93rd AFS Casting Congress in review.
More than 150 individual presentations from the ten AFS technical divisions comprised the program of the 93rd Casting Congress of the American Foundrymen's Society held May 7-11 in San Antonio, TX. Part two of our coverage looks at papers from the Aluminum, Cast Iron, Pattern, Steel, Melting Methods & Materials, Brass & Bronze and Investment Casting Divisions.
R&D Yielding Production Parameters for MMCs
Aluminum SiC Metal Matrix Composites (MMCs) were the subject of several papers presented in San Antonio. Work at the Univ of Alabama/Tuscaloosa, conducted by F. Rana, B. K. Dhindaw and D.M. Stefanescu, is aimed at optimizing SiC particle dispersion. The researchers examined the effects of four process variables--mixing time, temperature, speed and particle feeding rate, along with variation in the weight of SiC particles, their size and minimum agglomeration and maximum dispersion.
Based on an experimental design, two-level multivariate regression analyses yielded combinations which indicate that agglomeration and dispersion of the SiC particles is "highly dependent" upon the percentage of different particle weights in the system. They reported that higher stirring speed and duration, normally associated with lower mixing temperatures and feeding rates, led to enhanced dispersion of the particles in an Al-Mg alloy. While tensile properties of the MMC were comparable to the base alloy, wear resistance was found to be superior to the matrix alloy.
Research by G. Carman and J. T. Berry, also from Alabama/Tuscaloosa, was conducted to estimate the modulus of elasticity of MMC castings. An accurate estimate is necessary if MMCs are to be used in structural applications. The researchers used Finite Element Modeling (FEM) to examine the effects of changes in volume fraction, aspect ratio and component characteristics on the elastic moduli.
The FEM results were found to be "remarkably close" to experimentally determined values for 4.5% Cu/SiC, but did not correlate well for the 3.75% Mg/alumina system. Lack of correlation in their view, "may be associated with either the presence of voids in the continuous Al matrix, or the lack of adequate cohesion between the matrix and the imbedded fibers."
Progress Casting Group's experience in the production of SiC alloy castings was explained in a presentation by R. E. Carity. Progress uses a furnace equipped with an inert gas cover. Carity stressed that because of the decreased fluidity, the melt must be stirred. All types of fluxing or degassing must be avoided. Preliminary runs showed test bars "full of oxides and trapped gas."
According to Carity, there is no method currently available to clean or flux these alloys without removing the silicon carbide particles. Progress uses ceramic filters--about double normal size--located after a "bubble trap" but before the first ingate in the gating system. A skimmer core in combination with an open riser prevents blockage of the ceramic foam filters. Future tests will help determine if the size of the riser (height or volume) is related to its cleaning ability. Carity said the current system is about 90% efficient in removing bubbles, and yields castings of grade B quality in precision dry sand and permanent molds.
The effects of strontium as well as phosphorous on hypereutectic Al-Si alloys were studied by a team at McGill University, Quebec, Canada: N. Tenekedjiev, D. Argo and J. E. Gruzleski.
A synthetic 17% Si-Al alloy was prepared using argon degassing; sample step castings were poured in both sand and permanent graphite molds. Results led the researchers to conclude that if refinement and uniform distribution of primary silicon are the main concern, then phosphorous is the best element to use. On the other hand, both strontium and sodium will modify the eutectic even in the presence of phosphorous, provided sufficient quantities are used and the resultant tensile properties are better than those obtained by phosphorous treatment.
Heat Treating MMCs
A study to determine the optimal solution time required to maximize the mechanical properties and to establish the aging curve of their SiC-Al composite was conducted by Dural Aluminum Composites Corp. D. Hammond of Dural reported that heat treating SiC reinforced aluminum alloys produces profoundly different effects than with non-reinforced Al alloys. Length of solution time had an insignificant effect on the average yield, ultimate strength and average modulus. But remarkably, continued aging up to 28 hrs led to a continual improvement in yield strength.
Heat treated MMCs showed decreased ductility compared to unreinforced alloys. According to Hammond, modifiers yielded a 2% improvement in low ductility, but the fracture toughness of the MMC castings is not dramatically lower than those of the parent alloy.
D. Apelian and S. Shivkumar, Drexel Univ, and G. K. Sigworth, Reading Foundry Products, presented an analysis of research on optimizing the heat treatment of MMC castings. In solution heat treatment, they report that the eutectic Si morphology plays a vital role in determining mechanical properties. Additions of small amounts of Na or Sr lead to rapid spheroidization, not the coarse acicular needles which act as crack initiators in unmodified alloys. Because the rate of spheroidization is extremely rapid, the authors note that "...the long solution times used in most foundries may not be necessary when the alloy is properly modified."
