US3110943A - Production of metal ingots, castings and the like - Google Patents

Description

United States Patent 3,110,943 PRODUCTION OF METAL INGOTS, CASTINGS AND THE LIKE William Kelsey, Sheffield, England, assignor to Inca Steel Company, Limited, Shefiield, England, a company of Great Britain No Drawing. Filed Sept. 30, 1959, Ser. No. 843,372 Claims priority, application Great Britain Oct. 2, 1958 8 Claims. (Cl. 22-192) This invention relates to the production of metal ingots, castings and the like and is particularly concerned with the production of ingots, castings and the like in improved physical condition.

It is well known that, in the casting of molten metal into a mould, and particularly when steel and steel alloys are cast in ingot moulds, segregation is liable to occur around the axis of the resultant solidified ingot. This is due to the fact that the molten metal in the (region of said axis, i.e. towards the middle of the cast metal, is the last to solidify. In high speed tool steels, for example, large crystals and associated segregation of massive carbides occur 'at the core of the resultant ingot and consequently the metal of the core is of undesirable structure. This major segregation occurs in high speed tool steels due to the tendency of the alloying additions (tungsten, molybdenum and chromium) to form massive carbides during solidification, particularly when there is a high percentage of such alloying additions present in a high carbon steel.

Various proposals have been made to overcome this disadvantage in the casting of ingots, with a view to minimizing temperature gradients across the molten metal section and thus to produce a near-level temperature curve throughout the cast metal during the whole cooling period. Thus it has been proposed to preheat the moulds, and in another proposal to effect rapid cooling of the cast metal by cooling the mould, e.g. by casting into copper ingot moulds provided with water-cooled sleeves.

However, even using the aforesaid techniques cooling of the cast metal takes place in a direction inwardly from the mould, i.e. from the outer suriiace of the cast metal adjacent the mould wall, towards the centre of the casting or ingot. No matter how rapid the cooling is effected, a concentration of segregated carbides is always found at the core of the casting or ingot. It is to be noted, however, that the more rapid the cooling the less is the diameter, or cross-sectional dimensions, of the core of segregated crystals.

It is an object of the present invention to provide a method of castings metals, especially high-speed tool steels, which minimizes or prevents the occurrence of the above mentioned undesirable features and which at the same time produces an ingot or casting which has an equiaxed structure and is free from shrinkage cavities. Castings, ingots and the like produced by the method of the present invention have a very high degree of homogeneity and uniformity of physical properties, and objects ultimately produced from such cast metal are more uniform in structure and thus less likely to suffer from weak spots and faults.

According to the present invention there is provided a method for producing from molten metal castings, ingots and the like having when solid an equiaxed crystal structure free from major segregation which comprises pouring 3 ,119,943 Patented Nov. 19, 1963 ice the molten metal into a mould which is itself formed of an exothermic composition or which has a lining within the mould formed of an exothermic composition, which exothermic composition on contact with the molten metal generates by its exothermic reaction sufficient heat to prevent, or substantially prevent, initial chilling of the molten metal and to delay cooling of the metal.

It is found that by the prevention of the initial chilling which normally occurs when molten metal cont-acts the mould walls any tendency for solid metal nucleation at the mould wall surfaces which usually initiates crystallization throughout the mass of molten metal, is prevented and the liquidus state is maintained for an extended period. This maintenance of the liquidus state allows the whole mass of metal to cool substantially uniformly and give to the solidified metal the advantageous equiaxial characteristics referred to above.

The heat supplied at the mould walls must be at least suificient to prevent entirely, or at least almost entirely, any transmission of heat from the molten metal to the mould at the time of pouring and for an extended period thereafter. If incipient solid nucleation should set in initially, the maintenance of the supply of heat remelts the solid nucleates very rapidly.

According to one preferred form of the invention the supply of heat is provided by the location at the mould walls of a composition of which the ingredients react together exothermically when raised to the temperature of the molten metal. Such compositions and others which burn exothermically are referred to for brevity herein simply as exothermic compositions. This exothermic composition may constitute the whole mould but generally it is preferred to provide it as a lining to the mould Walls. In that case, in order to minimize loss of heat from the exothermic composition by transmission to the mould, it is preferred that the mould itself should be made of a heat-insulating material or that there should be a layer of heat-insualting material between the exothermic composition and the mould.

