US 3295346 A
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Jan. 3, 1967 H. B. BOMBERGER, JR 3,295,346
METHODS FOR THE ELEVATED TEMPERATURE PROTECTION OF METALLIC SURFACE, AND COATINGS THEREFOR Filed Feb. 11, 1964 2 Sheets-Sheet l INVENTOR Howard B. Bombelrger ATTORNEY United States Patent O 3,295,346 METHODS FOR THE'ELEVATED TEMPERATURE PROTECTKON OF METALLHQ SURFACE, AND COATINGS THEREFOR Howard B. Bomberger, J12, East Liverpool, Ohio, assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey Filed Feb. 11, 1964, Ser. No. 346,057 9 filaims. (Cl. 72-41) The above-identified application is a continuation-in part of co-pending application Serial No. 747,454 filed July 9, 1958 for Protective Coating for Metallic Surfaces and Method for Applying the Same, in the name of said Howard B. Bomberger, Jr., and now abandoned.
This invention relates to methods for preventing surface contamination of metals by furnace and atmospheric gases during hot processing, to coatings therefor, and to metal articles provided with said coatings. More particularly, the invention relates to coatings for refractory metals which also serve as a lubricant to improve the hot working characteristics of the metals.
As is well known, most refractory metals and alloys must be processed to mill forms at elevated temperatures well above 1500 F. Unless special processing techniques are employed, the reactive metals, such as titanium, zirconium, molybdenum, tungsten, columbium and tantalum react rapidly with furnace gases and air during heating and hot working operations. This results not only in a loss in material from scaling but also in an impairment of the mechanical properties of the material. The principal contaminants of titanium, for example, are oxygen and hydrogen. Oxygen reacts with the hot metal and forms both a scale and a subscale which must be removed by an expensive and cumbersome operation involving a caustic treatment at 800 to 900 F. Alternatively, the descaling may be accomplished by grit blasting or by grinding. None of these methods, however, is very satisfactory and particularly so for formable alloys which age rapidly at hot working temperatures. Similarly, hydrogen gas also impairs the properties of the metal and can be removed only by a costly vacuum annealing operation.
A problem concurrent with that of gas contamination is the great difiiculty of hot working the higher strength refractory metals. Titanium alloys, for example, are very difiicult to produce by a strip process because of the tremendous mill power requirements. Therefore, an ideal coating is one that not only offers a higher order of protection from gas contamination but also one which serves as a lubricant to enhance hot working.
Many attempts have heretofore been made to minimize gas contamination of titanium and other refractory metals and to improve the hot working characteristics of these metals. However, no good, practical coating has heretofore been developed which gives the necessary protection against gas contamination while providing a lubricant to enhance hot Working. As an example, one coating which has been used in the past, in an effort to prevent gas contamination, is aluminum. In this process a refractory metal is immersed in a molten bath of aluminum to produce a layer of relatively pure aluminum, up to 0.003 inch in thickness, plus a thin diffusion layer. This coating provides at least some protection against the gross penetration of oxygen, hydrogen, and nitrogen during hot working operations and also serves as a lubricant during forming. However, it has certain undesirable characteristics which limit its usefulness. For example, one deficiency of an aluminum coating is that in areas of severe deformation during hot working the coating ruptures, thereby exposing the refractory metal to the atmosphere; Once an aluminum coating is ruptured in this Patented Jan. 3, 1967 manner, it is not readily self-repairing. That is, the aluminum does not easily flow and does not tend to cover up the ruptured portion so that the refractory metal remains exposed to the contaminating gases. Still another deficiency of aluminum is that it is porous to hydrogen gas, and, thus, contamination may occur even though the refractory metal is completely covered with the aluminum coating. Furthermore, the refractory metal must be carefully cleaned before coating with aluminum, and the cleaning process is relatively lengthy and expensive.
Another method heretofore employed for coating refractory metals was to paint the metal with a silicon base aluminum paint. This coating offers no lubricating properties, is very suspectible to scratching, and has very poor strength characteristics. Furthermore, cleaning of the refractory metal before painting has been found to be very critical in that the sheet must be absolutely clean and must pass a water-break test. A coating of this type does virtually nothing to stop hydrogen and has been found to be extremely diflicult to remove from the surface of the metal after a hot working operation is completed. Generally speaking, this method of protecting the metal has been found to be entirely unsatisfactory.
A further method, although not actually a coating, in volves seam welding the refractory metal inside envelopes of steel which are then evacuated. Although this method appears satisfactory for preventing gas contamination, it is relatively expensive and cumbersome and does not in any Way enhance the hot working characteristics of the metal.
It is, of course, advantageous to prevent the undesirable effects of contamination, and to provide hot working lubricity, in respect of metals other than the so-called refractory metals, and the prior art, in response to such needs, has provided coating compositions directed to these ends. Individual prior art coatings have been designed for use over a wide range of hot working tempenatur-es. Thus, various ceramic coatings, for example, porcelain enamels, have been devised having low fusion temperature, as low as, e.g., 1000 F., but such coatings are difficult to remove in many applications.
Oxide coatings, as alumina and zirconia, have been used at very high temperatures, e.g. over 3000 F., but the difficulties of application and plasticity limit their usefulness to only a limited number of very high temperature applications, and, moreover, such coatings are not readily self-healing.
