Ladle stirring corrects steel casting misrun defect at NACO.
Last year, the Cicero, Illinois foundry of National Castings, Inc. (NACO) was having a problem. The 500-employee steel foundry was experiencing 1524 incidents per month of misrun defects on cast-in numbering and lettering on a 1300 lb railroad car bolster casting.
The defect would show up as missing or very faintly legible characters. In many cases, with one or two figures missing, they could be welded on in the finishing room. If the entire sequence was missing, the casting would have to be scrapped. In either case, the defect was expensive and labor intensive, and had to be corrected.
This article recounts the steps taken to address the problem, and details how NACO implemented ladle stirring as the ultimate solution.
A Function of Fluidity
The misrun defect appeared in the ID sequence on the cope side of the green sand-molded, electric arc furnace-melted carbon steel casting. Because the cavities for the figures are the last part of the mold to fill, NACO officials realized the defect must be a function of the molten steel fluidity, and assumed the metal was partially solidifying before it reached that point.
On that assumption, the gating system was adjusted to shorten the distance for the metal to travel to the numbering. This, however, seemed to have no impact on the problem.
Next, pouring temperature was addressed to increase the steel's fluidity. The true liquidus temperature of each heat was 2753F (1511C). Yet after increasing the pouring temperature to as high as 2935F (1613C), the misruns continued. What was more, the extreme metal temperature was breaking down the molding sand and causing scabbing and other defects. Clearly, something else had to be done.
A Lesson From Steel Mills
Searching for the solution, NACO looked to the experience of the steel continuous casting industry. A lion's share of the that industry's quality concept in recent years has been built on inert gas stirring in the pouring ladle, a practice found in a majority of steel mills. The foundry decided that ladle stirring would provide an answer without incurring heavy capital expenditure.
Before NACO took action, however, a clear set of objectives for ladle stirring was laid out.
1. Obviously, the misruns had to be improved.
2. Lower pouring temperature by 60F - Benchmarking against continuous casting practices, which pour at a maximum of 60F above liquidus, foundry officials determined their own pouring temperature should eventually be at 2813F (1545C). Allowing themselves some leeway, they targeted 2840F (1560C).
3. Introduce stirring without causing pouring stream flaring or sacrificing clean shutoff on the foundry's bottom pour, stopper rod ladles.
4. Achieve cost savings in the form of improved alloy recovery.
The introduction of inert gas into the bottom of the ladle produces a stirring action that homogenizes the melt [ILLUSTRATION FOR FIGURE 1 OMITTED]. In an unstirred heat, there is a significant temperature gradient within the ladle, with the coldest metal at bottom and the hottest in the middle. The stirring action of the gas helps to erase this gradient and produce a consistent temperature throughout. This eliminates the need for excessive superheating. It also helps to blend the alloy additions more completely into the steel.
Also, as the inert bubbles rise toward the surface, they attract and trap oxides and unwanted gases within the melt, floating them up into the slag cover at the top. The result is a substantially purified heat.
A number of inert gases can be used in stirring/purging. Nitrogen is a common gas. However, in a heat with a high aluminum content, nitrogen can react with the aluminum to form a nitrate that can be damaging to steel casting structure. Carbon dioxide offers the advantage of lower cost, yet it can cause excessive plug wear. Weighing these factors, NACO officials decided to use the more expensive, but more benign, argon gas.
Converting the Ladle
Gas is introduced into the bottom of the ladle via a cone-shaped nozzle fitted into a cylindrical refractory plug [ILLUSTRATION FOR FIGURE 2 OMITTED]. The style chosen by the foundry is a porous plug (Labyrinth Purging Cone from Intertec, Cincinnati). Common plug installation practice is to cut a 10 in. hole in the bottom of the ladle and install the plug from the outside. The refractory floor of the ladle is then rammed around the plug to the same height. With this method, the larger rear end of the nozzle cone is exposed and can be replaced from the outside, which is preferable in continuous casting operations where there isn't time to cool down the ladle to change the nozzle. The drawback to this style is that it requires a system of wedges and plates to support the nozzle in place.
NACO, on the other hand, cut a 1 in. hole in the ladle, just large enough for the connector on the back of the nozzle to protrude through. Installation and nozzle changing is performed from the inside [ILLUSTRATION FOR FIGURE 3 OMITTED]. This lowers the chances of plug failure, and also doesn't require a high operator skill level for plug assembly and insertion. The method better lends itself to foundry operations.