Concerning quenching, they note that impact strength, elongation and UTS all vary inversely with duration of room temperature exposure prior to quenching. Small amounts of Cu, Sn, In or Cd have been shown to minimize the effect of room temperature aging on strength properties.
The higher elongation and greater impact strength typical of rapid quenching (lower water temperature) was, in their view, particularly noteworthy. A quench time of less than 10 sec is recommended for premium quality castings.
Another paper presented by the same authors entitled "The Influence of Molten Metal Processing on Mechanical Properties of Cast Al-Si-Mg Alloys" reviewed recent research on a number of processing parameters, including: melt composition, degassing, inclusion removal, grain refinement, modification and control of melt quality through thermal analysis. Recent mathematical models of the degassing process shows that it primarily depends on three factors: metal temperature, purge gas bubble size and purge gas composition. Low metal temperatures and small purge gas bubbles results in the most efficient purging.
A number of different filtration processes were reviewed, leading the authors to conclude, "Filtration in the mold cannot usually clean otherwise dirty metal. The lack of an inexpensive, reliable and reasonably accurate analytical technique to measure inclusion contents in aluminum foundries has greatly hindered our understanding of inclusion removal."
Research into the effects of phosophorus additions on 11.8% Si-Al A390 castings produced using the Evaporative Pattern Casting (EPC) method was the subject of a paper by B. E. Carlson and R. D. Pehlke of the University of Michigan/Ann Arbor. The influence of microstructural changes on the mechanical properties of the material and the quality index factor (relating UTS and EL) were assessed.
A parabolic trend resulted relating primary silicon size to phosphorous additions: an increase up to about 0.006% P resulted in a decrease of the silicon particle size; further increase resulted in overmodification and an enlarging of the silicon particle size. Percent elongation and yield strength were shown to be strongly related to %P.
Research at Drexel by S. Shivkumar, L. Wang and B. Steenhoff focused on the metallurgical qualities of EPC produced A319 castings. Indications are that the grain size and dendrite arm spacing vary inversely with the distance of the casting from the downsprue.
Three significant types of porosity were found near the surface of some castings: large spherical bubbles, elongated needle-shaped porosity and interdendritic. The evolution of large amounts of gaseous products in the mold promotes increased porosity levels. In addition, there are products of liquid degradation formed in the mold which can be trapped.
Research indicates that the distribution and morphology of silicon and iron-rich phases vary within castings. The concentration of Fe rich phases decreases gradually with the distance from the downsprue. The local solidification time has a strong influence on the size and morphology of the various phases present in a casting.
A paper discussing "Recent Developments in Thermal Analysis of Aluminum Casting Alloys," was authored by S. L. Backerud, University of Stockholm, and G. K. Sigworth, Reading Foundry Products. The authors reviewed research which led to the integration of the peaks of the derivative of the cooling curve corresponding to the nucleation of the aluminum phase. The research confirmed earlier findings which suggested that the integrated peaks are related to the grain size in the casting. It was reported that continued work on higher order derivatives is under investigation at GM.
Also presented was an example of an analysis of the cooling curve from the center of the sample and its derivative; the calculated relative rate of solidification and solid fraction versus time; temperature versus solid fraction; and the relative rate of phase formation versus solid fraction.
Four speakers presented papers on different research involving A356 alloy. Q. T. Fang and D. A. Granger, Alcoa Laboratories, conducted research into porosity formation in modified and unmodified castings made from the alloy. Their goal was to quantitatively define the effects of hydrogen content, solidification conditions and grain refining on pore volume fraction, pore size and morphology in A356 alloy.
A mathematical model was developed to explain variations in pore size--the main factor that determines pore volume fraction in a given alloy--in relation to variations in cooling rate, hydrogen content and grain refining. The model led the researchers to a number of interesting conclusions: the hydrogen solubility limit does not change due to segregation during solidification; the metallostatic pressure is negligible compared with atmospheric pressure; the cooling rate remains constant during solidification in each calculation; and the eutectic has the same physical properties as the matrix alloy.
"A Study of the Influence of Processing Parameters on the Microstructure and Properties of A356 Aluminum Alloy" was conducted by E.N. Pan, M. W. Hsieh and S. S. Jang, National Taiwan University and C. R. Loper, Jr., University of Wisconsin/Madison. The parameters studied included the effects of degassing treatment, grain refinement and modification, filters, precipitation heat treatment and others. A standard grain refiner, Al-Ti-B, and modifying agent Al-Sr-Si were used in a series of heats all involving chemically bonded sand molds. Grain size decreased with increasing titanium content; optimum tensile strength and elongation were achieved at about 0.20-0.25% Ti, with a marked difference in grain size in metal molds compared to furan and green sand molds at low titanium levels--little difference at higher levels.
