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Melting and pouring.

Continuous refinements to proven technology best describe the newest developments in metal melting and pouring operations.

"The first moments of creation of the new casting are an explosion of interacting events; the release of quantities of thermal and chemical energy trigger a sequence of cataclysms." From "Castings" by John Campbell.

This is how this author in his new book about metalcasting describes the enormously complex and violent birthing pangs that are the culmination of the metalcasting process. He graphically recounts how the molten metal attacks and is attacked by the turbulent interaction of the furnace and mold environments. There is the furious agitation caused by the chemical reactions taking place as the hot metal surges from the pouring ladle into and through the mold. These forces are depicted by Campbell as volcanic as they diffuse through the cooling metal, producing prodigious stresses and strains in mold and casting alike.

These tremendous physical and chemical reactions contained in the melting and pouring processes impose a special burden on the foundry to select melting and pouring system components best suited to its current and forecasted molten metal needs. Every melting situation must be analyzed to define first the foundry's production needs in terms of what is expected from its melting and pouring equipment. How can it be expected to meet customer quality and quantity requirements and still meet the foundry's profit objectives? What is the foundry's product mix and what are its economic expectations for the equipment in the near and long terms?

A thorough study of all of these factors that influence operating and product costs should be made and tabulated so that a reasonable comparison of the alternatives can be made for furnace and ladle requirements.

Factors that influence melting and pouring include energy availability and costs, the acquisition expense and physical accommodation of equipment. Process flexibility and melt rates, types of charge materials available, refractories in use and environmental controls required also must be carefully weighed.

Helping foundries meet their manufacturing needs and competitive pressures are a host of equipment and services suppliers. It is largely through their technological advances and refinements in melting and pouring that increased productivity and manufacturing economics are made possible.

Melting metal and casting it into molds is the essence of the foundry industry. It has been thus for centuries. The means to accomplishing both tasks has been the preoccupation of foundry suppliers for nearly as long. The whole process, however, of defining how castings are made, particularly as it concerns melting and pouring metal, has accelerated in the last decade. Some of the latest advances and refinements are highlighted below.

Furnace Technology

Recently, aluminum casting users have sparked the drive for higher quality molten aluminum. Formerly, the need for "aircraft quality" metal was the domain of only high-quality sand, permanent mold and investment casting foundries. This metal normally was not required in large quantities and shops that ran these types of castings could afford the luxury of batch treating the molten metal to remove inclusions and gas. Today, demand is for high-quality metal in production quantities in diverse markets. This demand recognizes that improved metal quality results in significant property improvements in aluminum castings.

Extensive development work in recent years has led to melting systems that go beyond simply providing molten metal. Newer systems can now combine melting with other metallurgical processes like metal filtering and degassing.

For example, one system consists of two furnaces operated in tandem. One is a highly efficient wet bath reverberatory furnace (fossil fuel fired, electric or dual energy) and the other is a crucible-type furnace (fossil or electric) with a specially modified crucible. The larger reverberatory furnace provides efficiency and a large volume of metal with low metal loss. A design modification is a stand alone crucible that provides a metal cleaning system and precise temperature control. Some of the advantages of these designs are discussed below:

* A smaller quantity of metal in the filter furnace and an independent controller stabilize metal temperature fluctuations that might occur in the reverberatory furnace as it is charged with cold metal.

* Depending on the crucible size used in the filter furnace, the metal level can be drawn down 5-1/2 to 8-1/2 in. in both the filter and reverberatory furnaces. This provides the user with a large quantity of metal available between charges if continuous charging is impractical.

* The high-capacity filtration media requires changing only once or twice a year. Depending upon metal cleanliness, it offers an economical, clean and low maintenance means to obtain high-quality metal.

* The lining walls and furnace top use a high-temperature ceramic fiber that provides excellent insulation, low heat storage and low maintenance. The lining is not subject to thermal shock, allowing the furnace to be brought on-line quickly.

* A solid-state power control unit assures full proportioning that continuously meters only that power required at a given time. It avoids the widely fluctuating pitfalls of the "on-off" or "high-low" control systems. The full proportioning controls allow excess power for rapid temperature recovery, but only what is needed. The elimination of contactors assures smoother control and less maintenance at lower cost.

* The furnace filter bed, consisting of various mesh sizes of tabular alumina balls and chips, allows proper metal filtration and optional degassing.

This type of furnace design, provides a filter well and a silicon carbide-bonded particle filter to clean the molten metal as it flows through the filter. The inclusions are trapped and held in the filter element as the molten metal passes through to the hot metal discharge.