Glass coatings, containing iron oxide and. iron sulfide, have been used in the hot Working of tool steels and show good lubricity but poor heat conductivity.
Other oxides, as well as carbonates, e.g. those of magnesium, zinc and beryllium have been applied, in a phosphoric acid solution, as coatings, to metals, as titanium. However, these compounds tend to react with the basis metal at temperatures of about 1150 F., to form therewith more complex oxides.
Sodium silicate has been used for the aforesaid purposes, but is limited to a maximum temperature of about 2000 F. Mixtures of sodium silicate and stainless steel powder have also been used as a protective coating for metal bases, but tend to weld to the base metal at about 1700 F., and does not flow at temperatures of 2000 F.
Coatings comprising an easily reducible ceramic oxide are not satisfactory for the purposes contemplated herein, e.g. hot working protection of titanium and base alloys thereof, because of the oxidation that occurs on the titanium surface upon fusing the ceramic coating thereto.
Other non-ceramic coatings which have been used in the prior art, include electrodeposited coatings of chromium and nickel, for example, in the protection of molybdenum, at temperatures up to about 2000* F. How- J) ever, nickel coatings on titanium have been found to actually increase, rather than to decrease, hydrogen absorption in the base metal.
Metallizing by vapor deposition is not a practical, economical, large-scale mill procedure.
Silicone paint coatings have been used but are difficult to remove, and are hydrogen-permeable.
Glassy coatings have also been utilized as protective coatings for metals during hot working thereof. For example, US. Patent No. 2,962,808 teaches the use of a water-soluble boron oxide coating during hot working, and US Patent No. 2,658,834 teaches the use of glass coatings consisting of sodium metaborate, with or without the addition of a clay filler, plu a reducing agent. The borate component can be used alone, but coatings thereof, as Well a coatings consisting only of boron oxide, are of strictly limited utility, because of excessive fluidity at elevated temperatures. The addition of increasingly large proportions of clay confers ceramic-like, high-temperature stiffness, but neither the clay-free nor the clay-containing coatings exhibit the desirable combination of properties, as proper fluidity or stiffness, together with gas impermeability which are required for est results.
Other glasses have also been used, as a mixture of boron oxide, silica and sodium oxide, as taught, for example, in U.S. Patent No. 2,715,765, directed to the hot working of vanadium and base alloys thereof. However, I have found that both sodium oxide and silica should be avoided in coating compositions for hot working. Silica-containing coatings do not have best protective properties and, further, silica is reduced by hot titanium which thereupon suffers substantial contamination by elemental silicon. Alkali metal oxides lower fusion temperature of boron oxide-containing coatings.
Accordingly, it is an object of the present invention to provide a group of compositions useful as coatings for the protection of metal articles from contamination by environmental gase during elevated temperature working of such articles.
It is another object of the invention to provide protective coating compositions which also serve as a lubricant during hot working of an article coated therewith.
It i a further object of this invention to provide an article for hot working comprising a metal base and an overlying protective and lubricating coating.
It is still another object of the present invent-ion to provide improved methods for protecting and lubricating a refractory metal article during hot working thereof.
In accordance with the foregoing objects, a first preferred embodiment of the invention comprises an article for hotworking comprising a metal base and bearing on the surfaces thereof a water-soluble coating containing a major amount of boron oxide and a minor amount of feldspar, together with a minor amount of an inorganic filler and binder agent.
A second preferred embodiment of the coated article aspect of the invention comprises a refractory metal base, as titanium, or a base alloy thereof, bearing on the surfaces thereof a coating containing a minor amount of boron oxide and a major amount of feldspar, together with a minor amount of an inorganic filler and binder agent.
A preferred embodiment of the inventive method comprises applying to a refractory metal base, preparatory to hot working, a coating comprising, by weight percent of dry ingredients, from about 20 to about 80% boron oxide, from about 20 to about 70% feldspar, up to preferably about 3 to 8% of an inorganic filler and binder, as bentonite, and up to about 12%, preferably about 6 to 10%, of an organic binder, as a vinyl stearate, the coating being applied in suspension in a volatilizable, preferably water-free vehicle, drying the coating to volatilize the vehicle and to have a residue about 2 to 20,
preferably about 2 to 4, mils in thickness on the base, and then hot working the coated article.
Coating compositions preferred for a first, lower range of hot working temperatures constitute a slurry of a nonaqueous, volatile vehicle containing a suspension of particulate boron oxide, in amount of about 50 to by weight of solid contents, and feldspar, in amount of about 20 to 50% of solids, together with effective amounts of bentonite and a substantially non-volatile organic binder. Similar compositions, wherein the relative proportions of boron oxide and feldspar are reversed, i.e., about 30 to about 50% boron oxide and about 50 to 70% feldspar, are preferred as coatings for use at highest temperatures.
The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification and in which:
FIGURE 1 is a photograph showing specimens of refractory metals with and without the coatings of the present invention after an exposure of four hours at 2300 F.;
FIGURE 2 is a photomicrograph, at 75 magnification, of a bar coated with the ceramic material of the present invention showing the lack of gas contamination after heating for four hours at 2300 F.;
FIGURE 3 i a photomicrograph, at 75 magnification, showing the contamination and surface oxidation which occurs when a bar is not coated with a protective material; and,
FIGURE 4 is a graph illustrating the effect on surface hardening of coated and uncoated refractory metals after heating at 2300 F. for four hours.
