US2757445A

US2757445A - Hard surface composite article and method of making

Info

Publication number: US2757445A
Application number: US153999A
Authority: US
Inventors: Earl M Anger
Original assignee: Duraloy Co
Current assignee: Duraloy Co
Priority date: 1950-04-04
Filing date: 1950-04-04
Publication date: 1956-08-07
Anticipated expiration: 1973-08-07

Description

United States Patent Earl M. Anger, Scottdale, Pa., assignor to The Duraloy Company, Scottdale,

Pa., a corporation of Delaware No Drawing. Application April 4, 1950, Serial No. 153,999

24 Claims. (Cl. 29-195) The present invention relates generally to the art of coating metal articles and is particularly concerned with novel metal articles having surface portions of exceptional hardness and durability, and with novel methods by which said articles can be produced.

Commercial rolls, dies, guides and similar devices, which are employed at elevated temperatures in steel handling and shaping operations rapidly pick up metallic particles which firmly attach themselves to said devices. For example, the rolls on the shafts in roller bottom heating furnaces pick up metal from the sheets which they convey through the furnace and such picked up metal takes the form of minute sharp pointed cones which adhere tenaciously to the rolls and form pits or depressions in the sheets subsequently moving over the rolls. It is prohibitively expensive to free the rolls of picked up metal and the rolls are too expensive to be replaced whenever they have picked up such metal. There has consequently been a long-standing urgent demand for a solution to this problem, which has not to my knowledge hitherto been met.

There has also been a long standing, urgent and heretofore, unsatisfied demand for articles which would resist over protracted periods the corrosive effects of molten aluminum, such as melting pots, skimmers and thermowells employed in the handling and treatment of molten aluminum. Ordinarily, iron, steel and steel alloy articles of this type are rapidly attacked and destroyed in ordinary commercial use. Likewise, in galvanizing operations there has been a serious and heretofore unsolved problem of rapid corrosion and destruction of such metal equipment by molten zinc.

In accordance with the present invention, all these problems can be solved. The articles of my present invention, for example, not only do not pick up metal particles in high temperature metal working conditions, but they are so resistant to corrosion by molten aluminum and molten zinc that they can be used for periods many times longer than iron, steel and steel alloy articles heretofore employed. In fact, in contact with molten zinc, rather than being eroded or corroded, the articles of my invention are built up to a measurable but not undesirable degree, a blue powder being formed on them which tends to even further increase their wear-resistance and normally long useful lives. Furthermore, the articles of my present invention have the surprising characteristic of being useful in the production of plate glass, where they solve the old serious problems of pickup and sticking. My articles in the form of glass forming rolls do not either pickup particles of raw hot glass stock or stick to it. Thus, this invention promises to save vast amounts of plate glass production expenses, the frequent replacement and cleaning of glass processing rolls heretofore always being essential to the production of satisfactory glass plates.

In addition to the foregoing advantages, the articles of this invention arerelatively inexpensive to produce, compared to the devices of the prior art, and their manufacice ture in accordance with the method of this invention is relatively simple and free from critical circumstances which are difficult to establish and maintain.

Briefly stated, a typical article of this invention comprises a suitable metallic base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of layers of aluminum alloys of different compositions, the alloy portion being between about 0.005 inch and 0.040 inch thick and having an oxide-rich surface approaching diamond hardness and having a diffinity for metal particles, said surface containing alumina as a preponderant oxide constituent and also containing an oxide of metal of the base portion in a matrix of aluminum alloy.

The metallic base portion of the article of this invention may be of any desired form as, for example, a roll or skimmer, a melting pot, a thermo-well, or the like as mentioned above. Furthermore, this base portion may be covered entirely or only partially with the aluminum alloy portion, depending upon the: portion of its area that is to be subjected in use to circumstances conducive to corrosion or undesirable accretion or sticking, as above stated.

