Chill testing: the effect of carbon equivalent.
Chill testing provides the opportunity to evaluate and change the properties of the gray iron melt to make it consistent with expectations. "Reading" the chill fracture is a quick way of integrating the combined effects of carbon and silicon and correlating them with the expected properties of the castings.
The inclination of an iron to solidify as a very hard white iron is referred to as the chilling tendency. Control of the chilling tendency is an important aspect of gray iron metallurgy. Measurement of chilling tendency is done by pouring a test casting against a chill plate (or into a triangular shaped core mold) and measuring the cooling differential as an indicator of iron composition.
In Part 1 of this series, we considered pouring practices and iron cooling as factors in chill testing. Now, let's take a look at the effect of carbon equivalent (CE) on chill testing. Carbon, silicon plus a small amount of phosphorous are necessary when producing gray iron. A balance of these elements limits white iron formation and subsequent machining problems.
The combined effect of carbon and silicon (combined, these serve as both chill reducers and graphitizers) can be represented by a CE value, where CE = C + 1/3 (Si + P). Thus, if the total C of a melt is 3.3%, the Si is 2. 10% and the P is 0.06%, the CE would be 4.02%. The foundry would correct high chill by increasing inoculant added to the ladle to adjust the iron's chemistry and chill testing.
Important considerations in the production of gray iron are the carbon/ silicon ratio, pouring and cooling temperatures, and the alloying and residual elements in the iron melt. The carbon content influences the fluidity of molten iron and affects the machinability of gray iron castings. Silicon strengthens and hardens ferrite, thereby increasing the strength of the iron. Because Si and C are both graphitizers and chill reducers, they are key factors in the manufacture of thin-section gray iron castings. CE control is essential for the development of more uniform properties between thick and thin section gray iron castings. Melt Properties and CE
Various aspects of melting practice are important in determining chilling tendency and mechanical properties. With any cupola mix, the first metal tapped is frequently higher in total carbon than subsequent taps taken after melt equilibrium conditions have been reached. To compensate for this, it is common practice to add more steel in the first few charges.
However, the added low-carbon scrap may be great enough to require the addition of extra silicon to maintain the desired silicon/carbon ratio. Normally, the lower the CE, the greater the chill. Undermost circumstances, higher CE cast irons will have less shrinkage than lower CE irons.
For example, an iron with a CE of 3.6% will solidify over a larger temperature range, while one with a CE of 4.3% will solidify over a much smaller range. The higher CE iron solidifies as a eutectic with virtually no primary austenite and a larger graphite percentage at solidification. The short freezing range eliminates a "mushy" zone and the additional graphite expansion serve to reduce shrink tendency.
Residual or other alloying elements, as well as the CE value, affect chill readings. Alloys can act as either carbide stabilizers, which increase chill tendencies, or graphitizers, which tend to reduce chill tendencies. Pouring delays, temperature loss and melt oxidation also can affect the chill depth of test specimens,
Table 1 and 2 illustrate the approximate effect of CE on the chill values of cast gray iron. In Table 1, the carbon is constant and the CE is varied by changing the silicon. In Table 2, the Si is held constant and the CE varied by altering the C content. It can be concluded that silicon is slightly more effective than carbon in reducing the chill value of a given CE gray cast iron. This information was excerpted from: Patterson, V. "Foote Foundry Facts," July 1980). Cupola Handbook, American Foundrymen's Society, Inc., 5th Ed. (1984).
Table 1. Effect of CE on Chill Width in Gray Iron When Carbon Remains Constant Carbon W-3 Chill Equivalent Carbon Silicon Depth % % % 1/64 in. 3.8 3.50 0.90 14 3.9 3.50 1.20 12 4.0 3.50 1.50 10 4.1 3.50 1.80 8 4.2 3.50 2.10 6 4.3 3.50 2.40 4 275oF (1510C) pouring temperature Table 2. Effect of CE on Chill Width in Gray Iron When Silicon Content Remains Constant Carbon W-3 Chill Equivalent Carbon Silicon Depth % % % 1/64 in. 3.8 3.00 2.40 9 3.9 3.10 2.40 8 4.0 3.20 2.40 7 4.1 3.30 2.40 6 4.2 3.40 2.40 5 4.3 3.50 2.40 4 2750F (15100C) pouring temperature
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|Title Annotation:||Cast Facts; Part 2 of 3|
|Date:||Jun 1, 1991|
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