Coatings for protection of metal articles are useful, not only in respect to protection of oxidation and/or corrosionor contamination-susceptible metals, as the above-mentioned refractory metals, during very high temperature working thereof, but also in respect to lower temperature working of metals, as tool steels, which are heated, for working or for special heat treatment, to temperatures in the range of l5002l00 F. At such temperatures, unprotected tool steels scale (oxidize) and, also importantly, they decarburize.
Decarburization is very troublesome and requires expensive conditioning of mill products and very elaborate controls for its prevention on finished tools and machine parts. In some cases carbon pickup as well as loss must be controlled to very close limits.
Certain of the coatings and methods provided by this invention are adapted for use at such temperatures and effectively prevent decarburization.
I have found that composition containing boron oxide and feldspar, as the principal effective ingredients, are admirably useful and effective to overcome the abovementioned defects of prior art protective coatings, in that they are very resistant to oxidation and corrosive effects of the hot working environment and are impervious to penetration of gases deleterious to the underlying base metal.
Moreover, by varying the relative proportions of the aforesaid principal ingredients, a desired fluidity of the compositions can be maintained, for effective self-healing purposes, over a wide range of temperatures, e.g., from about 1200 F. to about 2300 F. Maintenance of proper coating viscosity also serves to maintain an effective lubricating function to assist in hot working.
All of the compositions of the invention fuse readily, at the particular, intended hot working temperature, into a very viscous non-porous liquid which wets and adheres to metal surfaces and is self-repairing. That is, when a break occurs in the coating, the viscous liquid will immediately cover this break to prevent any gas contamination. The compositions of the present invention are essentially inert to refractory and other metals and they are impervious to oxygen and hydrogen so that no scale forms on the metal protected by these materials. At the time of 6 70%, boron oxide, and from about 20- to about 70%, preferably about 30 to about 60%, feldspar.
More restricted ranges of these ingredients are chosen, within the aforesaid broad ranges, in accordance with the tion, the compositions of the present invention serve as 5 hot working temperature at which the coatings are to be excellent lubricants for hot rolling. Tests on titanium at used, the boron oxide and feldspar proportions being se- 17000 for p Show that 50% greater reduction looted, for a given temperature, so as to be productive of is possible when rolling at fixed mill settings when coma coating which, taking into consideration other composiposition containing a preponderant proportion of boron tional ingredients as hereinafter more fully described, will oxide is used as a lubricant. Thus, the use of these com- 10 the sufiiciently free-flowing, at the hot working temperapositions in a production operation will permit faster rollture, t be readily lf-heali but hi h ill al o be ing, greater reductions, lower rolling temperatures and sufficiently viscous to insure against loss of coating and wider sheet than is now possible with some of the tougher exposure of the metal base to undesirable, deleterious atalloys. mospheric conditions.
A principal ingredient of my new coating composition Thus, within the aforesaid broad ranges, several more is, as aforesaid, boron oxide, B 0 (or a compound prolimited ranges of boron oxide and feldspar are productive viding boron oxide), sometimes referred to as boric acid of coating compositions, each of which is especially useful or boric anhydride. Although boron oxide can, and, as within a restricted temperature range and which, together, aforesaid, has been used alone as a protective coating, constitute a limited series, e.-g., three, of coating composithe relatively low fusing point thereof limits the usefultions which cover the entire hot working temperature ness of such coatings to hot working at lower temperarange from about 1200 F. to about 2300 F. or even tures than are required in many applications. higher.
I have found that the advantages of a boron oxide coat- For a temperature range of about 1200 to about ing can be utilized and greatly supplemented by the use, 1600 F., especially 1200 to 1500 F., I have found that in conjunction therewith, of certain amounts of feldspar, a, composition containing about 20 to about prefa complex mineral consisting principally of albite erably 20 to 25%, feldspar and about 50 to about 80%,
preferably 60 to 70%, boron oxide constitutes an effective (NaAlSi3O8) protective and lubricating composition, especially when the balance of the composition is comprised as hereinafter 30 disclosed. Such compositions, particularly those conflflolthlte 2 2 s) anorthoclase a 8 taining relatively large amounts of boron oxide and relacelsian 2 2 a) hya'lophane 2 2 4 12 tively small amounts of feldspar, within the aforesaid mlcmclme a s) and olthoclase B ranges of each, are water-soluble-those containing larg- Y Y these sfparate QP PP P may est amounts of feldspar having greater water-solubility comblnfifi t0 SYnthB1Ze the feldspar lngredlent of my than those wherein the relative proportions of these in- COFHPOSIUOHS but, 9 I have found that F natural gradients are reversed. This feature of this class of my mineral feldspar admirably serves the aforesaid purposes, Howl compositions is, of course, of great advantage in use of such a synthetic material 18 obviously economically removing the coatings Subsequent to the hot Working undesirable. erations N only the combination of feldspar, t boron 40 For a hot working temperature range of about 1400 F. oxide, productiye of a means for controlling coating con1- to about 20000 F especially 150O 1800w F my Coating position viscosity over a wide temperature range in ac- Compositions Contain about 30 to about 50% each of cordance will} the P PQ F of these ingredients used, boron oxide and feldspar (the total maximum amount of but the resulting compositions have beenfound to possess these two ingredients being about 90% in Such COmPOSiexceedingly favorable and enhanced utility in the protections) tion of the underlying, basis metal from oxidation and For a hot working tfimpsrature range of about 18000 contammatton dunng Workmg of coated f to about 2300" F., especially about 2000 2300 F., a
The particular proportions of the principal ingredients, boron oxide and feldspar, may be chosen, within certain Coatmg composmon contammg about 20 to .about ranges of each, to produce compositions having a variety wefembly about to about 'boron oxlde together of viscosities at a given temperature-or a particular viswl'th from about J0 to about preferably about cosity or a viscosity within a desired rangewhile preto feldspar w bfist Serving the other desirablfi composition attributes, as Table I hereinbelow is illustrative of the aforesaid types permeability to environmental, as atmospheric, gases. of boron 0X1de-f1d$Pa1*CntaiI1ing Coating Composltions,
Thus, my coatings contain these principal compositional 55 and ShOWS the Outstanding uses and Properties of each- TABLE I Composition Application Characteristics A .r to 80% B503, 20 to 30% Coating for sheet products heated up to about; Water s0lub1eean be easily removed in warm feldspar. 1,800 F. for periods of 30 minutes or less. Water-has good lubricating; ChiraOtGllStiCS. gll ill'itggili lsgd where heating at or below 1,800 B About 50% B203, about Coating for heavymillproduets such as barsand Maintains highviscosity atintermediatetempcra- 50% feldspar. billets which are subsequently hot worked. tures in the range l000-2, 000 F. Has advantage of being more readily self-repariing and giving better coverage than. composition B.
