Magnesium recovery is the all-important end result of the ductile iron conversion/modification process. Many details need to be considered in evaluating the overall process, all of which can lead to lower magnesium alloy usage. Initial results using deoxidized base iron suggest possibilities of substantial reductions in magnesium alloy for the DI conversion process.
This discussion delves into the theoretical aspects of the chemical reactions involved in ductile iron processing; the measurement of the resulting magnesium recovery; and the stabilization of the DI process. It does not involve equipment design for the conversion/modification process, nor the selection of magnesium-nearing material.
The mechanical equipment design for introducing magnesium to molten iron is such that recovery of magnesium is fairly repeatable, and values for recovered magnesium can be anticipated. The "wild," non-uniform high- or low-magnesium levels, which one hopes seldom occur, will not be considered here. Common sense directs that the conversion/modification process must be strictly controlled to prevent abnormal low magnesium recoveries.
This discussion is split into six parts:
- Magnesium level measurement
- Free magnesium level - isolate from MgO
- Free oxygen in DI base iron causing magnesium loss
- Elimination of graphite nodularity fade
- Magnesium alloy reduction possible with deoxidation
- Iron deoxidation - overall impact on DI technology
1) Magnesium level measurement
Magnesium found by spectrographic OES testing is the total amount of magnesium present in the iron matrix. Unfortunately, magnesium is present as pure magnesium and also as MgO. Only the pure magnesium form produces graphite nodularity. MgO is basically inert, falsely affecting the OES magnesium level and detrimentally affecting the physical strength properties as the MgO particles act as crack-initiation sites, lowering the iron’s strength and ductility.
MgO is formed by chemical reaction with oxygen. When it remains in the iron matrix, a false indication of magnesium level is created. MgO is inert regarding graphite nodularity but it is present in most OES level determinations. Because of this an inflated level of magnesium is referenced by DI technologists to ensure adequate pure magnesium is present, to produce the needed graphite nodularity.
Pure magnesium levels within the iron result from the direct transfer from magnesium added in the conversion process, with it physically existing in solid solution in the molten iron. Only its reaction with free oxygen in the iron and its partial pressure expulsion into the atmosphere affect its presence once it has gained entry into the iron matrix's solid solution.
When they first form MgO particles are suspended throughout the iron's matrix. After forming they tend to agglomerate into oxide masses and float out when given adequate time and the absence of violent stirring action, which counters the buoyancy process. In steel processing, deoxidation, which is a standard step in all steel melting, is followed by a two-minute minimum quiet idle period that allows for the oxides produced in the deoxidation process to agglomerate, attain buoyancy, and float out. Iron deoxidation encounters less oxide volume when compared to steel deoxidation. Less oxide volume reduces the time needed between deoxidation and casting to result in a cleansed, suspended oxide-free, iron matrix.
With DeOX metal treatment, deoxidation typically occurs during the melt cycle, which introduces basic time delays facilitating self-cleaning of the iron matrix.
The key to meaningful and useful magnesium OES measurement is to determine the pure magnesium level, not an artificially enriched value that includes MgO. This requires DI processing to be consistent and repeatable so that the MgO level in the spectrographic sample is minimized. Every DI producer differs, so individualized time delays must be established. Once the MgO level in the typical spectrographic sample is minimized, the magnesium level needed for full nodularity can be established.
The standard 0.030% Mg minimum for full nodularity is not a representative "red line". Full nodularity has been achieved at 0.010% Mg, which suggests substantial reduction in magnesium addition is possible but the amount of the reduction can't be established precisely without much more investigation. The DI technologist has the significant challenge to uncover the precise minimum level of magnesium needed for his or her specific foundry.
2) Free magnesium level - isolate from MgO
The simplest method to isolate MgO from the overall magnesium level is to allow the "cleansing" of the molten metal of the precipitated nano-size oxides. MgO oxides will agglomerate, eventually reach buoyancy, and float out of the molten mass. The trick in DeOX metal treatment is to allow adequate time between cutting off the free-oxygen atom supply and OES sampling of the molten iron. As noted, steelmakers adopted two minutes at minimum as the established delay for the precipitated oxides to exit the liquid steel. A similar time span is needed for the iron deoxidation process, with the time delay established in tandem with the specific DI conversion process in use.
Several techniques are available to remove free oxygen from DI base iron before modification. The advantage of DeOX metal treatment versus deoxidation by rare-earth additives, such as Preseed and the zirconium materials, is that DeOX D-1 is typically added at the beginning of the melt cycle and chemically reduces iron oxide as it forms, maximizing the time delay for cleansing action to occur in the iron matrix. The additions of rare-earth materials to molten iron inherently creates a "dirtier" iron matrix due to minimal time delay for oxide floatation, exposing the molten DI casting process to downstream oxide defects.
Deoxidization by addition of rare-earth materials, if carried out during the melt cycle, is hindered by iron oxide contacting the iron bath continues to form free-oxygen atoms throughout the process. With DeOX metal treatment, the source of free-oxygen atoms – iron oxide – is chemically reduced and eliminated.
