Controlling heat transfer with refractory coating.
Inside This Story
* The cast iron industry is facing higher demands to produce cleaner, thinner-walled castings with closer tolerances.
* Refractory refractory
Material that is not deformed or damaged by high temperatures, used to make crucibles, incinerators, insulation, and furnaces, particularly metallurgical furnaces. coating manufacturers have been pressed to produce coatings capable of enhancing the performance of resin/sand systems.
* This article details a study to determine the affect of two refractories, diatomaceous earth diatomaceous earth: see diatom.
Light-coloured, porous, and friable sedimentary rock composed of the frustrules (silicate cell walls) of diatoms. and graphite graphite (grăf`īt), an allotropic form of carbon, known also as plumbago and black lead. It is dark gray or black, crystalline (often in the form of slippery scales), greasy, and soft, with a metallic luster. , on thermal heat transfer.
Higher demands have been placed on the cast iron industry to produce cleaner, thinner-walled castings with closer tolerances. In order to produce these, it is necessary to make cores that have tighter tolerances and are stable under all pouting pout 1
v. pout·ed, pout·ing, pouts
1. To exhibit displeasure or disappointment; sulk.
2. To protrude the lips in an expression of displeasure or sulkiness. conditions. Regardless of the tolerances achieved in a sand core at room temperatures, the distortion distortion, in electronics, undesired change in an electric signal waveform as it passes from the input to the output of some system or device. In an audio system, distortion results in poor reproduction of recorded or transmitted sound. of the core upon exposure to molten metal and rapid heating may introduce additional variances to the finished casting.
While new resins resins,
n.pl complex, insoluble, sticky substances secreted by plants. Used as astringents, antimicrobials, and antiinflammatories, and are burned as incense. Can cause oral ulcers and epidermal irritations. and sand systems are being explored to meet these requirements, the metalcasting industry has pressed refractory coating manufacturers to produce coatings capable of enhancing the performance of resin/sand systems. To date, refractory coating's primary function has been to enhance casting surface finish and minimize metal penetration defects.
In most U.S. core applications, the final dry coating deposit typically is limited to 0.004-0.019 in. (0.10-0.25 mm) in order to compensate for dimensional changes associated with coating cores. In Europe, heavier dry coating deposits ranging from 0.10-0.15 in. (0.25-0.4 mm) have demonstrated some anti-veining characteristics, which implies that heavier coating deposits may reduce the thermo-mechanical stress development in resin-bonded sand systems.
One approach to help prevent thermal distortion is to reduce the heat transfer through the coating and into the core. The primary method for this is to lower the thermal conductance thermal conductance
A measure of the ability of a material to transfer heat per unit time, given one unit area of the material and a temperature gradient through the thickness of the material. It is measured in watts per meter per degree Kelvin. of the refractory coating (increase the insulation insulation (ĭn'səlā`shən, ĭn'sy–), use of materials or devices to inhibit or prevent the conduction of heat or of electricity. ).
Refractories with insulating characteristics are necessary to reduce heat transfer. The insulating characteristics of the refractory particles in a coating are controlled by multiple factors, including:
* particle chemistry, shape, crystalline Like a crystal. It implies a uniform structure of molecules in all dimensions. For example, phase change technology, widely used for rewritable optical discs, uses crystalline spots (bits) to reflect the laser beam. Amorphous, non-crystalline bits do not reflect light. structure and density;
* thermal conductivity thermal conductivity
A measure of the ability of a material to transfer heat. Given two surfaces on either side of the material with a temperature difference between them, the thermal conductivity is the heat energy transferred per unit time and per unit of the refractory;
* the thickness and alignment of the particles in the deposit.
Investigations were performed to compare diatomaceous earth (DE), a low thermal conductance refractory (high insulation coating), to graphite (G), a high thermal conductance refractory (low insulation coating), at temperatures and pressures that simulated pouring cast iron against a coated phenolic phe·no·lic
Of, relating to, containing, or derived from phenol.
