Gas from Green Sand Molds and Organically Bonded Cores

Transcription

1 Gas from Green Sand Molds and Organically Bonded Cores by Leonard Winardi, Graduate Student University of Alabama at Birmingham And Charles E Bates Founder, AlchemCast LLC cbates@alchemcast.com 1

2 Gas from Green Sand Molds and Organically Bonded Cores Leonard Winardi and Charles E. Bates, Abstract Research was conducted on gas evolution from several green sand and dry sand molding systems. The purpose was to develop methods for predicting gas pressure in local areas of molds and cores and to understand and be able to eliminate blow-holes and pinholes. Variations were made in the water, clay, and sea coal contents of the sand to determine the major factors affecting gas evolution rates and volumes. Gas evolution rates and volumes from green sand are dominated by the water content. The gas evolved from sands containing % water was 3 cm 3 after 5 seconds contact with iron. The moisture evaporation and condensation greatly increased heat transfer into the sand as evidenced by a low rate of temperature rise in the core when the moisture content was high. A sand containing 2% Seacoal evolved about 35 cm³ in 5 seconds compared to 3 cm³ from a comparable sand containing 1 % Seacoal. Organic additives such as Seacoal, cereal, and clay do have measurable effects. The uncoated epoxy acrylic sample evolved less total gas compared to the green sand, but the gas was evolved very quickly after metal contact. Unless the metal head pressure is quickly developed, the high initial evolution rate can cause blows in the casting. The presence of water, clay, and Seacoal in green sand significantly decreases sand permeability. A no-bake resin bonded sand with an AFSGFN of 63 containing 1.25% PUCB binder had twice the permeability of the same sand containing clay, water and Seacoal. 2

3 Introduction Surface and sub-surface gas porosity is a significant problem in the casting industry. High local gas pressures produced by pyrolysis of organic materials and the evolution of water as steam drive the gases into the molten metal. This gas produces blow defects and allows hydrogen absorption which causes pinholes when the hydrogen is rejected during solidification. The gas evolved is usually entrapped at or near the cope surface and under cores, as illustrated in Figure 1. Since gas bubbles may float some distance before becoming entrapped, identifying the source is sometimes difficult. 1 Proper sand permeability, gating and venting is crucial for producing sound, high quality castings free of porosity. Figure 1. Gas Defect Beneath the Surface of a Gray Iron Casting. Venting molds and cores is currently a combination of foundry experience, and art. Core gases in simple castings can usually be vented through core prints. But as casting complexity increases, venting becomes more difficult, and this results in scrap and lost production time. 2 Green sand, with insufficient permeability to allow moisture to exit through the back face of the mold can cause moisture and hydrogen to bubble through the metal, especially where portions of the mold protrude to form a pocket in the casting. In severe cases, substantial amounts of water can be driven into the casting. Various investigators have measured the rate of gas evolution from binder systems with various degrees of success. 3,4,5 The Dietert Gas Pressure Test has been used for many years and involves placing a sand sample in a boat, placing the boat in the cold end of a tube furnace, sealing the furnace, sliding the boat into the hot zone, and measuring the pressure developed as a function of time. The pressure of the gas is a function of the number of moles of gas formed, furnace temperature, and furnace volume. The rate of gas evolution is controlled by the heat transfer from the tube to the sample, and the rates are substantially below the rates experienced when metal impacts a mold or core surface. The central issues associated with mold/core gas evolution and associated gas defect formation are: 3

