Another approach to iron casting: the permanent mold process.
The ferrous permanent mold (FPM) process was patented by Eaton Corp. in 1932. Although this method of pouring molten iron into an iron or steel mold was born on U.S. soil, the process has been more widely embraced overseas. In Europe, for example, 6-8% of all iron castings are made in FPM, and the growing use of the process is also seen in China, Japan and India.
The FPM casting process is predominantly a mass-production method that becomes cost-effective only when used for properly selected parts. Matched with the proper casting design, there are definite advantages in cost, quality, energy reduction and environmental issues.
In addition to superior casting finish and dimensional tolerance, there are several main advantages of producing gray iron castings via FPM.
First, the process is able to ensure dense, gas- and shrink-porosity-free structures for leak-free castings needed in hydraulic and gas components' applications. Pressure tests are routinely performed on FPM castings with little or no rejections.
Second, FPM castings have a history of exceptional machinability due to the absence of sand or chilled corners and edges. Scrap after machining is negligible because FPM casting defects tend to be external and detectable by visual inspection. Machining time is improved by the process' ability to hold close casting dimensions.
Other advantages over sand casting include: reduced production time, reduced finishing costs, elimination of sand and sand handling, and improved dimensional accuracy and stability.
FPM gray iron castings can obtain a 30,000 psi tensile strength and a 143-207 Bhn with a fully ferritic matrix containing predominately type D graphite. Basically, FPM castings can be strong yet machinable.
Moreover, for ductile iron, the amount of magnesium that must be added in FPM is less than in sand casting, resulting in a lower residual content in the finished casting due to a higher undercooling rate. This results in controlled shrinkage, improved nodularity of the ductile iron, enhanced mechanical properties and better overall casting quality.
The European FPM market (excluding Eastern Europe and countries of the former Soviet Union) reportedly includes 15 foundries with an estimated annual production of 35,000 tons. Markets include automotive (brake components), machine tools (small gears), air compressors (cylinders and crankshafts) and hydraulic components. In the Eastern European countries (Czech Republic, Slovakia, Poland, Hungary and Bulgaria) and countries of the former Soviet Union, the reported annual production of FPM castings is about 650,000 tons. Table 1 shows typical applications of FPM castings in the former USSR.
A newly reconfigured German-owned foundry in Brazil specializes in automotive brake, hydraulic and compressor components. Annual production is about 12,000 tons of gray iron and 6000 tons of ductile iron. Figure 1 illustrates the two production lines for the foundry. It is equipped with nine, 12-station turntables with automated mold coating, closing and casting ejection. Molds are made of gray iron, water-cooled and feature an ejection system.
The Japanese FPM market, specializing in high volume air and gas compressors for the refrigeration, air conditioning and automotive industries, is comprised of at least six FPM foundries with an annual capacity of 18,000 tons. Figure 2 shows the layout of a Japanese foundry specializing in high-volume components for air conditioners. The foundry is equipped with programmed single head FPM machines able to produce 5000 tons of castings/year.
Liquid iron is transferred from an 8-ton induction coreless furnace to two 3-ton induction holding furnaces. The FPM floor consists of two production lines, each consisting of 14 semiautomatic indexing single-head machines. All molds are water-cooled and have an automatically operated ejection system. The typical 6.5 min casting cycle includes:
* pouring time, 7-8 sec;
* solidification time before knockout - 45-50 sec;
* molding cooling and castings ejection;
* mold air blowing, gating system refractory and soot coating.
Quality control consists of precise chemistry and microstructure testing (one casting per heat treatment) as well as casting quality monitoring. Total scrap is 4% (includes 3.5% internal scrap).
Recently, Japan built two FPM foundries in China and Malaysia to produce castings for compressors used in air conditioners and refrigerators. Combined annual capacity is 6000-8000 tons.
In India, two foundries specialize in the low- to high-volume FPM casting of simply-shaped, low-weight industrial components in gray and ductile iron.
A foundry in Canada employs the FPM process to cast various fittings and elbows for the sanitary pipe industry.
In the U.S., besides a few small iron FPM foundries, Grede Foundries, Inc., is the largest producer of gray iron FPM castings. Its Perm Cast Div. plant in Cynthiana, Kentucky, (the original Eaton Corp. foundry) serves industrial markets such as compressors for air conditioners, automobile anti-lock brake systems, belt drives, automobile brake systems, automobile air conditioners, and hydraulic and home heating fuel pumps.
Perm Cast is equipped with 12, 12-station semiautomatically-operated carousels - turntables that automate mold opening and coating in a preset cycle. It operates two shifts during 17.5 hr. Four carousels producing low-weight, thin-wall castings, rotate continuously in 2.5-3.5 min cycles. The remainder are indexing turntable FPM machines with cycles varying from 4-7 min for pouring, mold coating, core setting, solidification and casting ejection. The rotation and knockout time can be altered depending on the job mix, but for a 4 min rotation time, knockout time is typically 45 sec.
