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Metal Casting Technologies : March 2009
TECHNICAL FEATURE Ferrous casting alloys for heat resistance Dr. John Pearce Introduction H eat resistant cast parts are needed for efficient operation of high temperature processes in many applications including power plants, refineries, furnaces and combustion engines. For example in the production of cement, as shown in Figure 1, cast heat resistant steel plates are used to handle and cool the hot cement clinker discharged from a kiln at around 1000o C. To withstand the demands of service at high temperatures candidate casting alloys require structural and dimensional stability, creep and thermal shock resistance plus oxidation, scaling and wear resistance. They may also need specific resistance to various combinations of damaging constituents, such as Carbon, Nitrogen, Halogens, Sulphur, molten salts and ash that can be present in high temperature working environments. Aside from cost considerations, as for all heat resistant materials, alloy selection will depend on the following: ? ? Temperature cycles & heating/cooling rates Mechanical and thermal stresses, effects of expansion and thermal conductivity ? Environmental factors: resistance to oxidation, carburization, wear, etc. ? Requirement for machining to final shape. This article provides an outline of the basic metallurgy of some cast irons and cast steels that can be considered for applications requiring heat resistance. Heat resistant cast irons In general unalloyed grey cast irons are resistant to scaling and growth at 36 www.metals.rala.com.au Figure 1(b). Worn and cracked cement clinker plates after about 150 days in service. Each plate is approximately 450 by 300mm in size temperatures up to around 350o 400o C. Above C microstructural changes and scaling begin to severely limit their performance although, where mechanical strength is not a major factor, normal grey irons containing 0.5-1% Phosphorus (for improved scaling resistance) can be used up to about 600o C. Specially alloyed cast irons are therefore needed to withstand higher temperature service conditions. The problem of growth When normal pearlitic grey (flake graphite) irons are cycled through the eutectoid temperature range (650-750o These are: ? C) they undergo volume changes caused by structural transformations. Pearlite + Ferrite ? Austenite on heating and the reverse reaction on cooling, and Figure 1 (a). Arrangement of cast heat resistant steel plates on cement clinker cooling table