The base may suitably be of any of a number and variety of metals and alloys. The first and foremost criterion of a suitable metal or alloy for the base is that this metal or alloy be of melting point temperature substantially above the melting point temperature of pure aluminum and preferably that it be above about 1400" P. which as will be explained later, is about the minimum heat treating temperature capable of producing the foregoing results regardless of the type or kind of metal or alloy present in the base. In the preferred practice of this invention, I employ a commercial alloy steel of chromium and nickel in which nickel content is approximately 35% and the chromium 15%, the balance being primarily iron of the usual carbon content of about 0.2% with additional minor amounts of the usual impurities such as manganese and silicon. It is, however, entirely possible to obtain the foregoing results by using other alloy steels, such as chromium steel containing approximately 12% chromium, and to use, for example, simple carbon steels ranging in carbon content from about .09% to 1.5%. Commercially pure iron, or even absolutely pure iron, is also suitable as are malleable, white and gray cast irons providing that the grain size of the graphite in the latter is not so unusually large as to interfere with bonding of the aluminum alloy portion to the base article and alloying across the interface of the base and aluminum alloy coating. In addition to the ferrous metals, copper, bronzes, nickel and nickel alloys such as Inconel, Nichrome and the like, and chromium-nickel alloys can be used to gain the advantages of this inventron.

The base article can take the form of a massive solid body or a mere coating on a core of some other composition, the important considerations being that the base have the ability to accept the aluminum coating and the ability to alloy with the deposited aluminum to produce the tenacious adherence of the aluminum alloy layer required in general commercial use of the final article.

In the preferred practice of this invention, the base article comprises a core of ferrous metal and a coating of suitable alloy as described above. Accordingly, the first step in the method of this invention in the preferred practice is the spraying of the ferrous metal. core with an oxidation-resistant alloy steel selected from the group consisting of nickel steels, chromium steels and chrtr mium nickel steels and thereby coating the core.

Preliminary to producing the aluminum alloy portion on the base, the base, regardless of its composition or form is suitably prepared to assure tenacious adherence of the aluminum alloy produced in accordance with the subsequent steps of my process. This preparation, preferably involves roughening the base surface and thoroughly cleaning it, as by sand-blasting, pickling, etching or some equivalent operation. In some instances, however, satisfactory preparation of the base surface may be accomplished by merely heating it to between about 200 F. and 500 F., providing the aluminum coating is applied while the temperature of at least the surface portion of the base is within this temperature range.

The production of the aluminum alloy portion or coating is accomplished by metal spraying molten of heat-softened aluminum on the base article prepared as described above. The metal spraying is done in accordance with the usual commercial practice and is followed by a heat treatment step, which is essential to the production of an article having the qualities and advantages set forth above. These circumstances of this step are important, particularly so far as minimums are concerned, but they need not be varied in accordance with such factors as film thickness so long as the thickness of the aluminum film deposited on the base article is within the range between about 0.005 inch and about 0.040 inch. Films which are outside this range, however, are not desirable as they cannot be relied upon to produce consistently the foregoing results. These heat treatment circumstances are preferably varied somewhat with variations in the constitution of the metal base, although generally speaking, satisfactory results can be obtained where a deposited aluminum film of the requisite thickness is subjected to a temperature between about 1400 F. and 1800 F. for a period between about five hours and about seven hours under an oxidizing atmosphere, that is, an atmosphere containing about 15% to 30% oxygen. If the base is of copper or a bronze, I prefer to limit the temperature of this treatment to a maximum of about 1600 P. But where the base is of ferrous metal, i. e. iron, steel, steel alloy or cast iron, I prefer to subject the coating to a temperature between about 1500 F. and about 1800 F. Also in the better practice of this invention, the objects regardless of the constitution of the base article are furnace-cooled gradually to room temperature under the same atmosphere as that to which they are subjected in the previous heating period. It will be understood, however, that satisfactory results and articles of my invention may be obtained when the heat treating step is carried out at temperatures substantially above those stated above and/ or for periods substantially longer than five to seven hours, providing the base is not softened or melted or distorted too much.

The circumstances of the heat treatment are designed to produce extensive alloying of the aluminum of the coating with the metal or metals of the base bearing the coating. It is also designed to produce suflicient oxidation of the aluminum of the coating and the metal or metals of the base alloyed with the aluminum and present in the surface of the aluminum deposit so that an oxide rich surface portion is produced and is present on the finished article as stated above. Thus, since copper and tin alloy more readily with the aluminum than does iron, chromium or nickel, the temperature range in this heat treatment may be and preferably is lower where the base article is essentially of copper or bronze than where it is of ferrous metal, nickel, nickelchromium alloy or chromium-nickel alloy.

