Improve the performance of your ductile iron pressure pour.
With ductile iron casting shipments forecasted to reach an all time high in seven of the next 10 years, foundries of all sizes are paying much more attention to the sought-after metal in an effort to capitalize on demand. However, a booming market doesn't necessarily guarantee profit. Performance has a very definite effect on the size of your bottom line. A foundry that employs a pressure pour unit should take a good look at operations to make sure the system is running as smoothly - and as efficiently - as it can.
Statistics show that the majority of U.S. foundries produce ductile iron that has been treated with magnesium ferrosilicon (MgFeSi). If you belong in this group, you should design your pressure pour to accommodate the special composition of the metal. Years of experience have shown that a pressure pour is able to operate for up to 47 months without plugging the inductor - as long as it is designed with metallurgy in mind and receives the proper care.
By understanding the dynamics of oxide and sulfide deposition, cleaning your pressure pour on a daily basis and respecting the unique metallurgical requirements of MgFeSi-treated iron, you can optimize the capabilities of your pressure pour system. You also should pay special attention to vessel buildup, plugging of the inductor, Mg fade resulfurization of the iron, restarting your pressure pour and changing alloys.
The Treatment Process
MgFeSi is added to ductile base iron to reduce the sulfur (S) level so that the iron will solidify with the graphite in a nodular shape. The reactive elements in the treatment alloy combine with the dissolved S to form their respective sulfides. If the ductile base iron contains 0.02% S before treatment and 0.004% S after treatment, there will be 300-350 g of metal sulfides formed in each ton of treated iron.
Since the oxygen (O) content of the base iron is relatively low, few metal oxides form during the treatment process However, the treatment alloy introduces considerable oxides. MgFeSi disperses 400-450 g of oxides including Mg, iron, Si, calcium (Ca), and aluminum (Al) in each ton of treated iron.
While a portion of these oxides and sulfides float to the surface of the molten iron in the treatment vessel. numerous microscopic particles remain suspended in the iron and enter the pressure pour. It has been reported that the remaining oxides are in the order of 250 g/ton of iron, while the remaining sulfides are in the order of 100 g/ton of iron.
Even though the magnesium sulfide (MgS) has a lower density than the molten iron, it does not readily float to the surface because small particles take longer to float than larger particles. Since the flotation process takes time, it is beneficial to provide for some dwell time in the treatment ladle before pouring the treated iron to minimize the amount of sulfides and oxides entering the pressure pour.
Those suspended particles that remain in the treated iron eventually separate from the iron in the pressure pour. Some will deposit on the side walls of the vessel, on the bottom of the vessel, in the fill siphon, in the pour siphon and in the throat of the inductor [ILLUSTRATION FOR FIGURE 1 OMITTED].
Side Wall Buildup
The deposits on the side walls and bottom of the vessel reduce the capacity of the pressure pour. For example, the capacity of one foundry's 10-ton pressure pour was reduced by 50% in just 10 weeks because it was not cleaned. In other words, there was a total of 25 cu ft of metal oxides and metal sulfides in this vessel. Since the loss of capacity affects the operation of the pressure pour the vessel should be cleaned on a daily basis.
In order to facilitate cleaning, the lid of the pressure pour must be equipped with a removable hatch to allow the side walls of the vessel to be scraped and to remove the slag. Since the vessel cannot be totally cleaned through this hatch, the lid also must be removable to allow access to the entire vessel. If the scraping operation is done correctly, there will be a loss of refractory and the capacity of the vessel will increase.
The deposits in the siphons restrict the rate of metal flow into and out of the pressure pour. A restriction in the fill siphon increases the time to pour the molten iron from the treatment ladle into the vessel. A restriction in the pour siphon increases the response time to raise or lower the metal in the pour launder. A partial restriction in either siphon will hamper the operation of the pressure pour. while a total restriction in either siphon will shut down the pressure pour. Since the fill siphon tends to build up faster than the pour siphon, it must be cleaned at least once an hour. The pour siphon must be cleaned on a daily basis.
The diameter of the siphons must be large enough to accommodate any buildup that occurs between cleanings so that the metal flow will not be affected. The siphons should be no less than 8 in. around and preferably larger. The use of a refractory containing silicon carbide in the siphons has been found to facilitate removal of the buildup.
Likewise, a siphon should not be designed with a curved section at it base as is common on gray iron pressure pours. The curved section establishes a region that cannot be cleaned and will shut down a pressure pour on treated ductile iron in 10-12 weeks. The siphons should be designed with a straight passage to facilitate cleaning.
