Alloys for High-Temperature Use

Improper materials selection and processing Inadequate component design Component misuse 

Fracture in response to tensile loading and at relatively low temperatures may occur by ductile and brittle modes. 

Preventive measures may be taken inasmuch as evidence of plastic deformation indicates that fracture is imminent. 

More energy is required to induce ductile fracture than for brittle fracture. Cracks in ductile materials are said to be stable (i.e., resist extension without an increase in applied stress). 

For brittle materials, cracks are unstable—that is, crack propagation, once started, continues spontaneously without an increase in stress level. 

---

Actually, in many cases strength and mechanical properties become of secondary importance in process applications, compared with resistance to the corrosive surroundings. All common heat-resistant alloys form oxides when exposed to hot oxidizing environments. Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film. Thus, mild steel is seldom used above 500°C because of excessive scaling rates. Higher temperatures require chromium. This is evident, not only from Table 5, but also from Table 8 which lists the important commercial alloys for high-temperature use. 

The intermetallic alloy NiAl is discussed as a potential base alloy for high temperature structural materials. Its use is currently limited by low room temperature ductility and fracture toughness. Consequently, substantial research efforts have been directed towards understanding its mechanical behaviour [1, 2] so that detailed experimental [3, 4, 5] and theoretical [6, 7, 8] analyses of the deformation of NiAl are available today. 

The applications of arsenic as a metal are quite limited. Meialluigically, it is used mainly as an additive. The addition of from to 2% of arsenic improves the sphericity of lead shot. Arsenic in small quantities improves the properties of lead-base bearing alloys for high-temperature operation. Improvements m hardness of lead-base battery grid metal and cable-sheathing alloys can be obtained by slight additions of arsenic. Very small additions (0.02 - 0.05%) of arsenic to brass reduce dezincdfication. 

FIG. 1—Diagram of burner rig used to test alloys for high-temperature hot corrosion. 

The requirement for a strip rollable alloy suitable for high-temperature use in metal matrix composites (MMC s) led to the development of TIMETAL-21S (Beta 2IS). The alloy has creep resistance equivalent to Ti-6A1-4V combined with the advantages of a strip rollable, cold formable p alloy. It is finding many uses where its corrosion and oxidation resistance at elevated temperatiire are more important than the ability to withstand high stresses at elevated temperature. Applications up to 600 °C are anticipated. The alloy s oxidation resistance and mod-... 

If SCMV4 (2.25Cr - IMo alloy steel plate commonly used for boilers and pressure vessels and suitable for high temperature use) is used as the separator material, the maximum shell thickness is taken as 0.28 m and the number of the separators required is 4. The design pressure of the separator is 27.5 MPa (1.1 times 25 MPa). The total weight of the separators required is about 162 tons. 

Specialized alloys are used for high temperature appHcations on turbine blades, furnace parts, thermocouples, etc. These coatings can be as simple as iron—silicon—chromium or as exotic as chromium—aluminum—hafnium (36,41,52). 

The contact ends of printed circuit boards are copper. Alloys of nickel and iron are used as substrates in hermetic connectors in which glass (qv) is the dielectric material. Terminals are fabricated from brass or copper from nickel, for high temperature appHcations from aluminum, when aluminum conductors are used and from steel when high strength is required. Because steel has poor corrosion resistance, it is always plated using a protective metal, such as tin (see Tin and tin alloys). Other substrates can be unplated when high contact normal forces, usually more than 5 N, are available to mechanically dismpt insulating oxide films on the surfaces and thereby assure metaUic contact (see Corrosion and corrosion control). 

---