The ingredients of a furnace charge, whether it is an electric furnace or a cupola melter, are formulated to produce the final chemistry required for the castings to be poured. Unfortunately, formulating the charge does not determine the final chemistry or quality of the molten iron. Melting is not simply the process of re-melting existing metallic materials: Slag-induced influences during the melting process — caused or produced by the slag/metal chemical reaction — have a consequential role in iron chemistry and finished metal quality
Molten iron chemistry variations result from two primary sources:
(1) Accuracy of the weight of individual metallic and alloy ingredients in the charge; and,
(2) Chemical reactions (slag/metal reactions) that occur during the melting process and cause unpredictable and widely varying loss of C, Si, MN and other necessary elements.
Oxidation losses cause 99% of all chemistry variations in molten iron. Unwanted weight variations in charge ingredients, which frequently are assumed to lead to chemistry variations, in fact are a minor influence in most melting operations. You must experience melting without oxidation loss to appreciate the significance of this.
The wide variations in metal chemistry faced by some ferrous foundries are caused by oxidation loss of the key elements. It is a simple analytical comparison: Chemistry will vary by 50% when a 50% oxidation loss occurs. Oxidation must be controlled in order to attain “straight-line” chemistry.
Can carbon be controlled to produce straight-line chemistry? Unequivocally, yes. One-hundred-ton-per-hour cupola furnaces have been operated for entire daylong campaigns with carbon variation of 0.01% C, and such exceptional chemistry control is possible with any melting operation.
Tuyere injection can be used to counter oxidation losses in a cupola, in addition to supplementing carbon and silicon in molten metal exiting the cupola. First, oxidation must be resolved. Then, silicon and carbon can be injected in any amounts needed to trim the chemistry.
The materials to be injected must be "injection-grade" and "injection-quality": Standard-grade silicon carbide (SiC) and graphite do not qualify. Simply, lower-quality materials do not work and using them discredits tuyere injection as a reliable melting tool.
SiC must possess a high dissolution rate in the molten iron, and only a few grades of SiC qualify. Carbon must possess an equally high dissolution rate in molten iron and no commonly available graphite carbon raisers meet this qualifying standard.
Trimming chemistry — Both carbon and silicon-carbide can be injected to trim the cupola metal chemistry. Mastermelt engineers spent two years developing the technology and skill needed to determine the specific materials that can be injected effectively.
It is pointless to inject SiC and carbon materials that do not provide full carbon or silicon recovery. Without full recovery, chemistry control deteriorates further.
Both carbon and SiC are commonly tuyere-injected, and both materials are unique: They do not melt but enter into molten iron via an atomic exchange at the molten-metal interface. The governing forces controlling the rates these materials enter into molten iron are complicated, best left to scientists and crystallographers. Carbon and silicon-carbide’s entry rates into molten iron are inherent properties of the material. The properties develop during production of the material and are reset when production concludes.
When Mastermelt first introduced SiC tuyere injection many suppliers and foundries followed suit and start injecting; none of the competing SiC injection systems proved successful. Many of those systems injected “injection carbon,” which turned out to be coke breeze. Unfortunately, many (if not all) of those users proved unequivocally coke breeze is ineffective for controlling iron chemistry, though some suppliers still recommend it.
When injected carbon or silicon carbide produces full recovery in molten iron, melting personnel are afforded a very viable tool for trimming chemistry precisely. One melting supervisor reported Mastermelt DeOX tuyere injection puts the cupola on “cruise control” for chemistry control throughout the melt campaign.
Misunderstanding chemistry — Many explanations surface when molten iron chemistry is out-of-spec. In cupola melting, the classic excuse for low carbon and silicon levels is “the cupola is oxidizing today” or a "a double charge must have happened." In electric furnace melting, “poor carbon” or “low-purity silicon carbide" are the standard explanations.
None of these excuses are accurate. Chemistry variations, heat to heat, result from oxidation loss. DeOX stops both carbon and silicon oxidation by stabilizing chemistry in both EF and cupola operation. Neutralizing all iron oxide in the cover slag stabilizes chemistry because Iron oxide is converted into inert by-products. Oxidation processes are stopped.
Small amounts of iron oxide (e.g., 1.5% FeO) appear to be inconsequential for causing negative effects on molten iron, but that is far from true: FeO content must be less than 0.2% FeO.
A typical acceptance level of 1.5% FeO in cupola slags produces 20-30% silicon oxidation loss, which is not acceptable. At the 0.2% FeO level, silicon-oxidation losses are eliminated and unimaginable iron quality results.
Iron-oxide content must be reduced to near zero for oxidation losses to be stopped. Many foundries check slag chemistries, but few if any realize iron oxide must be controlled at less than 0.2% FeO.
Slag color indicates the level of iron-oxide contamination: the typical black or very dark slags indicate trouble. A greenish color indicating lower levels of FeO begins to appear at the 1.0% level and progresses to light green at 0.2% FeO content.
