I'm a metallurgist and was superintendent of the melt department at the Tonawanda metal casting plant. Jon's statement was true. We ran all 4 of our cupolas with the same chemistry base iron. We were producing about 2,500 tons of iron a day - feeding 7 molding lines, so as a practical matter, it would have been impossible to route different base irons to different lines in production. Alloying with ferro-chromium and silicon was performed as the iron was poured into the hot metal crane ladles (these cranes traveled on monorails that transported the molten iron to the individual molding lines). All lines received silicon (a late alloying agent designed to refine the grain structure of the cast iron by providing more grain nucleation sites and reducing carbide formation). Ferro-chromium was only alloyed into the lines that produced blocks (mold line 1 & 2 at Tonawanda). It was added to promote pearlite formation which increased the tensile strength of the cast iron. You may ask - then why wasn't ferro-chromium added to metal being delivered to all of the lines? the reason is that chromium can cause carbides to form in thin sections of casting. The cutting tools at the motor plants would not like this, so chromium was only added to blocks where increased tensile strength was required (class 30 gray iron). The chrome content in class 30 iron was targeted at .032 %.
Just food for thought - nickel is also a strong pearlite stabilizer and would increase the tensile strength of cast iron (similar to chrome, but without the tendency toward carbide formation). Ferro-chromium (not metallic chromium) is a relatively cheap commodity, and, therefore, a good choice for alloying in a high production environment. In smaller job shops - nickel alloying could be used to increase tensile strength in cast iron. In these low production environments - the shop may not want to risk the carbide formation tendency of ferro-chromium. So - this is just conjecture - over the years people familiar with job shop practices may have assumed that nickel was being used in high volume environments.
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