The exothermic composition may be any of those known for use in the sink-heads and risers usually employed on the moulds of ingots and castings. Compositions based on metal oxide, e.g. iron oxide and a reducing agent, e.g. aluminium, of the type known as Thermit compositions, are suitable but are vigorous in action and it is preferred to employ an exothermic composition of the general character of the types sold under the trade mark Feed-ex. Such compositions may consist essentially of an easily oxidisable metal such as aluminium, as powder, granules, cuttings or the like, or as ball mill dust, together with one or more oxidising agents, e.g. alkali metal, nitrates and/ or iron oxide or manganese oxide. The quantity of oxidising agent may be itself insufficient to oxidise completely all the aluminium present, and a small proportion of a fluoride, e.g. 0.1 to 15% by weight of the composition, ,may be included. Suitable fluorides are the alkali metal fluorides, sodium aluminium fluoride, potassium aluminium fluoride, silicofl-uorides and borofluorides. These compositions will usually also contain a filler material, such as sand or grog, and a binding agent, such as sulphite lye, gum arabic or a clay. These compositions burn to leave a coherent residue of high heat-insulating properties.

However, other types of exothermic composition may 3 be employed, eg, those based on carbonaceous material such as charcoal or coke, or sand impregnated with oil, which latter is ignited on contact with the molten metal on application of an oxygen-bearing gas.

Since most types of exothermic composition generate gases (and some may require the injection of oxygencontaining gas, e.g. air, in order that they should burn freely) it is desirable to provide vents through which gas may escape and/or be injected.

According to one method of carrying out the process of the present invention use is made of a mould, the Wall or Walls of which are provided with vent holes, the mould is lined with a layer of heat-insulating, refractory material, e.g. sand, the thickness of which layer can be varied according to the conditions required. This sand lining is provided with transverse vent holes, communicating wit-h the vents in the ingot mould wall, as well as with vertical vents which extend from or near to the base of the lining to the top surface thereof. Superimposed on the face of the sand lining is a lining of exothermic composi tion, preferably an exothermic material which can be rammed in position, the layer of exothermic composition forming the actual mould cavity into which the molten metal is cast. This exothermic lining instead of being formed in situ may be preformed, either as a one piece unit or as segments or sections which are assembled together in the mould.

Whilst generally the sand and exothermic layers will be formed separately, they may be made as an integral multilayer unit, or formed from multilayer sections and seg ments, each multilayer piece comprising a layer of exothermic composition and a layer of sand, or other heatinsulating material, formed and bonded together.

When molten metal is cast into the mould, it ignites the exothermic material which rapidly reaches a high temperature and in so doing provides sufficient hea t to and around the cast metal to prevent initial chilling thereof at the mould Walls and to retard the solidification of the metal. When the exothermic material is completely burnt out, it generally leaves a residue which acts as a very hot insulator; as it keeps hot for a long period it only allows for very slow cooling of the molten metal. The gradual cooling prevents any major segregation at the core of the resultant cast ingot and gives rise to a solid body of metal having a fine equiaxed structure.

The thicknesses of the layers of exothermic composition and heat-insulating material can be varied between wide limits and the thicknesses will depend to some extent on the heat-insulating proper-ties of the latter layer. Thus with a heat-insulating layer having good insulating properties a thinner layer of exothermic composition can be used than when a poorer heat-insulating layer is used. The essential consideration is that there must be sufiicient exothermic composition used to provide a layer of sufiicient thickness and sensitivity to prevent initial chilling of the metal on contact with the mould wall and a combined thickness of exothermic composition and heatinsulating material to cut down the rate of heat transfer from the metal to the mould to a level which will prevent the normal growth of columnar crystals firom the mould wall inwards to the centre of the cast metal.