Additions of 0.03-0.04% Sr resulted in a fully modified eutectic silicon structure with a fibrous morphology. They found that optimum Sr addition depends upon the initial P content, the size of the melt and the type of furnace.
A water simulation of the introduction of a purging gas into molten aluminum indicated that a semi-spherical graphite head with radial holes produced the best bubble distribution.
Solution heat treating of A356 alloy followed by aging for 10 hr at 160C or for 3 hr at 180C produced higher strengths, while elongation was highest after a 3 hr aging at 140C.
Research conducted by a team from Drexel University and G. Sigworth, Reading Foundry Products, on optimizing the heat treatment of A356 alloy, found optimum solution times to be 50 min for permanent mold castings and 200-400 min for sand castings. Solution temperatures of 540C, used in most foundries, may be reduced following recent observations that small amounts of liquid during the solution treatment leads to extremely high spheroidization and coarsening rates.
F. Chiesa, Centre de Metallurgie, Quebec, Canada spoke on the development of a method to evaluate decreases of the temperature of permanent molds when casting aluminum. An analytical expression for the heat losses was developed by applying a multiplicative correction factor to the known solution in one dimension. Using an explicit finite difference method, predictions were made of the metal temperature as it reached different parts of a simplified 3-D permanent mold.
Chiesa noted that the analytical expression of mold heat loss was shown to consistently predict potential misruns of the plate casting. Four different gating arrangements were later studied and, according to Chiesa, "The considerable impact of temperature losses on production operating conditions and productivity was clearly demonstrated."
Cast Iron Division
Ductile Iron Highlights Cast Iron Program
The growing importance of ductile iron to the entire family of cast irons is becoming increasingly evident. Along with basic research into the area of the effects of alloying elements on the microstructure and properties of cast iron, developments in ductile iron dominated the cast iron program this year.
The second phase of AFS-sponsored research on the effects of graphite flotation upon the tensile and impact properties of ductile iron castings was presented by T. N. Blackman, BCIRA, Birmingham, England. This preliminary investigation was aimed at quantifying the effects of graphite flotation upon the impact and tensile properties of ferritic ductile iron castings.
Blackman reported that the research did show that the presence of flotation caused a large reduction in impact properties and ductility of the castings. Tensile strengths were reportedly lowered by 20% but offset yield strengths and hardness were relatively unaffected.
Initial results of the research suggest that for optimum casting properties and performance, it is desirable that graphite flotation should be prevented by control of metal composition and casting practices.
In his work on the magnesium requirement of ductile iron, R. W. Heine, Univ of Wisconsin/Madison, turned up some interesting and practical findings. He pointed out that "ductile iron produced from automatic pouring furnaces with stream inoculation suggests that ductile iron with suitable nodularity can be produced at significantly less than 0.04% residual percent Mg when the residual percent sulfur is very low."
Therefore, Heine said, there are several potential benefits in producing low sulfur, low magnesium ductile iron. Among these may be: less casting dross and slag defects; more consistent feeding behavior and fewer shrinkage defects; less carbides in light sections; decreased consumption of MgFeSi and Mg; riserless castings in lighter sections; better endurance limit; and more tolerance for manganese.
"These benefits would come at the expense of producing lower sulfur base iron and controlling residual aluminum and cerium," Heine added.
The effectiveness of graphitic pretreatment of ductile iron in actual production foundry settings was studied by C. R. Loper and B. Y. Hur, Univ of Wisconsin/Madison, and T. H. Witter, Superior Graphite Co.
Graphitic pretreatment is of interest because the addition of graphite does not contribute to the formation of inclusions in the iron. This procedure is described as the treatment of the base iron with a graphitic inoculating agent before treatment of the base iron with magnesium.
To determine the effectiveness of pretreatment, tests were conducted during the production of castings at three foundries that used substantially different melting, treatment, molding and pouring techniques. The effect of pretreatment was monitored in three different castings.
The authors reported that "even with the inability to control the production variables as carefully as they might be controlled in a laboratory experiment, utilization of graphitic pretreatment for producing ductile iron can result in several benefits," including a significant reduction in the formation of carbides, increased nodule counts, more uniform nodule size and nodule distribution. Conventional postinoculation with FeSi can be continued with the pretreatment.
Austempered Ductile Irons
The growing interest in austempered ductile irons was the impetus for research work by Y. H. Lee, Research Institute of Industrial Science and Technology, Korea, and R. C. Voigt, Univ of Kansas/Lawrence.
Citing what they termed the present lack of comprehensive data on the hardenability and austemperability of ductile irons, the authors set about to develop a fundamental understanding of the subject and to identify the important factors affecting hardenability.
