Ferrous foundries that are not deoxidizing molten iron are missing out on some of the latest advancements available for process and product improvement. Deoxidized iron delivers such major improvements in material quality that it can be understood as a new metallurgical grade, one that yields better quality castings and improves the bottom-line results for those plants that have adopted it.
Improved ductile iron
Consider deoxidation of base ductile iron and the reported behavioral changes of the carbon atom as ductile iron cycles from the molten state to finish castings. Using the established technology, ductile iron is produced by melting carefully regulated metallic scrap, to prevent unwanted "carbide " formation. Careful regulation of melt metallics have settled foundries on the use of pig iron, plate, and structural steel grades, very "clean" alloy-free busheling steel grades, and minimal use of machine-shop borings, to stay within the control limit of max. 0.05% Cr.
The carbon-atom behavior change that results from deoxidation, i.e., reduction of iron's sensitivity to carbide formation, completely changes ductile iron manufacturing. That standard of max. 0.05% Cr residual level for carbide-free ductile iron – maintained by foundries at least since Keith Millis' invention of ductile iron in 1943 – is shown now to be an unfounded limit on productivity. Deoxidation opens the door for major changes in ductile iron manufacturing.
Now ductile iron can be produced with 90,000 psi tensile and 17-19% elongation, as-cast, with high-chrome residual levels (0.30% Cr); 120,000 psi tensile and 9% elongation, as-cast, with 0.50% Cu. Chrome can replace copper in producing the elevated-strength ductile irons, which is a major shift in ductile iron manufacturing: Copper alloying is no longer needed. Deoxidized ductile iron melting technology changes everything.
Counting the benefits
A partial list of the changes in ductile iron melting made possible by iron deoxidation, all proven in production applications, includes:
- Magnesium alloy requirements for ductile iron conversion can be reduced 30% or more.
- Improvements to DI material strength mean that there’s no need for downstream heat treatment or annealing.
- Alloying for property regulation can be changed away from copper to chrome, saving the expense of copper alloying.
- Steel scrap grades can include elevated chrome residual levels typically produced by "shredded auto scrap" steel grades
- Special steel scrap grades (e.g., P&S and busheling) are no longer needed.
- Pig iron is no longer needed.
- Boring grades of metallic scrap can be melted. Electric furnace charges containing 40% or more of borings are melted without concern using deoxidation technology.
- Charge metallics containing low levels of manganese are no longer required for ferritic ductile iron grades.
- All grades of ductile iron can be produced "as-cast".
- Furnace slag formation and downstream control of slag become "non-issues."
- Nodularity fade of treated ductile iron becomes a "non-issue."
- Graphite morphology, graphite size and distribution become "best ever" thanks to iron deoxidation.
Note that all of these deoxidation benefits come without any associated risk.
Improved cast iron, reduced scrap
Deoxidizing cast iron elevates the material’s strength, changing Class 30 base iron to near Class 40 with no other change to chemistry – only deoxidation. Full Class-40 elevated strength and other higher-grade strength levels can be attained with only small amounts of chrome alloying. No other alloys (e.g., moly, nickel, copper, etc.) are required.
Chrome becomes the alloying element of choice due to its low cost, and this is made possible by the coupling of carbide insensitivity which deoxidation imparts into this new molten iron grade. Carbide insensitivity is a major revelation and advantage of iron deoxidation: It's "no- carbides" formation covers both gray iron and ductile iron, which defies long-regarded but now unfounded metallurgical theories.
Iron oxidation creates oxides that transform into slag. Deoxidation can produce slag-free iron melting which has never before been possible.
Casting surface defects are caused by the continuous precipitation of oxides in the molten iron matrix as the iron solidifies. Because oxides form up to the point of solidification, many oxides take shape after the molten iron enters the mold cavity, after the strategic placement of gating system ceramic filters. The precipitated oxides agglomerate, attain buoyancy, and float to the surface.
If solidification is not complete, the oxide defects form surface defects. If partial solidification has occurred, the oxide defects remain hidden as sub-surface inclusions, which are later exposed during the machining process.
The oxides formed can be solid or gaseous. The solid oxide defects are called "oxide flotation defects" and easily identified with SEM evaluations. These cannot be avoided if the base iron contains free oxygen atoms. The only solution is to deoxidize the molten iron prior to casting.
Every foundry that has adopted iron deoxidation has achieved casting scrap rate reductions. Several foundries proudly report achieving the coveted goal of less than one-percent casting scrap rate. One foundry that is a long-time advocate of the process claims customer casting scrap returns have been "zero'' ever since it started deoxidation. Surface defects and sub-surface pinholes virtually vanish.
Low-magnesium ductile iron
Deoxidation removes the free-oxygen atoms normally present in conventionally melted iron. If they are not removed, these atoms instantly combine with magnesium added in the ductile conversion process, reducing the magnesium available to initiate nodule formation. The resulting magnesium oxides, if they are not allowed to agglomerate and float out during processing, remain in the matrix and contribute to magnesium levels determined by spectrographic analysis of the treated ductile iron.
However, the magnesium-oxide portion of the overall magnesium level does not contribute to nodular formation. The MgO oxides thereby create a false impression of the amount of pure magnesium available in the molten iron matrix. This long-standing confusion is evident in ductile iron being produced today. Iron deoxidation is changing ductile iron analysis.
Technologists have found good nodularity in deoxidized iron with magnesium levels reported as low as 0.010%. More investigation is needed to determine the true level of magnesium needed for full nodularity. With deoxidation, both gray and ductile iron achieve a "cleaner" matrix, which greatly reduces the detrimental effects to machinability, castability, and fluidity that are produced by nano-sized suspended oxides.
In ductile iron, the primary suspended oxide is magnesium oxide. Decreasing the magnesium oxide's presence in the ductile iron's matrix yields elongation levels exceeding 25%, as-cast, which exceeds what is normally achievable with specially designed full-anneal ductile-iron heat treatments.
Iron deoxidation is proving to be the start for new metallurgical processing techniques, and is making available very significant savings to all foundries embracing the new technology.
The changes to manufacturing processes and improvements in operating standards and production results produce millions of dollars in savings for the foundries that implement deoxidation. Typically, the cost savings jump off the page on the first day of deoxidation process evaluations. There is no "down-side " to iron deoxidation.
Ron Beyerstedt is the president of Mastermelt LLC.