The present disclosure relates generally to casting, and more particularly, to a lost foam casting of a low alloy steel having carbon content in a range from about 0.1 to about 0.4 percent.
Generally, sand casting requires a plurality of cores for casting complex structure such as turbine shells, turbochargers, crankcases, blowers and the like. The usage of plurality of cores increases material and labor cost, and may also result in long lead time in casting.
Lost foam casting may be used to address the problems related to cost and lead time. However, the casting obtained through the lost foam casting may have excessive carbon content. Further, the lost foam casting uses green bonded sand as backup medium within a sand casing, which may produce gaseous product or bubbles when a molten metal is poured into the mold, thereby entrapping the gaseous product within the casting. The carbon pickup and gas entrapment in the lost foam steel casting are caused due to incomplete foam removal before the molten metal solidifies within the mold. The retained foam generates carbon black and the entrapped gases redistributed inside the casting causes generation of higher local carbon content than the required limit.
Further, the molten metal poured in the mold may also react with the green bonded sand resulting in the fusion of the sand to the casting, thereby creating sand burns which may degrade the surface of the casting. The process of removal of the sand burns from the casting may further add to the process cost.
Thus, there is a need for an enhanced casting process for producing a low alloy steel having a very low carbon content.
In accordance with one exemplary embodiment, a method of casting a low alloy steel is disclosed. The method includes receiving a mold having a foam pattern provided with a permeable refractory coating. The foam pattern is disposed within a sand casing and compacted sand is disposed between the foam pattern and the sand casing. The method further includes pouring a molten metal including a low alloy steel having a carbon content in a range from about 0.1 to about 0.4 percent, into the mold so as to vaporize the foam pattern to form a low alloy steel casting. Further, the method includes removing a gasification product through the permeable refractory coating during the casting process. The method further includes removing the low alloy steel casting from the mold.
In accordance with another exemplary embodiment, a mold is disclosed. The mold includes a sand casing filled with compacted sand. Further, the mold includes a foam pattern having a cavity, disposed in the sand casing such that the compacted sand is disposed between the foam pattern and the sand casing. The foam pattern includes a permeable refractory coating having a permeability in a range from about 10 to about 100 μm2 and a permeance in a range from about 2000 to about 24000 μm3. The compacted sand has a permeability in a range from about 100 to about 1000 μm2. The foam pattern has a bulk density in a range from about 13 to about 28 kg/m3 and a surface density in a range from about 13 to about 35 kg/m3.
In accordance with yet another exemplary embodiment, a method of manufacturing a mold and casting a low alloy steel using the mold is disclosed. The method includes forming a foam pattern having a cavity and applying a permeable refractory coating on the foam pattern. Further, the method includes disposing the foam pattern within a sand casing and filling unbonded sand between the foam pattern and the sand casing. The method further includes compacting the unbonded sand to form compacted sand so as to generate the mold. Further, the method includes pouring a molten metal into the mold to vaporize the foam pattern so as to form the low alloy steel casting. The method further includes removing a gasification product through the permeable refractory coating during casting. The molten metal includes the low alloy steel having a carbon content in a range from about 0.1 to about 0.4 percent. Further, the method includes removing the low alloy steel casting from the mold.
These and other features and aspects of embodiments of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
While only certain features of embodiments have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as falling within the spirit of the invention.
Embodiments discussed herein disclose a method of casting a low alloy steel. More particularly, certain embodiments disclose receiving a mold having a foam pattern disposed between compacted sand and a sand casing. Further, the method includes pouring a molten metal of low alloy steel into the mold so as to vaporize the foam pattern to form a low alloy steel casting. The method further includes removing the low alloy steel casting from the mold.
More particularly, certain embodiments disclose a method of manufacturing a mold. The method includes forming a foam pattern having a cavity and applying a permeable refractory coating on the foam pattern. Further, the method includes disposing the foam pattern within a sand casing and filling unbonded sand between the foam pattern and the sand casing, to form the mold. Further, the method includes compacting the unbonded sand to form compacted sand within the mold.
The foam material includes at least one of a polystyrene, a polymethylmethacrylate, and a polystyrene and polymethylmethacrylate copolymer material. In one embodiment, the process of forming the foam pattern 104 may include the step of injecting pre-expanded beads of the foam material into a preheated mold (not shown in
In the illustrated embodiment, the foam pattern 104 has three legs 104a, 104b, 104c and a body 104d connecting the legs 104a-104c. The foam pattern 104 shown in the embodiment is for illustration purpose only and should not be construed as a limitation of the invention.
The method 100 further includes a step 106 of forming a plurality of venting ports 108a in the foam pattern 104. Each venting port 108a removes a gasification product from the foam pattern 104 during a casting process. The method 100 further includes a step 110 of applying a permeable refractory coating 112 on the foam pattern 104. The step 110 further includes a step of preparing a permeable refractory coating material 114 having a predefined rheology. The permeable refractory coating material 114 includes an inorganic binder and a back bond material including alumina and/or zircon.
