Lost foam processes for casting ferrous metals

Abstract: A lost foam casting process for forming low carbon ferrous metal (such as stainless steel) parts (18) utilizes a high vacuum (67 to 98 kPa) during the molten metal pouring stage of the lost foam process. The high vacuum speeds up the pouring operation and also draws off undesired volatile impurities such as carbon.

Description: The present invention relates to lost foam processes for casting ferrous metals such as stainless steel. Casting processes using lost foam are known and a description of such a process may be found in US Patent No. US-A-2 830 343 (H.F. Shroyer). This casting process utilizes a cavity-less casting method wherein a polystyrene foam pattern is embedded in sand. The foam pattern left in the sand is decomposed by molten metal that is poured into the foam pattern. The molten metal replaces the foam pattern thereby precisely duplicating all of the features of the pattern. In a similar manner to investment casting using lost wax, the pattern is destroyed during the pouring process and a new pattern must be produced for every casting made. The process thus utilizes the following basic steps. Firstly, a foam pattern and gating system is made using some sort of mould. Secondly, the mould or foam pattern and gating system are usually assembled into a cluster of individual parts to facilitate large volume production. The cluster is then coated with a permeable refractory coating. The prepared cluster is then placed into loose unbonded sand that is packed around the foam cluster by vibrating the entire mould assembly. The molten metal is then poured directly into the foam cluster decomposing the foam in the cluster and replacing it with the poured metal. The cluster is then removed, separated and the individual parts finished off by well-known methods. The previously-described lost foam process has been used to produce gray iron and non-ferrous material parts. To date, it has been impractical to pour certain ferrous metals such as stainless steel utilizing the above procedure. The stainless steel molten metal generates carbon when it is volatilized and the carbon is absorbed into the liquid metal thereby raising the carbon level of the finished stainless steel product. Certain applications for stainless steel have ASTM Standards for carbon content that are within the ranges of 0.06% to 0.08% carbon. One such application for such stainless steel parts that have to be made according to this ASTM Standard is the production of tube hangers for nuclear reactors which require the parts to be produced from ASTM grade material A-297HH. Attempts have been made to manufacture these tube hangers from stainless steel according to the above-described lost foam process, with unsatisfactory results. The sand surrounding the foam forms was even subjected to vacuums between 4 and 12 in. of mercury (13.5 and 40.5 kPa) applied to the flask holding the sand and the parts to maintain the sand around the part. Even using these vacuum ranges, which are suggested in the prior art to maintain process integrity, the results were unsatisfactory. Thus, it will be seen that a need existed for a lost foam process for manufacturing ferrous metal (such as stainless steel) parts which require a low carbon level according to application standards, such as those set by ASTM. US Patent No. US-A-3 842 899 discloses a lost foam casting process in which the mould chamber is formed with a perforated support for the mould-forming sand, and which is mounted on a turntable with other mould chambers for stepping through various stations. Means are provided for pressurising the mould chamber and for alternately subjecting the chamber to suction. According to the present invention there is provided a lost foam process for casting low carbon ferrous metal parts, the process comprising the steps of: forming a sand filled chamber having a plastic form of the part contained therein; covering the top of the sand filled chamber; applying a vacuum to the chamber; and pouring molten ferrous metal into the plastic form area to replace the plastic form; characterised in that: the vacuum applied to the chamber is in the range of 67 to 98 kPa; and the temperature of the poured molten ferrous metal is in the range of 1,350°C to 1,595°C ± 15°C; whereby any generated carbon is vacuum extracted to produce a low carbon ferrous metal version of the part. A preferred embodiment of the invention overcomes the problems associated with the previously-proposed lost foam production methods as well as others by providing a lost foam process that is able to manufacture stainless steel parts with minimal or very low carbon content. The preferred process utilizes a high vacuum applied to the apparatus during the pouring of the stainless steel at a predetermined volume and temperature to allow the carbon generated during this moulding process to be vacuum extracted, resulting in low carbon stainless steel parts. Accordingly, the preferred lost foam casting process will draw off any undesired volatile elements formed during the pouring process. