Pouring stream shrouding at Harrison steel castings.Still largely a unique process for steel founders, shrouded shroud n. 1. A cloth used to wrap a body for burial; a winding sheet. 2. Something that conceals, protects, or screens: under a shroud of fog. 3. a. pouring trials showed a significant increase in metal cleanliness Cleanliness See also Orderliness. Cleverness (See CUNNING.) Berchta unkempt herself, demands cleanliness from others, especially children. [Ger. Folklore: Leach, 137] cat continually “washes” itself. and a total cost savings. Over the years, the Years, The the seven decades of Eleanor Pargiter’s life. [Br. Lit.: Benét, 1109] See : Time "Clean Steel" projects by the Steel Founders' Society of America (SFSA SFSA Steel Founders' Society of America ) have established that a majority of the inclusion defects present in steel castings are caused by reoxidation. This occurs when liquid metal is exposed to oxygen after the metal has been fully deoxidized. The major sources of this oxygen are entrainment entrainment /en·train·ment/ (en-tran´ment) 1. a technique for identifying the slowest pacing necessary to terminate an arrhythmia, particularly atrial flutter. 2. of air into the pouring stream and exposure to oxygen in the mold cavity. The idea of protecting the pouring stream from oxidation via mechanical and gas systems has been successfully used in the continuous casting Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. It allows lower-cost production of metal sections with better quality, due to finer control through automation of the casting industry for decades. The idea behind shrouding shroud n. 1. A cloth used to wrap a body for burial; a winding sheet. 2. Something that conceals, protects, or screens: under a shroud of fog. 3. a. (the process of enclosing the metal stream as it is poured) is to prevent oxygen from reacting with liquid metal in the pouring stream. The SFSA has been working to develop this technology for use in the foundry industry. Early trials emphasized inert (argon argon (är`gŏn) [Gr.,=inert], gaseous chemical element; symbol Ar; at. no. 18; at. wt. 39.948; m.p. −189.2°C;; b.p. −185.7°C;; density 1.784 grams per liter at STP; valence 0. ) gas shrouding of the pouring stream and mold cavity. While this method was successful in reducing the amount of reoxidation that occurred during pouring, the process also led to high levels of porosity in the castings due to the entrapment entrapment, in law, the instigation of a crime in the attempt to obtain cause for a criminal prosecution. Situations in which a government operative merely provides the occasion for the commission of a criminal act (e.g. of inert gas inert gas or noble gas, any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon. in the metal. Experiments using mechanical shrouding at Wisconsin's Pelton Casteel and Falk Corp. in the mid-to-late 1990s revealed that mechanical shrouding alone held some promise. In particular, a mechanical shroud on a 1000-lb production part at Falk reduced pouring time, and improved the cleanliness of the casting and subsequent machining. The process was concluded to be cost-effective due to the replacement of the gating system and increased part quality. With this in mind, Harrison Steel, which used a conventional bottom-pour method, began an experiment to determine the prospects of shrouding. This article provides an overview of the process (including equipment needed), the process changes and the problems encountered. A study on the impact of casting cleanliness and a basic cost analysis are also included. Harrison's Experiment Steel for these experiments was melted in 20-ton acid-lined arc furnaces. All the ladles were bottom-pour types (lined with bloating bloating Vox populi A lay term for post-prandial abdominal fullness or swelling fireclay brick), and fireclay nozzles were used. Most of the work for this study was done on a group of four parts (Fig. 1) for off-road mining applications. Molds were made in green sand with nobake cores. The consumable A material that is used up and needs continuous replenishment, such as paper and toner. "The low-tech end of the high-tech field!" equipment used in the shrouding process included the shroud itself and gasket materials for sealing the nozzle and shroud to the ladle base. The holder is also lined with a refractory cloth in order to reduce the chances of damaging the shroud during the attachment process. Additional durable equipment includes the shroud holder assembly and the ladle base-plate that allows the attachment. These materials are shown in Fig. 2. A station to hold the ladle during shroud attachment was also built for the process (Fig. 3). No modifications were required to the nozzle or stopper assemblies. While most continuous casting applications using mechanical shrouding now utilize an alumina-graphite material, the short campaign of foundry practice makes this material economically prohibitive. For foundry use, the material of choice for shrouds is fused silica fused silica n. See quartz glass. . To produce an airtight air·tight adj. 1. Impermeable by air. 2. Having no weak points; sound: an airtight excuse. airtight Adjective 1. seal, the gasket materials used between the bottom of the ladle and the shroud are made of plasticized clay. The shroud itself is shaped like a tube connected to a truncated truncated adjective Shortened cone. This cone shape is standard among the different lengths and is used to mate with the holder collar. The internal