The deposits build up in the inductor and close over the throat. A conventional throat-type inductor will generally plug in 10-12 weeks. One of two inductor locations is generally selected: the side of the vessel or the bottom of the pressure pour.
The European designs favor mounting the inductor on the side of the vessel. The advantage of this location is that the inductor is readily accessible and easily hot-changed. The disadvantage of this inductor location is that the throat opening is close to the minimum metal level, resulting in suspended oxides and sulfides being sucked into the throat where they remain as buildup.
The American designs favor mounting the inductor on the bottom of the pressure pour. The advantage of this location is that the inductor throat opening is farther from the minimum metal level, reducing the tendency to suck suspended oxides and sulfides into the throat. The disadvantage of this inductor location is that it is not readily accessible for a hot-change.
While a bottom-mounted inductor performs better than a side mounted inductor, neither mounting location is free from buildup. Rather than using an inductor that needs to be hot-changed due to buildup, the pressure pour for treated ductile iron requires an inductor that is free of this problem. In order to design such an inductor, we need to understand what causes the conventional throat-type inductor to collect deposits.
Examination of spent inductors shows that most of the buildup occurs in the upper portion of the channel between the 10:30 and 1:30 position, while little or no buildup is found in the bottom portion of the channel between the 1:30 and 10:30 position. This happens because the rate of metal movement is slower in the throat than in the channel. Hence, the suspended oxides and sulfides separate from the molten iron in the throat, not in the channel.
Since the throat acts as a collection vessel for the oxides and the sulfides, a throatless inductor has been designed and built in an effort to avoid the problem of buildup. The throat was eliminated, allowing the channels to connect directly to the bottom of the vessel [ILLUSTRATION FOR FIGURE 2 OMITTED].
To ensure that the nonmetallics remain suspended in the metal, the inductor is pulsed from low power to high power. During the low power cycle, little or no nonmetallics are pulled into the loop. During the high power cycle, an aggressive stirring action keeps all particles in suspension and flushes unwanted nonmetallics from the loop. Thus, the nonmetallics remain suspended in the metal and exit the loop without building up on the refractory.
In an unheated pour box, the treated ductile iron must be pigged after 12-15 min. Since Mg has a high affinity for O and since an unheated pour box generally has no protective atmosphere, the Mg fades with time. Because a pressure pour for treated ductile iron uses an inert atmosphere, which is generally nitrogen (N), the Mg will still fade but at a significantly lower rate.
The physical state of Mg in molten treated ductile iron is a gas. Since the vapor pressure of Mg is 7.6 atmospheres at 2600F (1427C), and since the pressure in the vessel is less than one atmosphere, Mg will leave the molten iron as Mg vapor and establish a partial pressure with the pressurizing gas.
Each time the vessel is refilled with treated iron, the pressure must be lowered to make room for the incoming metal. Some of the Mg vapor will be exhausted with the pressurizing gas. Mg will be lost at the rate of 0.004-0.005%.
The operation of a pressure pour is not controlled by time but on a minimum/maximum Mg content, just like carbon and Si. As long as the Mg is within specification, the pressure pour can run on a continuous basis, eliminating the need to pig faded iron.
As Mg vapor leaves the vessel, it cools and builds up in the pressurization system. The first place where Mg accumulates is at the point where the N line attaches to the vessel. Likewise, the Mg will build up in the valve train. The closer the exhaust valves are to the vessel, the greater the buildup of Mg in them. For this reason, the valve train should be located 40-50 ft from the pressure pour.
The pressurization system must be cleaned on a regular basis to ensure that the vessel will pressurize or depressurize as needed. Care must be exercised when the pressurization system is opened to the atmosphere for cleaning, as the Mg will flare.
When a pressure pour is first used, the treated iron contacts the refractory lining. Then as MgS and magnesium oxide (MgO) builds up on the side walls and bottom of the furnace, the treated iron ceases to contact the refractory lining and, instead, contacts the buildup on the lining. As long as Mg is present in the molten iron and the atmosphere is N, the MgS buildup is stable. However, on the weekend, when the vessel is purposely at a low metal level and is opened to the atmosphere for cleaning, three reactions take place.
First, as the inspection hatch is opened to remove slag and buildup, air enters the vessel. The O in the air reacts with the exposed MgS on the side walls and above the molten iron in accordance with the following reaction:
MgS(s) + 3/2 [O.sub.2] (g) = MgO(s) + S[O.sub.2] (g)
Here. the MgS is oxidized to MgO and sulfur dioxide (S[O.sub.2]), which accounts for the foul S smell when the vessel is first opened to the atmosphere. Care should be taken not to breathe the exhaust gases.