I have obtained particularly good results where the heat treatment of an alloy steel base having an aluminum coating in the above prescribed thickness range has been carried out in several temperature stages. In the first stage of this treatment, the coating is subjected to a temperature of about 1400 F. and maintained at that temperature for about five hours. The coating is then subjected to a temperature of about 1600 R, which is maintained for about five more hours. Finally, the

temperature to which the coating is subjected is increased to about 2000 F. and maintained for about five hours and then the object is cooled in air to room temperature. The article may be cooled between these various heating steps, or any of them, but ordinarily no useful purpose is served thereby and it is obviously not economical to do this. When the heat treatment is carried out as I have prescribed, there is produced in the surface of the deposited aluminum coating or film a hard complex mixture of compounds. I am not certain as to exactly what takes place to produce these compounds, but I know that in general, three well-defined layers of aluminum alloys of different compositions are formed in this coating portion and that it is to these layers and especially the outermost one that the extreme hardness, the resistance to pickup, and other advantages of my articles are attributable. In all the samples of my present articles that I have examined and analyzed, the first or innermost layer which is bonded directly to the base article is primarily a complex solid solution or alloy of aluminum and the metal or the metals of the base, the aluminum being present in relatively small proportions. The second or intermediate layer is materially richer in aluminum than the first, but also is an alloy of aluminum and the metal or metals of the base article. The outermost layer is rich in aluminum compared with the others and is largely an alloy of aluminum with minor amounts of the metal or metals of the base article. I have previously hypothesized the existence of a very thin skin of aluminum overlying this outermost alloy layer but I have not been able definitely to establish the existence of this skin. Instead I have found that all the aluminum in this outermost part of the coating is in the form of aluminum-rich alloy, inter-metallic compounds with metal of the base and alumina, most of which is the alpha variety. I have further found that this outermost layer contains on its surface and immediately therebelow appreciable amounts of the oxide or oxides of the metal or metals of the base. The alumina and the said oxide or oxides are disposed in a matrix of the aluminum alloy of the outer layer and they are tenaciously retained by this alloy. The total amount of oxide in the surface of the article of this invention normally amounts to between about 30% and about 40% of the surface portion of the article. By surface portion I mean that part of the outermost alloy layer within about 0.0004 inch of the surface.

The hardness of the present articles is undefinable in exact absolute terms because I have not been able to discover a polishing medium which will produce a sat isfactory surface for an accurate hardness test. The working surfaces of these articles are substantially harder than the surface of similar articles heretofore known and used and they closely approach diamond hardness. Articles such as rolls prepared in accordance with this invention, in fact, rapidly wear out emery wheels and the like which have been applied to them in attempts to effect polishing, and these articles are sufficiently hard to withstand the most severe wearing uses as rolls in roller-bottom heating furnaces, for example, without such erosion or deformation as will require their frequent replacement.

The following illustrative, but not limiting, examples of the method of this invention are offered to assist those skilled in the art to understand and practice the present invention:

Example I An alloy steel roll for a roller-bottom heating furnace containing about 35% nickel, 15% chromium, 49% iron and 4% carbon was sand blasted and thereby thoroughly cleaned. Molten aluminum was sprayed on the thus prepared roll in accordance with conventional metal spraying practice and a deposit of aluminum of uniform thickness of, about .020 inch was produced. The coated roll was subjected to a temperature of 1600 F. for five hours in an ordinary, indirectly-fired heat treating furnace, under an oxidizing atmosphere containing approximately 15% free oxygen, the balance largely being free nitrogen. At the end of the heating period the article is permitted to cool in the oven to substantially room temperature.

Microscopic examination of the coating on the article revealed three well-defined aluminum alloy layers of varying aluminum contents from the innermost to the outermost layer. Hardness tests proved the deposited coat to approximate diamond hardness. Spectrographic analyses of the outer .0004 inch portion revealed the presence of aluminum oxide primarily as alpha alumina, iron oxide, chromium oxides and nickel oxides, all of which were contained in a matrix of aluminum-rich alloy which also contained substantial amounts of the AlzCr. Several unidentified constituents were detected in this coating surface portion, one of these being in substantial percentage and showing in considerable strength on the spectrum at the nine angstrom position. This constituent remains unidentified to date.

Chemical analysis of the skin portion of the coating of the aluminum alloy portion revealed that 22.08% of that parts was aluminum oxide, 12.16% was nickel oxides (as NiO and NiOz), 1.66% was F6203, 1.38% was chromium oxides, and the balance was metallic, 45.80% being aluminum, 11.74% being iron. Silica, silicon and manganese also were present in small amounts.