4 1. What are the volumes and compositions of gas produced when a molten metal contacts a mold or core prepared with a particular binder? The composition is important because it affects the gas viscosity. 2. What is the timing of the gas evolution after metal contact with the mold or core? The time of the gas evolution is important because if it occurs before the casting has a solid skin or the metal has sufficient head pressure, the gas may be blown through the metal. 3. What are the gas flow coefficients through the core and sand being used? The pressures in molds are sufficiently high that both Darcy and Forscheimer coefficients are needed to describe the gas volume that will flow through the pores in the sand. 4. If gas is blowing through the metal, how/where should the core or mold be vented or what binder/sand/additive/wash changes should be made to eliminate the defect? Gas blowing through gray iron poured around a inch diameter by 6 inch long solid shell core can be seen on the following web site: A bubble from a coated PUCB core formed during pouring an aluminum casting is also illustrated at that web site. The metal head pressure was raised fast enough in this case to push the gas bubble back into the PUCB core instead of allowing it to exit through the casting. In the current research, most of the experiments were conducted on green sand and dry sand cores. Deep pockets in green sand molds can behave like cores and may create pressures sufficiently high to eject gas into the casting. Variations were made in the water, clay, and sea coal contents to determine the major factors affecting gas evolution, and data on one organically bonded sand is included for comparison. All experiments were conducted with sand specimens in contact with iron. Experimental Procedures Techniques have been developed to measure the gas evolution rates from cores in contact with a variety of metals and measure the gas flow coefficients through sand-binder combinations used in foundries 1. The gas volume measuring procedure involves immersing a sample of the sand, typically 1 1/8 inch (2.85 cm)in diameter by 2 inches (5 cm) long, into the type of metal and at the temperature of interest to a particular foundry. In the current study, gray cast iron was used at a temperature of about 245 o F (1345 o C). Experimentally, a sample of sand was placed in an insulated metal print and immersed in molten metal. The gas formed during pyrolysis passed from the core or mold sample, through the print and into a preheated pipe. From the pipe, the gas passed into a preheated oil chamber and displaced hot oil from the chamber. The rate of oil displacement is a function of the rate of gas evolution from the core. 6 4

5 These procedures allow the effects of variations in additives and coatings to be systematically evaluated under conditions that simulate events in a mold. This paper presents the results of studies on green sand and one epoxy acrylic bonded sand. Results and Discussion A gas evolution curve from sand containing 2.9% water, 6% clay, and one percent sea coal is presented in Figure 2. The initial rate of gas evolution was about 25 cm 3 /sec five seconds after metal contact, and a value of cm 3 /sec was reached after 2 seconds. The total gas evolved during 5 seconds contact with iron was 3 cm 3 from a specimen weighing about 4 grams % Water 6% Clay Gas Evolution Rate (cm 3 /s) Gas Volume (cm 3 ) Figure 2. Gas Evolution Rates and Volumes from Green Sand Containing 2.9% Water, 6% Clay, and. It is well known that water is evaporated from near the surface of green sand after contact with metal and driven into the interior of the mold or mold pocket. The effect of the evaporation and condensation of moisture on the temperature profile in the specimen of green sand containing 2.9% water, 6% clay, and is illustrated in Figure 3. The temperature at the mid radius and core center as a function of time is illustrated. The heat from the iron evaporated water near the surface and probably caused some moisture decomposition. The water was evaporated and condensed as it was driven from the surface into the cooler portions of the sample. The mid radius position reached 1 o C about 5 seconds after metal contact and stayed at this temperature for about 15 seconds. The center position reached 1 o C about 7 seconds after metal contact and stayed at this temperature for about 4 seconds. 5

6 % Water 6% Clay 245F Mid Radius 35 Temperature ( o C) Center Figure 3. Temperature Profile at Half Distance and at the Center of the Green Sand Core Containing 2.9% Water, 6% Clay, and 1% Sea Coal. Gas evolution substantially increased as the steam and other gases were driven from the surface of the core about 5-6 seconds after immersion, and the rate of gas evolution stayed above 1cm 3 /sec from about 15 seconds after metal contact to 3 seconds after contact. The core center was still at 1 o C when the rate of gas evolution began to decrease. Sufficient heat passed through the core surface to evaporate the moisture at the mid radius position about 18 seconds after initial contact, but the center position did not rise above the water boiling point until about 45 seconds after metal contact. The peak rate of gas evolution, illustrated in Figure 2, was reached about the time the mid radius position reached the water boiling point. It is anticipated that a substantial amount of water was being driven from the sample at this time, but gas analyses will be required to confirm this idea. The rapid temperature rise to 1 o C in the core was a result of steam (mass) convective heat flow. Steam condensation drove the center temperature to 1 o C shortly after metal contact, and the steam kept the temperature at the center constant until sufficient conduction had occurred through the dry sand layer to evaporate the free water. Some experiments were conducted to validate this idea. Specimens were first prepared with 2.5% sodium silicate and dried at 9 o C for 4 hours in regular oven to evaporate the free water. After drying, these specimens contained only sodium silicate (with no additives) and perhaps a small amount of hydrated water. The gas evolution results are illustrated in Figure 4. The center location temperature rise curve is illustrated in Figure 5 with a green sand temperature rise curve superimposed. The volume of gas evolved from the dried sodium silicate bonded sand was about 4 cm 3 in 5 seconds. This volume was a result of some hydrated water 6