Not readily water soluble. C About 30% B203, about 70% Coating for heavy mill products such as bars and Maintains high viscosity for 4 to 8 hours at temfeldspar. billets which are subsequently hot Worked. ggllglglll'es up to 2300 F. Not readily water ingredients, in a broadest range, by weight percent, of
It will be noted from the foregoing table that the prefrom about 20 to about 80%, preferably about 30 to about ferred compositions for general coating application fall 7 within the range of 30 to 80% B and to 70% feldspar.
It will also be noted from the foregoing Table I that, as the percentage of boron oxide decreases, the solubility of the coating in water likewise decreases. However, as the percentage of feldspar increases, the viscosity of the compositions at higher temperatures correspondingly increases. That is, the coatings containing a high percentage of feldspar, being more viscous, are more suitable for high temperatures than those containing a low percentage of this constituent. Coatings containing a large percentage of feldspar such as compositions B and C are particularly adaptable for heavy mill products such as bars and billets which are subsequently hot worked. In these cases, the coated material may pass through several hot working stages without loss of the coating before it is formed into the final product. At the end of the processing operation, the coating may be removed as by sand blasting or some other suitable means well known in the art for removing coatings.
Composition A, on the other hand, is particularly adapted for light sheet products which are heated up to a maximum of about 1800 F. In this case, the coating is readily water soluble so that the sheet may be merely passed through a warm water bath after processing to remove the coating.
An essential requirement of good coatings is that they form a non-porous crack-free cover before the metal attains a temperature at which significant oxidation can occur. Otherwise, the metal will experience contamination. All of the coatings outlined above do provide an excellent cover for the metal before the oxidizing temperature is reached. As noted earlier, boron oxide is a most important ingredient in these coatings. It serves the purposes of not only forming a good coating and good coating base but it also wets and covers the metal surface at a rather low temperature (about 800 F.). Yet, unlike most low melting oxides, the coatings of the present invention maintain a high and useful viscosity at temperatures up to 2300 F. Furthermore, as viscous fluids they repair themselves if any small imperfections or ruptures occur.
Referring to FIGURE 1, two specimens of a titanium alloy with and without coatings of 70% feldspar and 30% boron oxide are shown after having been exposed for four hours at 2300" F. The shorter, uncoated specimens contain a heavy scale after heating. However, the coated bars are substantially unaffected. Futrhermore, it will be noted that none of the coatings ran off the coated specimens after heating for four hours at 23 00 F.
In FIGURES 2 and 3 photomicrographs are shown of the cross sections of the coated and uncoated specimens of FIGURE 1. FIGURE 2 shows the coating intact with no evidence of oxidation or surface penetration. The uncoated specimen of FIGURE 3, however, lost not only 19 mils of metal, but also was very deeply contaminated as indicated in FIGURE 3. The hardness curves of FIGURE 4 for the coated and uncoated specimens show that, due to the contamination of the uncoated specimen, the hardness of its surface was materially increased. Furthermore, it is apparent that this contamination extended to the very center of the uncoated bar. The curve for the coated sample, however, shows that the hardness of the alloy was substantially unaltered both at its surface and at the center.
In addition to boron oxide and feldspar, other components are also added to confer the properties necessary for the compositions of the invention to best function as combined protective and lubricating coatings.
The inventive compositions may be applied to the basis metal as a fine powder mixture or frit, as by dusting and fusing, or by flame spraying. However, for best results, the coatings are applied, under better control as to uniformity of coverage and thickness, by suspending the dry ingredients in a liquid vehicle, and applying the resultant mixture to the metal base, as by spraying, brushing or dipping.
