Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
  • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
  • B22C1/165—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents in the manufacture of multilayered shell moulds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C23/00—Tools; Devices not mentioned before for moulding
  • B22C23/02—Devices for coating moulds or cores
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/02—Sand moulds or like moulds for shaped castings
  • B22C9/04—Use of lost patterns

Definitions

• This invention relates to a process for the production of metal castings, in particular to a process of metal casting employing ceramic shell moulds.
• ferrous metals for example plain carbon steels, low-alloy steels and many ferritic and martensitic stainless steels
• the relatively porous structure of a ceramic shell mould can permit oxidation of the metal adjacent to the shell wall to occur.
• the nature of the resulting changes in the metal surface vary according to the conditions and the particular metal concerned, but they can be such that the appearance and general quality of the castings are adversely affected.
• the ceramic shell mould should be located in a non-oxidizing atmosphere during the pouring and cooling of the metal. It has also been proposed to surround the mould with carbon at a sufliciently high temperature to combine with the oxygen in the vicinity of the mould, thus preventing its access to the metal.
• the casting and shell are allowed to cool under these conditions to a temperature below the minimum at which exposure to air would have an adverse effect.
• the shell mould should be surrounded by the carbon before the metal is poured, but if this cannot be arranged, immersion in the carbon should be effected as soon as possible, for example during casting or immediately on its completion.
• Active carbons are carbons characterized by high porosity and correspondingly high surface areas.
• the production of active carbons usually involves a first stage in which the raw material, for example bone, wood, peat of nut shells, is carbonised, normally by heating in the absence of air.
• the second stage the resulting char is subjected to an activation process.
• suitable gases For example steam or carbon dioxide can be used at temperatures of 800900 C., or air can be used at 300-600 C.
• the oxidizing gases remove residual hydrocarbons and other volatile material and cause an erosion of the carbon surface.
• Active carbons generally have specific suface areas of at least square metres per gram, and for use in the present invention, active carbons having specific surface areas of at least 500 square metres per gram, and more especially in the range 1000-1600 square metres per gram are preferred. '(The surafce area is usually determined by gas absorption techniques based on the procedure of Brunauer, Emmett and Teller). A further preferred feature is that the active carbon should have a residual volatile content not exceeding 5% by weight. Active carbons derived from charcoals of vegetable origin have given particularly good results in the present process, especially an active carbon produced by the pyrolysis of coconut shells.
• substantially all the particles should pass through a 16 B.S.S. (British Standard Sieve) mesh, and more preferably substantially all should pass through a 30 B.S.S. mesh.
• material that contains any substantial proportion of particles passing a 200 B.S.S. mesh is rather dusty, and while effective for the process of the invention is inconvenient to handle, and tends to generate dirty working conditions. It is therefore preferred to use carbon substantially all of which is retained by B.S.S. mesh.
• the active carbons are available in various grades corresponding to different ranges of particles size; grades having particles size ranges of -30+80 and -52+l20 B.S.S. mesh have been used very successfully. A grade having a particle size range of 16+60 B.S.S. mesh has also been shown to be satisfactory.
• Mixtures of active carbon and refractory solids in particulate form can be used to surround the mould in the process of the present invention.
• Such mixtures are effective when containing as little as 5% by weight of the active carbon, although the minimum which it is possible to use in any. particular instance will depend on a variety of factors including in particular the susceptibility of the metal to oxidation.
• mixtures can be selected to give faster rates of cooling than can be achieved using carbon alone. This may be desirable if thick sections of metal are being cast and if the metal is such that a slow rate of cooling is associated with the development of surface porosity.