Any of various synthetic thermosetting resins, obtained by the reaction of phenols with simple aldehydes and used as adhesives. urethane urethane (yoor´ithān´),
n ethyl carbamate used as an anesthetic agent for laboratory animals, formerly used as a hypnotic in humans. coldbox (PUCB) disc (Fig. 1). The effectiveness of the two types and thicknesses of refractories in preventing heat transfer and thermal expansion thermal expansion
Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change. to the discs was measured through changes in the thermal distortion curves of the discs and by comparing the heat transfer ([T.sub.transfer]) from the thermal source to the experimental discs.
[FIGURE 1 OMITTED]
Calculating Heat Transfer
Coating thicknesses of 0.004 and 0.008 in. (0.1 and 0.2 mm) were selected as representative of industry practices and because they are the thinnest and heaviest surface deposits that could be applied with the experimental one refractory component coatings. As seen in Fig. 2, refractory layers are not strictly limited to the surface of the discs, but also penetrate into the interstitial space Interstitial space
The fluid filled areas that surround the cells of a given tissue; also known as tissue space.
Mentioned in: Lymphedema between the sand grains of the disc.
[FIGURE 2 OMITTED]
In addition, the transfer of heat from the contact point at the thermal hot surface used to heat the discs contributes a thermal joint conductance factor to the heat transfer.
As a consequence, no attempt was made to calculate the actual thermal conductance of the two refractories. Instead, the rate of heat loss from the thermal hot surface per unit of time was recorded during each disc heating cycle to determine and compare the amount and rate of the heat transfer. Heat loss from the thermal hot surface ([DELTA]T) was calculated by determining the lowest average temperature the hot surface recorded then subtracting the starting temperature of the host surface from the lowest temperature. The average time (t) to reach the lowest temperature was read directly from the recording, so the rate of thermal heat transfer was calculated as: [T.sub.transfer] = [DELTA]T/[DELTA]t
Laying the Groundwork
In the study, the refractory coating formulas were greatly simplified from production formulas to minimize interactions between typical coating components and the effects to those directly related to the refractories under evaluation. The non-refractory components of the dried coating were less than 1% by weight of the deposit. The median particle sizes Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. of the two refractories were matched as closely as possible, xantham gum was chosen as the suspension/rheological system for both coatings, the surface-active substance used was the same for both coatings, and surface tension of the two coatings was matched as closely as possible (Table 1).
The test cores were 2 x 0.35-in. (5.08 x 0.89-cm) transverse To cross from side to side. disc specimens made using a common two-part industrial phenolic urethane resin resin, any of a class of amorphous solids or semisolids. Resins are found in nature and are chiefly of vegetable origin. They are typically light yellow to dark brown in color; tasteless; odorless or faintly aromatic; translucent or transparent; brittle, fracturing . A commercial grade 4 screen washed lake sand of 48 GFN GFN Gone for Now
GFN Gay Financial Network
GFN Good For Nothing
GFN Glass Filled Nylon
GFN Group-Forming Network
GFN Grand Forks, North Dakota (border patrol sector)
GFN Goodbye for Now
GFN Global Futures Network was used to make the discs (Table 2).
The resin level was 1.25% based on sand (BOS) with a 55:45 ratio of part 1 to part 2, and technical grade TEA was used as a catalyst. A four-off corebox was used with a blow pressure of 50 psi PSI - Portable Scheme Interpreter (nitrogen) for 0.5 sec. and the catalyst gas purge To eliminate or delete. cycle (nitrogen) of 40 sec. at 20 psi.
The experimental discs were randomized ran·dom·ize
tr.v. ran·dom·ized, ran·dom·iz·ing, ran·dom·iz·es
To make random in arrangement, especially in order to control the variables in an experiment. by corebox activity and by coating. They were held at. a 30-degree angle and dipped 20 min. after stripping. Only one side of each disc was coated with a refractory coating. The wet coated discs were allowed to drain for 15-20 sec. before being transferred to a forced air oven for drying. Once cooled to lab temperature, the dry coated discs were weighed and then transferred to Western Mich. Univ. for final testing.
The net change in dry weight caused by refractory coating was considered indicative of increased coating deposit. The change in dry weight was calculated for every disc by subtracting the strip weight from the final dry weight of the coated disc (Table 3).
Reading the TDCs
In this investigation, PUCB specimens were coated with DE or G to two thickness levels while control cores were left uncoated. All specimens were tested at a molten cast iron temperature of 2,320F (1,2880 and a weight of 1.46 lbs. (663 g) for 3 min.