In degassing, hydrogen is absorbed, mechanically attached to or chemically reacted with the purging gas. The most common purging gases are nitrogen and argon. The porous refractory part of the plugs and wands produces a high quantity of small bubbles to use the purging gas more efficiently. Degassing not only reduces the hydrogen but also removes inclusions by floating them to the surface of the bath.

The flux injection/rotary degasser system is a relatively new development for aluminum foundries. It is a combination of technologies involving the coupling of flux injection with rotor dispersion to improve molten metal treatment. The mechanically injected flux salts provide significant improvement over the manual or lance addition methods by reducing material handling and waste while improving flux use. Rotor dispersion enhances the kinetics and efficiency of the fluxing reaction and can be used with a variety of aluminum alloys.

The combined flux injection/rotor dispersion process is suitable for degassing, flux cleaning, grain refining, modifying and mini-alloying. It also can reduce aluminum dross and increase metal recovery.

Another, less obvious, refinement of induction melting systems is the use of fiber optic cables. The furnace utilizes this development, which helps to reduce noise associated with induction furnace power supplies. The use of single logic board control, isolation from other power components, fiber optics for interfacing, dual digital and analog metering combine to aid in controlling noise and improving melt accuracy. Even when the furnace is only one-third full, the system automatically delivers full power at the highest efficiency.

This system has a power supply that is UL-listed and is available from 100-750 kW. In addition, these cooling systems require no high maintenance targets. Inlet/outlet differential pressure, water conductivity and ambient temperature are monitored automatically to prevent any catastrophic damage.

This power supply is suited for new installations as well as a replacement for existing power supplies. It is available from 50 Hz to 10 kHz in power ratings up to 8000 kW.

Pouring Technology

Pouring molten metal into molds links the melters' labor with that of the casting finishers. It is the linchpin of the whole casting process. While manual pouring remains the standard method of operation for most foundries, advances in automated mold filling technology are leading to greater accuracy and increased flexibility in the pouring process.

Currently used on vertically parted molding machines worldwide, the system is a closed-loop automatic pouring control system. Built around a laser-based distance measurement sensor, it provides real-time, infinite pouring control that eliminates overpours, short and interrupted pours, and sand and slag inclusions. It increases metal yield, pouring consistency and productivity.

Four major components that make up the system include:

* a laser sensor mounted in a water-cooled jacket with an air purge system for protecting the sensor's lens;

* a computer controller housed in an environmentally sealed, free-standing cabinet;

* a remote operator's panel;

* a servo-driven stopper rod actuator and control software.

The system automatically positions the pouring ladle into the proper location above the sprue cup without any delay in the pouring process. It also regulates the flow rate sufficient to maintain a constant level of molten metal in the sprue cup.

As the molding line indexes, the system monitors the top surface of each mold. As the mold passes under the laser sensor, the leading edge of the sprue cup is detected due to a sudden change in the measured distance to the mold surface. At this moment, a position reading is made on the mold indexing device by way of a position sensor.

The indexing stroke is programmed by a controller that calculates the distance the mold will move before it stops. Once the pour position is determined, the vessel carriage adjusts its position within a tolerance limit. By the time the mold line stops indexing, the ladle is properly positioned and pouring can begin.

During pouring, the laser sensor continuously measures the level of molten metal in the sprue cup and transmits level measurement data to the controller. The controller compares it to the programmed set level value, establishing a continuous error value. Any set point deviation causes the controller to signal a servo-driven actuator that opens or closes the stopper rod, as necessary to maintain a consistent sprue cup level.

At the end of the pour, the user can select any pouring profile program. A low finishing metal level can be set into the controller to minimize excess metal in the sprue cup, or it can be set high for large castings requiring high ferrostatic pressure.

Other laser-based molten metal control systems range from applications for maintaining the level in a pour box on a pressure pour furnace to controlling furnace tilt on a continuous casting line to a level control system used in continuous strip casting.

An even newer development is a coreless induction-heated automatic stopper pouring vessel that prevents temperature losses during mold line delays. This eliminates costly and time-consuming pigging of molten metal and also prevents low temperature pouring and resulting scrap castings. As an extra benefit, the induction heating also will compensate for marginal temperatures of molten metal delivered to the vessel.

The horizontal coreless induction heated section is built into the fully enclosed pouring vessel as a flow-through section between the fill and the stopper pouring sections. Because it is an integral part of the pouring vessel, it adds little or no additional length to the overall dimensions of the standard unheated pouring vessel. It allows the replacement of most unheated vessels with the coreless heated vessel without altering existing pouring routines.

