Changing Assumptions, Adopting New Practices for Molten Iron

All iron that is conventionally melted contains a small volume of free-oxygen atoms. Typical free-oxygen atom levels range from 0.0003% to 0.00010% which is expressed as 3 to 10 parts per million (PPM.) This is a very tiny amount but it creates huge consequences during melting and casting, and it alters the true governances or principles affecting the iron's metallurgy.

The inert free-oxygen level, which is the level at which free-oxygen atoms play no part in downstream chemical reactions, nears 1.0-2.5 PPM, depending on the silicon content of the iron. In near-pure iron (such as in steel alloys), the inert oxygen level equals 1 PPM.

Oxygen atoms, present in very small amounts above the inert level, have profound effects on the overall iron matrix. Mastermelt deoxidation research is slowly uncovering these effects, and most are remarkably interconnected with long-established theories.

In steelmaking, oxygen blow-down of pig iron elevates the oxygen content, at times to over 2,500 PPM. When the steel heat is “killed," free-oxygen content is reduced below 100 PPM. In iron, we are talking about a minute fraction of that amount of free oxygen, and then further reducing that by deoxidizing treatments of molten iron.

The small but very critical amount of free oxygen makes it very difficult for foundry technologists to comprehend that such a small amount of anything can be an issue, or that such a small amount can cause major physical property changes within the solidified iron. But, to the frequent dismay of those foundry operators, it makes a huge and very profound difference.

Our research and testing recently uncovered oxygen atoms' influence when free-oxygen levels are reduced to the inert level. Well-known principles of iron casting, principles thought to be scientifically based, are proven to be totally wrong.

One example of an unsupportable metallurgical principle is ductile iron’s sensitivity to carbide formation – the universally accepted level of 0.06% Cr is exceeded.

With deoxidation, 0.30% Cr levels produce no carbides and generate elongation values exceeding 20.0%, as-cast. This defies existing metallurgical theory. It also eliminates the need for high-temperature heat treatments, dramatically eliminating the need for the hard-to-get, low-alloy steel scrap charge materials. The result is a startling reduction in the cost of metallic charge materials, reducing overall manufacturing costs for ductile iron thanks to deoxidation technology.

Think about the ways that deoxidation affects the cost considerations for ductile iron producers:

•  No special steel metallic grades;
•  No special need for pig irons;
•  Full recovery of all auxiliary alloy additives;
•  Full precipitation of carbon onto the graphite nodules, with minimal carbon tied up as solid solution pearlite;
•  Optimization of metallurgical structure within the ductile iron (nodule count and size, distribution, shape, quality);
•  Physical strength and elongation elevated since the typically suspended, nano-sized MgO oxide levels are dramatically reduced;
•  15-45% less magnesium alloys for conversion treatment;
•  Full nodularity at lower magnesium levels since all magnesium spectrographically reported in deoxidized iron is the pure magnesium and not a portion being MgO, which is included in the reported magnesium level in conventionally melted ductile base iron;
•  Extended nodularity fade time in deoxidized iron since the iron oxide (which supplies free-oxygen atoms to conventionally melted ductile base iron) is not present, so nodularity fade is only caused by the very slow magnesium partial pressure decay mechanism, releasing magnesium into the atmosphere.

The only explanation for this abrupt physical change in ductile iron manufacturing and the accompanying cost reduction is that many long-held metallurgical theories and standards are simply wrong.

The reduced tendency to carbide formation in deoxidized ductile base iron has been replicated in multiple foundries, each one reporting similar results regarding the carbide-forming tendencies of deoxidized iron. Once free-oxygen atoms cease to exist in the iron matrix, new material properties emerge. An almost new grade of iron results.

Think about a pipe manufacturer casting molten ductile iron into rotating metal molds. The rapid in-mold cooling combined with conventional melting practices produces massive carbidic structures. Now, the possibility exists for casting without the carbides — and without downstream high-temperature anneal heat treatments. Much verification work remains to change this age-old process but exciting breakthrough concepts have emerged.