The vehicle used to form such liquid or slurry products must be relatively highly volatile in order that it may be relatively quickly (for the sake of economy) and thoroughly removed at relatively low temperatures, i.e., substantially below the hot working temperature, so that the coating, when in use at the operating temperature, is not impaired by evolution of gaseous or vapor bubbles. The presence of water in the coating compositions, particularly at elevated temperatures, as the hot working temperature, is highly undesirable in view of undesirable oxidation effects thereof upon the basis metalas well as the aforesaid undesirable effects caused by evolution of Water vapor. Accordingly, the dry ingredients of the coating are preferably used in an anhydrous or substantially anhydrous form, and the liquid vehicle is preferably non-aqueous. Hydrated solid ingredients are particularly objectionable, for the water of hydration thereof is, to a more or less great degreedepending on the nature of the hydrated compounddifficult to remove and evolution of water vapor may, therefore, be delayed until relatively high temperatures are reached, e. g., in the operational, hot working temperature range of the coating. This resutls, not only in spot cooling of the coated article and temporary rupture of the coating, but, most importantly in unwanted chemical reactions, as oxidation, including reactions with the basis metal for which protection against such reactions is sought. Comparative tests have showed me that better protection of the basis metal is afforded by coatings wherein anhydrous solids are used than by the coatings, otherwise similar, but containing substantial amounts of hydrated solid constituents.
Additionally, the liquid vehicle must be chemically nonreactive with respect to both the basis metal and the other components of the coating composition. Suitable liquid vehicles include volatile halogenated hydrocarbons,
as trichloroethylene, tetrachloroethylene and trichloroethane which have the additional important advantage, for large-scale mill operations, of being substantially nonfiammable. Other suitable vehicles are acetone and the various volatile, economically commercially available alcohols. Still other vehicles, having the aforesaid requisite properties, are apparent to those skilled in the art.
When a liquid product form is used, it is necessary, for the sake of coating area and thickness uniformity, to incorporate a suspending agent which is effective to maintain uniform product consistency, but which is chemically inert with respect to the basis metal, and the other ingredients of the composition. Various clay-like materials are useful for this purpose, but I have found that bentonite is particularly effective. Bentonite is a wellknown, identifiable colloidal clay comprising appreciable amounts of the mineral montmorillonite containing principally aluminum silicates, with some magnesium and iron. I have found that addition of bentonite to my coating compositions markedly decreases the settling rate of the solid components in the liquid vehicle and facilitates and enhances the mixing and spraying of such materials. Bentonite is therefore incorporated in the coating compositions in a minimum amount effective to accomplish the above-mentioned functions, e.g., about 1 or 2 percent, and up to about 6 or 8 percent, by weight of dry ingredients. Larger amounts are not desired since the aforesaid desirable properties are obtainable within the mentioned percentage range, and also because substantially greater amounts tend to interfere with other requisite properties as viscosity and lubricity of the coating compositions at operating temperatures.
It is also a requirement that the protective, lubricating com-positions of the invention, when applied to the base metal article, as in a liquid slurry form as aforesaid, adhere thereto with sufiicient tenacity after application and before elevation of the temperature of the coated article to the desired hot working temperature, and that the coating does not peel or flake so as to leave portions of the underlying metal exposed.
This latter objective is achieved in the present compositions 'by incorporation of a binder which provides a coating of sufiicient tenacity, toughness and green strength, that the coated article, both before and after drying to remove the liquid vehicle, can be subjected to considerable rough handling without damage to or deterioration of the coating. Although bentonite adds strength to the coating compositions, and, in that regard, can be considered to serve a binder function, it is not sufficient, alone, to meet the above property requirements. Gum rosin is useful as a binder, and I have found that such material can conveniently be added, for example, in amount of about 100 grams of rosin per 200 liters of (non-aqueous) vehicle and such a stock solution then added to the vehicle-solids slurry in an amount of from about 4 to about 8% of the total liquid volume,
A more useful organic binder, giving a coating having greater air-dried toughness and strength, comprises the group of commercially available chlorinated hydrocarbons, as 5460 Aroclor, which are non-flammable, only slightly volatile (but will readily burn off at the high temperatures reached before hot working, and are thermoplastic and non-drying). I have found particularly useful binders to comprise a mixture of a chlorinated hydrocarbon, as aforesaid, and one or more organic resins, as a polyvinyl acetate resin e.g., Vinac, or a polyvinyl s-tearate, as B-100.
Such organic binders are added to the compositions of the invention in an amount sufficiently great that the toughness and adherence of the coating to the metal base is substantially enhanced to withstand the required handling procedures. A few percent, e.g., 3 to 4 percent, is suflicient for minimally required coating adherence, and about 8 to 10 percent confers maximum property advantages for most rigorous conditions. A particularly useful binder has been found to consist of about 4% each of the above-mentioned chlorinated 'hyrocarbon and vinyl stearate.
Still other materials may be optionally added to the coating compositions of the invention. Thus, I have found that additions of up to about 10%, preferably about 1 to about aluminum oxide is very beneficial for increasing rolling friction, i.e., to overcome excessive slippage during rolling of the coated products. I therefore contemplate incorporation of this material in my novel compositions, particularly in those instances where the hot working temperature is great and coating lubricity is otherwise high or where, for any other reason, an increase in Work pressure against the workpiece is needed.
Powdered aluminum is also a useful ingredient in the latter case, since it oxidizes to aluminum oxide upon exposure to air at elevated temperatures.