diameter of the tube is slightly larger than 2 in. and has an outside diameter Outside diameter is the diameter of the addendum (tip) circle. In a bevel gear it is the diameter of the crown circle. In a throated wormgear it is the maximum diameter of the blank. The term applies to external gears.1 Notes 1. of 4 in. Shrouds have been tested with overall lengths of 27, 36 and 48 in. Shrouds up to 72 in. in length are currently available, but would require different holder geometry than the shrouds currently in use. In order to be shroud-poured, the part must feature a side riser large enough to allow insertion of the shroud, and enough room below the bottom of the riser for a pouring well and an impact pad in the mold. The mold height must also allow the ladle-shroud assembly to be lifted above the cope surface. The smallest riser diameter used in the shroud pouring process is 12 in. The bottom of the riser is extended 2-4 in. deeper, and the bottom of the well area is lined with alumina alumina (əl  `mĭnə) or aluminum oxide, Al2O3, chemical compound with m.p. about 2,000°C; and sp. gr. about 4.0. firebrick firebrick, brick that can withstand high temperatures, used to line flues, stacks, furnaces, and fireplaces. In general, such bricks have high melting points that range from about 2,800°F; (1.540°C;) for fireclay to 4,000°F; (2,200°C;) for silicon carbide. to prevent erosion.When the nozzle for a shrouded heat is installed, a baseplate baseplate /base·plate/ (-plat) a sheet of plastic material used in making trial plates for artificial dentures. base·plate n. 1. with shroud attachment hooks is used. A 0.25-in. layer of gasket material is inserted between the ladle shell and the baseplate. For each new batch of shrouds, the clamping assembly and shroud is given a dry run on an empty ladle to check for proper adjustment of the clamps. A line of tape on the outside of the shroud marks the depth that the shroud must be lowered into the mold. Shroud depth could also be measured by attaching a short length of chain to the bottom of the ladle with bolts at the proper depth. Implementation Once the heat is tapped and the ladle treatment is complete, the heat is brought out to the pouring floor. After pouring 250 lb in an ingot mold a box or mold in which ingots are cast. See also: Ingot to preheat pre·heat tr.v. pre·heat·ed, pre·heat·ing, pre·heats To heat (an oven, for example) beforehand. pre·heat  er n. the nozzle, the stopper rod is secured and the
ladle is placed in the shroud attachment stand.Some changes were required to the molding procedure in order to allow shrouded pouring. Pouring basins! cups, tile lining in the gating system and specialty sands within the gating system are not required. Meanwhile, the pouring riser needs firebrick at the bottom of the well and must be masked during the refractory coating process in order to avoid entrapment of spalled wash in the casting. A typical entry riser is shown in Fig. 4. The shroud holder is lined with a refractory cloth to prevent excessive clamping force from damaging the shroud. A 0.5-in, gasket is placed on the top of the shroud, and the tube is lowered into the holder assembly. Once the ladle is in the stand, the shroud holder is secured to the bottom of the ladle by clamping onto the J-hooks that are part of the ladle base-plate. The clamps are wire-closed as a safety measure, and then the ladle is lifted clear of the holder. The shroud attachment operation takes 1-3 min for a trained crew to complete. Once the shroud is attached, the stopper bar is unlocked, and the ladle is aligned above the mold to be poured. Once the ladle is correctly aligned above the mold, it is lowered until the preplaced mark is even with the top of the mold. The stopper bar is then moved immediately to the full open position, and the mold is poured. Once the mold cavity is completely filled, the stopper is shut off. The ladle is then lifted to move the shroud clear of the mold and over to the next box. When the last shroud-ready mold is finished, the shroud is lowered into a box of sand for removal. The wire holding the clamps is cut, and the clamps are released. The ladle then either continues pouring the heat in the conventional manner, or it is ready to be slagged off. Problems Encountered As with all new processes, some unforeseen events occurred during initial testing and startup. One problem encountered early in the testing process was that having a shroud attached to the bottom of the ladle made clearing the nozzle by oxygen lancing impossible. Also, it was not possible to pour an entire heat through a shroud using the initial process because the nozzle became too constricted con·strict v. con·strict·ed, con·strict·ing, con·stricts v.tr. 1. To make smaller or narrower by binding or squeezing. 2. To squeeze or compress. 