Examining bottom gating at AG Anderson.
* Bottom gating is presented as a possible solution to turbulent fill in steel casting applications.
* Several examples where bottom gating has solved defect problems at AG Anderson are explored.
* An example of the adverse effects of bottom gating is detailed.
Many studies about casting quality have concluded that properly designed gating systems play an important role in reducing or eliminating re-oxidation products. The same studies show that liquid surface turbulence is the phenomenon that is directly responsible for the formation of oxides in the casting.
Bottom gating has been used as a solution to turbulent flow. Most castings benefit from bottom gating in which the molten metal falls under controlled conditions to the lowest point of the casting. At this point, the metal reaches the highest speed. After that, it flows into the casting through a runner system intended to slow the metal speed to acceptable limits and minimize turbulence.
This article focuses on bottom gating for steel casting production and details this technique as a practical solution for controlling molten metal turbulence and eliminating flow-related defects. The advantages and disadvantages of this technique are explored along with production observations made during gating trails. Last, the article provides practical solutions for molding and patternmaking to develop efficient gating systems.
AG Anderson Ltd., London, Ontario, Canada, is a 125-employee jobbing facility producing a wide variety of steel and iron castings from 2-5,500 lbs. Carbon, low alloy and stainless steel represent 60% of production, with gray, ductile and high alloy iron making up the balance.
Melting is performed in three medium frequency induction furnaces with capacities of 600, 2,500 and 4,000 lbs, each lined with a spinel bound fused alumina-based refractory. Pouring is performed using lip pour ladies ranging from 600-6,100 lbs. Pre-cast disposable ladles are frequently used for pouring smaller castings.
In an effort to reduce non-metallic inclusions and improve metal flow, all steels are filtered using 10 PPI ceramic foam filters. Pressurized gating systems capable of accommodating ceramic filters also have been developed and are used on a regular basis.
Practice has shown that when using proper de-oxidation techniques combined with gating systems designed to reduce turbulence, a significant amount of re-oxidation during mold filling still takes place. Post-filter re-oxidation can occur when the metal falls for long distances or travels through intricate mold cavities. As a result, bottom gating philosophy has been adopted as an added remedy to the existing solutions already in place.
To accommodate bottom gating, AG Anderson has developed a three-part mold technology. The mold has two parting lines and the drag consists of two parts. This design requires the main runner bar to be placed in the bottom part of the drag. A schematic representation of a bottom gating system using the three-part mold technology is illustrated in Fig 1.
[FIGURE 1 OMITTED]
As shown in Fig. 1. the filter basin and runner bar are both located in the bottom part of the drag ("Drag B") while a section of both the sprue and the ingate are in the top part of the drag ("Drag A"). The casting is in the cope.
Figure 1 also shows the actual casting represented in the above drawing--a 2,600-1b. open impeller in a CF-3M. In this case, the bottom gating technique was implemented to achieve a more uniform temperature distribution throughout the casting and to improve the efficiency of the top riser lay supplying hotter metal into fine riser toward the end of the pour. The result was remarkable.
Defects caused by temperature distribution such as burned-on sand on localized areas, hot tears at the casting/ ingate junction and shrinkage left unassisted by an inefficient, cold riser were completely eliminated. Other types of defects such as cold laps, cold shots and re-oxidation products, previously caused by metal flow, also were significantly reduced.
Even though the improvements in the casting were remarkable, when the mold filling was simulated in casting process modeling software, a "fountain effect" was noticed at the top of the ingate. In this particular case, the gating system was not modified because the defect was not detrimental to the application. However, the fountain effect can be significantly reduced in an un-pressurized gating system where the flow is controlled by the bottom of the spree rather than the ingate.
Un-Pressurized Bottom Gating
The low-alloy steel sprocket carrier casting illustrated in Fig. 2 is an example of a component that required extensive trial and error studies to determine the best gating systems. Initially, a conventional pressurized gating system with filters at the parting line was selected. The results were unsatisfactory as numerous sub-surface, non-metallic inclusions were uncovered by machining.
[FIGURE 2 OMITTED]
The first attempt at a solution was to replace one of the top risers with a riser sleeve for direct pour. This time, modeling showed a significant turbulent flow on the opposite side of the mold cavity.
At the end of all the experiments, the job was gated using an un-pressurized bottom gating system with a ratio of 1:3:2. with this design, the computer simulation shows that the sprue backfills quickly, and the flow after the filter appears to be controlled without significant jetting of the metal into the casting cavity. The flow throughout the casting was quiescent and the casting was filled evenly. The sprocket carrier is now molded in a three part flaskless mold as shown in Fig. 2.
The bottom of the "Drag A" section of the mold is a strike-off face so a step parting line is needed on the top of the "Drag B" section to locate the two pints together. The gating system works well. supplying a non-turbulent flow and good temperature distribution throughout the casting. Even though molding time, labor and materials increased due to the change, the work in the cleaning, welding and inspection departments was reduced by more than 30%.
While the bottom gating technique has solved many problems for AG Anderson, it isn't always the solution.
For the melt pot casting (alloy AISI430) illustrated in Fig. 3, bottom gating resulted in some internal and surface defects. During production, bottom gating resulted in top risers that were too cold and incapable of properly feeding the main body of the casting, resulting in macro-porosity and shrinkage. Excessive cold lapping along the top flange also was caused by the cold front of metal delivered toward the end of the pour.
[FIGURE 3 OMITTED]
As a solution, the gating system was changed to a more conventional system with the filter basin, runner bar and legates that were at the parting being moved to the top of the flange.
Bottom or Not
Experiments and computer simulations show that there are numerous advantages to bottom gating. First and foremost, the number of re-oxidation defects is significantly reduced. In addition, other defects, such as sand inclusions as a result of mold erosion, burned-on sand in hot spots, casting distortion caused by non-uniform temperature distribution throughout the casting, hot tears and even broken cores, can be eliminated or considerably reduced by adopting this technique. Shop trials and simulations also confirmed that bottom-filling systems with filters provide the least amount of defects related to metal surface turbulence.
The key with applying the system is understanding your costs. Since bottom gating carries with it a higher cost than traditional casting, facilities must factor in volume, alloy, part configuration and maximum head height to determine the total component cost from molding to the finishing department and machining to determine if bottom gating is the correct solution.
A properly designed gating system only works well when proper melting, de-oxidation and pouring practices are followed.
This article was originally presented at the 2003 Steel Founders' Society of America Technical & Operating Conference.
For More Information
"Gating Design via Computer Fluid Flow Modeling," R. Nariman, Proceedings from the 1997 Steel Founders' Society of America Technical and Operating Conference, Steel Founders' Society of America, Barrington, Illinois.
"The Effect of Gating and Pouring Practice on Re-oxidation of Steel Castings," P. Scarber, Jr., J. Griffin, and C.E. Bates, Proceedings from the 2001 Steel Founders' Society of America Technical and Operating Conference, Steel Founders' Society of America, Barrington, Illinois.
Vasile Ionescu and Gil Catherwood AG Anderson Ltd., London, Ontario, Canada
Vasile Ionescu is a metallurgist for AG Anderson with 23 years in the steel and iron metalcasting industry. Gil Catherwood is the manager of production engineering for the firm.
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|Date:||Mar 1, 2004|
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