Many ferrous foundries operating electric furnaces include pig iron in their furnace charges, although many of those operators responsible for preparing those charges have only vague ideas to explain why it is so. This is hard to understand, particularly as pig iron is a major expense in iron melting. A higher-level evaluation of the pig iron’s inclusion would reveal its ineffectiveness and its exorbitant cost in melting iron.
Among the various assumed benefits of pig-iron addition to EF charges, the only true advantage has been that it favors carbon precipitation as graphite during iron solidification. Pig iron offsets free-oxygen atoms' tendency to form carbides. In fact, pig iron is merely high-carbon iron with at times elevated silicon contents. It is cast iron with low residual elements.
But the advent of iron deoxidation has offset that advantage. Iron deoxidation far surpasses the graphitization enhancement of pig iron. Deoxidation eliminates free oxygen's presence in the iron matrix, and without free oxygen present graphite precipitation is unhindered. Cast iron and ductile iron properties rise to new standards of quality, practically resulting in a new engineering material.
Iron deoxidation has changed melting technology for iron foundries. The desired one-percent casting scrap rate becomes easily attainable. Many foundries report a total lack of surface and subsurface defects with deoxidized iron – a claim that is supported by thousands of tons of deoxidized iron castings.
Basics of ferrous metallurgy
As noted, pig iron’s primary benefit in metalcasting operations is that favors the carbon present in molten iron to precipitate as graphite during solidification, as opposed to carbon atoms remaining in solid solution within the iron matrix, or forming carbides. The "chill" of the iron is reduced.
Carbon precipitating as graphite forms flakes in cast iron and nodules in ductile iron. Up to 0.8% carbon can remain in solid solution within the matrix.
Solid-solution carbon favors pearlite formation upon cooling, and the amount of precipitated graphite controls the volumetric expansion that occurs when molten iron solidifies. The portion of carbon atoms in solid solution directly affects the volumetric shrinkage characteristics. Increasing the solid-solution carbon level depletes that available for graphite precipitation, which leads to increased shrinkage at times. The best solution to this situation is to stabilize solid-solution carbon levels through and with deoxidation.
The level of free-oxygen contamination in conventionally melted iron varies, depending on melting practice. Typically, free oxygen is present at the 5-10 PPM level (0.0005-0.0010%.) Free-oxygen atoms are always present in conventionally melted iron.
Deoxidized iron contains 1-2 PPM free oxygen, which is the inert level for oxygen. At that level, free oxygen does not affect the iron’s properties or characteristics – that is the significance of it being inert: no reactiveness.
Any free oxygen, even minute amounts above the inert level, produces major consequences. In primary steelmaking, 1 PPM above the inert level in steel heats is considered to result in two-percent defects in processed steel. Free-oxygen atoms significantly influence carbon precipitation, retarding graphite formation, favoring the solid-solution carbon in the iron’s matrix and carbide formation.
Pig iron offsets free-oxygen atoms’ influence, and it is used for that purpose, but that is all it can do. It will not increase or decrease free oxygen levels. Free-oxygen atoms end up being added to molten iron during EF melt cycles due to molten iron mixing action within the furnace – exposure of the fresh molten iron to oxygen in the atmosphere.
Some foundry technologists suggest that the purity of pig iron will dilute residual alloys’ influence on the solidified iron. In fact, the small quantities (e.g., 10 percent) included in an EF charge will simply dilute the residual alloy levels by a comparable amount, which is of no consequence to the quality of the iron melted and cast by a foundry.
Check the chemistry
Another contradicting factor is that the “inoculation” effect for flake or nodule graphite precipitation attributed to pig iron is no longer needed. Iron deoxidation removes the free oxygen atoms that retard carbon precipitation as graphite. Once free-oxygen atoms are gone, graphite precipitation is un-hindered – meaning there is no benefit added by pig iron.
Graphite precipitation during solidification influences the volumetric expansion that occurs during solidification. Foundries casting heavy section castings report no change in volumetric shrinkage after iron is deoxidized and pig iron removed. And there is no casting scrap due to shrinkage when pig iron is removed.
One foundry with a 90% casting yield reported no issues after removing pig iron from the furnace charge. Ninety percent casting yield is near the benchmark for efficient “feeding,” so that the effect of removing pig iron from the charge is quickly noticeable if the shrinkage tendency of the iron changed. None was noted on numerous pig iron removal evaluations.
All the foundries deoxidizing iron report no issues when removing pig iron from gray and ductile iron heats. None of the recent adoptees that are still evaluating the no-pig-iron practice have reported any adverse effects to date, from any operating foundry – large or small.
Deoxidation improves iron’s physical strength, which allows foundries to raise the iron’s carbon level without jeopardizing tensile strength.
Higher carbon levels greatly improve iron’s volumetric expansion upon solidification, which will result in less “risering” during molding and improved casting yield. The physical properties of deoxidized cast iron are elevated, almost an entire grade-level increase. Castings’ volumetric shrinkage during solidification decreases after deoxidation, for example, deoxidized Class 40 iron shrinkage characteristics are similar to standard Class 30 iron.
Evaluating deoxidation treatment
Along with the metallurgical evaluations, foundries should consider the cost value of adding pig iron to the EF charge. As noted, pig iron is cast iron with low residual elements – and considering the cost of pig iron a foundry might save considerable expense by charging lower-cost steel to the furnace.
For cost and quality, the emerging alternative to charging pig iron is melt deoxidization. The standard rate for adding Mastermelt’s DeOX D-1 to the EF is at 0.3%. Excess DeOX D-1 addition to the EF is shown to cause carbon migration from the iron’s matrix to the flake or nodule, causing depleted solid solution carbon levels and resulting in ferrite formation upon cooling.
Fully ferritic microstructures are possible with elevated manganese in ductile iron, and deoxidation will remove the carbide-forming tendency produced by chrome in DI. Chrome levels in DI exceeding 0.30% Cr are carbide-free. Deoxidation changes the carbon atoms’ behavior in both cast iron and ductile iron, demonstrating again that pig iron no longer adds any benefit favoring “ferritic” grades of DI.
Ron Beyerstedt is the president of Mastermelt LLC.