US3440112A - Composition for protective treatment of metals - Google Patents

Description

US. Cl. 148-22 2 Claims ABSTRACT OF THE DISCLOSURE A protective composition for applying to the surface of slabs, billets or other articles of metal which are to undergo heat treatment which comprises 950% by weight silicon carbide, 660% by weight ferrosilicon, and 8-50% by weightsilica flour.

This invention relates to the treatment of metals and moreparticularly to means for preventing or reducing oxidation of the surface of metal objects such as billets slabs and castings during heat treatment operations applied thereto.

It is commonplace in the metal-working industry that metals are subjected to various heating operations, eg, for annealing and/ or stress relieving or in conjunction with hot working-operations. During such heating the surfaces of the metal are especially liable to suffer the effects of oxidation phenomena, e.g. the formation of a layer of the metal oxide on the surface of the metal object and/ or the selective oxidation of an alloying constituent. The latter phenomenon is particularly exemplified by the surface decarburisation of steels or other ferrous alloys. The effects may be exhibited by the formation of an oxide scale or, in the case of decarburisation by the creation of a surface layer which has changed chemical and physical characteristics and which subsequently must, in consequence, be machined away. Metal losses incurred in these ways can be severe and, therefore, very costly. Thus, for example, a steel billet or slab, prior to rolling into rail or sheet, is is commonly placed in a so-called heat-treating furnace in which it is uniformly heated. The furnace must of course be opened to remove the billet or slab. Oxidation and possibly decarburisation there takes place and the loss of metal from such causes may be as high as 3% by weight of the metal. It is, therefore, of prime importance that some means should be found of shielding the metal surfaces from an oxidising atmosphere during any heat-treating operation.

It is well known that, in order to reduce these oxidation losses, small casting may be subjected to their necessary heat treatment in a controlled inert or non-oxidising atmosphere. Although this expedient has been fairly successful in many instances, and indeed, is widely practiced, there are certain serious objections to its general application. An obvious non-oxidising atmosphere for use is an atmosphere of nitrogen but this may give undesirable side effects e.g. nitriding of steel when heated in nitrogen atmosphere. Argon may be used instead of nitrogen but is of course very expensive. In any event, expensive equipment'is necessary, e.g. gas flow control apparatus and specially constructed furnaces in which air may be prevented from entering by induction. Furnaces in which the atmosphere may be controlled are suitable for heating small objects such as tools but it is impractical to make use of such apparatus to heat large objects such as billets and slabs which may weigh several tons. Moreover, if the metal object is required to be removed from the furnace 3,440,112 Patented Apr. 22, 1969 whilst still in a heated condition (eg, a steel billet or slab for rolling), fairly severe oxidation can still take place on exposure to the atmosphere.

Alternative means of protecting the surfaces of metal articles to be heated have therefore been developed. Thus, it is known to apply a paint to the surfaces of the metal before subjecting it to the heat treatment. Such paints may include materials which are preferentially oxidised (e.g. powdered aluminium, ferrosilicon or silicon) and are thus intended to function in a sacrificial manner. Other types of paint depend upon the physical exclusion of the atmosphere from the surface by the ability of their constituents to form a glaze when heated. Materials used in such paints are mixtures of various refractory oxides, slags, silica and ground glass. The preferentially oxidised materials mentioned above have also been included in the glaze-forming preparations. Varying degrees of success may be achieved by the use of these known paints which are generally applied to the metal surface as a layer of the order of However, none of them has been found to be capable of consistently reducing oxidation losses to a satisfactory degree although, in some instances, the amount of metal lost as oxide scale has been reduced by as much as 70% A reduction in losses even of this magnitude is however considered unsatisfactory in that the degree of oxidation still suffered is inevitably accompanied, in the case of steels, by surface decarburisation, thus necessitating a machining operation to remove the decarburised layer.

It may be observed in this connection that where a mechanical protection is to be achieved, as by a glaze, it is important that there should be no cracks or pinholes in the protective layer for if there. are the oxidation and decarburisation effect tends to spread beneath the glaze far beyond the crack or pinhole itself.

It is an object of the present invention to provide a composition which, applied to metal articles to be treated, affords a substantially improved measure of protection against surface oxidation and surface decarburisation during heat treatment, so that the extent of residual metal oxide formation is at most 20% by weight of that which occurs when no preventive measures are taken, i.e. at least four fifths of the normal loss is avoided. The residual oxidation is then at so low a level that it may be wholly unnecessary to carry out any machining of the surface for removal of oxidised or decarburised strata.

It has been found, as a result of considerable research and experiment that a protective layer, in order to be capable of achieving this degree of success, must be able to form a continuous film on the surface of the metal article 'which is impervious to gases, and must adhere firmly to the metal surface. It has also been discovered that the coating composition must be formulated such that the applied coating does not completely fuse to form a glaze since, in such a case, the film acts as an oxide ion carrier and scaling can become very severe indeed; worse, in fact than on a similar, untreated specimen. Moreover, as a practical matter the composition used to apply the layer should have such fluidity that it may be applied by spraying, dipping, brushing or any other conventional method.

It is an object of the present invention to provide an improved method of treating metal slabs and billets to minimise or inhibit surface oxidation which makes use of novel compositions for application to the metal surfaces.