Another deterrent for rare-earth deoxidation is cost. Rare-earth materials are in short supply and are very expensive.
With alternative deoxidation methods and materials, ever-present free oxygen continuously produces fresh MgO in the iron matrix and creates an entirely different scenario for OES magnesium measurement.
Standardizing deoxidation time after casting minimizes the amount of MgO affecting the overall Mg spectrographic level. The amount of MgO included in the spectrographic result remains constant once the deoxidation process is standardized, which establishes the precise level of the magnesium needed for full nodularity.
Previously, it was noted 0.10% Mg produced full nodularity, which is much lower level than current theory suggests. The 0.010% Mg level measured in a deoxidized DI could have been an anomaly, but it alerts the technologist to the possibility of significant reductions in magnesium alloy treatment amounts. Careful correlation between known free magnesium levels and graphite nodularity must be carried out by the foundry's technologist to establish the optimal magnesium alloy treatment. It is not difficult task to accomplish, and big savings are the potential reward.
3) Free oxygen in DI base iron causes magnesium loss
Conventionally melted iron contains some level of free-oxygen atoms. Oxygen atoms present in the DI base iron instantly combine with magnesium added in the DI conversion process, removing a portion of the free magnesium added to produce graphite nodularity. Iron deoxidation effectively removes all free oxygen from the DI base iron, eliminating this magnesium oxidation loss.
Magnesium loss can instantly return and affect the overall DI conversion process if re-oxidation of the DI occurs once the deoxidation and/or conversion process completes . The DI technologist must carefully examine molten iron processing to assure re-oxidation is prevented.
Surface slag chemistry checks for iron oxide determine if re-oxidation is occurring. Iron oxide content should be minimal, less than 5% iron oxide but preferably less than 1% iron oxide. 10%-20% iron oxide in surface slag indicates advanced re-oxidation is underway.
4) Eliminate graphite nodularity fade
When the level of free magnesium declines in DI, graphite nodularity decline (aka “fade”) may follow – a scenario is faced by most DI producers. Some add extra magnesium simply to prevent nodularity declining when the magnesium level falls, an expensive alternative.
The simple fact is magnesium levels decline in all conventionally treated and processed DI. A better solution is to prevent magnesium level decline, i.e., magnesium lost to oxidation. DeOX metal treatment is proven to achieve stable nodularity.
The primary cause of magnesium loss from modified DI is chemical reaction with oxygen atoms, i.e., oxidation. Molten DI reacts with the atmosphere to form iron oxide on its surface, mainly due to turbulence exposing fresh molten iron to the atmosphere. For this reason, molten DI handling practice must be controlled. "Bottom pouring" is used in steelmaking to minimize atmospheric contact.
Iron oxide contacting molten iron must be eliminated, and adding small amounts of DeOX D-1 to molten iron's cover slag will do that. DeOX D-1 chemically reduces the iron oxide present and forms an effective barrier to further atmospheric contact, preventing new iron oxide from forming.
Eliminating free-oxygen entry into molten DI very effectively stops nodularity fade. Some magnesium may be lost due the partial pressure differences between the DI and the atmosphere but that loss is trivial.
5) Magnesium alloy reduction possible with deoxidation
A minimum of 30% magnesium alloy material reductions have been demonstrated in every iron deoxidation application. It is critical to determine the expected recovery level for free magnesium before the magnesium alloy amount is reduced. Some foundries have reduced the magnesium alloy after 2-3 deoxidized heats, but that is not normal or recommended.
A critical detail is that magnesium alloy added in the DI conversion/modification process should be reduced in all iron deoxidation applications, since this reduction offsets the added material costs incurred during iron deoxidation. Higher magnesium levels after deoxidation provide no added quality to DI, so reducing mag alloy addition is the only sensible follow-up step.
Determining the minimum free-magnesium level needed for full graphite nodularity is a major factor as this determination can add to the amount of magnesium alloy reduction. It is prudent for each foundry to carry out nodularity reviews aligning nodularity with the OES magnesium level reported. Such an effort will establish the OES magnesium level into the foundry's process – which contains a certain but minimized level of MgO needed for full nodularity – and remove the pre-conceived magnesium level thought to be needed.
In one unusual DI foundry magnesium alloy reduction approached 70%, which shows that the overall DI conversion process needs to be reviewed in every foundry considering the new iron deoxidation technology.
6) Iron deoxidation's overall impact on DI technology
Iron deoxidation is exposing technology – and this is forcing changes in established traditions and DI theories. Iron deoxidation is new, and much technology is yet to be revealed. Significant investigations are underway.
Iron deoxidation has been cost effective in every DI iron deoxidation application when proper oversight is employed. Magnesium alloy savings have played an important part in creating the overall savings. Unfortunately, there have been foundries that resist reducing their magnesium additions, and because of that philosophy deoxidation cost effectiveness has been affected. The negative vibes created by these misguided technologists must be overlooked. Keith Millis invented ductile iron in the 1950s (or earlier), and now iron deoxidation is taking DI to a new plateau.