A further improvement can be realized by the use of aluminum powder or other similarly reactive metal, as titanium (in an equivalent amount), for such additives, being readily ox idizable, elfectively act to' remove oxygen and moisture from the coating upon heating, thereby providing added protection of the basis metal.
Coating compositions, in slurry form, are readily made by mixing the dry, powdered, solid ingredients and adding the mixture to the liquid portion wherein the solids are effectively dispersed and suspended. A slurry, particularly useful for spray application, comprises about one pound of dry solids per quart of liquid. A somewhat heavier slurry has been found better adapted for app-lication by dipping, In any event, the components are thoroughly blended before application to the base metal.
The resulting mixture may then be sprayed onto the metallic surface with a conventional paint spraying gun 1 Registered trademark of Monsanto Chemical Co. Registered trademark of Colton Chemical Co. 3 Product of Colton Chemical Co.
or other suitable spraying or painting device. The type of spraying or painting technique which is best suited for a particular application may, of course, be determined 'by experiment. The spray should be such as to completely cover the surface of the metal while avoiding the formation of large droplets.
The coated article is then dried, conveniently by an air-drying step, although higher temperatures may be usedand are preferred in any case wherein the composition contains free or hydrated water or wherein the liquid vehicle is insufficiently volatile at room temperature within feasible time limitations. Unless the coatings are thoroughly dried 'before exposure to high, hot working temperatures, blistering and peeling of the coating and basis metal contamination will occur. In the case of the preferred, highly volatile vehicles, and with water-free solids, a few minutes at room temperature suffices to remove volatile matter. If an aqueous vehicle is used, several hours are required for drying, For best results, particularly in the case of water-containing; coatings, the coated articles are baked, e.g., at temperatures of at least about 1400 F., for times of, for example, 30 minutes or more.
Further, specific examples of particularly useful and effective coating compositions in accordance with the invention are set forth in Table II.
In the case of each of the Table II compositions, the powdered solids were mixed together and added to a liquid vehicle consisting of a chlorohydrocarbon containing methylchloroform (Chloroethane Each slurry composition, intended for spray application, was thoroughly blended, and consisted of one pound of solids per quart of liquid vehicle.
Coating No. is water-soluble and is principally useful for hot working applications at temperatures of about 1200-1600 F. for times up to about one-half hour.
Coating No. 50 is principally useful at temperatures of about 1400-2000 F. for times up to about 6 hours.
Coating No. 72 is principally useful at temperatures of about 18002300 F. for times up to 6 hours (about 4 hours maximum at 2300 F.)
Water-insoluble coatings such as Nos. 50 and 72 are removable from the basis metal articles, rafter hot working, by suitable mechanical means, e.g., by grinding or grit blasting. My intermediate temperature coatings, as No. 50 of Table II, are soluble in hot caustic and may be removed from the basis metal article in that manner.
Best results are obtained by the use of the protective coatings of the invention if the coatings are applied to a clean scaleor oxide-free basis metal surface. Thus comparative tests, using coating No. 72 of Table II, showed that, when the coating was applied to a test specimen of titanium-13% vanadium-11% chromium 3% aluminum, measuring 0.50 inch x 2.5 inch x 3 inch, and having a surface oxide film (produced by a 1 Registered Trademark of Dow Chemical Co.
4 hour anneal at 1900 F. followed by grit blasting), and the specimen subjected to a 50% gage reduction, resultant surface cracking of the specimen was about the same as for another specimen similarly treated but free of the protective coating. On the other hand, a third specimen, similarly treated, but wherein the same coating was applied to a previously deskinned, clean surface, showed an excellent, smooth surface, Without cracks.
temperature range, e.g about 1600 F., a thickness of from about 5 to about mils gives best protection, whereas coatings, as No. 72 (Table II), should be at least about 14 mils in thickness, all when used in their respective temperature ranges for about the maximum The above-mentioned factors, i.e., surface cleanli- 10 times permissible. My tests have shown that, under nessbefore coating application, and drying the coating the most severe of the contemplated temperature conbefore subjection of the coated article to a high, hot ditions, coating thicknesses up to about 20 mils are working temperature, are not the only factors of imeffective. portance in the practice of the inventive method and Limitation of coating thickness is of further importance in the provision of the novel coated articles. Limitain respect to the extent of protection afforded against the tation of the coating thickness to a restricted value is embrittling effect of hydrogen absorption by the basis also of great importance if the full advantages of the inmetal. If the coating is too thin, excessive amounts of vention are to be realized. Thus, I have found that, hydrogen are allowed to pass into the basis metal during if the coatings are too thin, adequate protection of the elevated temperature treatment thereof. Surprisingly, I basis metal is not obtained. Moreover, if the coatings have also found that excessively thick coatings tend to are excessively thick, blistering and peeling will occur contribute hydrogen, to the basis metal, originating, I (by evolution of volatiles, principally from the organic believe, from volatile matter in the coatings. This effect binder and absorbed moisture), exposing the basis metal of coating thickness upon hydrogen content of the basis to atmospheric contamination. Supporting the foremetal is clearly manifested by the results of experimental going, tests were conducted wherein test panels of a tests wherein 2 inch x 3 inch x 0.090 inch test coupons Ti-13%V-1l%Cr-3%Al alloy were provided with the of a substantially hydrogen-free alloy comprising 13% inventive surface coatings, of various thicknesses, exvanadium, 11% chromium, 3% aluminum, balance subposed in air to an elevated temperature and then visually stantially titanium, were coated with the inventive coatinspected for appearance of coating and metal surface. ing compositions, in several thickness of each, and sub- The results are given in Table III hereinbelow. 3O jected to an elevated temperature treatment. The hydro- TABLE III Test Conditions Coating Coating Composition Thickness, Panel Condition, After Testin 3 Panel No. Temp, I Time, Hrs. Mils No. 80, Table II 1 1, 500 1 2 Substantially unaffected.