7 evolution and expansion of air contained in the pores of the sand. The temperature rise in the sodium silicate core, illustrated in Figure 5 was gradual without any isotherms. It took about 25 seconds for the core center to reach 1 C. The green sand sample reached a temperature of 1C in about 7 seconds, which supports the idea of mass conductive heat flow. This data indicates that the majority of the gas coming from the green sand is associated with water or the decomposition products of water such as hydrogen Gas Evolution Rate (cm 3 /s) Rate ρ = 9.8 lb/ft Rate ρ = 87.6 lb/ft Gas Volume Gas Volume Gas Volumes (cm 3 ) Figure 4. Gas Evolution Rates and Volumes from Dried Sodium Silicate Cores in contact with iron at 245F Temperature ( o C) Baked Sodium Silicate 2.5% Water Figure 5. Temperature Profile at the Center of a dried sodium silicate bonded core in contact with iron at 245 F. 7

8 Since water and hydrogen are considered to be the dominant factors influencing the volume of gas evolved, a similar experiment was conducted where green sand specimens were dried to.6% water. The samples contained 6% clay, and. The gas volume results are illustrated in Figure 6. This sand evolved about 7 cc gas in 6 seconds, with a peak rate of evolution of 15cm 3 /sec. Some of the gas may have come from clay dehydration, some from partial Seacoal pyrolysis, and some from water disassociation % Water Immersion temp: 245F Gas Evolution Rate (cm 3 /s) Gas Volume (cm 3 ) Figure 6. Gas Evolution Rates and Volumes from Green Sand Containing.6% Water, 6% Clay, and. Gas evolution curves from green sand containing 2.5% water, 4% clay, and 1% sea coal are illustrated in Figure 7. The temperature history in the center of the core is illustrated in Figure 8. The total gas volume evolved in 5 seconds was about the same as in the 2.9% water sample, with a value of about 3 cm 3. The peak gas evolution rate was also approximately the same with a value of about 125cm 3 /s, although the curves appeared to be flatter and to have a more extended peak compared to sands containing more clay. 8

9 % Water Immersion temp: 245F Gas Evolution Rate (cm 3 /s) Gas Volume (cm 3 ) Figure 7. Gas Evolution Rates and Volumes from Green Sand Containing 2.5% Water,, and Temperature ( o C) % Water 6% Clay 2.5% Water Figure 8. Temperature Profile at the Center of a Green Sand Sample containing 2.9% water and 6% clay compared to one containing 2.5% water and 4% clay. 9

10 Temperature ( o C) % Water 2.5% Water Figure 9. Temperature Profile at the Center of a Green Sand Sample containing 2.9% water and 6% clay compared to one containing 2.5% water and 4% clay % Water 2% Seacoal Immersion temp: 245F Gas Evolution Rate (cm 3 /s) Gas Volume (cm 3 ) Figure 1. Gas Evolution Rates and Volumes from Green Sand Containing 2.5% Water,, and 2% Seacoal. Both the clay and water content affected the heating rate in the sand prior to reaching 1C, as illustrated in Figure 9. The center temperature rose more rapidly in cores containing more water because the mass transport provided by the water was higher. Increasing clay content may require more energy for to dry the clay. The center temperature of the dried green sand 1

11 rose more slowly because of the reduced mass transfer caused by the lower water content. Data on gas evolution from a sand mixture containing the same water content, 4% clay, and 2% Seacoal is illustrated in Figure 1. The mixture containing 1% sea coal evolved about 3 cm³ (Figure 7) of gas during 5 seconds immersion, and the volume from the mixture containing 2% Seacoal was approximately 35 cm³ in the same time interval. This indicates that while the volume of gas from green sands is dominated by the moisture content, organic additives such as Seacoal, cereal, and other organic materials have measurable and possibly significant effects. For comparison purposes, typical gas evolution curves from an epoxy acrylic (EA) core and an EA core having a dried water based coating on it are illustrated in Figure 11. The epoxy acrylic core evolved about 44 cm 3 of gas within 5 seconds, and the coated core evolved about 61 cm 3. The peak rate of evolution was about 16cm 3 /sec in the EA core and 25cm 3 /sec in the coated and dried EA core. The peak rate of gas evolution was reached about 4 seconds after metal contact. This gas can blow into the metal unless the pouring rates are very high to quickly build metal pressure above the core. Gas Evolution Rate (cm 3 /s) Rate: Epoxy Acrylic Cores Coated WB#3 Fe at 1275C Rate: Epoxy Acrylic Cores Uncoated Fe at 1275C Vol: Epoxy Acrylic Cores Coated WB#3 Fe at 1275C Vol: Epoxy Acrylic Cores Uncoated Fe at 1275C Gas Volume (cm 3 ) Figure 11. Gas Evolution Rates and Volumes from washed and unwashed epoxy acrylic cores in contact with iron at 245F. The presence of water, clay, and Seacoal in green sand significantly decreases sand permeability and can retard the gas evolution from the mold or mold pocket, as illustrated by the data in Figure 12. The no-bake molding sand containing 1.25% PUCB binder exhibited an air flow rate at 2 PSIG of 6-68 cc 3 /cm 2 /s at a density of kg/m3 (98-1 pounds per cubic foot). The air flow rate through green sand was in the range of 3 to 35cm³/cm 2 /s at the same density in green sand. The permeability of the wet zone behind the mold metal interface could be even lower than the values reported. 11

