Carbides, Cemented

Abstract

Abstract Cemented carbides belong to a class of hard, wear‐resistant, refractory materials in which the hard carbides of Group 4–6 (IVB‐VIB) metals are bound together or cemented by a soft and ductile metal binder, usually cobalt or nickel. Although the term cemented carbide is widely used in the United States, these materials are better known internationally as hard metals. Cemented carbides are manufactured by a powder metallurgy process consisting of a sequence of carefully controlled steps. The carbides or the carbide solid solution powders are prepared and blended with very fine binder metal powder, cobalt or nickel, in ball mills, vibratory mills, or attritors using carbide balls. The powder blends are compacted, presintered, and shaped, and the carbide subjected to sintering and postsintering operations. Inhalation of extremely fine carbide, cobalt, and nickel powders should be avoided. Efficient exhaust devices, dust filters, and protective masks are essential when handling these powders. Recycling of cemented carbide scrap is of growing importance. The performance of a tool material in a given application is dictated by its response to conditions at the tool tip. High temperatures and stresses can cause blunting from the plastic deformation of the tool tip, whereas high stresses alone may lead to catastrophic fracture. In addition to chemical analysis, a number of physical and mechanical property evaluation techniques are employed to determine cemented carbide quality. Standard test methods employed by the industry for abrasive wear resistance, apparent grain size, apparent porosity, coercive force, compressive strength, density, fracture toughness, hardness, linear thermal expansion, etc, are set forth by ASTM/ANSI and the ISO. Among the physical properties, cemented carbide density is very sensitive to composition and porosity of the sample and is widely used as a quality control test. The properties and performance of cemented carbide tools depend not only on the type and amount of carbide but also on carbide grain size and the amount of binder metal. Information on porosity, grain size, etc, is obtained by optical and scanning electron microscopy. Among the mechanical properties, hardness and transverse rupture strength (TRS) are often evaluated for quality control. Early carbide metal‐cutting tools consisted of carbide blanks brazed to steel holders. Indexable inserts were introduced in the 1950s. In this configuration, the so‐called throwaway carbide insert is secured in the holder pocket by a clamp instead of a braze. When a cutting edge is worn, a fresh edge is rotated or indexed into place. For machining purposes, alloys having 5–12 wt% Co and carbide grain sizes from 0.5 to >5 µm are commonly used. Use of ultrafine‐grained carbides (<0.5 µm) is expected to grow not only for woodworking tools, wear parts, printed circuit board microdrills, and endmills, but also for metal‐cutting insert applications. The straight WCCo alloys have excellent resistance to simple abrasive wear and are widely used for machining short‐chipping materials such as gray cast irons and non‐ferrous alloys as well as for nonmetal‐cutting applications such as mining, oil and gas drilling, transportation and construction, metalforming, structural, and forestry tools. Steel cutting compositions typically contain WCTiC(Ta,Nb)CCo. Today coated carbides account for nearly 75% of all metalcutting tools used in the United States and chemical vapor deposition (CVD) accounts for ∼70% of coated carbides. The rest are coated with physical vapor deposition (PVD) process. CVD coatings have evolved from single layer TiC coatings to multilayer hard coatings (5–20 µm total thickness) comprising various combinations of TiC, TiCN, ZrCN, TiN, and Al 2 O 3 . CVD coatings are deposited at high temperatures (∼1000°C) or moderate temperatures (MT–CVD, ∼850°C) or by plasma assisted process (PA–CVD, ∼600°C). Multilayered CVD coatings on specially tailored substrates have widened the metal‐cutting application range of the coated tools. In the mid 1980s physical vapor deposition (PVD) emerged as a commercially viable process for depositing thin (2–5 µm) hard coatings on cemented carbide tools at <550°C. Typical commercial coatings include TiN, TiCN, TiAlN, TiB2, CrN, and multilayers of TiN–TiAlN. The PVD process offers unique advantages, including the ability to apply a fine‐grained, smooth, low‐friction, and thermal crack‐free coating over sharp edges. PVD coatings also feature compressive residual stresses, which are beneficial in resisting crack propagation and preventing premature tool failure. Nano‐layered PVD coatings (alternating very thin layers ∼20 nm thick) have been introduced. Nano‐composite coatings are in the development stage. Today almost one half of the total production of cemented carbides is used for nonmetal‐cutting applications such as mining, oil and gas drilling, transportation and construction, metalforming, and forestry tools. The majority of compositions used in these applications comprise straight WCCo grades. There are >200 cemented carbide producers in the world. A majority of hard metal production can be attributed to several of the larger producers. Many of the smaller producers have narrow manufacturing capabilities and a limited range of product offerings. Developments in materials, coatings, and insert geometries have claimed an increasing share of research and development budgets in the cemented carbide industry. Continuing developments in computer‐numerically controlled machining systems have placed a heavy emphasis on tool reliability and consistency.