Internal high-temperature alloys

Haynes High Temperature Alloys, Haynes International, Inc., Kokomo, Ind., 1984 to 1993. 

J. H. Park and W. D. Cho, Surface Modification of High Temperature Alloys A Protective and Adhesive Scale-Forming Process, Presented at 4th international Symposium on Processing and Eabrication of Advanced Materials IV, Cleveland, Processing and Fabrication of Advanced Materials IV, eds. T. S. Srivatsan and J. J. Moore, 1995, pp.287-304. 

C.D. Sorensen, T.W. Nelson, and S.M. Packer, Tool Material Testing for FSW of High-Temperature Alloys, Proceedings of the Third International Conference on Friction Stir Welding, Sept 27-28, 2001 (Kobe, Japan), TWl, paper on CD... 

In the case of carburisation, the carbon dissolution in high-temperature alloys leads to internal carbide formation. The carbides M23C6 and M7C3 (M = Cr, Fe, Ni) are formed at low carbon activities between 10 starting with M23C6. With increasing Oq, M23C6 takes up more Fe and Ni and later... 

Figure 5-13. Cross-sectional optical micrograph showing the subsurface region in a commercial, high-temperature alloy after long-term oxidation. The subsurface region consists of large voids and smaller oxide precipitates (courtesy of Haynes International).

Carburization involves the formation of internal carbide precipitates. For reasons discussed in Section 5.4, internal carburization is deleterious to the service life of a high-temperature alloy, particularly if the alloy is subjected to thermal cycling down to room temperature. Carburization is a problem mainly for components exposed to process environments above about 900 °C and containing a high content of CO, CH4, or other hydrocarbons. As an example, severe carburization occurs in the cracking tubes used in ethylene and other olefin plants (Kane and Cayard, 1995). Carburization can also be a problem in the heat-treatment of components associated with carbur-... 

Oxidation morphology is globally the same for other Cr-rich high-temperature alloys like Alloy 230 and AUoy X exposed to oxidizing helium but for the extent of processes. Fig. 3.13 plots evolution of the above-listed microstructural features with time. All oxidation-induced phenomena visibly follow parabolic laws with specific rate constants. It was shown that surface oxidation is the first contributor to the mass gain, with a measurable part issuing from internal oxidation for AUoy 617. 

P.J. Ennis, D.F. and Lupton D.F, Proc. of the Petten International Conference on Behaviour of high temperature alloys in a ressive environments. The Metals Society, London, 1980, 979. 

The role of the reaction equilibrium in the corrosive environment for internal corrosion (in this example internal sulfidation) under a defective (originally protective) oxide scale, (a) Oxygen not involved in the reaction equilibrium, no pS2 gradient, (b) Oxygen involved in the reaction equilibrium, pSj increases through the oxide scale in inward direction. (From Grabke, H. J. et al., in High Temperature Alloys for Gas Turbines and Other Applications, eds., E. Betz et al, D. Reidel, Dordrecht, the Netherlands, 1986, p. 245.)... 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

D02 G.E. Duvall, D.E. Davenport, and J.J. Kelly, Metallurgical Effects of Explosion-Induced Shock Waves, in Research Seminar on High Nickel Alloys for High Temperatures, Iron-Nickel Alloys, Stainless Steels (The International Nickel Co., New York, 1960). 

K. Higashi, T. Nakamura, T. Mukai, S. Tanimura, "High temperature deformation characteristics of extruded 2090 Aluminium - Lithium Alloys in a wide range of strain-rate" International Aluminium - Lithium Conference, Garmisch - Partenkirchen, aluminhium -Lithium vol 2. 1114-1116, 1992, Publ Deutsche Gesellschaft fur Materialkunde e.V. Oberursel, Germany. 

There are no significant high-temperature applications for alloys of nickel with iron. The scales formed in air consist of nickel oxide and iron oxide and the latter is usually present in the form of the spinel, NiO-FejOj . In the case of the more dilute nickel alloys, internal oxidation of nickel was Observed S. Substitution of a substantial proportion of nickel by iron results in a deterioration in the oxidation resistance of nickel-chromium... 

An excellent reference book for the high-temperature corrosion resistance of materials of construction is George Y. Lai, High-Temperature Corrosion of Engineering Alloys, ASM International, Metals Park, Ohio, 1990. 

Creep resistance will be important if the material is subjected to high stresses at elevated temperatures. Special alloys, such as Inconel (International Nickel Co.), are used for high temperature equipment such as furnace tubes. 

When rhodium is combined with platinum and palladium, the elements together form the internal metals of automobile catalytic converters, which convert hot unburned hydrocarbon exhaust gases to less harmful CO and H O. Similar alloys are used to manufacture high-temperature products such as electric coils for metal refining furnaces and high-temperature spark plugs. 

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