Do 2 1, 500 1 4 Very slight coating blistering;
very slight surface oxidation.
D0 3 1, 500 1 8 Extensive coating blistering;
uniform surface oxidation.
None 4 1, 500 1 0 Uniform surface 0 'idation. No. 80, Table II 6 1, 500 1 1 Coating unaffected; slight, uniform surface oxidation. None 6 Unonnealed comparison panel. No. 50, Table II 13 1, 600 4 2 Coating substanti illy unaffected;
Zery slight metal surface oxidaion.
D0 .1 14 1, 600 4 7 Substanti 11y unaffected.
D0 15 1, 600 4 11 Slight to mo lerate eotting blistering; uniform, moderate metal surface Oxidation.
Do 16 1, 600 4 17 Extensive co itizig blistering and extensive, uniform metal surface oxidation.
D0 17 1, 600 4 0 Uniform, extensive metal surface oxidation.
one 18 Unannealed comparison panel. No. 72, Table II 31 2, 200 4 4 Coating unaffected; slight metal surfzce oxidation.
Do 32 2, 200 4 6 Very slight coating blistering;
slight metal surface oxidation.
D0 33 2, 200 4 11 Very slight coating blistering;
slight metal surf ice oxid ition.
Do 34 2, 200 4 14 Getting substanti ally unaffected;
zery slight metal surface oxida- Do 35 2, 200 4 0 Drastic cotting impairment and metil surface oxidation.
None 3 t. Unannealed comparison panel.
1 Graded, on basis of comparison with untreated control P111615, and increasing severity of visually observable impairment of coating integrity and oxidation of metal surface.
From the Table II data, it will be seen that, on the gen content of each of the samples was then determined,
basis of coating blistering and metal surface oxidation, with results as given in Table IV.
TABLE IV Anncal- Anneal- Coating Hydro Coating Com- Sample ing ing Thickgen Abposition No. Temp, Time, ness, sorbed,
F. hrs. mils ppm None 1 1, 500 1 o 65 No. 80, Table II. 2 1. 500 1 1 47 D 3 1, 500 1 2 4 4 1, 500 1 4 9 5 1,500 1 8 22 6 1,700 1 0 97 7 1, 700 1 1 12 8 l, 700 1 2 28 9 1, 700 1 4 18 10 1, 700 1 9 27 11 1, 600 4 0 101 12 1, 600 4 2 12 13 1. 600 4 7 8 14 1.600 4 11 50 15 1. 600 4 18 46 It is evident, from the Table IV data, that, for the low temperature range coatings, as No. 80 (Table II), coating thickness must be between about 1 and 4 mils, preferably 2 to 4 mils, in order to have best protection against hydrogen absorption, over a temperature range of 1500- 1700 F. For similar reasons, the intermediate temperature range coatings, as No. 50 (Table II), must be limited in thickness to between about 2 to 10 mils and preferably at least about 5 mils.
A great number of prior art protective coatings comprise silica, or other silicon-containing materials, in sub stantial amounts. However, I have found such materials have a distinctly undesirable result in my coating composition, in respect to acquired hardness 0r embrittlement, after thermal treatment, of basis metals, as titanium and its base alloys, which alloys are profoundly affected by the presence therein of even very small quantities of silicon. Upon heating of articles bearing a silicon-containing substance, the latter are reduced and silicon diffuses into the underlying basis metal. Thus, even though coatings comprising boron oxide and feldspar, on the one hand, and boron oxide and silica, on the other, are otherwise somewhat similar in respect of the visible effects of thermal treatment on articles coated therewith, the latter are of definitely lesser value due to the aforesaid effect. This latter effect is clearly illustrated by the results of tests performed on a number of %-inch (0.187 inch surface to center) bar samples of a titanium base alloy comprising 5% aluminum, 2.5% tin. Some of these samples were coated with a 70% feldspar, 30% boron, oxide, and the others with a mixture of 80% silica, boron oxide heated for 4 hours at 2300 F., and then hardness measurements (Vickers Hardness Number, with an 0.1 kg. load) were made at several distances radlally inwardly of the sample surface. esults of such tests are given in Table V.
TABLE V Radial Dis- Sample tance from Vickcrs Coating Composition N o. Coated Sur- Hardness face, inches No.
70 ields ar boron oxide 1 0.005 240 p 2 0. 01 240 3 0. 02 228 4 0. 03 240 5 0. 04 230 6 0, 05 235 7 O 6 248 8 0 07 215 9 0. Q8 220 10 0.0 9 210 11 0. 1 87 d (sargple centero) 00 80 silica 20 boron oxi e 1 5 I (sample center) The markedly higher hardnesses-throughout the sample section-of the samples bearing the silica-containing coatings, clearly show the inferiority of such coatings to those of the present invention which, by virtue of their ability, in the aforesaid thickness ranges, to resist hydrogen penetration, obviate the necessity of expensive and time-consuming vacuum annealing treatments to remove hydrogen.