Example 11 A roll similar in form to that of Example I, but of different composition, being a chromium alloy steel containing about 12% chromium, was similarly prepared by sand blasting and was metal spray-coated with aluminum to a thickness of about .020 inch. The heat treatment was the same as that stated in Example I. Metallographic and spectroscopic analyses of the surface portion of the final article revealed that the coating was substantially like that of Example I. Chemical analysis of the coating revealed that the surface or skin contained 35% alumina existing primarily in the alpha form, 3.4% of chromium oxide and 1.5% iron oxide. The balance of this surface portion was metallic aluminum amounting to about 50% of the coating, chromium and iron being present in lesser amounts but alloyed with the aluminum as in the previous example.

Hardness of this article proved to correspond closely to that of the article of Example I.

Example III Still another roll like that of Example I, but of copper rather than alloy steel, was metal spray-coated with heat softened aluminum after being prepared by sand blasting. The aluminum coat was established uniformly at 0.010 inch and the roll was subjected to a temperature of about 1450 F. for about five hours under an oxidizing atmosphere as described in Example I.

The final article, after being cooled in air to room temperature, was examined microscopically, spectroscopically and chemically. The article had three welldefined aluminum-copper alloy layers in the aluminum alloy portion, the outermost layer being the richest in aluminum and the innermost being the richest in copper. The surface portion (to 0.0004 inch depth) of the article contained an alumina and copper oxide, the former being the preponderant oxide constituent of the surface portion, and contained aluminum-copper alloy as a matrix for the oxides. The total oxide content of this portion amounted to about 35%. Again hardness measured as accurately as possible approximated that of diamond.

Example IV A roll like that of the foregoing examples but of mild steel was sand blasted and then provided with an aluminum coating about 0.030 inch thickness by conventional metal spray procedure. After heat treatment involving subjecting the coated article to a temperature of about 1700 F. for six hours under an oxidizing atmosphere as defined in Example I, the article was examined as in the previous cases. Hardness proved to approximate the diamond. Three well-defined aluminum alloy layers were present in the coating and the aluminum content increased from the inner to the outer layer. The surface portion (to 0.0004 inch depth) proved to contain about 30% oxides, the large proportion of which was alumina (mostly in the alpha form). Iron oxides were, however, present in this surface portion in appreciable quantity and all these oxides were contained in and firmly bonded to an aluminum-rich alloy matrix.

Example V An Inconel roll in form like that of Example I was prepared by sand blasting and then metal spray-coated with aluminum in accordance with conventional practice, the coat being uniform and about 0.035 inch thickness. Heat treatment as described in Example I but lasting for about seven hours resulted in a final article of approximately diamond hardness in the aluminum coated portion. Again three Well-defined aluminum alloy layers existed in this portion and these layers as in the foregoing examples were of approximately the same thickness. The aluminum content of these layers increased from the inner one to the outer and the outer layer had an oxide-rich surface portion (0.0004 inch deep) in which the proportion of oxide to metal (alloy) amounted to about 40%. The preponderant oxide was alumina (again primarily in the alpha form) but oxides of nickel and chromium were also present in appreciable amounts. These oxides were securely held in a matrix of aluminumrich alloy which appeared to be primarily of aluminum and nickel.

As may be deduced from the foregoing: examples, the rate of cooling in this heat treatment is not critical, but I prefer to cool the articles in the furnace or in air, for reasons of convenience and economy. The hardness of the surface of the articles thus prepared is not materially improved by quenching, and while quenching may increase the rate of production, it also requires special materials and equipment and more operations than my preferred practice. Heat treatment of special type is not necessary prior to application of the aluminum.

This is a continuation-in-part of my copending application, Serial No. 737,709, filed March 27, 1947, now abandoned in favor of the present case.

Having thus described the present invention so that those skilled in the art will be able to understand and practice the same, I state that what I desire to secure by Letters Patent is defined in what is claimed.

What is claimed is:

l. A corrosion resistant metal article comprising a base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of aluminum alloys of different compositions, said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a ditfinity for metal particles, said surface portion containing alumina as a preponderant oxide constituent and an oxide of metal of the base in a matrix of aluminum alloy.

2. A corrosion resistant metal article comprising a steel base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of aluminum alloys of different compositions, said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a difiinity for metal particles, said surface portion containing alpha alumina as a preponderant oxide constituent and iron oxide in a matrix of aluminum alloy. ll

3. A corrosion resistant metal article comprising a copper base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of aluminum alloys of different compositions, said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a diffinity for metal particles, said surface portion containing alumina as a preponderant oxide constituent and copper oxide in a matrix of aluminum alloy.