"Because of the differences in the austenite transformation characteristics of steel and ductile irons, hardenability concepts that have been developed for steels must be re-evaluated when they are applied to the hardenability behavior of ductile irons," they reported. "The transformation behavior of ductile irons is considerably more complex than for steels."
In summarizing the experimental work from a three-year study, the two authors also included the development of reproducible hardenability testing procedures, the effects of alloying elements and other important variables on the hardenability of a large number of ductile iron compositions, and the development to hardenability predictive models.
Voigt and Loper, authors of other papers presented this year, collaborated on one involving the study of matrix structure development in ductile iron. In this study, the authors considered the factors affecting the matrix structure in both as-cast and heat treated ductile irons with respect to the solidification and transformation conditions imposed upon castings.
The future growth of ductile iron, some believe, may be tied to the development and application of austempered ductile iron. According to R. K. Buhr and G. Morin, CANMET/MTL, "The mechanical properties attainable (in ADI) are of great interest in a number of areas, notably the automotive and agricultural machinery industries, and will likely find many more applications as well."
The CANMET authors this year looked at the effects of certain residual elements on ADI. They reported that "The austempering heat treatment involves an austenitizing soak, at which time carbon is dissolved in the austenite. The amount of dissolved carbon increases as the austenitizing temperature increases. In general, the higher the carbon in the austenite, the more stable is this phase, and more will be retained following transformation at the austempering temperature to baintic ferrite. However, the presence of alloying elements affects the speed and temperature of this reaction."
In some cases this will produce adverse effects on mechanical properties of the castings. As a result, the authors investigated the effects of titanium, antimony, chrome and vanadium on ADI properties.
Based on a limited number of melts, they found that residual amounts of these elements, singly or in combination, had "a minor effect on the mechanical properties" of ADI castings, but some increased amounts of cell boundary carbides could be detected. Also, the samples in which Ti was present had decreased impact properties.
The effects of a variety of alloying agents also were the subject of various papers from the Cast Iron Div this year. Titanium was the subject of one prepared by J. R. Hitchings, Amenex Associates, and J. Klein, SKW Metals and Alloys.
According to the authors, "Titanium is routinely added to gray cast iron as a nitrogen scavenger and, to a lesser extent, as a hardness control agent. One of the best known uses of Ti in cast iron is its ability to suppress or eliminate porosity caused by an excess of dissolved nitrogen."
The authors' investigative work also revealed other conclusions about the use of Ti in gray iron. Among these were that "additions of Ti to cast iron melts for nitrogen control have a one-time, non-reversible reaction, after which new additions must be made if further effects are required. The Ti has usually completed its reactions and formed stable compounds within six to eight minutes after addition."
They also found that the three most effective Ti compounds used in cast iron are Ti metal scrap, FeTi and Ti silicide from silicon-based ferroalloys.
In his report on the development of abrasion-resistant, nickel-containing alloy white irons with high hardness, G. J. Cox, metallurgical consultant, United Kingdom, showed that various Ni-Hard irons have much to offer in "providing very hard, cost-effective, abrasion-resistant castings."
Cox demonstrated that hardness values of 700 Bh and above can be achieved in modified Type-1 irons without high-temperature heat treatment while regular Ni-Hards can be improved by either air hardening, cryogenic treatments and/or magnesium additions.
He also suggested that "high-chromium irons primarily alloyed with some 2.5% Ni show economic advantages and castings which will give an abrasion resistance equal to or better than more established alloys can also be made."
New Technology Moves Design to Product without Tooling
"The need exists for a mathematical algorithm that will allow the evaluation of various methods of parting a particular shape," said R. M. Kotschi, Kotschi's Software and Services, Inc. Factors to be taken into account include the fact that the technique must strive to reduce the number of required cores to an absolute minimum and any deviation from the integrity of the shape as designed must be considered.
Another important factor is the effect of productivity on the resulting tooling. Although the methods proposed by Kotschi are in the developmental stages, critical discussions by the manufacturing community should contribute to development of the successful algorithm.
T. Vorgitch, 3D Systems, Inc, discussed stereolithography, a new technology developed to fill a missing link in the computer aided design and computer aided engineering process. Combining three technologies--laser/scanner, chemistry, computer--it makes prototypes quickly and helps eliminate long delays between design and prototype.
Stereolithography enables you to move "from complex design to physical piece without tooling," according to Vorgitch.
"A Proven Metallic Coating that Protects Your Tooling Investment"--electroless nickel--was described by J. Henry, Wear-Cote [R] International, Inc, as a means for imparting wear resistance, reducing mold scrap, improving release and sand compaction, providing corrosion resistance and preserving tooling tolerances.
