conventional solid state synthesis to radical techniques, namely spray conversion and chemical vapor reaction and deposition methods. Huge and significant technology developments have also taken place in terms of sintering. However, nanocrystalline WC-Co powders lose their nanoscale characteristics upon sintering similarly to the sintering of most nanosized powders, due to extremely rapid grain growth during sintering. Although commercial processes are now available for producing sintered WC-Co with ultrafine grain sizes (<200 nm), controlling grain growth during sintering and producing bulk nanocrystalline hard materials remains a critical technology challenge.
The mechanical properties of these cemented tungsten carbide using nanosized WC-Co powders are also of immense interest. It is quite understandable that the hardness of the materials made from nanosized powders are significantly higher than what could be attained from conventional powders. The literature reports on the fracture toughness of these materials, however, lack much in agreement as those on the hardness. There is no clear picture yet on whether the materials using nanosized powders offer any advantage with regard to their fracture toughness. There is strong evidence, however, that sintered materials with ultrafine grain sizes have extremely high flexural strengths, or transverse rupture strengths, as it is known in the hardmetal industry. There are also strong indications of dramatic shift in the mechanical behavior when the grain size of WC-Co becomes progressively finer. It is noted that the potential of fully consolidated cemented tungsten carbide with true nanoscale
grain sizes (<30 nm) remains unexplored because no such materials have been made. For those materials, especially metallic structural materials, the inability to achieve nanoscaled grain sizes at the sintered state has also hindered efforts to characterize and understand their mechanical behavior as nanostructured materials. Cemented tungsten carbide is one of those materials.
This paper provides an overview of the development of nanocrystalline cemented tungsten carbide materials. The review will first summarize different methods of nanosized powder synthesis including both monolithic tungsten carbide (WC) and composite WC-Co powders. The review of the sintering and consolidation of the nanosized powders will emphasize the challenges and the progress toward achieving nanoscale grain sizes, or grain sizes that are as fine as possible, at sintered states. In the last section of this review, the mechanical properties of WC-Co materials made from nanocrystalline powders will be summarized.