Molten metal is maintained at any level from full to as low as six inches in the refractory-lined vessel and an inert gas atmosphere can be introduced above the molten bath to reduce inoculant fade time. A wall between the fill section and heated section combines with the stopper pouring mechanism to create a totally enclosed vessel for nitrogen containment.

An electro-magnetic pinch effect of the induction heating process is a quality control feature that creates a depression in the surface of the molten metal bath in the heated section. Metal oxides float to the top of the bath and are captured in this depression, preventing them from flowing through to the stopper pouring section.

The coreless induction heated section occupies only 26 in. of the vessel's overall length, yet is capable of delivering a full 200 kW into this limited space to provide temperature gains as rapid as 10|degrees~F per minute. Typically, the unit is operated at between 90-95 kW, sufficient to maintain bath temperature during normal mold line delays. Power input is adjusted automatically, according to bath level.

The bath temperature is monitored with an immersion thermocouple. The resulting temperature is used to determine the power setting of the continuously adjustable, coreless induction heating unit. The heated stopper pouring vessel is available for ferrous and some nonferrous metals. It also features video camera monitoring and control of each automatic pouring cycle.

A British development, referred to as a vacuum lift heated (VLH) automatic pouring system, incorporates a controlled vacuum to maintain a fixed head of metal above the pouring nozzle. The system includes control systems for both the vacuum lift and stopper operations. The unit consists of a refractory insulated pouring vessel, a refractory tube connected to a vacuum source, a coreless induction coil and power source. A probe determines the liquid metal level and a stopper and nozzle to control pouring rate. It uses a computerized controller to program pouring cycles.

A combination holding furnace and fully automatic pouring system and a channel induction furnace for holding and pouring a wide range of ferrous metals. Both are controlled by a microprocessor. The former unit can be used with aluminum die-casting machines, permanent mold and automated sandcasting lines.

Classed as a pressure tank, the furnace is lined with a special refractory resistant to aluminum alloys that can be easily cleaned and claims a useful life of up to 10 years. The channel furnace offers controlled pouring without slagging, exact metering of the required amount of liquid metal, more uniform pouring temperatures and metal analysis and increased, more consistent, productivity.

Another available automatic pouring system uses a stopper rod-based device and is used with gray and ductile irons. It uses a video-based computer controller for locating the sprue and starting and stopping the pour. Unlike laser controlled systems, this video controlled system does not require a special sprue configuration for reflecting laser light. Because the system's video camera is also linked to a standard TV monitor, molds can be poured by remote manual control if the automatic controller is offline.

Continuing Advancements

Pioneered in the 1960s, the modular furnace created the base platform for subsequent advances in solid-state power electronics, component miniaturization and microprocessor control. With these modular types of coreless induction melters, power ratings and densities have risen as physical systems have become more compact.

A packaged system now being readied for shipment to a major domestic car company will put an unprecedented 10,000 kW on a 20-ton furnace, doubling the usual power rating for this size furnace.

In all furnaces, electronic control and automation have grown progressively more comprehensive and user-friendly. State-of-the-art furnace design is represented by fully digital small melting system in ratings up to 1000 kW in which all measurement, control, protection and display functions are performed by an integral computer module. The computer is accessed by means of a liquid crystal display and a touch-sensitive keypad for the furnace operator.

Using a technique called synchronous melting and holding at temperature, holding power at the appropriate level is applied to the furnace in tapping mode, while the balance of the available power is applied to the second furnace for melting. Power utilization is maintained at 100% at all times, resulting in increased production from the same power rating at less cost. Metal can be held overnight without switching power back and forth between furnaces. Melting can be carried out simultaneously in two furnaces with the power level in each furnace matched to production needs. Systems from various manufacturers come in a complete range of furnace sizes and power ratings.

Melt shop productivity, quality and cost management all are augmented by systems that automatically cycle a furnace through sintering, cold starting and melting sequences for various alloys, temperature monitoring, fault detection and energy consumption tabulation.

Furnace utilization can be increased by interfacing it with support systems, such as charge makeup or hot metal delivery. Programming is available to integrate all melt department costs and functions into a total management information system. Operating data can be displayed graphically on a color monitor and can be fed to a printer and even transmitted via modem to remote sites for real-time status analysis.