3. to allow the pour to be completed. This buildup build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. is attributed primarily to the agglomeration ag·glom·er·a·tion n. 1. The act or process of gathering into a mass. 2. A confused or jumbled mass: of fine alumina particles in the low-pressure areas of the nozzle. The literature suggested that this problem could be overcome by calcium (Ca) treatment of the steel. Doing so would produce calcium aluminate a·lu·mi·nate n. A chemical compound containing aluminum as part of a negative ion. Noun 1. aluminate - a compound of alumina and a metallic oxide inclusions--which are still liquid at steel pouring temperature--and will, therefore, not collect in the nozzle. The typical aluminum content of the steel was 0.025-0.04 weight %. The standard process for this steel grade included a ladle treatment of 0.65 lb of Ca in the form of calcium silicide Calcium silicide (CaSi2), also called calcium disilicide, is an inorganic compound, a silicide of calcium. It is a whitish or dark grey to black solid matter with melting point between 700-935°C. cored wire per ton of mol ten metal, but clogging still occurred. Adjusting the process to include an addition of 1.2 lb/ton of Ca allowed the entire 20-ton heat to be poured without any noticeable nozzle restriction. While the initial tests produced an airtight seal between the shroud and the ladle, the second batch of gaskets could not maintain a seal. After consulting with the supplier of this material, it was found that the original gaskets had dried out while awaiting shipment, and the new ones were of a consistency regularly supplied to continuous casting users. After the new gaskets were baked at 350F (177C) for 30 mm, they were solid enough to ensure a good seal. This is now a standard practice. Several holder attachment processes were tested, but in the end, the original three-clamp was selected as the easiest to adjust and use. Results Several differences existed between the shrouded and gated castings. All the shrouded castings had a smaller gross weight due to the removal of the downsprue and runner systems. This change added an average of 1.5% to the casting yield. In all the shrouded parts, a marked reduction was seen in the amount of "dirt" found in the castings after shakeout Shakeout A situation in which many investors exit their positions, often at a loss, because of uncertainty or recent bad news circulating around a particular security or industry. Notes: During the dotcom boom and bust, numerous shakeouts occurred. . Typical results show a 60-80% reduction in the amount of reoxidation. This reduction in dirt levels is, of course, associated with a reduction in work in the cleaning room. The smaller amount of reoxidation present and the removal of the gates mean less repair welding and finishing are needed. Table 1 summarizes the results for four different castings, each of which showed significant improvement with the shroud-pouring method. The large reduction shown on the frame casting is most likely due to the fact that this part has a large cope surface and a reasonably flat geometry. There were no statistically significant differences in the amount or severity of cracks or shrinkage found in the castings. The pour time data did not show any statistically significant difference between the pouring methods. Any changes are most likely hidden by the large effect of variations in head height on pouring time. The small spindle spindle: see spinning. A rotating shaft in a disk drive. In a fixed disk, the platters are attached to the spindle. In a removable disk, the spindle remains in the drive. Laptops use spindle designations to indicate the number of built-in drives. undergoes extensive machining operations, which were monitored over a period of time to see if any changes occurred. While conclusions are as of yet incomplete due to the large amount of scatter in the data, a 15% reduction in the cost of tool usage was realized. Cost Analysis A cost analysis was performed to compare the data of the additional materials with the savings generated. A list of the factors included in the analysis is given in Table 2. Figure 5 shows the total savings vs. the cost for the process on the small spindle, which was fairly representative of the group. This analysis did not include any savings for the reduced cleaning costs related to the removal of the gate system; only direct labor and materials labor and materials (time and materials) n. what some builders or repair people contract to provide and be paid for, rather than a fixed price or a percentage of the costs. were included in the cost of quality numbers. In the foundry, only material costs were included, and no labor costs were considered. The analysis showed that the extra cost associated with the shroud pouring process is recovered by the reduced molding cost. The quality improvements showed a 25-40% reduction in the materials and direct labor spent repair-welding this group of castings. A similar reduction in finish grinding was assumed. In summary, the shroud pouring process has proven to be a success with larger parts and is currently employed on about 10% of Harrison's tons shipped. Benefits include a 50-80% reduction in dirt and an overall cost savings. As a general rule, the cost of the shroud and other consumables has been recouped in the reduced molding cost, and the quality improvements have led to significantly reduced costs in the cleaning room.
Overall Level of 'Dirt' Defects
Found in Castings (Inches)
Method Frame Small Medium Large
Spindle Spindle Spindle
Gate 86.25 16.10 12.03 15.85
Shroud 12.67 6.41 8.30 5.56
% Improvement 85% 60% 31% [*] 65%
(*.)no statistical differences at 80%
confidence level on medium spindle
Factors Considered in Shroud
Pouring Cost Analysis
Expense Savings
Fused-silica shroud Hot metal for downsprue and runner
Shroud gasket No tile, cores, basins, etc. for gate system
Baseplate gasket Reduced use of special sands (zircon)
Firebrick Reduced welding
Extra calcium treatment Reduced grinding
Extra sand to fill gate Reduced machine tool wear
(measured on small spindle only)
[Graph omitted] |
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