Iron from the cupola is tapped into a channel induction furnace, which is periodically tapped at 2580-2620F (1416-1438c) into 800 lb transfer ladles. The transfer ladles are then tapped into 220 lb pouring ladles and, finally, poured at 2420-2460F (1327-1349C) into vertically parted, soot-coated, air- or water-cooled permanent molds mounted on rotating 12-station carousels.
Grede also built a new module dedicated to high-volume production of air conditioner parts.
Honda of America, Anna, Ohio, began producing ductile iron steering knuckles on an automatic FPM line (Quick Cast Knuckle - QCK) in the fall of 1995. Honda pours 22 tons of iron per day via the process (see sidebar).
The typical chemistry of the gray iron FPM process is as follows:
3.45-3.65% Carbon (C) 2.45-2.65% Silicon (Si) 0.40-1.00% Manganese (Mn) 0.35% Phosphorus (P) max 0.15% Sulfur (S) max 0.02-0.10% Titanium (Ti)
Table 1. Gray from FPM Castings - Former USSR Industrial market Typical castings electric motors frame and flange power transmission equipment pump housing, piston, gear chemical equipment nozzles for chemical processing and cooling tower hydraulic equipment pump housing, piston, valves machine tool small wheel disc, pressing plate, flange, gear farm machinery vacuum pump body and cover, holder, bearing cage glass industry die metallurgical equipment ingot molds construction machinery brake shoe, bobbin, counterweight, pedestal sanitary industry fitting, elbow transport machines brake drum for trailers, brake rolls for conveyor crushing and pulverizing equipment grinding balls, slugs and billets
Ti is essential for providing the undercooling required to meet ASTM specification A823 - 84 that calls for predominately type D graphite with some type A graphite associated with the center line or around sand cores. However, if desired cooling rate can be obtained in the mold by using a more effective cooling system, the Ti content in the base iron may be in the lower side of the above-mentioned range. The high carbon equivalent (CE) is needed to regulate the chili depth and reduce sink/lap type defects.
The pouring temperature is colder than in the sand process to promote type D graphite and prolong the mold life while not sacrificing quality. Inoculation, if used, is strictly for chill control, as the promotion of type A graphite isn't desired. Basically, inoculation isn't needed for cupola-melted iron because the colder spout temperatures reduce the exposure to superheating conditions. On the other hand, inoculation is essential when melting via coreless induction to restore nucleation sites and control chill and shrinkage.
FPM iron is poured into gray iron, steel or copper permanent molds. Relatively thin steel permanent molds (0.75-1.0 in.) countered from the back with a welded water-cooling jacket offer advantages over iron or copper molds because they are weldable. Mold cavities and gating systems may be machined into the blank or precast using conventional sand or precise casting methods. The mold blanks can be designed to be cooled by air or water.
At one point in each full rotation, the molds are coated with acetylene soot. The soot acts as a protective coating for the mold, but if it's too thick, it will act as an insulator and dramatically slow cooling. Water-based graphite-containing coating is also used. All FPM mold castings are heat treated as per ASTM standard A823-84. Some are annealed at 1550-1700F (843-927C) for 1 hr and furnace-cooled to obtain fully ferritic metallic matrix, while the rest are normalized at 1500-1700F (816-927C) for 1 hr and air-quenched.
A microstructure of a normalized FPM casting usually obtains 10-30% pearlite. If a higher percentage is required, it may be obtained by small additions of antimony (Sb). The required microstructure calls for predominantly type D graphite, size 5-8, with some type A graphite, size 4-6, associated with the center or any sand cores. Tensile strength is proportional to section size: 30,000 psi is the minimum for a separately cast standard 0.5 in. test bar (ASTM A823-84) and 25,000 psi for a separately cast standard 0.75 in. test bar. Hardness ranges from 143-229 Bhn, depending on heat treatment.
The traditional FPM process does, however, have limitations in areas such as casting design, mold cooling, mold life and specific casting problems that have restricted further implementation. For small, low weight castings, for example, high productivity sand molding may be more economical than FPM.
One of the most significant problems is the mandatory heat treatment of gray and ductile iron industrial application castings. It is necessary to ensure and regulate the reproducibility of the microstructure and mechanical properties of the casting. It is also essential to develop the good machining qualities needed in finished castings. In some cases, however, it's possible to avoid heat treatment by applying heavy inoculation combined with the pouring into preheated permanent molds [up to 400 - 600F (204-316C)].
Of course, high temperature heat treatment brings increased energy consumption. This, in turn, leads to high energy costs that are ultimately reflected in the casting cost.