To prevent heat loss from the upper surface of the poured ingot, or casting, it is useful to provide an insulating cover. This may be a layer of a heat-insulating material or may be an exothermic anti-piping compound of the type commonly used in foundry practice. Such a material is applied to the surface of the metal immediately after pouring is complete. Products which are based on aluminium and oxidising agents and which therefore react exothcrmically have proved very satisfactory for this purpose.

Although the bottom of the mould may be formed only of sand, which is sufficient for many purposes, it is sometimes desirable to line it with exothermic compositions in a way similar to the mould walls. Care must be taken to ensure that any exothermic composition layer is Well supported by a substantial backing layer as the weight of metal it has to carry can be considerable. When no exothermic lining is provided on the bottom of the mould thickness of the bottom layer, e.g. of sand, should be greater than that of the Walls to cut down heat losses from the bottom of the cast metal and to support the metal.

It is generally desirable in the production of castings to achieve as good a surface finish as possible, and es pecially when casting metals such as high speed tool steels (where machining costs are very high) a good as-cast surface is an economic necessity. Accordingly a casting made with a finish which allows it to be used, or passed for further working, in the as-cast condition is very advantageous.

Where the molten metal makes direct contact with the layer of exothermic composition on the walls of the mould the surface finish of the resultant ingot or casting is conditioned by this exothermic material and in practice it is found that the quality of the surface finish depends on the particular exothermic composition used. Thus, ingots and castings made by pouring metal into a mould lined with an exothermic composition usually require surfiace grinding to improve their surface finish.

According to a further aspect of the present invention, where a layer of exothermic composition lines the walls of the mould, or the exothermic composition constitutes the entire mould, the surface of such exothermic composition which constitutes the inner surface of the mould is itself provided with a thin, metal-impervious surface lining for direct contact with the molten metal.

Such a lining can be obtained by painting the surface of the exothermic composition layer with materials such as mica, graphite or cement in "a suitable carrier, e.g. a solution of a resin in a volatile solvent, the resin acting as a binding agent, or by preformi-ng a lining of any of these materials, or of mica, graphite, hardboard, cardboard, paper, glass fibreglass, silica flour or thin metal sheet or any suitable refractory, any of these materials being treated if desired with a fire retardant. An especially good refractory lining can be produced by investment casting, e.g. lost wax or frozen mercury casting, and the subsequent finish obtained on the cast metal when using such a refractory lining is of first quality. A lining produced by this method or similar methods has good dimensional accuracy, extremely good surface finish, and resistance to the attack of molten metal, so that the cast metal body produced is already of the desired dimensions and can be used, or further worked, with only a minimum of cleaning.

This inner lining, formed by any method, may if desired be fonmed first and the exothermic layer moulded around it or the lining can be applied onto the surface of a pre-formed exothermic com-position layer. If the lining is made by investment casting, the exothermic material can be used as the second investment material once the first layer has been formed.

When a lining to improve surface finish is used care should be taken to prevent it acting as a chill when the metal contacts it. To this end it is desirable to use, when such a lining is present an exothermic material having a low ignition temperature so that it will ignite as soon as the metal contacts the thin innermost lining. be arranged that the exothermic material is pre-ignited before the metal is poured although this is generally not preferred.

Although reference is made above to moulds having vented walls it is to be understood that if the exothermic and backing layers are themselves provided with sumcient vertical vents, vents through the metal mould itself are not necessary.

The following example will serve to illustrate the invention:

It may Example Parts by weight Aluminium 23 Iron oxide 5 Sodium nitrate Fluoride 3 /2 Binder 2 Filler refractory 56 /2 The thickness of the layer of exothermic composition was about inch.

Molten high-speed tool steel was poured into the prepared mould and anti-piping compound was placed over the surface of the molten metal.

After allowing the metal to solidify, the resultant cooled ingot was removed and a cross-section taken from each end. These two sections showed under microscopic examination a fine, uniform, equiaxed structure free from the usual major segregation at the centre, there being crystals of uniform size, well dispersed and evenly distributed throughout the section of the ingot.

In a modification of the foreging example the layer of exothermic composition was formed around a thin refractory lining produced by investment casting. It was found that the ingot surfaces were materially improved.