Additions of alkali metal oxides, particularly sodium oxide, are ordinarily avoided in my novel coating compositions, for such materials lower the coating fusion temperature and also the coating viscosity, thereby narrowing the useful temperature range.
The coatings and coated articles of the invention, and the application thereof in the inventive method, make use of a flux boron oxide, of relatively high melting point, as contrasted, for example, to other prior art fluxes, as the sodium borates, together with a ceramic filler, feldspar, in contrast of relatively lower melting point than commonly used prior art ceramic hot working coating constituents, as clay. The result is a highly useful group of compositions which are useful, over a wide range of temperatures, as working lubricants as well as protective coatings.
It can thus be seen that the present invention provides coating compositions and methods affording excellent protection of metals, particularly refractory metals, against contamination at high temperatures, while being readily removable after the heating operation is completed. Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in form, composition and process steps may be made without departing from the spirit and scope of the invention.
1. The method of lubricating and protecting refractory and other metals from surface contamination and scaling during heating and working operations comprising: applying to the surface of such a metal a suspension of a powder mix consisting essentially of about 50 to boron oxide and about 20 to 50% feldspar in a vehicle of highly volatile liquid, volatilizing said vehicle whereby a substantially continuous coating is deposited on the surface of said metal, heating the thus-coated metal to a temperature up to 1800 F. for a period up to 30 minutes for coatings containing more than 70% boron oxide and to a temperature up to 2000 F. for a period up to 8 hours for coatings containing less than 70% boron oxide, whereby said coating fuses to form a glass coating which is suitably viscous and self-repairing.
2. The method of claim 1 wherein the suspension contains, by weight percent of total solids, from an effective amount up to about 8% of a clay-like suspending agent, and from an effective amount up to about 10% of a substantially non-volatile organic binder.
3. The method of claim 2 wherein the suspending agent is bentonite.
4. The method of lubricating and protecting refractory and other metals from surface contamination and scaling during heating and working operations comprising: applying to the surface of such a metal a suspension of a powder mix consisting essentially of about 20% to 50% boron oxide and about 50% to 70% feldspar in a vehicle of highly volatile liquid, volatilizing said vehicle whereby a substantially continuous coating is deposited on the surface of said metal, heating the thus-coated metal to a temperature up to 2300 F. for a period up to eight hours whereby said coating fuses to form a glass coating which is suitably viscous and self-repairing.
5. The method of claim 4 wherein the suspension contains, by weight percent of total solids, from an effective amount up to about 8% of a clay-like suspending agent, and from an effective amount up to about 10% of a substantially non-volatile organic binder.
6. The method of claim 5 wherein the suspending agent is bentonite.
7. The method of lubricating and protecting refractory and other metals from surface contamination and scaling during heating and working operations comprising: applying to the surface of the metal a suspension of a powder mix consisting essentially of, by weight percent of solids,
Boron oxide 30 to 50 Feldspar 30 to 50 Bentonite 1 to 8 in a vehicle of a highly volatile liquid, volatilizing said vehicle whereby a substantially continuous coating is deposited on the surface of said metal, and heating the thus-coated metal to a temperature in the range of about 16002000 F. whereby said coating fuses to form a glass coating which is suitably viscous and self-repairing.
8. The method of hot working light sheet metal product articles comprising the steps of: applying to the surface of such a metallic article a protective lubricant consisting essentially of, by weight percent, about 60 to 80% boron oxide, 20 to 30% feldspar in a vehicle of highly volatile liquid, volatilizing said vehicle whereby a substantially continuous coating is deposited on the surface of said metallic article, heating the thus-coated metallic article to a temperature up to 1800 F. for a period up to 30 minutes whereby said coating fuses to form a glass coating which is suitably viscous and self-repairing, working said metallic article at said temperature while said glass coating acts as a lubricant to minimize friction between said article and hot working means in contact therewith, cooling said Substantially non-volatile organic binder article and adherent glass coating, and removing said glass coating from the surface of said article by dissolution with warm water.
9. A method of hot working heavy mill metal product articles comprising the steps of: applying to the surface of such a metallic article a protective lubricant consisting essentially of, by weight percent, about 30 to boron oxide and about 50 to feldspar in a vehicle of highly volatile, non-aqueous liquid, volatilizing said vehicle whereby a substantially continuous coating is deposited on the surface of said metallic article, heating the thuseoated metallic article to a temperature up to 2300" F. for a period up to eight hours whereby said coating fuses to form a glass coating which is suitably viscous and selfrepairing, and working said metallic article at said temperature while said glass coating acts as a lubricant to minimize friction between said article and hot working means in contact therewith.
References Cited by the Examiner UNITED STATES PATENTS 1,091,678 3/1914 Locke 106-54 1,736,642 11/1929 Beaudry 10647 2,393,449 1/1946 Armistead 106-47 2,858,235 10/1958 Rex 29-528 2,934,444 4/1960 Smith 10647 2,962,808 12/1960 Cole et a1 29424 3,195,333 7/1965 McDaniel 72-46 CHARLES W. LANHAM, Primary Examienr.
R. D. GREFE, Assistant Examiner.