• While the process of the invention can be used for production of castings of high chromium steels, its particular advantages lies in the fact that it permits satisfactory casting of plain low-carbon steels, for example BS.1617A, BS3146 and CLA.9; of low alloy steels, for example those of the Fortiweld type and type EN36C; and high-carbon, high-alloy, tool steels generally containing around 12% Cr, and other alloys which have poor high-temperature scaling resistance.
• plain low-carbon steels for example BS.1617A, BS3146 and CLA.9
• low alloy steels for example those of the Fortiweld type and type EN36C
• high-carbon, high-alloy, tool steels generally containing around 12% Cr, and other alloys which have poor high-temperature scaling resistance.
• the ceramic shell mould for use in the process of the present invention will have been produced by conventional means.
• the normal process for the production of a ceramic shell mould involves the preparation of a pattern in wax or other material that is expendable; building up round the pattern a shell of refractory material by applying a number of coatings of a slurry made of powdered refractory in a liquid binding agent (such as one derived from ethyl silicate), usually with intermediate stucco coatings of a somewhat coarser refractory, and subjecting the dried assembly to a process such that the pattern is removed, for instance melted out. The shell is then fired.
• a liquid binding agent such as one derived from ethyl silicate
• the ceramic shell mould is preferably immersed in the active carbon immediately after withdrawal of the mould from the firing furnace, and usually the metal is poured into the mould without delay.
• the mould can be placed in a static bed of active carbon or particulate material including active carbon, but the use of a fluidized bed is usually preferred.
• a ceramic shell mould was produced by applying successive coatings of a slurry of sillimanite of particle size less than 200 B.S.S. mesh in hydrolyzed ethyl silicate solution and a stucco of sillimanite of particles size -40+80 B.S.S. mesh, to an assembly of wax patterns, each slurry coating being allowed to set before applying the next. A total of six slurry coatings was applied, and after the final coating had set the assembly was dried in a stream of warm air until excess alcohol and water had been removed.
• the wax assembly was then removed from the mould by treatment in a steam autoclave, after which the mould was prepared for casting in the usual manner by firing in a furnace at 1050 C. for one hour.
• the hot mould was withdrawn from the firing furnace and immediately placed in a mild steel cylindrical flask and embedded in activated carbon to within approximately one centimetre of the rim of the pouring cup.
• the activated carbon was material produced by the pyrolysis of coconut shells and commercially available under the name Ultrasorb. Its particle size distribution was as follows:
• the resultant castings were found to have an excellent smooth blemish-free surface following a light shot-blasting operation to remove remnants of mould material adhering after the knock-out operation.
• the surfaces of the castings were marred by pitting characteristic of cast ferritic stainless steel.
• EXAMPLE 4 This example describes an experiment providing a comparison between the use of active carbon to surround a ceramic shell mould according to the invention, and the use of a charcoal for the same purpose.
• a wax assembly was made consisting of two one-inch square bars of wax eight inches long joined at one end to a common runner-bar to the centre of which a conventional wax pouring cup was attached.
• the one-inch bars were parallel and separated by a distance of four inches.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Mold Materials And Core Materials (AREA)
• Compositions Of Oxide Ceramics (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)
• Carbon And Carbon Compounds (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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Citations (5)

• 1966
  • 1966-01-17 GB GB2116/66A patent/GB1132361A/en not_active Expired
• 1967
  • 1967-01-06 NL NL676700277A patent/NL150030B/xx unknown
  • 1967-01-11 SE SE419/67A patent/SE315366B/xx unknown
  • 1967-01-11 US US608496A patent/US3474851A/en not_active Expired - Lifetime
  • 1967-01-12 DE DE1558268A patent/DE1558268C3/de not_active Expired
  • 1967-01-16 FR FR91247A patent/FR1507960A/fr not_active Expired
  • 1967-01-17 CH CH64967A patent/CH454371A/de unknown
  • 1967-09-20 GB GB42787/67A patent/GB1184908A/en not_active Expired
• 1968
  • 1968-09-13 IL IL30699A patent/IL30699A/xx unknown
  • 1968-09-16 FR FR166329A patent/FR95744E/fr not_active Expired
  • 1968-09-17 CH CH1388568A patent/CH467640A/de unknown

Patent Citations (5)

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