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a perspective view of lost foam apparatus utilized in the present process; and Figure 2 is a schematic end view of the apparatus shown in Figure 1. In the present method as illustrated in the drawings, high alloy low carbon stainless steel boiler tube hangers are manufactured according to ASTM Standard A-297HH. The tube hanger shapes are first made from plastics foam shaped material, such as poly-methyl-methylacrilate (PMMA) available from Dow Chemical Company. These boiler tube hanger shapes are assembled into castable quantity assemblies typically consisting of 84 boiler tube hangers spacedly formed from a connecting element. The boiler tube hanger assemblies are then spray coated with a refractory coat of alumino-silicate approximately 4 mils (0.1 mm) in thickness. The coated assemblies are then allowed to dry for approximately 12 hours at a temperature of about 120°F (50°C) after which time the tube hanger assemblies are ready to be utilized in the vacuum foam process apparatus. The apparatus shown in the drawings is a standard lost foam type of apparatus wherein an open container or chamber 10 has a bottom film 12 consisting of a 5 mil (0.13 mm) thick film of ethylene vinyl acetate (EVA). A bottom chamber 14 located below the open container 10 and separated therefrom by the film 12 subjects the film 12 to a vacuum of approximately 18 in. of mercury (60 kPa) obtained by drawing the vacuum through an aperture 16 in the bottom chamber 14. The open container 10 is approximately 15 to 20 feet (4.5 to 6.1 m) square and is approximately 4 to 7 feet (1.2 to 2.1 m) high. The open container 10 is next filled with an approximately one inch (25 mm) layer of sand. Typically, two different types of sand may be used. One type is sand that has a nominal American Foundry Society (AFS) grain fineness number of 90 - 100 with a dry permeability of approximately 65. Another type is sand that has an AFS number of 34 - 38 and a dry permeability of 450 - 525. Different types of washes for these sands were evaluated with a proven wash developed for use in automotive engine plants for producing gray iron engine components using known lost foam processes. Next, four sets 18 of boiler tube hanger assemblies each consisting of 84 boiler tube hangers were placed into the open container 10 with each of the sets 18 being connected together by known gating systems 20, and a pour opening 22 provided for pouring the molten stainless steel into the sets 18 of gated tube hanger assemblies. The open container 10 was filled with loose dry sand of the type previously discussed; since the moulds are relatively delicate a controlled sand filling from a controlled hopper (not shown) is done to prevent mould destruction and/or individual tube hanger breakage. The open container 10 is filled with sand to a level 24 which will cover the tube hanger assembly sets 18. The filled open container 10 is then vibrated to densify the entire sand bed. The previous steps were undertaken with the application of a vacuum of approximately 18 in. of mercury (60 kPa) applied to the lower chamber 14 separated from the container 10 by the film 12. Next, the open container 10 is covered with a top film 26 of the same 5 mil (0.13 mm) thickness EVA material as the film 12, and a vacuum of approximately 22 in. of mercury (74 kPa) is applied to the container 10 through three 2 in (50 mm) vacuum hose lines connected to openings 28, 30, 32. These three vacuum lines draw approximately 500 CFM (0.25 m 3 /s) and during a pour will draw approximately 1500 CFM (0.75 m 3 /s) at a working vacuum range of approximately 20 to 29 in. of mercury (67 to 98 kPa). The molten stainless steel is then poured into the mould assemblies 18 by way of the pour opening 22 extending through the top film 26 to the assemblies 18. The molten stainless steel is typically poured at a temperature of approximately 2,450°F (1,350°C). An analysis of the required pour temperatures was conducted and using the standard alloy depressant factors on solidus/liquidus of multi-alloy steels, an average liquidus was calculated to be 2,650°F to 2,675°F (1,455°C to 1,470°C). On this basis, the desired pour temperature was selected at 2,875°F plus or minus 25°F (1,580°C plus or minus 15°C). The mould pouring was timed with an average pour time of 18 to 22 seconds for the large four tube hanger pattern assemblies 18 being placed in the chamber 10 and an average pour time of 12 to 18 seconds for smaller numbers of tube hanger patterns/moulds. This calculated out to a metal delivery rate of approximately 78 to 64 pounds per second (35 to 29 kg/s) and 74 to 50 pounds per second (34 to 23 kg/s) respectively. An effort was made