US3474851A - Processes for casting molten metal in active carbon coated ceramic shell moulds - Google Patents

Description

United States Patent Office Patented Oct. 28, 1969 US. Cl. 164-23 Claims ABSTRACT OF THE DISCLOSURE The process of the disclosure is one for the production of a metal casting, in which molten metal is poured into a ceramic shell mould, and the casting and shell are allowed to cool while surrounded by active carbon in particulate form.

This invention relates to a process for the production of metal castings, in particular to a process of metal casting employing ceramic shell moulds.

A problem which has for a long time hindered the manufacture of satisfactory castings from certain types of ferrous metals, for example plain carbon steels, low-alloy steels and many ferritic and martensitic stainless steels, stems from the fact that such metals are oxidized at high temperatures. Under normal conditions of operation, 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.

Various proposals have been made for dealing with this problem, for example that 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. While the forms of carbon proposed hitherto, for example graphite and anthracite, in general give reasonably satisfactory results in the casting of the ferritic-martensitic stainless steels containing in the region of 13% by weight of chromium, they do not function satisfactorily with all alloys in this group, and are generally inadequate to prevent surface disfigurati'on in castings of plain lowcarbon steels, certain low-carbon low-alloy steels, and high alloy but non-stainless tool steels such as the German specification X165 Cr Mo V12 (DIN 17,006).

We have now found that certain other forms of carbonaceous material are significantly more effective, giving improved results throughout the range of stainless steels mentioned above, making possible the production of high quality castings from plain low-carbon steels, low-carbon low-alloy steels, and high alloy tool steels, and also making possible the use of thinner ceramic shells which hitherto would have had unacceptably high porosity.

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. Whenever practicable, 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. In the second stage the resulting char is subjected to an activation process. Several such processes have been proposed, and one which is used extensively involves controlled oxidation of the carbon with 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.

In respect of particle size, we prefer to use somewhat finer material than that indicated as suitable where other forms of carbon have been proposed. Preferably, 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. At the lower end of the particle size range, 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. Commercially, 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, for example Molochite, zircon sand or metal ball-shot as used for shot blasting, can be used to surround the mould in the process of the present invention. In certain circumstances 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.

By suitable choice of components, 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.

It is contemplated that 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.

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.

The invention is illustrated by the following examples.

EXAMPLE 1 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:

BS. 481 sieve: Percent 30 rnesh+40 mesh 43 40 mesh+60 mesh 45 60 mesh+80 mesh 80 mesh+l00 mesh 2 A few seconds later a molten steel having the following composition:

was poured into the mould to make the castings, and allowed to cool to below red-heat before stripping from the mould.

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.

4 EXAMPLE 2 Shell moulds were surrounded by activated carbon as described in Example 1 during the casting and cooling of a ferritic stainless steel having the following composition:

Percent C 0.09 Mn 1.48 Si 0.23 Ni 1.31 Cr 12.25 Mo 0.61 V 0.34 Fe Balance The castings had excellent surfaces completely free from pitting.

In a comparative test, the activated carbon was replaced by graphite of the following particles size distribution:

Percent +10 mesh 4.5 +16 mesh 40.0 +20 mesh 46.0 +30 mesh 86.0 +40 mesh 95.0

The surfaces of the castings were marred by pitting characteristic of cast ferritic stainless steel.

EXAMPLE 3 By surrounding shell moulds with activated carbon as described in Example 1, castings having excellent surfaces were obtained from steels of the following compositions:

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.

This design enabled the ceramic shell mould made therefrom to have the two legs embedded in separate containers before casting. In one container Ultrasorb formed the embedding material and in the other container charcoal having a comparable particle size was used.

Metal of the following composition was cast in the mould:

The bar of metal cast in the charcoal bed showed some pitting and a considerable number of blow-hole defects, whereas the bar east in the bed of Ultrasorb had a satisfactory surface.