Explaining out-gassing

Counterweight castings cast in sand molds undergo an “out-gassing” after casting, which requires weeks and sometimes longer periods before it subsides. Premature painting of the cast surface prevents the gas from escaping, eventually building up and forming a “bubble” under the painted surface. This aging process has been in-place for over a century. Molten iron deoxidation finally solved the issue, and aging is no longer needed.

The technical explanation for the out-gassing involved carbon in the molten iron oxidizing during conventional melting. Because the molten iron density prevented the carbon-monoxide bubble from forming, the CO built up in molecular form within the molten iron. After solidification, the CO molecules slowly moved toward the surface and entered the atmosphere. Painting needed to be delayed until most of the CO molecules exited the casting's interior.

Counterweight castings, and similarly heavy-section iron castings, were “aged” as standard procedure, until deoxidation revelations pointed toward the need for new principles of iron metallurgy.

Consider that ferritic grades of ductile iron – which typically require stringent segregation of steel scrap to avoid manganese and other alloy contamination – use special melt stock, such as special grades of pig iron when casting in green sand molds.

Now, ferritic ductile iron can be produced without regard for alloy contamination. Twenty-percent elongation has been achieved in ductile iron, as-cast, melted with high amounts of shredded auto cast, no busheling, or P&S steel scrap grades included. Metallurgists encountering deoxidized iron for the first-time cannot believe the results.

Many comparable benefits are revealed when iron is deoxidized. Both cast iron and ductile iron end up “cleaner," free of most of the suspended nano-sized oxides that affect the iron’s physical properties. Casting fluidity changes too, and machinability improves manyfold. Benchmark machinability levels are established without regard for chrome levels. Casting surface defects – proven to result from oxide formation initiated by the free-oxygen atoms present in the molten iron – cease to occur. Note that casting scrap levels have been reduced in every deoxidation application to date, and iron foundries can (and do) save millions in production costs.

Melt technologist preparing for base iron deoxidation must first consider chemistry changes in the deoxidized heats that come about due to oxidation losses being stopped. Carbon, silicon, and manganese will no longer be lost during the melt cycle. Conventionally, technologists assigned “recoveries” typical of the various charge ingredients. Recoveries are fictional. They represent oxidation losses. Full recovery of all charge ingredients is obtained when deoxidation is properly employed.

Deoxidation analysis

Results from multiple deoxidation efforts at individual foundries demonstrate cost savings and outstanding molten metal quality. These examples include coreless induction and cupola melting, in furnaces of different volumes, and melting gray iron and ductile iron, .

Deoxidation eliminates the presence of iron oxide, which turns out to be the root cause for refractory erosion and oxidation losses. Cupola operation employing deoxidation produced iron oxide-free slag, changing normal operation. With deoxidation, refractory service life was extended from six days to three months in a large cast iron cupola operation. Slag-free cupola operation became a reality. No slag buildups occur in the cupola front box, and the cupola can be started and stopped at will.

Significantly, greenhouse gas production (GHC) is cut in half (or more) with deoxidation. The effort stops the carbon loss that always occurred but is seldom understood in cupola melting. The reduced carbon levels in the deoxidized iron allow substantial reduction in coke rates. And reduced coke rates increase cupola melt rates, leading to reduced cupola blast rates.

Then, major cupola blast rate reductions become possible. Simple calculations show cutting the blast rate go hand-in-hand with cutting GHC formation; 60% reductions in blast rate cut GHC production by that same amount. GHC formation becomes a non-issue.

Along with the iron’s increased carbon level, slag formed is nearly eliminated — reduced more than 80% due to elimination of oxidation in the cupola. Limestone, which is added to neutralize the normal high silica slag, can be all but eliminated. The only silica slag component produced in deoxidized cupola operation slag comes from melted coke ash. Considering the reduction in coke rates, the melted coke ash (SiO₂) becomes a minor portion of overall slag formation. Most cupola slag will come from dirt or other contaminations in the cupola charge.

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