Accoring to a first feature of the present invention there is provided, as a new composition of matter for application to the surface of slabs, billets, and other articles of metal, a composition comprising ferrosilicon, silicon carbide, finely divided refractory material and a fusible glaze-forming material. According to a further feature of the invention there is provided a method for the treatment of slabs, billets and other articles of metal, to reduce or inhibit surface oxidation during heat treatment thereof, which comprises applying to the surface thereof a layer of a composition containing ferrosilicon, silicon carbide, finely divided refractory material and a fusible glaze-forming material.

It is generally most convenient to apply the aforesaid compositions in the form of dispersions, suspensions or slurries and for that reason it is desirable to include with the composition as applied, in addition to a liquid carrier for the solid ingredients, a dispersing agent (i.e. an agent which will aid in holding the particulate ingredients in suspension in the carrier liquid). It is also frequently desirable to include an adhesive substance which will aid in anchoring the applied layer to the metal surface.

In a particular form of the invention the composition further includes ingredients which when the coated metal is heated react with one another exothermically. It is found that in this way the fusing of the glaze-forming material is facilitated and improved results. may be obtained.

Referring to these various ingredients.

The ingredients ferrosilicon and silicon carbide are well known and require no further comment.

The finely divided refractory material may be, for example, alumina, silica e. g. silica flour, magnesia or any other refractory oxide or a mixture of any of these, or any other refractory material such as a refractory silicate.

The fusible glaze-forming material may be powdered glass or any material known per se for use as a ceramic glazing material. These are usually mixtures of silicates, borates or phosphates 'with metal oxides, e.g. iron oxide, or lead oxide. Slagging agents known for use in the iron and steel industry may also be employed.

When, as is preferred, the composition is in the form of a suspension, dispersion or slurry in a liquid carrier, any liquid carrier can be used. Water is generally preferred from the standpoint of convenience and cheapness. Any other volatile or inflammable liquid can be employed, e.g. an alcohol, but the latter are generally less preferable as introducing fire and explosion hazards.

The dispersing agents may be any such compound known per se e.g. a montmorillonite gel. Adhesives present in the composition may be gums or resins, or bentonite clay or mixture of these.

Where it is desired that the composition should contain exothermically reacting ingredients, these will usually consist of an oxidisable substance and an oxidising agent. The former may be the ferrosilicon which is in any event present in the composition but there may additionally be present a proportion of finely divided aluminium. The oxidising agent may be most conveniently an alkali metal or alkaline earth metal nitrate or chlorate, iron oxide (Fe O or Fe O manganese dioxide and mixtures of any of these.

Where aluminium and an oxidising agent is present it is usually preferable also to include a proportion of a fluoride, e.g. alkali metal or alkaline earth metal fluoride, aluminium fluoride or a mixed fluoride such as sodium aluminium fluoride or potassium aluminium fluoride or a complex fluoride such as a silica fluoride, boro fluoride or titano fluoride.

Preferably according to the invention the composition employed contains the following ingredients in the ranges indicated. All the ingredients should be of fine particle size, preferably passing a 200 E58 Mesh and most preferably passing a 300 E55 Mesh.

A mixture of these materials may be readily slurried in water for use in dipping, brushing swabbing or spraying. For example 1 00 parts of weight of the solid mixture may be dispersed in 50 parts by Weight of water to make a brushable paint.

The following specific example will serve to illustrate the invention.

Example The following ingredients all passing a 200 E55 Mesh, are mixed in the percentages by weight indicated:

Sixty parts by weight of the dry mixture are added to 40 parts by weight of water and stirred to form a paint. This paint, when brushed onto steel ingot surfaces, affords protection against scaling at temperatures up to 1300l350 C.

The compositions of the present invention have the advantanges that they need no complicated mixing procedures, and the solids may be blended and stored in the dry state until required. There is thus no risk of the deterioration which sometimes ensues with previous water-based preparations. Moreover the compositions give greater protection to the surfaces of the metal which is to be subjected to heat treatment. The mechanism is not fully understood but it is believed that the refractory silicon carbide particles, having a plate-like structure, enable the coating to be applied in a more densely packed condition and thus present a greater barrier to the atmosphere surrounding the coated article.

It is to be appreciated that the compositions may b used to provide a barrier between a metal surface and any atmosphere in which the metal is to be heated. For instance, it may be required that a metal object should be subjected to a carburising treatment on only a part of its surface; the area which does not require carburising may be coated with the composition and the whole object may be then subjected to the appropriate temperature and atmospheric treatment. The area covered by the compositions will not be carburised.

The new protective compositions according to this invention are suitable, therefore, for application to metal billets, and slabs which are to be heated before hot working operations. The compositions are also useful for protecting the surfaces of finished metal articles which are to undergo heat treatment, e.g. tools which have been formed and machined to size but which must then be annealed, tempered or otherwise heat treated.

I claim as my invention.

1. A composition for application to the surface of slabs, billets and other articles of metal prior to heat treatment thereof which comprises the following ingredients in the following proportions by weight:

2. A composition for application to the surface of slabs, billets and other articles of metal prior to heat treatment thereof which comprises the following ingredients in the following proportions by weight:

Percent 2,806,801 9/1957 Leston 148-22 81110011 carblde 25-45 2,959,503 11/1960 Lindson 14822 Ferrosllicon 15-35 511% fl 15-35 L. DEWAYNE RUTLEDGE, Primary Examiner. Bentomte 3-5 5 Powdered glass 1-5 TERRY R. FRYE, Assistant Examiner. Sodium or potassium cryolite 1-5 US. Cl. X.R.

References Cited UNITED STATES PATENTS 2,272,038 2/1942 Morgan 1 106-44