Typical of tandem operations are two 20-ton units in operation at a large Midwestern iron foundry. Metal is heated by channel inductors designed for quick changeover. Patented systems assure tight seating of the stopper rod and prevent slag buildup in the pouring nozzle. Teapot filling and pouring insure slag-free metal delivery to the molds.

Gas pressure (compressed air or inert gas) controls the level of metal in the pouring spout. When the stopper rod opens and pouring starts, the gas pressure is increased automatically to keep the iron in the spout at a constant level. A stable pouring stream is maintained even when the vessel is being filled during pouring.

A teach-in system assures the proper filling of every mold. Using a joy stick, the operator programs the pouring system to the ideal sequence for a particular casting run. Locked in electronically, the pouring sequence repeats for the balance of the run, and again whenever the same mold is cast.

Extensive research and development work in the automated pouring field has resulted in the development of innovations like the dual power inverter, basically a single induction power unit that can supply energy to two furnaces simultaneously and continuously.

The basic unit allows a foundry to melt in one furnace while providing power to maintain a precise temperature in a second furnace for pouring and/or holding without electric or mechanical furnace switching. Because melting continues uninterrupted by any need to switch power for reheating, productivity gains of up to 20% are reportedly achievable.

The system makes it easy to melt in one furnace while sintering in the second, to heat up both furnaces at the same time or to cold-start or hold metal at specific temperatures in both furnaces. There are separate and fully-variable power controls for each furnace.

The general benefits of this type of system include: only one set of incoming utilities; less floor space; conservation of outside and operating power demands; greater equipment utility.

Development of the Plasma Arc Ladle for Foundries Under Way

Steel casting users continue to press foundries to improve casting quality. This insistence is of critical importance for steelcasters because of the continued nibbling away of their domestic market share by ductile iron and foreign casting producers. Clean steel technology is clearly a priority.

Oxide macroinclusions, tramp elements and gases are major factors responsible for the rejection of steel castings by design engineers. Clean steel means low levels of sulfur and phosphorus, low gas content and a minimum of both macro- and microinclusions. An important improvement factor is minimizing the metal's exposure to oxygen.

The steel foundry industry is meeting these challenges with a variety of ladle refining advances. However, previous ladle metallurgy processes have been difficult to transfer directly to small foundries because of the small ladles typically in use. That could change with an improved arc ladle design. Funded by the Electric Power Research Institute and directed by Process Metallurgy International, Inc., it promises cleaner steel with fewer inclusion defects.

The ladle uses a cored electrode through which argon gas is pumped into the domed, gas-tight ladle cover. Argon also is bubbled up through the melt from the bottom of the ladle to stir the bath and increase oxide inclusion flotation. The movable electrode produces a plasma arc to keep fluid the refining slag cover and maintain or raise the temperature of the molten bath, allowing lower melting furnace tap temperatures.

Ladle refining variables that are addressed by the new ladle configuration are:

Temperature Control--The relatively small foundry refining ladles and their high surface-area-to-volume tends to cause excessive and rapid temperature loss. Insufficient heat is available to melt and keep molten the essential refining slags. A moderate flow argon stirring system can cause a ladle bath temperature loss of 3-4|degrees~F per minute; a high flow rate, 6-8|degrees~F per minute.

Slags--A proper top slag is required for the refining process. It removes dissolved oxides from the bath, protects alloy additions from oxidation and removes sulfur. An effective refining ladle should be closed tightly to maintain a good reducing slag. Oxidizing melting slags must be excluded from the refining ladle or the remnant slags must be chemically modified by the addition of reducing agents to reduce amounts of FeO and MnO. A supplemental heat source also is effective in keeping slag in a fluid condition.

Stirring--Proper ladle refining reaction requires good slag-to-metal contact. This contact depends on the melt being vigorously stirred either by induction stirring or the more usual argon bubbling practice. The slag cover also must remain fluid.

Refractories--Most clean steel practices use high alumina refractories because they have more insulating value. Basic refractories, like dolomite or magnesite, are more stable chemically and may be preferred in refining ladles using auxiliary heat sources like the plasma arc.

Alloying--Alloying elements are added to steel to enhance the metal's properties. The treatment of the steel with calcium is considered to be essential for the reduction of a number of oxide macroinclusions. The addition of other alloying elements under the controlled conditions achieved in a plasma refining ladle leads to improved control and higher recovery.

All of these variables are addressed in a practical, controlled atmosphere ladle furnace that compensates for heat losses during refining, the efficient melting of alloy additions and pouring temperature fluctuation. The new design is a small (seven tons) direct current electric arc plasma ladle refiner now completing production testing.