"The coating also provides another important benefit by acting as a built-in wear indicator. When the original coating wears through, the pattern can be stripped and recoated," said Henry, adding that it has proven to be the most cost-effective approach toward pattern abrasion and wear.
In describing the important steps to building quality tooling for expanded polystryene, J. R. McIntyre, Anderson Pattern, Inc, said the first step is understanding the evaporative casting process, what your customers need and want and the equipment. The next step is to acquire detailed knowledge of design and function of molds.
Finally, study manufacturing processes and procedures for high quality and productivity when working in aluminum, put it all together and you'll be ready for the future, said McIntyre.
A panel presentation on "Value Added" featured speakers W. Day, Intermet Corp/Lynchburg Foundry Div; R. Wiertel, Donsco, Inc; and T. J. Krouth, Griffin Industries Corp. In "Getting the Most Out of Your Prototype Castings," Krouth described producing accurate parts with limited lead times. Customer and prototype shop first review the function of the part, discuss design and then approve the final design and cost. One of the most important aspects, Krouth said, is that "You must have proper documentation of all steps."
Wiertel then provided a case history of how his company instituted a value added program in respect to machining and supplying finished parts. "You must understand your markets and your customers' needs and wants--and which ones you can supply," he said.
In his discussion of "Foundry Machining Castings," Day expressed the opinion of all the panelists saying, "When you decide to incorporate value added, you must invest capital money for additional equipment and facilities. And, in today's market, you must rapidly achieve both short- and long-term capabilities."
Ladle Metallurgy Holds Promise For "Clean" Cast Steels
"No technology has contributed more to the cause of clean steelmaking than ladle metallurgy," said M. Liebman, Alternative Industrial Marketing Resources, in his report during a Steel Div session. His comment seemed to underline the primary focus of steel foundries in their drive to eliminate macroscopic inclusions from steel castings to improve overall part quality, reduce post casting repair and, generally, improve foundry productivity and costs.
Along these lines, several new developments in secondary metallurgy were described in achieving cleaner cast steels during the 93rd AFS Casting Congress.
"Clean steel means `clean' in terms of surface, internal micro-cleanliness and mechanical properties," Liebman said. "Cleaner steels require lower sulfur contents and closer chemistry and alloy control."
A major problem faced by foundries in adopting ladle metallurgy technologies is the heat loss suffered during refining due to the smaller sized ladles generally used in foundries. Liebman's report identified several promising technologies for solving this particular problem, and examined alternative heating technologies such as the plasma torch, induction heating, oxy-fuel burners and oxygen lance as a means of ladle reheating in smaller ladles.
"Plasma and induction heating are particularly promising new solutions to the problem of adapting ladle furnace technology to smaller heat sizes," he said.
K. J. Fioravanti, Air Products and Chemicals, and N. F. Griffing and P. D. Stelts, Bethlehem Steel Corp, described another development in ladle metallurgy called Reactive Element Heating.
This patented process, also called REHeating, was developed at four Bethlehem Steel locations between 1985-88. It involves introducing a metallic fuel, typically aluminum or silicon, into molten steel which is then oxidized from the bath using submerged oxygen injection. The exothermic reaction between the fuel and oxygen serves to heat the bath in a controlled and uniform manner.
According to the authors, "This technology is a low capital cost alternative to electrically powered reheating facilities which is appropriate for foundry use. It provides a repeatable process for reheating most steel grades at rates as high as 35F/min.
"Reheating allows for increased flexibility in current secondary metallurgical practices, including desulfurization. Also, it can provide better temperature control during pouring which helps improve final product quality."
Powder injection, another technique for desulfurizing molten steel, was described by A. E. Dainton, Foseco, Inc. He discussed experimental work that was conducted to determine the effectiveness of injecting a synthetic lime/fluorspar flux for desulfurizing steels.
Dainton concluded that this ladle injection process "allows foundries to produce low sulfur steels without the need to invest in considerable capital equipment. Low alloy steel properties, with particular reference to Charpy impact properties at --40F, were significantly improved in comparison to the standard basic arc melted heats."
Gating practices for steel casting also received emphasis in this year's steel presentations. In addressing the historic dilemma of gating steel, N. Wukovich and G. Metevelis, Foseco, Inc, said that while producing high quality castings is becoming more of an art than a science, "The gating of the mold itself still remains a hit or miss proposition."
During their in-depth discussion of the many variables that must be controlled to produce an effective gating system, the authors offered several suggestions. Among these were the manipulation of the velocity of liquid metal and flow rate; use of tapered sprue cups; clean metal which improves fluidity and reduces turbulence; and other new techniques including the use of filters.
