US3415691A - Method of protecting metal surfaces during heat treatment - Google Patents

Description

United States Patent 3,415,691 METHOD OF PROTECTING METAL SURFACES DURING HEAT TREATMENT Ramesh Vyas, Birmingham, England, assignor to Foseco International Limited, Birmingham, England No Drawing. Filed Nov. 3, 1964, Ser. No. 408,672 Claims priority, application Great Britain, Nov. 7, 1963, 44,025/63 15 Claims. (Cl. 148--6.11)

ABSTRACT OF THE DISCLOSURE A method of treating metal surfaces to protect the surfaces against scale formation during heat treatment such as annealing including applying to the surface a composition of a vitreous material such as powdered glass, a refactory filler such as silica and ferrosilicon.

This invention relates to the treatment of metals and more particularly 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, e.g., 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 commonly placed in a socalled 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 castings 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 in 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 whilst still in a heated condition (e.g., a steel billet or Patented Dec. 10, 1968 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 therefor 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 perparations. 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 A2. 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 eifect tends to spread beneath the glaze far beyond the crack or pinhole itself.

It is an object of the present invention to provide a method of protecting metal surfaces which comprises applying a composition thereto which, when 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 imperivious 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 acase, 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.

Accordingly, 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, 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 ingredient ferrosilicon is well known and requires no further comment.

The finely divided refractory material may be, for example, alumina, silica, 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 agent may be any such compound known per se, e.g., a montmorillonite gel. Adhesives present in the composition may be gums or resins, r 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. However it is also suitable to use the fluoride in the absence of the aluminium and/or the oxidising agent.

The following formulations are illustrative of protective compositions useful in carrying out the novel method of this invention:

Example 1 Parts by weight Refractory (e.g. alumina or silica) (l00 mesh B.S.S.) Ground glass (-200 mesh B.S.S.) 3 Ferrosilicon (200 mesh B.S.S.) 34 Aluminium powder (100 mesh B.S.S.) 14 Fluoride (60 mesh B.S.S.) 4 Oxidising agent (60 mesh B.S.S.) 15 Bentonite 7 Rosin pitch 3 Alkyl ammonium montmorillonite gel The whole of the foregoing ingredients are mixed to a paste and diluted with isopropyl alcohol in the proportions 100 gm. paste: 100 cc. isopropyl alcohol.

The gel referred to is prepared by thorough kneading together 8.7 parts by weight of alkyl ammonium montmorillonite and 4.3 parts by weight of methyl alcohol. The dough thus formed is added to 87 parts by weight toluene whilst stirring gently. Finally the whole mixture is stirred vigorously for about three minutes, after which it is transferred to an air tight container and allowed to stand for 24 hours before use.

Example 2 Parts by weight Refractory material (eg. alumina or silica) (-l00 mesh B.S.S.) 3O Powdered glass (-200 mesh B.S.S.) 2 /2 Ferrosilicon (200 mesh B.S.S.) 6 Sodium fluoride (-l0() mesh B.S.S.) 2 /2 Bentonite 2 Vinsol resin 2 /2 Alkyl ammonium montmorillonite gel 11 The gel is prepared in the same way as in Example 1. The 11 parts of the prepared gel were mixed with 66 parts by weight of isopropyl alcohol until all lumps were removed and the remaining dry ingredients were added in the proportions stated.

Example 3 Parts by weight Refractory material (eg. alumina or silica) (-lO0 mesh B.S.S.) 18 Powdered glass (60 mesh B.S.S.) 2 Ferrosilicon (200 mesh B.S.S.) 28 Powdered aluminium (l00 mesh B.S.S.) l2 Fluoride 100 mesh B.S.S.) 5 Oxidising agent (nitrate, iron oxide or the like (60 mesh B.S.S.) l2 Bentonite 7 Water soluble resin binder (e.g. urea formaldehyde, phenol formaldehyde resin) Aqueous suspension agent (e.g. a gum) 4 Sodium silicate solution (4.8% solids) 4 Buffer (eg. boric acid or mono sodium orthophosphate) All the ingredients except sodium silicate, resin and the suspension agent are mixed together in the proportions stated (88 parts by weight) and added to parts weight of water. The remaining ingredients are then stirred into the mixture.

Example 4 Parts by weight parts of above mixture are mixed with 25 parts of isopropyl alcohol exactly as in Example 1.