Whilst the present invention is of particular value in the production of ingots, it is to be understood that the method may be applied to the production of any form of casting of metal where a product of the indicated uniform structure is required. It is of particular importance in the production of high-speed tool steels but may be used with metals of varying melting points and chemical compositions, e.g. non-ferrous metals such as aluminium, copper, nickel and the like and ferrous metals and alloys generally.

I claim as my invention:

1. A method of producing, from molten metal, castings and ingots having when solid an equiaxial crystal structure free from major segregation which comprises pouring molten metal into a mould of which at least an inner layer of the whole of the walls and the base thereof is formed of an exothermic composition, which composition on contact with the molten metal generates by its exothermic reaction sufficient heat to prevent, at least substantially, initial chilling of the molten metal and sufiicient heat to delay cooling of the metal, contacting the molten metal with the exothermic composition by said pouring operation thereby initiating the exothermic reaction, and applying to the exposed upper surface of the poured metal a composition selected from the group consisting of exothermic material and a heat-insulating composition which delays cooling of the molten metal.

2. The method of claim 1 wherein the said exothermic composition comprising at least an inner layer of the whole of the walls of the mould and the base thereof is backed by a layer of heat-insulating material.

3. The method of claim 1 wherein the said exothermic composition comprising at least an inner layer of the whole of the walls of the mould and the base thereof is formed of an aluminothermic composition.

4. The method of claim 1 wherein the said exothermic composition comprising at least an inner layer of the Whole of the walls of the mould and the base thereof has a basis of carbonaceous material.

5. The method of claim 1 wherein the said exothermic composition comprising at least an inner layer of the whole of the walls of the mould and the base thereof has a basis of a refractory material impregnated with oil.

6. The method of claim 1 wherein the exothermic composition comprising at least an inner layer of the whole of the walls of the mould and the base thereof has a smooth facing layer of a metal-impermeable material.

7. The method of claim 6 wherein said smooth facing layer is of refractory material.

8. The method of claim 6 wherein said smooth facing layer is a thin metal sheet.

References Cited in the file of this patent UNITED STATES PATENTS 1,793,672 Bridgman Feb. 24, 1931 1,923,769 Wood Aug. 22, 1933 2,045,576 Bedilion June 30, 1936 2,160,645 Cooper May 30, 1939 2,631,344 Kennedy Mar. 17, 1953 2,679,080 Olsen May 25, 1954 2,791,816 Pletsch et al May 14, 1957 2,951,272 Kiesler Sept. 6, 1960 FOREIGN PATENTS 652,022 Great Britain Apr. 11, 1951 661,727 Great Britain Nov. 28, 1951 743,635 Great Britain Ian. 18, 1956 743,634 Great Britain Jan. 18, 1956 743,636 Great Britain Jan. 18, 1956

Claims (1)

1. A METHOD OF PRODUCING, FROM MOLTEN METAL, CASTINGS AND INGOTS HAVING WHEN SOLID AN EQUIAXIAL CRYSTAL STRUCTURE FREE FROM MAJOR SERGERATION WHICH COMPRISES POURING MOLTEN METAL INTO AMOULD OF WHICH AT LEAST AN INNER LAYER OF THE WHOLE OF THE WALLS AND THE BASE THEREOF IS FORMED OF AN EXOTHERMIC COMPOSITION, WHICH COMPOSITION ON CONTACT WITH THE MOLTEN METAL GENERATES BY ITS EXOTHERMIC REACTION SUFFICIENT HEAT TO PREVENT, AT LEAST SUBSTANTIALLY, INITIAL CHILLING OF THE MOLTEN METAL AND SUFFICIENT HEAT TO DELAY COOLING OF THE METAL, CONTACTING THE MOLTEN METAL WITH THE EXOTHERMIC COMPOSITION BY SAID POURING OPERATION THEREBY INITIATING THE EXOTHERMIC REACTION, AND APPLYING TO THE EXPOSED UPPER SURFACE OF THE POURED METAL A COMPOSITION SELECTED FROM THE GROUP CONSISTING OF EXOTHERMIC MATERIAL AND A HEAT-INSULATING COMPOSITION WHICH DELAYS COOLING OF THE MOLTEN METAL.