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Steelmakers recognize that finished products’ surface defect rates are associated with any oxygen level above 1 PPM (0.0001%), yet in iron melting and pouring no correlation has been established to pin the free-oxygen atom's presence to an elevated casting scrap rate.

What Drives Iron Casting Scrap Rates?

March 3, 2023
Ferrous foundries’ results can be improved once metallurgists and operators understand the role of oxygen – and how and why casting defects form.

Since the beginning of the Iron Age 2500 years ago, all molten iron has contained free-oxygen atoms and those atoms have significantly corrupted its melting and pouring properties. Free oxygen contamination occurs in tiny amounts but those few parts per million (PPM) alter the iron’s behavior and properties.

Deoxidized iron's properties are so different – and so much improved – that an almost-new material grade results. Few metallurgists understand the role free-oxygen atoms play in iron melting and pouring. Oxygen atoms provide no benefit, only harm, and that harm is far-reaching.

After 35 years of continuous research and testing, the role of the free-oxygen atom in molten iron is beginning to be revealed and understood. Steelmakers are ahead of ferrous foundry technologists in this because oxygen has long been a factor in steel production, and much emphasis is placed on its role in the process.

In ferrous foundry operations oxygen's interactions are overlooked, mostly because operators lack the technical resources to measure free oxygen’s presence. Even free oxygen's influence on carbide formation is overlooked.

Steelmakers recognize that finished products’ surface defect rates are associated with any oxygen level above the steel's inert level of 1 PPM (1 PPM = 0.0001%). For example, 2 PPM = 2% defects; 3 PPM = 5% defects. Yet in iron melting and pouring, no correlation has been established to pin the free-oxygen atom's presence to an elevated casting scrap rate. Iron technologists have avoided the subject.

If you have undergone a tutored scrap-reduction program directed at your foundry's iron melting and processing, you may recall no mention of the oxygen atoms’ role in producing defects as part of that training.

High-cost scrap reduction programs have been missing a major cause of casting scrap, one that – if not controlled – will mean that scrap rates cannot be reduced. There are no technical publications decrying the importance of controlling free oxygen levels. Iron technologists avoid the subject, most likely due to a combination of factors: the inability to measure free-oxygen levels that can be used to judge iron quality; absence of technical information concerning the oxygen atoms' interaction throughout iron melting and pouring; and oxygen atoms' entry mechanism into the molten iron bath. (More detail on oxygen atoms’ role in iron melting and pouring is presented at www.Mastermelt.com)

The core of the problem is that most technologists do not understand what oxygen atoms affect, how the atoms enter the iron, and, most important, how to get rid of them.

Coming to the surface

In steel melting and processing, the molten metal surface is shrouded to the maximum possible extent in order to prevent steel contact with the atmosphere. In iron melting little emphasis is directed at stopping atmospheric contact. In fact, electric furnace manufacturers promote keeping the molten bath clear of all slag. This leads to the molten iron surface having direct atmospheric contact, which produces excessive iron-oxide content in the cover slag.

Steelmakers attempt to limit exposure to atmospheric contact but they know that is nearly impossible. Consequently, aluminum is added to heats of high-quality steel. The aluminum combines with all free-oxygen atoms, forming inert oxides. 0.03% AL is added to control the minute quantities (1 PPM = 0.0001%) of free-oxygen atoms always present. A hundred-fold excess of aluminum is needed to ensure – by the laws of probability – physical contact between the aluminum and oxygen atoms.

In iron melting and pouring, aluminum cannot be used to tie up free-oxygen atoms: Residual aluminum causes pin-holing defects in iron above 0.010% Al levels.

In iron melting, iron oxide forms during the melt cycle. More iron oxide forms in cupola melting than in electric melting. Mixing action in the EF produces the iron oxide, and high-mixing action furnaces produce higher iron-oxide levels. The level of iron oxide in cover slags controls the level of free oxygen added to the iron bath.