Savings and Improvements
The foundry began stirring in November 1995, and the results were immediately apparent. Though only 9% of heats were stirred that month, the number of misruns plummeted from 23 in October to just three in November. The next month that number was just one. NACO's misrun problem was solved [ILLUSTRATION FOR FIGURE 4 OMITTED].
The next task was reducing pouring temperature. November's average temperature was 2902F. In December, officials tried 2890F (1587C). Emboldened by good results, they dropped it all the way to 2850F (1565C) in January and it hasn't been above 2853F since [ILLUSTRATION FOR FIGURE 5 OMITTED]. With pouring temperatures down by 60F, the foundry has plans to drop it yet another 20F. In fact, heats have been poured as cool as 2772F (1522C) with excellent results.
Pouring colder means tapping colder, since the need for excessive superheat has been eliminated. This factor has cut kWh consumption by 21 kWh per ton, translating into real energy cost savings. Furthermore, furnace productivity is enhanced. With ladle stirring, the furnace is no longer used to refine steel, which it does inefficiently. Instead, it is free to go back to doing what it does very efficiently - melting metal.
Ladle stirring is a highly desirable refining medium. By tapping colder, alloy recovery is higher to begin with, and results are more predictable. Since its introduction, the foundry has seen 12% better alloy recovery, with no manganese fade at the end of the heat. With the option of trimming in the ladle, it is possible to control steel chemistry to very close tolerances, and additions can be on the low side of the recommended amount to save on alloy costs.
Not surprisingly, the overall quality of the steel has been enhanced. The foundry has better control of undesirable gases such as oxygen, and nitrogen parts per million have been reduced by 6%. Also, many of the oxide inclusions within the melt are floated to the top of the bath and trapped in the slag. Obviously, this makes for cleaner castings, but it also means that there is almost no buildup of oxides around the ladle's pouring nozzle. Even at the lower pouring temperatures, the ladle shuts off clean. The pouring stream is compact, without flaring, which lowers the amount of possible reoxidation during the pour and improves casting surface finish. These factors, combined with the absence of the misrun defect, have improved overall yield 4%.
Other benefits the foundry has seen since the introduction of ladle stirring include an increase in ladle life from 118 heats per campaign to 160, due to the lower metal temperature. Similarly, rotorod life has been extended from 4.1 to 5.9 heats per rod, and the amount of necessary gun material has decreased by 20%. In real monetary terms, ladle stirring's price, including the cost of the argon and the plugs, is $1.45 per ton of steel. However, after all the above benefits have been factored, ladle stirring has produced a savings of $6.50 per ton - no small number for a shop that melts 24,000 tons each year.
Thus, with a very low operating cost and no major capital commitment, NACO has defeated the misrun problem and improved its overall quality, productivity and cost-effectiveness.
RELATED ARTICLE: Stirring Step by Step at NACO
To optimize the benefits of argon ladle stirring, NACO's melt department follows a set procedure for each heat.
Prior to tapping the furnace, the argon supply line is hooked up to the nozzle at the bottom of the ladle. As tapping begins, the argon pursuer is turned on to 40 psi and left at that setting during the tap. At the end of the tap, the operator should be able to see the stirring action in the bath and make adjustments to the gas pressure accordingly. On the surface of the bath there will be a bright space in the slag directly over the plug. The size of this space should not exceed half the diameter of a basketball (or smaller, depending on the slag condition). This is a good indication of whether or not the appropriate gas pressure is being used. The rest of the slag cover should show a slight waving action.
After tapping is finished, the heat is stirred for another 2 min. At that point, a lab test sample is taken and temperature is checked. Stirring continues as the ladle is removed from the tapping pit and brought to the pouring area. There, alloys are adjusted if necessary, and stirring continues until the target steel temperature is reached, when the metal is immediately poured. The number of minutes each heat is stirred are carefully recorded.
One of the important factors in effective stirring is the pressure of the argon released into the metal. Again, the objective in stirring is to produce a more homogenous pouring temperature and alloy distribution while removing impurities and undesirable elements into the slag cover. To achieve these objectives, everything depends on the action of the gas within the melt.
The ideal situation is the introduction of many small argon bubbles, which are produced by a carefully maintained, moderate gas pressure level. The more bubbles for oxides and unwanted gases to attach to, the cleaner the melt will be. Also, while the small bubbles are sufficient to stimulate the proper stirring action in the bath, the larger bubbles and rougher motion caused by inordinately high gas pressure disturbs the slag cover, breaks it up and causes the slag to fold back into the bath - defeating the quality advantages of ladle stirring.
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|Title Annotation:||includes related article on ladle stirring; National Castings Inc.|
|Date:||Jan 1, 1997|
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