"Until more easily adapted computerized gating design systems are available," they added, "gating will remain more a religion than a science."
Also on the subject of gating large steel castings, J. M. Svoboda, Process Metallurgy International, described the findings of a recently completed research project sponsored by the Steel Founders' Society of America and several companies, which firmly established that "the major source of oxide macroinclusions in steel castings is reoxidation of the metal during pouring and flow through the gating system into the mold cavity."
Svoboda reviewed current steel bottom gating practice in production foundries and the basic principles of fluid mechanics. He then described a performed refractory gating system insert that has been developed to incorporate both the centrifugal and flotation mechanisms of inclusion removal.
According to the author, "Various refractory compositions are available and the surface chemistry of the interface may be modified by the appropriate choice of a mold coating."
Reducing inclusions in steel castings also was the subject of a presentation by T. V. Johnson, Corning Glass Works; H. C. Kind, Foseco, Inc; and J. F. Wallace, C. Y. Nieh and H. J. Kim, Case Western Reserve Univ. But rather than ladle metallurgy, the authors discussed the use of cellular ceramic filters to reduce inclusions.
"Studies have been conducted that show that filters and improvements in pouring practice, refractories, molding materials and gating design can reduce inclusions in steel castings," according to the authors. "Based on the demonstrated benefits to cast iron foundries of using ceramic filters to reduce or eliminate inclusions, filters show the most promise for favorably impacting steel foundry practice and reducing costs with little or no capital investment or marked changes in foundry practice."
Results of their testing showed that these filters will prime (get the metal through the filter) a standard grade of steel at a temperature as low as 1540C (2800F) and will survive a pour as high as 1675C (3050F). Use of the filter also improved the fatigue strength of as-cast test bars and significantly improved the cleanliness of castings made in the laboratory and commercial foundries.
Other areas covered during the steel casting program included the effects of sulfide inclusion on the tensile properties of microalloyed cast steels; the supply and demand status of molybdenum, nickel, ferrochromium and ferrous scrap; and the development of international cast steel specifications.
Melting Methods & Materials Division
Desulfurization Methods Dominate Division's Program
Advantages of the porous plug process for desulfurization of iron, and limespar as the desulfurization agent in place of calcium carbide, were explained by W. J. Peck, Central Foundry Div/GMC. Efforts to replace calcium carbide started as early as 1978, he said, to eliminate use of a hazardous material, process odor and to reduce desulfurization costs.
Results of his work show that the use of lime-spar has improved the working conditions and general environment in the melting department and vessel repair area. Vessel maintenance and refractory rebuild maintenance have not changed using lime-spar and desulfurization results have been satisfactory.
In another paper from Central Foundry Div/GMC, T. R. Wirth compared the advantages and disadvantages, from an environmental standpoint, of ductile iron desulfurization using calcium carbide (Ca [C.sub.2]) and calcium oxide/calcium fluoride (CaO/Ca [F.sub.2]).
Because Ca [C.sub.2] as a desulfurizing material has high potential for creating a hazardous waste slag, which will require treatment, it is undesirable and uneconomical to use on a large scale. Using CaO/Ca [F.sub.2] requires somewhat more effort in the area of process control but has advantages in both materials handling and waste management control.
CaO/Ca [F.sub.2] has no potential to generate acetylene gas, Wirth said, and is therefore not a potentially reactive waste. Leaching results have indicated that the slag does not release significant concentrations of metals and is typically not an EP toxic waste.
S. Katz and C. F. Landefeld, General Motors Research Labs, described research demonstrating the factors that control agglomeration, a key to efficient desulfurization. Because CaO/Ca [F.sub.2] is a composite material containing both liquid and solid phases, there is a tendency for the desulfurizer to agglomerate, which in turn reduces the lime-spar/iron interfacial area for reaction.
According to the authors, "The study provided ample evidence that in foundry continuous reactors, desulfurization with lime-spar must be carried out under nonagglomerating conditions. Three factors have a major influence on defining these conditions: CaO particle size, Ca [F.sub.2] concentration and iron temperature.
"This study has shown that the optimum mean particle size is about 1.3 mm. In addition, the Ca [F.sub.2] concentration must be maintained below a critical threshold value for agglomeration. The threshold decreases with increasing temperature and sulfur concentration of the iron, and lime-spar/iron contact time," they concluded.
M. Farrabee, American Cast Iron Pipe Co, speaking for the author C. Kern, compared desulfurization with the use of lime-spar and Ca [C.sub.2] in a mechanically stirred ladle. Although there are operational problems with lime-spar, they believe these can be overcome making this a more viable process because of the environmental disadvantages of Ca [C.sub.2].