The thermal distortion curves (TDCs) for all systems showed undulations indicating thermo-mechanical and thermo-chemical changes in the binder binder: see combine.
An earlier Microsoft Office workbook file that let users combine related documents from different Office applications. The documents could be viewed, saved, opened, e-mailed and printed as a group. system at an elevated temperature. During the first 2-5 sec., the control cores and the [G.sub.0.008], [DE.sub.0.004] and [DE.sub.0.008] thermal distortion curves all demonstrated a slight downward deflection deflection /de·flec·tion/ (de-flek´shun) deviation or movement from a straight line or given course, such as from the baseline in electrocardiography.
1. between 0.0002-0.0004 in. (0.005-0.01 mm). After this initial deflection, the [DE.sub.0.008] held constant, while all other test groups demonstrated an upward deflection between 5-60 sec. The order of upward deflection, from smallest to largest, was [DE.sub.0.004], control, [G.sub.0.004], [G.sub.0.008]. After reaching their peak, these four samples then began a downward trend out to the 180 sec. tested. After 180 sec., the order of least to most downward deflection of the test specimens was:
Ultimately, both G refractory coated systems demonstrated significant expansion during the first 65 sec., followed by plastic deformation plastic deformation,
n any irreversible deformation of tissues. for the remainder of the test period. The uncoated control system had a similar TDC TDC Top Dead Center
TDC Time-to-Digital Converter
TDC Tabular Data Control
TDC Total Development Cost
TDC Texas Department of Corrections
TDC The Discovery Channel
TDC Torpedo Data Computer
TDC Theater Deployable Communications (Fig. 3). The G refractory systems increased the thermal distortion of the discs by 38.2% at the 0.004-in. (0.01-mm) deposit and by 57.8% at the 0.008-in. (0.02-mm) deposit.
[FIGURE 3 OMITTED]
The DE systems were thermally stable. After the first 5 sec., the 0.004-in. (0.01-mm) DE demonstrated slight expansion for 110 sec. with a slight plastic deformation at the end of the test. [DE.sub.0.008] showed only slight plastic deformation. The thermal distortion was reduced by 42.2% at the 0.004 in. (0.01-mm) deposit and by 62.2% at the 0.008 in. (0.02-mm) deposit.
With respect to systems tested, there was a significant difference in thermal distortion (Table 4, Fig. 3). The G specimens had more thermal distortion than the controls, and the DE specimens had less thermal distortion than the control discs.
Comparing Hot Surface Temperature
Certain trends were observed while comparing the data for average hot surface temperature. The [G.sub.0.004], [G.sub.0.008] and control samples caused a thermal gradient gradient
In mathematics, a differential operator applied to a three-dimensional vector-valued function to yield a vector whose three components are the partial derivatives of the function with respect to its three variables. The symbol for gradient is ∇. at the hot surface and, after 20 sec., were reduced to 2,039, 2,075 and 2,102F (1,115, 1,135 and 1,150C). The temperatures then began the return to the steady state condition (Fig. 4). The G refractory increased the heat transfer (relative to the control) by 10.6% at the 0.004-in. (0.01-mm) deposit and by 12% at the 0.008-in. (0.02-mm) deposit. Both G samples demonstrated a maximum heat loss at 19 sec. with a change in temperature of -276 and -245F (-171C and -154 C). The average calculated heat transfer for graphite was -8.55C[s.sup.-1]. The graphite systems demonstrated at 12% increase in heat transfer from the hot surface when compared to the uncoated control disc.
[FIGURE 4 OMITTED]
The thermal gradient at the hot surface after 20 sec. for the DE specimens showed an insulative in·su·la·tive
Serving to insulate or keep safe: the insulative value of an animal's fur; insulative packing materials. property and only dropped to 2,246 and 2,210 F (1,230 and 1,210C) before beginning their return to the steady state condition. The DE refractory reduced the heat transfer (relative to the control) by 4.2% at the 0.004-in. (0.01-mm) deposit and by 5.6% at the 0.008-in. (0.02-mm) deposit. The average calculated heat transfer for diatomaceous earth was--2.56C[s.sup.-1]. The DE systems demonstrated a 66% decrease in heat transfer from the hot surface when compared to the uncoated control disc.
