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Metal Casting Technologies : March 2009
Heat resistant cast steels In many applications low alloy Chromium-Molybdenum steels can be successfully used at temperatures up to around 650o C. Above this temperature they exhibit sufficient high temperature corrosion resistance but are not recommended for use since they have limited creep resistance. Creep refers to the progressive plastic deformation that occurs with time when alloys are under stress at elevated temperatures. Face centred cubic metals such as Iron, Nickel and Cobalt strengthened by solute and precipitation hardening provide higher creep resistance than body centred cubic matrices. Hence austenitic Chromium- Nickel- Molybdenum steels are selected for higher temperature usage up to around 1150o this level advanced ceramics must be considered . Heat resisting steels are used in the C. As well as having superior creep resistance due to their low stacking fault energy FCC matrix, they also have much better spalling resistance during thermal cycling than lower alloy steels. A number of 25%Cr-20%Ni and 17%Cr-25%Ni type alloys have been especially developed for high temperature applications . Superalloys which are based on the binary systems of Fe-20%Cr , Ni-20% Cr, or Co-30%Cr are designed for severe environments, e.g. Nimonics, Inconels, Hastelloys, etc. These can be used at up to 1200o C, but above Grade Ni-Resists D2 D2B D3 D4A D5S ACI Cast Steels HH 25Cr-12Ni HK 25Cr-20Ni HT 15Cr-35Ni HU 19Cr-39Ni HN 20Cr-25Ni C 3.0max “ 2.6max “ 2.3max 0.20-0.50 0.20-0.60 0.35-0.75 “ 0.20-0.50 as-cast condition and are not usually heat treated. Typical microstructures  are shown in Figure 5. View (a) is typical of an as cast austenitic heat resisting steel and shows a network of divorced eutectic carbides in the interdendritic regions. In service at elevated temperatures the structure cannot remain stable and over a period of time, depending on the exact composition and service temperatures etc., secondary carbides, as shown in Figure 5(b), and other phases such as sigma can precipitate out both within the grains and at grain boundaries. These changes will progressively lower both heat resistance and mechanical properties. Sigma phase is a hard brittle Fe-Cr compound, during service its formation at grain boundaries and especially as a coarse needle-like precipitate will lower the creep rupture strength and will also result in severe embrittlement when the steel is cooled to ambient temperatures. Heat resistant cast steels and alloys for general applications are covered by ISO 11973:1999 and the classification of austenitic cast irons is covered by ISO 2892:2007. The heat resistant steels are usually described with reference to their Ni 18.0-22.0 “ 28.0-32.0 “ 34.0-37.0 11.0-14.0 18.0-22.0 33.0-37.0 37.0-42.0 23.0-27.0 Cr 1.75-2.75 2.75-4.0 2.5-3.5 1.5-2.5 1.15-2.25 24.0-28.0 “ 15.0-19.0 17.0-21.0 19.0-23.0 Alloy Casting Institute (ACI) designation. Some examples of composition for austenitic irons and steels are listed in Table 1. As cast structure: austenite matrix with interdendritic network of eutectic carbides. (x500) After soaking at 980o C for around 75 days. Precipitation of secondary carbides within the matrix and coarsening of the interdendritic/grain boundary carbides.(x500) ¦ References: 1. A.R. Bailey & L.E. Samuels. “Foundry Metallography”. (1971) Metallurgical Services, England, 211pp. 2. “Properties and Applications of Ni-Resist and Ductile Ni-Resist Alloys”. (1998) Nickel Development Institute Publication, 38pp. 3. J.W. Grant and J.C. Morrison. “Production of castings in flake and spheroidal graphite austenitic irons”. The British Foundryman (1972) Vol.65. pp. 172-185. 4. J.T.H Pearce. “Abrasive wear behaviour of alloy cast irons”. British Foundryman (1985) Vol. 78, pp. 13-23. 5. J.W. Boyes. “High chromium cast irons for use at elevated temperatures”. Iron and Steel (1966) Vol. 39 March pp.102-109. 6. G.J. Cox. “Some effects of composition on the properties of cast austenitic heat-resisting steels”. British Foundryman (1980) Vol.73, pp1-21. 7. J.R. Davis. “Heat Resistant Materials”, (1997) ASM Specialty Handbook, 591pp. “Heat Resistant Casting Alloys”. ASM Handbook Vol. 9, Metallography & Microstructure, pp.330-350. Si 1.0-3.0 “ 1.0-2.8 4.0-6.0 4.9-5.5 2.0max “ 2.5max “ 2.0max Table 1. Examples of alloy compositions (in wt.%) for some austenitic ductile irons and austenitic steels. METAL Casting Technologies March 2009 41 Mn 0.7-1.25 “ 1.0max 0.5-1.5 1.0max 2.0max “ “ “ “ Other 0.5max Cu “ “ “ “ 0.5maxMo “ “ “ “