The compositions thus produced are in the form of slurries which may be applied, preferably by spraying, to the surfaces of the metal to be subjected to heat treatment. Adherence of the resulting layers to metal surfaces is very good, cohesion being given to the applied coating by the resin and bentonite content. The gel is included in the composition as a suspension agent and, even though some settling of the solid constituents may occur, redispersion is effected very readily.

The anti-oxidising efliciency of the foregoing compositions is clearly demonstrated by the results of the following tests. Steel specimens of cylindrical shape, 1" diameter and weighing approx. 100 gm. were heated for 1 hour in a furnace at 1100 C. This resulted in surface oxidation of the specimens to the extent that of the metal was lost by scaling. It was found that these losses were consistently reduced by at least 85% when identically similar specimens were subjected to the same heating after spraying the surfaces with any of the compositions shown in the foregoing Examples 1, 2 and 3. (Naturally, appropriate precautions must be taken with the first two compositions, which contain isopropyl alcohol.) Similar steel specimens, when coated with the composition according to Example 4, were able to Withstand a heat treatment at 1300 C. for a similar period without undergoing a more severe oxidation. It is remarkable that such a degree of protection is afforded by as little as a 0.025" thickness layer of these novel compositions. In fact, increasing the thickness of the layer above 0.025 confers very little extra protection against surface oxidation.

It should be pointed out that, in the case of formulations utilising an aqueous medium, the self life of the composition is liable to be short because of chemical reaction between constituents. Such reactions may be avoided or minimised by the addition of further ingredients to control the pH of the aqeous phase. For example, Example 3 employs boric acid and monosodium orthophosphate as buffering agents. Corrosion of such ingredients as ferrosilicon may be further inhibited by precoating them, e.g., with insoluble resins.

The novel method of this invention is suitable, therefore, for protecting metal billets and slabs which are to be heated before hot working operations. The method is also useful for protecting the surface 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. In a method of protecting metal surfaces against scale formation during heat treatment the step of applying to such surfaces a composition cousitsing essentially of vitreous material, a refractory filler selected from the group consisting of aluminia, silica, magnesia, refractory silicates and mixtures thereof, and ferrosilicon, in proportions, per one part by weight vitreous material, of from about 5 to about 30 parts by weight refractory filler, and from about 1 to about parts by weight ferrosilicon.

2. The method of claim 1 wherein the vitreous material of the composition is a glass.

3. The method of claim 1 wherein the refractory filler of the composition is silica.

4. The method of claim 1 wherein the composition contains up to about 10% by weight of an oxidising agent selected from the group consisting of alkali metal nitrates, alkaline earth metal nitrates, alkali metal chlorates, alkaline earth metal chlorates, iron oxide, manganese Oxide and mixtures thereof.

5. The method of claim 4 wherein the composition contains up to about 10% by weight of a fluoride.

6. The method of claim 5 wherein the composition contains up to about 10% by weight a fluoride.

7. The method of claim 1 wherein the composition contains up to about 10% by weight finely divided aluminium.

8. The method of claim 1 wherein the composition is in the form of a suspension of the ingredients in a liquid carrier.

9. The method of claim, 8 wherein the liquid carrier is water.

10. The method of claim 1 wherein the composition contains up to about 10% by weight of a suspending agent 11. The method of claim 9 wherein the composition contains up to about 10% by weight of an adhesive substance.

12. The method of claim 8 wherein the composition contains up to about 10% by weight of an oxidising agent selected from the group consisting of alkali metal nitrates, alkaline earth metal nitrates, alkali metal chlorates, alkaline earth metal chlorates, iron oxide, manganese oxide and mixture thereof.

13. The method of claim 8 wherein the composition contains up to about 10% by weight finely divided aluminium.

14. The method of claim 1 wherein the refractory filler of the composition is alumina.

15. The method of claim 1 wherein the refractory filler of the composition is magnesia.

References Cited UNITED STATES PATENTS 1,359,281 11/1920 Scott 148-14 2,686,134 8/1954 Wooding et al. 148-24 X 2,806,801 9/1957 Leston 14826 X 2,865,798 12/1958 Hindson 14826 2,898,253 8/1959 Schneider et al. N 148l3.1 X 2,951,000 8/ 1960 Kennedy et al 148-26 3,037,878 6/1962 Cowles et a1 10648 X 3,167,450 1/1965 Koibuchi et al. 117206 3,178,322 4/1965 Schneider 148-13.1 X 3,196,537 7/1965 Groman et al. 1l7206 X 3,253,950 5/1966 Wasserman et al. 117206 RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R.