Another major obstacle restricting the widespread adoption of FPM is the relatively short mold life encountered in casting high temperature alloys. This makes it nearly impossible to produce steel castings, since mold deterioration would be a significant factor.
Some of the Commonwealth of Independent States (CIS) nations have advanced the FPM process through the use of lined permanent molds (LPM). In this process [ILLUSTRATION FOR FIGURE 3 OMITTED], the face of the cast iron or steel permanent mold is lined with a thin layer of sand slurry or sand mixture, depending on the alloy being poured.
The sand slurry is applied by pouring the slurry into the gap between the mold halves, resulting in an inside configuration that follows the contour of the pattern.
The sand mix is applied by blowing it into the clearance between the mold and pattern. Directional solidification and solidification microstructure are regulated by sand lining thickness variations. If wear-resistant chilled iron microstructure is desirable, like in automotive camshaft applications, FPM in the surfaces of eccentrics won't be lined, and solidified iron is exposed directly to the permanent mold.
Another advantage of LPM is that its thin layer of sand mix or refractory eliminates direct contact between the solidified casting and the mold, thus resulting in an as-cast iron structure without carbides and unlimited mold life. The technique is used to pour parts such as diesel engine crankshafts, machine-tool spindles and artillery projectiles of ductile iron and automotive crankshafts of gray iron. The annual production using this technique for pouring iron and steel is about 45,000 tons.
FPM experiences quality problems typical of any casting method. The difference lies in causes and preventive actions. Typical causes of the major problems are listed below.
Pinholes - pouring too cold; soot coating too thin; cold mold.
Slag inclusions - pouring too cold; worn chokes in mold; high oxide content; poor pouring technique.
Sinks/laps - low CE, soot coating too thick; pouring too hot.
Hot tears - pouring too hot; soot coating too thick; uneven cooling rate; hot spot in the mold.
Recent research and developments have led to significant process enhancement through quality and cost improvements.
Mold design - One of the major factors affecting the quality of FPM castings is a controlled and uniform cooling system. Controlled solidification was optimized by:
* a high efficiency water cooling system with cast-in-the-mold blank pre-fabricated steel pipe that allows progressive solidification to regulate cooling in different areas of the casting;
* air-cooled molds with air or water-cooled heat sinks that help eliminate hot tears in thick hub pulleys [ILLUSTRATION FOR FIGURE 4 OMITTED]. In this example, the thick hub solidified longer than thin spokes, which caused hot tearing.
* air- or water-cooled metal core design using high thermal conductivity and thermal resistance materials such as chromium copper or graphite.
Mold material - The thermal effects of liquid metal flow in the mold are the major factors in determining mold life and casting quality. Permanent molds typically operate at 400-600F (204-316C) and require a uniform heat transfer. Gray iron with type A graphite is recognized as a mold material that combines good thermal conductivity with acceptable heat resistance.
Research to improve mold life showed that the highest resistance to thermal shock was exhibited by chromium-molybdenum (Cr-Mo) hypereutectic gray iron with uniformly distributed medium-size flakes of type A graphite. Type A graphite raises thermal conductivity, while alloying elements increase metallic matrix heat and thermal fatigue resistance.
Gray iron chemistry - To optimize FPM gray iron base chemical composition, hypereutectic irons with a 0.05-0.15% Ti content were investigated. Microstructure studies showed Ti is relatively potent in controlling solidification structure of gray iron, increasing undercooling and promoting type D graphite. The effectiveness of Ti addition depends on carbon equivalent (CE) with more pronounced changes in iron with lower CE. Tensile strengths in iron increased fairly rapidly at Ti levels of 0.05% to 0.075% with 4.50% CE, and to about 0.085% Ti with 4.45% CE. At higher Ti levels, relatively slower increases in tensile strength were seen.
Ti-containing compounds were identified by scanning electron microscope. Even though some of the Ti nitrides and/or carbonitrides were found in the ferrite/graphite region, most of the Ti compounds were located in the metallic matrix. Results of gas analyses confirmed that as Ti increases in low soluble nitrogen base iron from 0.09% to 0.12%, the excess Ti produces mostly Ti carbides. The presence of Ti compounds significantly reduced machinability and decreased tool life in all machining operations. For the optimal combination of required properties with good machinability, the Ti content in gray iron FPM castings shouldn't exceed 0.075%.
Nondestructive testing - Results from a study on the use of nondestructive ultrasonic velocity (USV) testing showed that the microstructure/tensile strength of gray iron FPM castings can be determined. These tests may be completed in 1 min, eliminating the need for costly machining and testing of test bars and preparing microstructure samples. For the first time in gray iron FPM practice, USV testing has been implemented for microstructure/property evaluation and casting certification.
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|Author:||Lerner, Yury S.|
|Date:||Nov 1, 1996|
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