The ladle increases melting/pouring productivity 20-30% by handling a major part of the total heat cycle and operates as a separate, more efficient refining vessel. It can produce bearing-grade steel castings when linked to an adequate metal delivery system. Because it is also the pouring ladle, an additional metal transfer step is eliminated, reducing the opportunity for reoxidation to occur.

The ladle is undergoing extensive testing at Maynard Steel Casting, Milwaukee, Wisconsin, where it is being used to refine commercial grades of steel containing very low levels of sulfur and oxygen (|is less than~10 ppm). Results indicate the production of steel castings of superior quality. In addition, the ladle allows the production of two different alloy chemistry compositions from one furnace heat, thereby increasing production flexibility.

Over the last 20 years, the integrated steel industry has made significant progress in reducing the amount of fuel used in the production of hot metal from iron ores and pellets. By carefully controlling operating parameters, raw materials charged and exploring coke characteristics, ratios of coke to tons of hot metal produced have been reduced from 1200-1400 lb of coke per ton of hot metal to as low as 850 lb.

Coke and Cupola Outlook

While electric melting seems to capture most attention these days, the cupola remains the mainstay for the continuous production of cast iron. And while some have expressed concern about the availability of metallurgical coke, new data indicates that these concerns are unfounded. This, together with new research on the cupola, portends a bright future for this venerable foundry furnace.

Like much of the U.S. foundry industry during 1991, the foundry coke industry suffered through a major downturn. Foundry coke shipments totaled 1,210,073 net tons compared to 1,574,068 in 1989 and 1,441,455 in 1990. Despite the closing of two merchant coke plants during the year (Detroit Coke and Terre Haute Coke and Carbon), there was plenty of high-quality foundry coke available for cupola iron melting.

To satisfy the requirements of rapidly increasing technology in the foundry industry, all merchant coke producers have active and successful programs for quality control, statistical quality reporting and sophisticated regimens for problem solving.

Most watchers of the foundry industry expect better production rates in 1992 and better yet in 1993 and '94. Current estimates of the foundry coke market in 1992 indicate a shipping level of about 1,350,000 net tons with 1993 and 1994 looking at 1,500,000 to 1,700,000 net tons. The current capacity of the eight producers very active in foundry coke production is estimated at 2,600,000 net tons, thus providing a capability far in excess of industry needs.

Cupola Modeling

A research project, conceived by the Cupola Committee of the American Foundrymen's Society, is under way to try to emulate progress in computer modeling as it applies to cupola melting. It is hoped that the foundry cupola process could achieve the same type of result as has been achieved with induction melting modeling. The project was financed by the U.S. Dept. of Energy, the foundry industry and major suppliers to the industry. At this time, 27 supporters have given time, personnel, use of facilities and money to help complete the project.

An extensive literature search defined the thermochemical reactions of the current cupola process, which were converted to mathematical equations solvable with the aid of a PC. The goal of this segment of the project was to develop a single dimension steady state model of the cupola process. These computerized formulas were then checked against actual operating data provided by cooperating foundries to benchmark the accuracy of the equations.

When all calculations were completed, the project was assigned to a contractor to improve the equations and algorithms to optimize their use and speed of response. The programs are being designed to function on a workstation-type desktop computer. It is this phase that is under way with a tentative completion date in the second half of 1992. After the analytical portion of the project is completed, work will begin on the problem-solving aspect.

The project so far predicts the cupola output chemistry for a given set of input values that include scrap makeup and chemistries, coke analyses and properties, size of cupola, blast rates and temperatures and other operating parameters. This last phase of the project will allow a cupola operator to specify the output chemistry and properties desired. The program will respond with charge makeup and the necessary operating characteristics to produce the desired result at the cupola spout. This work is expected to be completed by the end of 1992 with the final distribution of the resultant data through AFS.

With fuel costs, both coke and electricity, subject to steady increases caused by industrywide efforts to meet ever more stringent pollution regulations, the project has enormous potential benefits for the foundry industry. Project managers are enthusiastic about the results so far and there is no question that the result will be better cupola operations at lower cost to the foundry industry.

This information is reprinted with permission from Foundry Facts, the newsletter of the American Coke and Coal Chemicals Institute.
COPYRIGHT 1992 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Equipment Review '92; includes related articles; latest techniques and innovations in foundry processes and equipment
Author:Bex, Tom
Publication:Modern Casting
Article Type:Cover Story
Date:Aug 1, 1992
Previous Article:Thixomolding promises savings.
Next Article:Sand preparation affects quality.

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