Second, O from the air will dissolve in the molten iron and react with the residual Mg, causing it to fade in accordance with the following equation:
[%Mg] + [%O]: MgO(s)
Once the residual Mg level in the molten iron is reduced to zero, the O will then dissolve in the faded iron to 4-6 ppm.
Third, the dissolved O will then react with the metal sulfides on the side walls and on the bottom that are in contact with the faded iron in accordance with the following equation:
MgS(s) + [%O] = MgO(s) + [%S]
Here, MgS is oxidized to MgO by releasing the S, which dissolves in and resulfurizes the iron. The amount of resulfurization depends upon the amount of metal sulfides in the vessel. It is for this reason that the vessel must be cleaned on a daily basis. Typically the bath can resulfurize to 0.04% during cleaning and to 0.12% after a lengthy hold.
To reduce the amount of resulfurization and minimize the foul S smell during cleaning, limestone should be added to the pressure pour. The calcium oxide (GaO) ties up the S in the slag in accordance with the following equation:
CaO(s) + [%S] = CaS(s) + [%O]
Theoretically, 1.05 lb CaO/ton of faded iron is required to reduce the S level by 0.01%.
Restarting a pressure pour after resulfurization requires that the S level in the vessel be lowered and restored to the normal treatment level. The higher the S level in the bath and the larger the molten heel, the more difficult this task becomes.
The S level is lowered by reaction. Here, overtreated iron is added to the pressure pour. Since over-treated iron contains higher-than-normal Mg, the excess Mg will react with and lower the S level before dissolving in the iron to its desired level.
Since the Mg is introduced as MgFeSi, the over-treated iron will contain higher-than-normal Si, causing the Si level in the vessel to increase above specification. The Si level is lowered by dilution.
Consider the case of a 20-ton pressure pour that holds 55,000 lb of iron and has a minimum heel of 15,000 lb. Assume that the iron has resulfurized to 0.12% S, that the treatment size is 5000 lb, and that the iron is to contain 2.0% Si and 0.04% Mg after treatment. The percent Mg in the over-treated iron is 0.10%.
The Mg, S and Si levels vary as overtreated iron is added to the pressure pour [ILLUSTRATION FOR FIGURE 3 OMITTED]. Note that the S level is with in specification after three ladles of over-treated iron, while it takes eight ladles of over-treated iron for the Mg to reach its desired level.
During this time, the Si level increased from 2.20% to 2.52%. Since the Si level in each ladle of normally treated iron is 2.2%, the Si level in the pressure pour will correct itself by dilution. Because the Si level will be high on the first metal poured after startup, non-critical castings may have to be poured until the iron is within chemical specifications. The alternative to pouring high-Si iron is to restart the pressure pour by over-treating a base iron with a lower-than-normal Si content.
Another problem with restarting a pressure pour is that the first metal into the pour launder is cooled as energy is transferred from the treated iron into the refractory. Likewise, any oxides on the surface of the pour launder react with and lower the residual Mg in the treated iron in the pour launder. The result is defective castings due to low modularity and carbides in the first metal poured.
While pressurizing and depressurizing to flush hot metal into and out of the pour launder helps, it does not solve the problem. since the molten iron in the pour siphon is low in Mg and cold. A better corrective action is to back the pressure pour away from the pour line, pressurize to fill the pour launder, hold the iron for a given period of time to heat the pour launder refractory and reduce surface oxides before dispensing a given amount of metal into a ladle. Not only does this flushing of the pour launder correct the problem, but the metal can be salvaged by returning it to a holding furnace.
Changing alloys in a pressure pour requires that the percentage of some alloying elements be lowered while others are raised. Those alloying elements that are too high are lowered by dilution while those elements that are too low are increased by addition.
The problem with changing alloys is similar to restarting the pressure pour. Since dilution of and additions to a molten metal heel are made easier as the molten heel gets smaller, it is desirable to decrease the metal level to its lowest level to minimize restarting and/or alloy change problems.
In the case of a pressure pour that does not tilt, the lowest level to which the metal can be lowered is the burp level, at which point the pressurizing gas escapes via the fill and poursiphons. Unfortunately, there is still 25-35% of the total weight of faded iron in the pressure pour at the burp level.
In the case of a pressure pour that can be tilted, the metal level can be lowered well below burp level by pouring metal from the vessel via the fill spout. It is then possible to reduce the amount of metal in the vessel to 10-15% of its total capacity. Thus, a pressure pour that can be tilted has an advantage over one that cannot, since less faded iron has to be retreated or less iron has to be alloyed.
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|Author:||Duca, William J.|
|Date:||Jul 1, 1998|
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