Machinery's Handbook, 31st Edition
1546 Powder Metallurgy Materials Advanced Materials.— Advanced materials outperform conventional materials with superior properties such as toughness, hardness, durability, and elasticity. There is al- ways a real need for better materials; the questions are how much better and at what cost? An applied scientist, with a particular application in mind, will scour lists of known ma- terials looking for one that meets his or her needs. The development of advanced materi- als can even lead to the design of completely new products, including medical implants and computers. If existing materials are unsuitable, applied and basic scientists must work together to develop new materials. The area of advanced materials research is very broad in scope and potential applications, and synergism between what is available and what needs to be developed reflects the important and complementary roles of the basic and applied sciences in materials science. While some advanced materials are already well known, it will take a few more years for others to appear in products. Here, we describe some advanced PM materials that are man ufactured using advanced processes such as pressing, metal injection molding, and spray forming. In addition, certain alloys are produced nearly exclusively by PM techniques. Superalloys : A superalloy, or high-performance alloy, is an alloy that exhibits specific desirable properties related to mechanical strength, stability at high temperatures, and surface resistance to oxidization and corrosion. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. The use of high alloy PM materials, notably superalloys, is most common in the production of near-net shapes and the forg- ing of preforms for aircraft turbine engines. Economic benefits have been the prime driv - ing force in the use of PM for the manufacture of these high cost alloys. These superalloys are processed from highly controlled metallurgically alloying ele ments, usually nickel, cobalt, or nickel-iron. Clean powders are produced by inert gas atomization methods that minimize surface oxidation during processing. After powder screening and cleaning, the powder is compacted using hot isostatic pressing (see Iso- static Pressing on page 1539). Thermochemical processing may be performed to en- hance mechanical properties or microstructure. Hot extrusion of the atomized powder is an alternate consolidation method (see Powder Extrusion on page 1543). PM processing offers the advantages of a homogeneous microstructure and near-net- shape configuration for a lower cost. As a result, countless pounds of PM superalloy components are now flying in both military and commercial aircraft engines. Additional applications of superalloys include parts for space vehicles, submarines, nuclear reactors, military electric motors, racing and high-performance vehicles, chemi- cal processing vessels, and heat exchanger tubing. Tool Steels: Tool steel refers to a variety of carbon and alloy steels that are particularly well suited to be made into tools. Their suitability comes from their distinctive hard- ness, resistance to abrasion, their ability to hold a cutting edge, and their resistance to deformation at elevated temperatures. It is well known that powder metallurgy provides a means of making new alloys and composites of tool materials, which cannot be made by conventional metallurgy, casting and hot-working methods. Powder metallurgy processing of high-speed steels has gained considerable significance over conventional ingot metallurgy processing. High-performance tool steels and high-speed steels are produced in mill shapes using PM process techniques. PM tool steels offer these advantages: • finer grain structures • improved homogeneity and distribution of secondary and carbide phases
• elimination of secondary stringers • less distortion in heat treatment • improved grindability • greater wear resistance • improved toughness and fracture strength.
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