Issue #2, 2007
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“HOT TOPICS” Issue #2, 2007 Fading of Magnesium in Treated Ductile Iron Introduction There are several factors that affect the nodularity these furnaces by using nitrogen under pressure of treated ductile iron. Magnesium residual level as well as keeping the bath relatively clean. Even is one of the important factors that should be in these furnaces magnesium loss can not be carefully controlled to avoid substandard prevented. Magnesium levels in the iron tend to nodularity in the castings. Knowing the minimum diminish due to constant heating of the iron to level of magnesium necessary for a particular maintain temperature, as well as being lost to the chemistry and set of operating variables in the slag accumulating inside the furnace. foundry, along with the rate of fading, is Depressurizing the furnace repeatedly will necessary for determining how long the iron can increase the Mg loss. The amount of magnesium be used for meeting the standards set up. Back loss should be more or less constant for a given issues of hot topics that deal with related topics furnace at operating level. In a heated pressure include: pour furnace, about three lbs of magnesium is Issue 3, 2003 Effect of magnesium in ductile iron lost per hour, regardless of the amount of iron in Issue 4, 1999 Magnesium treatment process the furnace. Hence, when the furnace is full, the Issue 2, 1999 Nodularity in ductile iron castings magnesium loss has lesser effect on the residual Issue 7, 2001 Effect of temperature in ductile than when the furnace is at heel level. This is iron processing illustrated in figure 1. Once the magnesium level goes below 0.01%, sulfur starts to increase in the Elemental data iron. After a long delay, such as weekends, the Atomic number 12 iron will have very low Mg and high sulfur. To Atomic weight 24.3 restore the bath to an acceptable level of Density 1.74g/cc magnesium, batches of iron at high Mg level Melting point 1200oF (649oC) have to be supplied to lower the sulfur and raise Boiling point 1994oF (1090oC) the Mg level. Leaving sulfur rich slag in the furnace will lead to accelerated depletion of Factors magnesium in the iron once the fresh treated iron Here we will look at the factors that affect the supply is interrupted. fading of magnesium in treated iron, many of which are interconnected, as we will see below. Mg loss with furnace size Temperature 0.035 At higher holding temperatures the rate of Mg 0.03 loss, as vapor, is higher since the Mg vapor 0.025 pressure exceeds the pressure above the metal 0.02 bath (if it is atmospheric). Rate of loss of Mg is 0.015 0.01 lower at a lower magnesium residual level for the 0.005 same holding temperature. Rate of loss %Mg/Hr 0 Holding treated iron in pressure furnaces is 0 5 10 15 20 25 30 35 common with high production lines. Iron is Iron in furnace tons exposed to higher temperature in the inductor loop than the bath temperature. Mg may tend to Fig1. Loss of Mg in pressurized holding furnaces vaporize and become part of the atmosphere over (relatively clean) under nitrogen atmosphere at the iron bath. Magnesium loss is minimized in various iron levels Al Alagarsamy, Citation Corporation, Birmingham, AL Mg loss with time in afull furnace Slag Slag or dross from Mg treatment contains oxides 0.07 of silicon, manganese, iron and magnesium, as 0.06 0.05 well as Mg and rare earth sulfides. Sulfides will 0.04 breakdown in the presence of oxides and an 0.03 oxidizing atmosphere. Any slag that is not 0.02 removed can build up on the ladle sidewalls %Mg residual 0.01 along with iron droplets that readily transform 0 into iron oxide with time. These oxides will 0 50 100 150 constantly release sulfur from the sulfides, which Holding time, minutes is free to mix with the iron. It is very important for this reason to keep the iron bath and ladle Fig2. Loss of magnesium in a full walls clean. Any slag build up in the ladle will pressurized furnace under nitrogen with time adversely affect the Mg loss and recovery during treatment. When the treatment ladles stand idle or Treated iron that is heated or held in open under torch during work stoppages, sulfur induction furnaces will lose magnesium at a generation is increased (higher oxidizing level). higher rate than covered or shrouded baths The next one or two treatments will see reduced (Figure 3). As the iron is heated and stirred by recoveries and increased rates of loss. During the power input, magnesium is brought into these times it is