For this reason, iron foundries encounter different casting scrap rates. Deoxidizing iron always lowers the casting scrap rates. The amount of reduction depends on the existing level of free oxygen in the normal iron. Deoxidation has reduced casting scrap levels from 7.5% to 2% with deoxidation in a mid-sized foundry, to reduction from 3% to 1% in a very large foundry. Deoxidation has successfully reduced scrap rates in every foundry where it has been adopted.

A foundry technologist is helpless to stop oxide flotation defects generated by free-oxygen atoms present in the base iron. Some casting defects always occur.

It is for these reasons that ductile-iron casting scrap rates are lower than the rates for gray iron. Ductile iron contains magnesium which very effectively removes all free-oxygen atoms from the metal's matrix. (Watch for more detail on ductile-iron casting scrap in future presentations.)

Mastermelt's past deoxidation efforts uncovered the correlation between iron oxide contacting the iron bath's surface and free-oxygen levels within the iron bath. High iron-oxide levels contained in cover slag generated high levels of free oxygen in the iron bath. Eventually, the mechanism for oxygen-atom entry into iron was exposed; the equilibrium equation – FeO = Fe + O, which iron oxide instantly establishes when fresh iron contacts the atmosphere – is the supplier of the dreaded oxygen atoms. Further investigation into oxygen atom entry proved the FeO equilibrium reaction was the only important source of free-oxygen atoms.

Cutting off the supply

The current deoxidation technology, which is improving iron melting and pouring results throughout the industry, concentrates on cutting off the supply of oxygen atoms. Mastermelt's DeOX D-1 deoxidation material effectively chemically reduces iron oxide contacting the metal’s surface before it can contaminate the iron bath. DeOX D-1 eliminates iron oxide – takes it out of action – forever. With DeOX D-1, the source of oxygen atoms is very effectively eliminated.

Oxygen atoms aggressively interact within the iron bath. They have a life expectancy of two minutes or less, meaning no further nano-size oxide formation once the supply has been cut-off, which leads to stable iron chemistries and a "cleaner" iron matrixes.

Precipitated oxides continually agglomerate, self-cleaning the matrix. The iron's physical properties increase accordingly, rising from class 30 to near class 40 without alloy addition. Metal hardness does not increase, just the tensile strength due to the lesser number of crack initiation sites provided by the suspended oxides.

The cleaner deoxidized-iron matrix enhances the castings' machinability. Benchmark gray iron machinability is obtained despite high residual chrome levels due to high frag steel levels in the charge. Residual levels of 0.30% Cr did not deter the benchmark level award because the matrix had been cleansed of suspended oxides, which obviously had more of a deleterious effect than the chrome residual. The same observance has been noted in ductile iron.

Iron fluidity increases spectacularly. Iron runs like water. No phosphorus additions are needed.

One foundry that adopted Mastermelt deoxidation reported all surface and sub-surface defects were eliminated once free-oxygen levels were controlled. That same customer saw total elimination of all casting returns – defects found after machining – for the past two years.

As mentioned, free-oxygen atoms continuously precipitate oxides throughout the iron matrix. If oxygen is present, oxides will be formed. Slag will continually rise to the surface. The only way slag formation can be stopped is to eliminate the free-oxygen atoms producing slag oxides within the bath. If the iron is not deoxidized, the iron technologist cannot stop casting defect formation.

Gating system ceramic filters are designed to trap slag when it forms. Unfortunately in conventionally melted iron, some of the slag forms in the mold cavity downstream of the filter. This was proven in a large foundry that saw the casting scrap rate reduced from 3% to 1% with deoxidation. In that case, all castings were poured with ceramic filters that had reduced the scrap rate from 5% to 3%, but no further. When deoxidation occurred, the scrap rate instantly reduced to 1%, proving that a portion of the slag defects had formed downstream of the filters.

The fact that iron deoxidation stops slag formation has so impressed several Mastermelt foundry iron deoxidation customers, they are considering removing the filters since no slag forms in the very clean iron.

Ron Beyerstedt is the president of Mastermelt LLC. Contact him at [email protected]