12 A combination of low sand permeability, high density and a high gas evolution rate causes green sand molds to be prone to producing blow defects. Gas bubbling through molten iron in risers of high density green sand molds has been observed on some occasions. Density (Kg/m 3 ) Air Flow Rate (cm 3 /cm 2 /s) Silica Sand 63 GFN Green Sand 63 GFN Density (lb/ft 3 ) Figure 12. Air Flow Rate through Sand at 2 PSIG. Core and mold samples had a Diameter of inch and Length of 1.5 inch. Summary and Conclusions Experiments were conducted on green sand, dry sand, and one organically bonded specimen in contact with gray iron to determine the rates and volumes of gas produced during contact with molten metal. Variations were made in the water, clay, and sea coal content to determine the major factors affecting gas evolution. The initial rate of gas evolution from green sand containing from 2.5 to 2.9% water,, and 4 to 6 % clay was about 25 cm 3 /sec five seconds after metal contact. The maximum rate peak was reached 25 seconds after contact or as the transport zone reached the specimen mid radius with a value of cm 3 /sec. The total gas evolved after 5 seconds contact with iron about 3 cm 3. The gas volume evolved from green sand is dominated by the moisture content, but organic additives such as Seacoal, cereal, and clay do have measurable effects. The Seacoal content of green sand affected the volume of gas evolved with a 2% Seacoal mixture evolving about 35 cm³ in 5 seconds compared to 3 cm³ from a comparable mixture containing 1 % Seacoal in the same time interval. The effect of mass convective heat transfer produced by moisture was clearly seen in comparisons of the temperature history in green sand and dry sand and dry sand samples. The rate of temperature rise in the core center was much lower when the moisture content was low. 12

13 The uncoated epoxy acrylic sample evolved less total gas compared to the green sand, but the gas was evolved shortly after metal contact. Unless the metal head pressure is quickly developed, the high initial gas evolution can cause blows in the casting. The presence of water, clay, and Seacoal in green sand significantly decreases sand permeability of sand. A no-bake sand containing 1.25% PUCB binder exhibited a gas flow rate at 2 PSIG of 6-68 cc 3 /cm 2 /s and green sand at the same density had about half the gas flow rate at the same density. Further research is directed in developing a model for predicting the pressure inside sand core and mold and the occurrence of blow defects. This model requires input from gas evolution measurements, sand permeability coefficients, and gas composition. Once the gas evolution rate and sand permeability coefficients are known, the pressure can be calculated 7. Bibliography 1. Worman, R.A. and J.R. Nieman, A Mathematical System for Exercising Preventive Control over Core gas Defects in Gray Iron Castings, AFS Transactions, vol. 81, pp , (1973). 2. Caine, J.B. and R.E. Toepke. Gas Pressure and Venting of Cores, AFS Transactions, vol. 74, pp (1966). 3. Dietert, H.W., A.L. Graham, and R.M. Praski, Gas Evolution in Foundry Materials Its Source and Measurement, AFS Transactions, vol. 84, pp (1976). 4. Bates, C.E. and R.W. Monroe, Mold Binder Decomposition and Its Relation to Gas Defects in Castings, AFS Transactions, vol. 89, pp , (1981). 5. Scott, W.D., P.A. Goodman, and R.W. Monroe, Gas Generation at the Mold-Metal Interface, AFS Transactions, vol. 86, pp (1978). 6. Scarber, Jr. P., Bates, C.E., Griffin, J., Effects of Mold and Binder Formulations on Gas Evolution When Pouring Aluminum Castings, AFS Transactions, vol. 114, paper no. 13 (26) 7. Winardi, L., Littleton, H.E., Bates, C.E., Gas Pressures in Sand Cores, AFS Transactions, vol. 115 (27) 13