R. Nelson and A. Powdrell, Harbison-Walker Refractories, described several improvement approaches that were investigated to reduce brick porosity and alleviate problems associated with slag penetration into high alumina and alumina-chrome brick. Densified matrix technology, they said, has been used to decrease brick porosity, decrease brick permeability and increase brick density without increasing impurity levels in refractory compositions. In contrast, porosity may be lowered by the addition of phosphorous pentoxide at the sacrifice of system purity in high alumina or alumina-chrome brick.
E. J. Hopkins, ABC Coke/Div Drummond Co, Inc, described testing done on individual heats of various cupola charge materials to determine melting loss with the resultant data, when applied, allowing the melt department to more closely control their over the spout melting costs.
All heats were taken with the company's 24 in. inside diameter cupolas and three 300 lb charges of each charge material were used. Among the conclusions, Hopkins said, are the fact that even the best charge material loses 5-8%; steels have a higher yield due to the higher carbon pickups; and a yield can be predicted by the individual percentage of components of the charge.
K. Nyamekye, Auburn Univ, then detailed research undertaken to develop a mathematical model to predict the performance of a coldblast cupola using heat and mass balance criteria. Inputs to the model, he explained, were blast rate, blast temperature and iron-to-coke ratio. Output data generated were spout temperature, gas composition profiles and temperature profiles within an operating cupola.
Results from the model were compared with those from experiments done to measure the temperature and gas composition profiles in the combustion zone of a coldblast cupola and they showed good agreement. Thus, Nyamekye concluded, "The model offers the possibility of predicting cupola performance for different operating conditions."
Brass & Bronze Div.
Investigations into Improving Alloy Properties Continue
A visiting scholar, Dr. M. H. Kim, assistant professor from the department of Metallurgical Engineering, Chungbuk National Univ in Korea, presented the first paper of the three sessions devoted to studies of brass and bronze metal castings. Along with J. T. Berry, Univ of Alabama/Tuscaloosa, Kim explained that there have been many studies concerning mechanical fins, which can increase the efficiency and life of machines, such as heat exchangers, internal combustion engines, transistor assemblies, etc.
In their research, the quantification and application of the cooling fin effect were studied by adopting pure lead, zinc and copper as the casting medium, and silica sand with a quick-setting organic binder as the molding medium.
Results showed that the fin effect in these castings can be quantified by measuring the enhanced movement of the solidification front in the locality of the fin ([A.sub.i]) and by relating A/S with S/T, S being the instantaneous thickness of the solidification skin in the absence of the fin and T, the fin thickness. The technique, they concluded, appears to have considerable potential for the local control of solidification rate.
M. Sahoo, CANMET/MTL, then discussed his investigation undertaken to establish the effect of antimony (Sb) on the mechanical properties of bronze journal bearings, which through de-babbitting had resulted in cumulative increases of Sb content in alloy C94100 to beyond the maximum specified level of 0.80%. He also wanted to determine if the level of Sb could be reduced to acceptable levels.
In an attempt to find a process that is economically more viable than diluting with fresh ingots the scrap containing high antimony, various additions were made to the melt with the hope of forming a compound of antimony that would be lighter than the melt and could be skimmed off prior to pouring.
Addition of elements such as Mg, Mg + Sr, Ca, Mg + Ca, Ni + Mn are not efficient in reducing the antimony content, concluded Sahoo, and the only sensible method is dilution with fresh ingots.
In another study conducted at CANMET, Sahoo discussed how the modifications required to reduce underside shrinkage problems in sand cast ZA-27 were tested for their effect on corrosion resistance. The ZA-27 alloys containing strontium, calcium and sodium additions were subjected to accelerated steam corrosion testing.
According to Sahoo, strontium additions do not affect mechanical properties and may cause a small increase in the intergranular corrosion. It minimizes the problems of underside shrinkage with no deleterious effects. Calcium additions at levels required to reduce underside shrinkage may reduce intergranular corrosion but cause pitting. Sodium additions cause a deterioration to both mechanical properties and corrosion resistance and are, he determined, less desirable.
"Practical Aspects of Iron Permanent Mold Casting of ZA Alloys" as they relate to design criteria were given by F. DeHart, Stahl Specialty Co, and D. Argo, Noranda Technology Center. A critical factor in producing premium quality ZA permanent mold castings is maintenance of directional solidification and, according to the authors, this is best controlled using top gating in conjunction with tilt pouring.
It was found that these ZA alloys may be cast satisfactorily in molds designed for the permanent mold casting of Al alloys but their unique properties must be considered in conjunction with such effects as mold and melt temperatures.