Slag color provides a great rating system for judging melting processes. In EF melting, slag color indicates the oxidation loss that has occurred. In cupola melting, slag color suggests iron oxide forming tendencies of the blast air in addition to severity of oxidation loss occurring. Slag color checks are an integral part of all molten iron-quality systems.
Metal fluidity — In the past, metal fluidity typically would be controlled by regulating the pouring temperature. Adding "superheat", meaning the iron’s temperature above the solidification temperature, increased the fluidity of the metal. When the temperature increase proved inadequate to control fluidity, small amounts of ferro-phosphorous were added to the iron.
Poor fluidity creates many headaches for foundries. Increasing pouring temperatures causes many quality issues in the casting process. Things like burn-on and increased shrinkage lead to big problems.
The metal’s chemistry is important to establishing melt fluidity. Iron develops a eutectic, its lowest melting point, as chemistry nears the 4.3% CE (C and Si percentage calculated in a unique manner). Iron chemistry varying above and below 4.3% CE have higher solidification temperatures. At times, minor chemistry variations can reduce fluidity enough to cause “mis-run” defects.
Up to now, chemistry control or change and phosphorous additions, along with pouring temperature increases, were the only tools available to the casting engineer to fight the mis-run issue. Now, DeOX provides an entirely new and effective way to improve metal fluidity.
As noted in a previous report, removing free oxygen atoms from molten iron stops the oxidation process. The oxidation process produces “oxides” that end up suspended in the metal matrix. Solid oxides like SiO2 and MNO, and gaseous oxide CO, accrue in the matrix and eventually combine with other like oxides, achieving critical mass, which allows the oxide particle to “float out” of the melt.
No more free oxygen — This coalescence process of the precipitated oxides produces cleaner metal. An important discovery by Mastermelt engineers was that once the supply of free oxygen atoms is cut off, it takes near two minutes for the suspended oxides to float out of the molten iron bath. This fact correlates with steel melting practices, which allow the same two-minute interval between de-oxidation and pouring.
One significant feature of lower-oxide-containing, much cleaner metal is that fluidity improves dramatically. An almost unbelievable increase in fluidity occurs when the metal is cleaned: It is an almost night-and-day difference. Phosphorous additions are eliminated, pouring temperatures can be reduced, and mis-run defects disappear.
In one casting application producing two-inch diameter cast iron pipe, which is cast in long thin-walled sections, mis-run or lack of filling the entire length of the metal mold was eliminated as a scrap defect. The casting supervisor of the pouring floor could not believe his eyes. Full-length, two-inch pipe castings were completed consistently.
There is no need for a “fluidity spiral” test when casting two-inch-diameter, cast iron pipe in a spinning metal mold. The casting application serves as the best test of all, but it is costly when fluidity is poor.
The improved fluidity referenced in the two-inch diameter, cast iron pipe transferred to ductile-iron pipe manufacturing, too. For years, the plant manager of a Midwestern pipe foundry touted the improved fluidity of Mastermelt tuyere-injected cupola iron in all sizes of ductile iron pipe.
In another application, the need for the phosphorous supplement was eliminated with Mastermelt SiC tuyere injection. When that injection technique stopped after seven years and a different injection material was injected, mis-run defects immediately appeared and phosphorous additions resumed. The replacement SiC material did not neutralize iron oxide within the cupola, hence free-oxygen levels were higher, producing higher levels of suspended oxides and reduced fluidity.
Thus, metal cleanliness becomes a new and very important molten iron property.
Examining how your favorite cook makes gravy can simulate metal cleanliness. Gravy starts as a clear, very thin and fluid broth to which a thickener is added. The end result is a thickened, slow-moving liquid. Nano-sized oxides suspended in the molten-iron matrix are the thickening agent in molten iron. The metal’s cleanliness correlates to fluidity. “Dirty” iron is difficult to cast successfully, but when the free-oxygen atom supply is cut-off, cleanliness can improve by a 10X factor or more; fluidity skyrockets accordingly.
Metal cleanliness can be measured and compared by determining the oxygen content in a solidified metal sample. All oxides contain the oxygen atom with total oxygen content representing the overall oxide level. Total oxygen results can be compiled to establish a fluidity rating system for the specific foundry’s casting applications.
It is necessary to determine the level of oxide contamination in the molten metal, which produces fluidity-induced defects. Oxygen content readily relates to iron fluidity and can be assigned limits similar to chemistry and metal temperatures in a foundry's overall quality-control program.
Now temperature, metal chemistry, and cleanliness can be used to prejudge molten iron’s fluidity, substantially reducing scrap-casting risk potential. Every iron foundry's quality program must consider this technology in its overall quality-control practices.
Ron Beyerstedt is the president of Mastermelt LLC. Contact him at [email protected]
This is the third in a series of reports addressing the effects of oxygen in iron melting.
- Harmful Effects of Molten Metal Oxidation, FM&T June 2020.
- Controlling Iron Oxide to Stop Carbon and Silicon Losses, FM&T July 2020.