In one embodiment, the permeable refractory coating 112 is applied on the foam pattern 104 by dipping process or flow-coating process. The dipping process may include dipping the foam pattern 104 in a container (not shown in
The permeable refractory coating 112 has a permeability in a range from about 10 to about 100 μm2 and a permeance in a range from about 2000 to about 24000 μm2. Permeability may be defined as an ability of the coating 112 to allow the gasification product to flow through the permeable refractory coating 112. Permeance may be defined as a product of permeability and thickness of the permeable refractory coating 112. The permeable refractory coating 112 having the permeability in the aforementioned range enables preventing metal penetration to obtain a desired surface finish of a low alloy steel casting (as shown in
The method 100 further includes a step 118 of disposing the foam pattern 104 within a sand casing 120 and filling unbonded sand 122 between the foam pattern 104 and the sand casing 120, to form a mold 124. In some embodiments, the sand casing 120 may include two halves which are clamped together to form the mold 124. The foam pattern 104 may be held within the sand casing 120 via a plurality of supports 126 so as to provide structural support and stability to the foam pattern 104. Further, a pouring basin 128, runner 130, and a riser 132 are coupled to the foam pattern 104. A molten metal is fed sequentially via the basin 128, the riser 132, and the runner 130 to the foam pattern 104. The mold 124 also includes a plurality of venting ports 108b extending from the foam pattern 104 to the atmosphere through the unbonded sand 122. The plurality of venting ports 108b is used to remove the gasification product from the foam pattern 104 during casting process. In one embodiment, the plurality of venting ports 108b is made of ceramic material. In the illustrated embodiment, the plurality of venting ports 108b are disposed downstream of the foam pattern 104 so as to enhance venting of the gasification product.
The method 100 further includes a step 134 of compacting the unbonded sand 122 disposed between the foam pattern 104 and the sand casing 120 to form a compacted sand 136. The compacting of the unbonded sand 122 is performed using a compaction device 138. In one embodiment, the compaction device 138 applies vibration of variable frequency and amplitude to the unbonded sand 122 so as to form the compacted sand 136. In another embodiment, the compaction device 138 applies vacuum force to the unbonded sand 122 to form the compacted sand 136. The compacted sand 136 has a permeability in a range from about 100 to about 2000 μm2. The permeability of the compacted sand 136 in the aforementioned range enables controlling of integrity of the low alloy steel casting dimension and rate of removal of the gasification product from the foam pattern 104. The compacted sand 136 provides structural stability to the foam pattern 104 during the casting process. Further, the compacted sand 136 of the embodiment is dry in nature and does not contain binders or additives for binding and supporting the foam pattern 104.
The method 140 includes a step 142 of pouring a molten metal 144 into the mold 124 via the basin 128, the runner 130, and the riser 132. The molten metal 144 may be stored at high temperature and then poured from a ladle 143 to the mold 124. The molten metal 144 includes a low alloy steel having a carbon content in a range from about 0.1 to about 0.4 percent. In one embodiment, the molten metal 144 has a temperature in a range from about 2900 to about 3100 degrees Fahrenheit. Further, the molten metal 144 is fed at a rate from about 0.04 to about 0.8 kg/sec/cm2. The feeding rate of the molten metal 144 in the aforementioned range enables complete removal of the foam pattern 104 from the mold 124 and also diligent removal of the gasification products 148 from the foam pattern 104. The temperature of the molten metal 144 in the aforementioned range enables complete vaporization of the foam pattern 104.
In one embodiment, the molten metal 144 at a temperature range from about 3000 to about 3100 degrees Fahrenheit is fed at a rate in a range from about 0.1 to about 0.8 kg/sec/cm2 into a cavity 146 of the foam pattern 104. In such an embodiment, the foam pattern 104 includes a polystyrene and polymethylmethacrylate copolymer material having a bulk density in a range from about 16 to about 28 kg/m3. In another embodiment, the molten metal 144 at a temperature range from about 2950 to about 3000 degrees Fahrenheit, is fed at a rate in a range from about 0.1 to about 0.3 kg/sec/cm2 into the cavity 146 of the foam pattern 104. In such an embodiment, the foam pattern 104 includes a polystyrene material having a bulk density in a range from about 14 to about 20 kg/m3. In yet another embodiment, the molten metal 144 at a temperature range from about 2900 to about 2950 degrees Fahrenheit, is fed at a rate in a range from about 0.04 to about 0.2 kg/sec/cm2 into the cavity 146 of the foam pattern 104. In such an embodiment, the foam pattern 104 includes a polymethylmethacrylate material having a bulk density in a range from about 13 to about 18 kg/m3.
The molten metal 144 vaporizes the foam pattern 104 and forms a gasification product 148. The gasification product 148 is removed through the permeable refractory coating 112 and the plurality of venting ports 108a, 108b. The permeable refractory coating 112 also prevents reaction of the molten metal 144 with the compacted sand 136 so as to avoid formation of sand burns. The method 140 further includes a step 150 of removing a low alloy steel casting 152 from the mold 124. At step 154, the low alloy steel casting 152 having a carbon content in the range from about 0.1 to about 0.4 percent and having a shape of the foam pattern 104 is obtained. The low alloy steel casting further has a carbon pick-up in a range from about 0.12 to about 0.16 percent, a surface defect (for example, sand burns) of less than 1 percent, and a gas entrapment of less than zero percent.
The exemplary lost foam casting process discussed herein provides required machined dimensions due to the elimination of a pattern draft angle, parting lines, and the ability to have dimensional tolerances. The utilization of unbonded dry sand reduces generation of gases and reaction with the molten metal having the carbon content in the range from about 0.1 to about 0.4 percent, resulting in formation of a casting having relatively reduced sand burns and entrapped gases within the casting. The type of foam material, flow rate and the temperature at which the molten metal is poured into the mold results in complete removal of the foam pattern from the mold resulting in formation of the casting having a reduced carbon content or pickup.
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