to reduce pour times by raising the temperature of the poured molten stainless steel to 2,900°F plus or minus 25°F (1,595°C plus or minus 15°C). The increased temperature showed a corresponding decrease in the pour times and a lower incidence of misruns. A large pour which consisted of approximately 1400 pounds (635 kg) of molten stainless steel took an average pour time of 10 to 14 seconds as opposed to the 18 to 22 second pour time at the lower molten metal temperature. The average pour rate was thus increased from the 78 to 64 pounds per second (35 to 29 kg/s) range to a range of 140 to 100 pounds per second (63 to 45 kg/s) at the elevated molten metal temperature. As was discussed earlier, all of these pours were done at a vacuum of approximately 20 to 29 inches of mercury (67 to 98 kPa) applied to the container 10 with no vacuum being applied to the lower chamber 14 during the pouring process. It is hypothesized that the high vacuum applied to the container 10 during the pouring of the stainless steel not only helps the pour of the molten metal by drawing the molten metal into the mould assemblies 18 but also allows the evacuation of carbon fumes from the container 10 during the pouring process. It was noted during one of the tests that whereas approximately 1400 pounds (635 kg) of metal was poured into the mould assemblies 18 within a time period of ten seconds under the application of the high vacuum, the same amount of molten metal required approximately 25 to 30 seconds to be poured into the moulds 18 without the application of any vacuum. The castings produced from the high vacuum lost foam process were analyzed and showed minimal to no carbon pickup. Differential metallography from the surface showed a worst case of 0.03% carbon pickup and a best case of slight decarburization. By way of contrast, samples made from regular lost foam processes without the use of high vacuum during the pour showed significantly higher levels of carbon pickup with a worst case of 0.23% and a best case of 0.09%. As was discussed earlier, carbon pickup is a significant problem in stainless steel applications such as boiler hangers since high levels of carbon affect subsequent attachment welds for these hangers and the hangers must be produced according to the ASTM Standards which require a low carbon content for the stainless steel.

Claims: A lost foam process for casting low carbon ferrous metal parts, the process comprising the steps of: forming a sand filled chamber (10) having a plastic form (18) of the part contained therein; covering (26) the top of the sand filled chamber (10); applying (28, 30, 32) a vacuum to the chamber (10); and pouring molten ferrous metal into the plastic form area to replace the plastic form (18); characterised in that: the vacuum applied to the chamber (10) is in the range of 67 to 98 kPa; and the temperature of the poured molten ferrous metal is in the range of 1,350°C to 1,595°C ± 15°C; whereby any generated carbon is vacuum extracted to produce a low carbon ferrous metal version of the part. A process according to claim 1, wherein the molten ferrous metal is poured at a temperature of 1,595 ± 15°C. A process according to claim 1 or claim 2, wherein the vacuum is applied to three sides of the chamber (10) at a flow volume of 0.25 m 3 /s. A process according to any one of the preceding claims, wherein the chamber (10) is covered with a plastics film (26) approximately 0.13 mm thick. A process according to any one of the preceding claims, wherein the plastic form (18) of the part is manufactured from poly-methyl-methylacrilate. A process according to any one of the preceding claims, wherein the plastic form (18) of the part is coated with alumino-silicate. A process according to claim 6, wherein the coated plastic form (18) of the part is dried for a period of approximately 12 hours at a temperature of approximately 50°C. A process according to any one of the preceding claims, wherein the plastic form (18) of the part is made as an assembly of a plurality of boiler tube hangers. A process according to any one of the preceding claims, wherein the sand filled chamber (10) is formed with a bottom plastics film floor (12) to which a vacuum of approximately 60 kPa is applied. A process according to claim 9, wherein the vacuum applied to the film floor (12) is relieved when the chamber vacuum is applied. A process according to any one of claims 1 to 10, wherein the sand filled chamber (10) is filled with sand having a grain fineness of 34 - 38 and a dry permeability of 450 - 525. A process according to any one of claims 1 to 10, wherein the sand filled chamber (10) is filled with sand having a grain fineness of 90 - 100 and a dry permeability of approximately 65.

Inventors: HAND BRYAN, HAND, BRYAN
Applicants: BABCOCK & WILCOX CO, THE BABCOCK & WILCOX COMPANY
Application year: 1993
Main classification: B22C9
IPC codes: B22C9, B22D27