common to compensate by intimate contact with oxygen, in the atmosphere adding extra alloys to balance the increased loss. as well as oxides in the slag, An oxidizing Similarly this happens when there is a change in environment destabilizes Mg Sulfide and frees up the amount of iron treated (increased tapped the sulfur to combine with more Mg. This weight due to job changes etc.) and the iron level reaction is quite rapid due to the violent mixing covers the slag line, increasing Mg loss. In of the iron with oxygen sources. In contrast to stopper rod pouring ladles during down times the heating iron in a furnace, iron kept in an un- iron level is maintained at a lower level. Slag that heated covered ladle loses Mg at a much slower is adhering to the walls is exposed to oxidation rate as shown in the figure 2. When the pouring and frees up accumulated sulfur. When ladle is tilted back and forth new surfaces are production starts again the new iron covers this exposed to air and when iron covers the slag the slag contaminated material absorbing the sulfur Mg fade rate increases. which then combines with Mg. Whenever there is an opportunity for slag and iron oxide to be Mg Loss with time present, there is a risk of increased Mg loss. Ind. furnace covered ladle Linear (covered ladle) Standard free energies of formation of sulfides 0.06 and oxides for key elements (excerpts from 0.05 Physical Chemistry of Metals by Darken and 0.04 Gurry) are given in Figures 4 and 5. When the 0.03 free energy of formation is highly negative that 0.02 reaction is favored over the ones the free energy Percent Mg 0.01 of formation is less negative. It is evident from 0 the figures 4and 5, that the magnesium oxide 0 5 10 15 20 formation is favored over the magnesium sulfide Time in minutes formation. Also if MgS is already present, in the presence of oxygen, MgO will form releasing Fig3. Loss of magnesium with time in a heated sulfur back into iron. open induction furnace and a covered insulated ladle full of iron (not heated) Free energy of sulfide formation Exposure to oxygen The loss of magnesium due to oxidation is much 0 higher and of more concern than the loss of 0 500 1000 1500 2000 -50 magnesium due to evaporation due to high partial -100 pressure. If the liquid metal is brought in contact -150 -200 with oxygen – either due to turbulence in air or in Free energy -250 contact with oxides in the slag – dissolved -300 magnesium in iron reacts with oxygen to form Temperature deg C stable oxides. Oxygen can also react with MgS to form MgO (At all operating temperatures as Mg Ca Ce Mn shown in Figures 4 and 5), freeing up sulfur. By minimizing the contact with oxygen and slag the Fig4. Free energy of formation of sulfides Mg fade rate can be controlled and greatly with temperature reduced. Inoculation fade is different from Mg fade and is Free energy of oxide formation also important to recognize. Even if the magnesium level is adequate the loss of nuclei 0 -50 0 500 1000 1500 2000 will affect nodularity unless it is re-inoculated. -100 -150 -200 Free energy -250 -300 Tempertature degC Mg Ca Mn Si Fe Fade time (Useful time) Fig5. Free energy of formation of oxides Many factors should be taken into with temperature account prior to establishing a maximum time that iron can be held and still result Quantity of metal in good nodularity. As we have seen in fig1. the amount of 1. Temperature of melt magnesium loss is about the same regardless of 2. Type and amount of slag furnace level. However, when the furnace is full, 3. Exposure to oxygen the reduction in Mg percentage is much lower 4. Amount of metal in the ladle compared to a near empty furnace. Rate of loss Clean ladles with minimum disturbance increases rapidly as the lower iron level exposes decreases fade rate and extends the more oxidizing surfaces in the furnace. The same process time window. Post inoculation is true for unheated manual pouring ladles or also plays a role in setting this time limit. stopper rod tundish ladles. The ratio of slag to Late inoculation containing shape active iron continuously increases when the ladle goes to heel level or empty. In addition, the quantity of elements can lengthen the useful time. slag increases with time. As this ratio increases Mg loss rate increases. Magnesium in a near empty ladle with slag will fade faster than a full clean ladle kept undisturbed. If a small quantity of iron is left to solidify in the ladle, upon remelting of this iron with fresh treated iron, higher magnesium fade rate can be expected.