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Metal Casting Technologies : June 2007
51 Chills Chilling can be used to neutralize hot-spots such as isolated heavy sections, fillets and junctions that cannot be directly fed. It should be noted that whilst chills can be a useful tool in eliminating hot-tears, their use can also initiate hot-tears. As the steel solidifies rapidly on the face of the chill, it also begins to contract there. At the sand surface near the edges of the chill, the metal is still mushy and often unable to withstand the contraction pull across the face of the chill and so hot-tears are formed near the edges of the chill. An example of this mechanism is shown in the computer simulation of hot-tear potential in figures 2, 3 and 4. Care should be taken to ensure the chills are not too severe for the section involved and if multiple chills are employed, the buffer of moulding material between the chills should be approximately equal to the length of the chill. Chromite or zircon sand can be effective in preventing tears in thin sections, particularly in fillets and junctions. Cooling brackets can help reduce tears in fillets, however to be effective, their thickness should be less than 0.25 the thickness of the casting section to which they are attached. Casting Design Good casting design with uniform distribution of metal thickness can reduce the incidence of hot-tearing. Abrupt changes of section should be avoided, wherever possible section changes should be gradually blended. Internal corners should be adequately filleted. Casting Flash & Fin Casting flash or fin can often initiate hot-tears due to the rapid solidification time and subsequent restraining effect of the flash or fin. Ensure pattern equipment is not contributing to hot-tears through poor fitting core-prints which cause flash and fin. METAL Casting Technologies June 2007 Figure 4. Predicted hot-tears near edges of chill. References: 1. R. Wlodawer. Directional Solidification of Steel Castings -- Pergamon Press, 1966 2. W. J. Jackson, M. W. Hubbard. Steelmaking For Steelfounders -- Steel Castings Research and Trade Association, 1979 3. Heine, Loper and Rosenthal. Principles of Metal Casting Second Edition -- McGraw-Hill, 1967 Figure 2. Steel Housing casting with external chill on bottom face Figure 3. Contraction pull of solid steel across chill face.