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HOT TOPICS Issue # 2 2004

Chill Testing of Cast Iron

Chill testing of cast irons in general, which includes ductile base irons, is the old, but still used, method to measure the nucleation potential (or chilling tendency) of the melt prior to casting. The term nucleation potential is just another way of saying how the graphite (or lack of it) will form upon solidification. Since this graphite form or type and amount are critical to the mechanical properties obtained in a particular cast section, it is wise to check it beforehand and adjust the furnace chemistry/nucleation if necessary. The standard methods for chill testing of cast irons is described ASTM A367. There are two basic methods; the Wedge test and the Chill test (see figure 1). Wedge test method The wedge test is made in a sand test core and is generally used for higher strength irons > 25,000 psi tensile, ductile base iron and for castings with sections of typically 3/8 inch and over. This method is sometimes used for final ductile iron ductility and microstructure testing as well. Chill test method The chill test method, where the metal is cast against a chill plate is generally used for higher C.E. (hypereutectic) & high Si irons, thin sections and where a specified minimum or maximum amount of chill must be measured. Both of these methods utilize different wedge sizes that correspond to the

typical chill values normally encountered from the section cooling rates and pouring temperatures used. The wedge size is usually adjusted upward (to next larger size) when the chilled area (clear chill) typically exceeds more than the total width of the fracture face of the specimen. Of course the size will depend upon whether the test is used for base iron (often called the constitutional wedge) or the final (inoculated or white) iron. The sizes and shape of each type are well described in the standard. Measurement of the chill value is made in increments of 32nds of an inch of width at the location where clear chill separates from the mottled gray structure in a wedge test. A 3/32 inch wide measurement is called a 3 wedge and may be also related back to the wedge size, if more than one size is used in a particular foundry. Note when measuring chill depth on a chill test fractured specimen, the depth of chill is measured from the chilled face in 32nds of an inch. The practice of chill measurement Consistency is important when performing these tests. The pouring temperature should be as high as possible and uniformly controlled. Lower pour temperature can produce higher chill levels in the same test giving erroneous measurements due to more rapid cooling. Sample ladles should be either of the insulating type or preheated sufficiently so that the test can be done

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quickly at a high temperature. They should also be clean. Mixing (stirring) of the melt must be done to insure that a representative sample is obtained. Try to dip and stir deeply into the metal bath. This is especially true when testing inoculated and/ or final irons. The sample may be water cooled as soon as it is solidified or left to air cool. The value will not change with either method. However the slower the cooling, the better the definition of the chilled section (see figure 2) becomes. Also test piece must be dry before fracturing to give a good reading. Finally the measurement and recording of the chill width or depth, depending upon which test is done, is made. The base melt may then be adjusted by additions of silicon and/ or nucleation materials (SiC, graphite or pig iron) to obtain the correct value.

Figure 2 Actual chill wedge James Mullins, Mullins Professional Services

Figure 1 Two methods of chill testing