Because Ni-Al bronzes and Mn-Ni-Al bronzes have a number of marine applications, such as castings for large ship propellers, fracture toughness becomes an important mechanical property. Results of a study done by J. Hallen-Lopez and J. I. Dickson, Ecole Polytechnique, and M. Sahoo, CANMET/MTL, of the fracture toughness of Ni-Al bronze alloy C95800 as a function of cooling rates using the [J.sub.Ic] testing technique were presented by Sahoo. The influence on [J.sub.Ic] of a heat treatment, which improves the corrosion resistance of the alloy, was also evaluated. Another aim was to determine the influence of microstructure of fracture toughness.
Fracture toughness was found to depend primarily on the size, type and distribution of Kappa precipitates present in the microstructure. Heat treatment to improve the corrosion resistance resulted in a considerable decrease in the fracture toughness as a result of the precipitation of numerous fine K precipitates within the a phase.
Investment Casting Division
Gas Pickup, Degassing Are Topics of Interest
A report on "SPAL for the Investment Casters," by S. H. Anderson of Liquid Air Corp and R. Nagan and B. Jhala, Arwood Corp, was based on testing conducted at Arwood Corp. The patent-pending process uses argon, [N.sub.2] and [CO.sub.2] to prevent contact of the melt with the atmosphere during the melting and processing and transfer.
Testing for various mechanical properties, chemical composition, gas analysis, fluidity and zyglo were conducted on different alloys. SPAL was developed to reduce pickup of [N.sub.2] up to 95% and [O.sub.2] up to 270%, leading to reduced porosity, pitting and surface cracking.
The initial tests at Arwood also indicate that the use of deoxidants, "increases the total gas pickup regardless of crucible material."
Simplified approaches to degassing common cast alloys were presented by T. Klemp III, Cannon-Muskegon Corp. Various deoxidizing elements and compounds were tested using closely controlled heats of AOD refined 17-4 stainless steel, with .04% addition of a reactive element. Aluminum was found to be the deoxidizing agent of choice, zirconium and titanium for dentrification. "Many of the proprietary, complex deoxidizing agents result in significantly higher [sic] levels of oxygen when compared to the use of no additions. This is attributed to both the `sponage' effect of reacting with atmospheric oxygen and the inherent uncleanliness (high oxide content) of the additions themselves," said Klemp.
According to Klemp, argon degassing requires considerably higher superheat and necessarily reduced productivity and shortened refractory life.
H. Huang, Univ of Alabama/Tuscaloosa, presented a summary of research involving the use of protective coatings during heat treatment of investment cast martensitic stainless steel. The glass-like refractory coating melts during treatment preventing air from reaching the casting surface, minimizing oxidation said Huang. If the glass powder was present at levels of more than 50%, at 1050C, the glass did not adhere to the casting, he noted.
Experimental results indicate that refractories of alumina-glass and gangue-glass have higher levels of fusible oxides, providing the sintering protection. Shakeout is also improved by the refractory.
The microstructure of IC austenitic stainless steel, as-cast and heat treated was the subject of an investigation by a number of researchers at Oklahoma State Univ. They found (paper 88-118) vermicular and lacy delta-ferrite morphologies, and determined the rate of delta-ferrite dissolution and changes in morphology.
Efforts to reduce Volatile Organic Compounds (VOCs) led the Duriron Co to consider converting its slurry system from alcohol- to a water-base according to a paper by M. E. Armstrong and D. R. Stickle. Initial tests showed that the water-base shell had relatively high retained strength after casting--making shell removal difficult. Some cracks were also found in a low ductility, high silicon cast iron. Stucco clumping and shell cracking during autoclaving were also encountered.
Duriron made changes in their filled wax and shell formulations, and now runs a water-base binder system at, "essentially the same rate as with ethyl silicate." Duriron is satisfied that the lowered VOC emissions, safer and more pleasant working environment definitely warrant the investment.
PHOTO : J. E. Gruzleski, McGill University
PHOTO : R. Warda, QIT-Fer et Titane, detailed an ongoing program for marketing ductile iron
PHOTO : castings in his presentation called "Marketing Castings: Beyond Survival."
PHOTO : P. C. Matejcik, session chairman, thanks Dr. R. Kotschi, Kotschi's Software, for his
PHOTO : presentation.
PHOTO : A. Dainton, Foseco
PHOTO : A. Powdrell, Harbison-Walker Refractories
PHOTO : W. L. Rudin (ctr), Wm. L. Rudin Associates, Ltd, cochairman of one of the Brass & Bronze
PHOTO : sessions, is flanked by session authors F. DeHart (l), Stahl Specialty Co, and M. Sahoo,
PHOTO : CANMET/MTL Energy, Mines & Resources.
PHOTO : T. Klemp III, Cannon-Muskegon Corp.
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|Title Annotation:||part 2|
|Date:||Aug 1, 1989|
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