TUNGSTEN CARBIDE AND COBALT
Tungsten carbide’s (WC) highly Solubility in the solid and Cobalt as a liquid binder at high temperatures provides a very good wetting of WC and results in an excellent densification during liquid phase sintering and in a pore free structure.
Particle size Variations in Tungsten Carbide lead to an astonishingly broad band of property relationships and possible applications, which range from high-strength steels on the tough side (Vickers Hardness: about 800 HV10) to hard Ceramics on the hard and wear resistant side (Vickers Hardness: 2800 HV10). However, compared to hard Ceramic materials such as aluminum oxide or silicon carbide which show brittle behavior during loading, cemented carbides always exhibit a considerable toughness, due to the part metallic nature of the composite.
This remarkable relationship between high hardness and considerable toughness, and the high flexibility of the materials in terms of property combinations which make cobalt based cemented carbide tools so successful in the machining industry. A too low toughness (resistance against crack propagation) would easily result in premature material failures during working. Beyond that, even hard materials, such as hardened steel, high strength titanium alloys and ceramics or fiber reinforced composites, can be machined at a high level of productivity.
These straight grades of united wolfram (containing WC and Co only) have maintained their unique position in the tooling industry, due to their outstanding properties. Additions of other hard carbides or carbo nitrides (TiC, Ti(CN), TaC) or alternative binder materials (Ni, NiCr, FeNiCo) have widened the application range in certain directions, for example in the machining of steel (TiC, TaC) or corrosion and oxidation resistant environments (NiCr), but the two phase materials (WC-Co) still demonstrate their predominance in numerous applications.
Cemented carbides are produced by Powder Metallurgy (PM). The respective powders (WC, Co, but also other metallic carbides or carbo nitrides as well as Fe and Ni are at first ball milled or attritor milled to form a powder mix. Then, a part is formed by different shaping technologies. In the case of large lot sizes and comparatively simple geometries, as with cutting inserts or mills, the part is formed by die pressing to its final shape (direct forming). Cylindrical shaped parts or parts with large length-to-diameter ratio are formed by extrusion. Plastifiers (e.g. waxes) are added prior to extrusion to render a smooth flow of the powder mix through the die. Small parts with complex geometries can be shaped by PIM (Powder Injection Moulding). In this case the plastified material is pressed into a mould, which is subsequently opened to remove the shaped part (indirect forming).
Large parts, such as rolls, hobs, anvils or rotary cutters for the hygiene industry are mainly produced by cold isotactic pressing (CIP). A “green” (isostatically pressed) block is formed at first and subsequently machined to the desired shape, either in the pressed or presintered stage, to improve the strength of the still porous part (40 to 50 vol%) for the shaping.
After shaping, the materials undergo a thermal treatment, called sintering, to form a dense, near-pore free body (residual porosity commonly below 0.02 vol%). Sintering is done either in vacuum or under hydrogen. Pressure-aided sintering (sinter hip) has become a standard technology to produce defect-free materials of outstanding strength.
During sintering, the body shrinks as a result of pore elimination (lateral shrinkage between 17 and 24%) but retains its shape. The better is the manufacturing process (milling, granulation, pressing, sintering), the more the part can be sintered to its final geometry (near-net-shape technology), and the less material has to be removed by subsequent precision grinding by diamond tools.
The result of sintering is a material with varying shape and composition (depending on the respective formulation for subsequent application), outstanding strength properties, high hardness, and high modulus of elasticity at a still considerable toughness level.
Cemented carbide wear parts are used in wire and section drawing, cold and hot rolling, stone-working, working of wood and plastics, in the textile, magnetic tape and paper industries, in the food and medical industries, the glass industry, for stamping and punch drawing (e.g. can making) and a large number of structural components, including plungers, boring bars, compacting dies and punches, high pressure dies and punches, seal rings, pulverizing hammers, needles, carbide feed rolls, chuck jaws, and others.
Cemented Carbide’s Very Important Application which is highly used in lots of Sector where Machining takes place and necessary. In terms of worldwide turnover, this segment is by far the largest. Cemented carbides in this field exhibit WC grain sizes from 0.5μm to 5μm and cobalt in the range of 3% – 12 %. which exhibit WC and Co only are used for the machining of cast irons, hardened steel, stainless steels, Nonferrous metals, nickel-based high strength alloys, wood, plastics or composites. WC-(W, Ti, Ta, Nb)(C,N)-Co grades (so-called steel cutting grades) are used in machining of steel, especially for long chipping alloys.
Cemented carbide indexable inserts with complex geometries are applied in all kinds of machining operations, such as turning, milling, grooving, threading, drilling, etc. Individual tools are equipped with up to around 300 inserts.