I have been casting Aluminum for 20 years and have a crucible propane furnace that I routinely pour 14lbs at a time from. It has the heat capability to melt cast iron. My question lies in controlling the cast iron quality and formulation. I want to cast knuckles for 4x4 drive axles. I can't seem to find much on how to control the iron content. Anybody have any references?
I've been casting iron commercially for 25 years. I've known shops, now out of business, that would buy cast scrap and just remelt it with no check of the compostion. For gray cast iron that would be acceptable for most hobbyist. You are looking to cast drive line knuckles however and with the loading that those part experience, gray cast iron will not last since there are no elastic properties to that material (poor impact resistance). Commercially those types of parts are cast from nodular or ductile iron and require treating of the molten iron with a magnesium alloy, usually a NiMg or MgFeSi +/- 6% Mg. (Do not use scrap magnesium unless you have a death wish) The problem with small furnace and post treating is that you will loose so much heat during the treat that your casting will misrun when poured.
But to answer your question as to composition control; In the days prior to thermal and spectral analysis the Meehanite Company made a fortune showing their licensed foundries how to maintain control by casting a test wedge from the metal prior to pouring the molds. You would wait for the wedge to solidify, then quench it to cool and break it in half. There will be a white area at the tip of the wedge and it would turn to a gray color as it got thicker. Measuring the thickness of the wedge at the point where the color changed from white to gray would give you an estimate as to the tensile strength of the iron, very little white would be a low tensile, low hardness iron... alot of white would equate to very hard casting that would be difficult to machine.
Marcellus Metalcasters, Inc.