The investigation results suggest that refractory coatings have a more significant impact on core distortion and, consequently, on casting tolerance than had been thought previously. Those actively exploring thin wall iron castings and precision iron castings may want to investigate the contributions that refractory coatings can make to their efforts.
Future investigations are planned to evaluate the impact of refractory coating heat transfer on total core distortion and the subsequent potential for vein formation. Research into the effects that refractory coating penetration between sand grains has on thermal distortion and thermal conductance also is planned.
There are numerous other chemical binder systems from which additional data could be gathered to learn about thermal properties, and additional work could be performed at different loads and different temperatures simulating other alloys This is a list of alloys for which an article exists in Wikipedia (or is proposed but not yet written).
They are grouped by base metal, in order of increasing atomic number. Within these headings they are in no particular order. and pressures representative of larger or smaller castings.
For More Information
"Evaluating Refractory Coatings: A Practical Approach," S.G. Baker, MODERN CASTING, Oct. 2002, p. 21-23.
Orville Guyer is a lab manager and Robert Emptage is a product development chemist (jargon) chemist - (Cambridge) Someone who wastes computer time on number crunching when you'd far rather the computer were working out anagrams of your name or printing Snoopy calendars or running life patterns. May or may not refer to someone who actually studies chemistry. at HA International, Toledo, Ohio
forced expiratory flow
forced expiratory flow rate. Key Professor at Western Michigan
Western Michigan, also known as West Michigan, is a region of the U.S. state of Michigan. Univ., Kalamazoo, Mich.
Table 1. Refractory Coating Properties Relative to Coated Discs. Brookfield Thixotropic Coating % Solids Visc. (cP) Index A [G.sub.0.004] 22.13 330 1.74 B [G.sub.0.008] 24.72 609 1.74 C [DE.sub.0.004] 23.32 259 1.76 D [DE.sub.0.008] 25.07 460 1.76 Median Particle Surface Tension Coating Size ([micro]m) (dyne/[cm.sup.2]) A [G.sub.0.004] 9.38 31.64 B [G.sub.0.008] 9.38 30.94 C [DE.sub.0.004] 9.28 34.32 D [DE.sub.0.008] 9.28 34.58 Table 2. Properties of Sand. Source AFS/gfn Shape Screens % Resin MI 48 subangular 4 1.25% Roundness/ Source Sphericity/Krumbein pH MI 0.5/0.7 7.2-8.4 Table 3. Changes in Disc Weight Caused by Dry Coating. Refractory Coating G High Thermal Conductance Dry coating thickness (in.) 0.004 0.008 Mean coated disc wt. (g) 25.967 26.123 Refractory penetration (sand grains) 5 2 Refractory Coating DE Low Thermal Conductance Dry coating thickness (in.) 0.004 0.008 Mean coated disc wt. (g) 26.151 26.260 Refractory penetration (sand grains) 5 2 Refractory Coating Control (Uncoated) Dry coating thickness (in.) 0 Mean coated disc wt. (g) 25.616 Refractory penetration (sand grains) 0 Table 4. Physical and Thermo-Mechanical Properties of the Refractory Coated and Uncoated Discs Systems. System Initial Temp Lowest Recorded [degrees]C([T.sub.0]) Temp [degrees]C(T1)) [G.sub.0.004] 1,288 1,117 [G.sub.0.008] 1,288 1,134 [DE.sub.0.004] 1,288 1,234 [DE.sub.0.008] 1,288 1,209 Control 1,288 1,151 System Change in Temp Time to Lowest [degrees]C([DELTA]T) Temp (sec., [DELTA]t) [G.sub.0.004] -171 19 [G.sub.0.008] -154 19 [DE.sub.0.004] -54 26 [DE.sub.0.008] -79 30 Control -137 18 System [T.sub.transfer] Thermal Distortion ([degrees]C/sec. Range (in.) [G.sub.0.004] -9 0 [G.sub.0.008] -8.11 0.01 [DE.sub.0.004] -2.08 0.02 [DE.sub.0.008] -2.04 0 Control -7.61 0