4. A corrosion resistant metal article comprising a bronze base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a pluraliy of aluminum alloys of different compositions, said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a diffinity for metal particles, said surface portion containing alumina as a preponderant oxide constituent and an oxide of metal of the base in a matrix of aluminum alloy.

5. A corrosion resistant metal article comprising a base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of aluminum alloys of different compositions, said base portion consisting essentially of a metallic material selected from the group consisting of pure iron, commercially pure iron, steels, copper, bronzes, nickel and nickel alloys, and said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a diflinity for metal particles, said surface portion containing alumina as a preponderant oxide constituent and an oxide of metal of the base in a matrix of aluminum alloy.

6. A corrosion resistant metal article comprising a chromium-nickel alloy-steel base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of aluminum alloys of different compositions, said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a difiinity for metal particles, said surface portion containing alpha alumina as a preponderant oxide constituent and oxides of iron, chromium and nickel in a matrix of aluminum alloy.

7. A corrosion resistant metal article comprising a base portion and an aluminum alloy portion bonded tenaciously thereto and comprising three well-defined aluminum alloys of different compositions and substantially the same thickness, the aluminum content of the alloys of said layers increasing from the inner layer contacting the base portion to the outer layer, said alloy portion being between about .005 inch and about .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a diflinity for metal particles, said surface portion containing alumina as a preponderant ox-- ide constituent and an oxide of metal of the base in a matrix of aluminum alloy.

8. A corrosion resistant metal article comprising a base portion and an aluminum alloy portion bonded tenaciously thereto and comprising a plurality of aluminum alloys of different compositions, and increasing aluminum content from the inner layer contacting the base portion to the outer layer, said alloy portion being between about .005 inch and about .040 inch thick and and having an oxide-rich surface portion approaching diamond hardness and having a difiinity for metal particles, said surface portion having an oxide content between about and about 40% and containing alumina as a preponderant oxide constituent and an oxide of metal of the base.

9. A corrosion resistant metal article comprising an alloy steel base portion containing about nickel and 15% chromium, and an aluminum alloy portion bonded tenaciously to said base portion and comprising three welldefined aluminum alloys, the aluminum content of the al loys of said layers increasing from the inner layer contacting the base portion to the outer layer, said alloy portion being between about .005 inch to .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a diffinity for metal particles, said surface portion containing about 22% alpha alumina, about 12% nickel oxides, about 1.5% chromium oxides and about 1.5 iron oxides, the balance of the surface being essentially metallic and the metal thereof serving as a matrix for the said oxides.

10. A corrosion resistant metal article comprising a chromium-iron base portion containing about 12% chro mium, and an aluminum alloy portion bonded tenaciously to said base portion and comprising three well-defined aluminum alloys, the aluminum content of the alloys of said layers increasing from the inner layer contacting the base to the outer layer, said alloy portion being between about .005 inch to .040 inch thick and having an oxide-rich surface portion approaching diamond hardness and having a diffinity for metal particles, said surface portion containing about 35% alpha alumina and about 3.5% chromium oxides and about 1.5% iron oxide, the balance of the surface consisting essentially of metal serving as a matrix for the said oxides.

11. A metal article for use in high temperature metal working operations comprising an alloy steel body having a surface portion containing aluminum and approaching diamond hardness and having a difiinity for metal particles, produced by metal spraying an object having a chromium-nickel-iron alloy surface with aluminum and there by producing on said object an aluminum coating between about 0.005 inch and about 0.040 inch thickness, heating the object to about 1400 F. and maintaining it at that temperature for about five hours, raising the temperature of said object to about 1600 F. and maintaining it at about that temperature for about five hours, raising the temperature of said object to about 2000 F. and maintaining it at that temperaure for about five hours, and cooling the object in air to room temperature.

12. The method of producing a metal article for use in high temperature metal working operations having a working surface approaching diamond hardness and having a ditfinity for small metal particles which comprises the steps of metal spraying a metal object with an oxidation resistant alloy steel selected from the group consisting of nickel steels, chromium steels and chromium nickel steels and thereby coating said object, metal spraying the thus coated object with aluminum and thereby coating the object with aluminum and thereby producing on said object an aluminum coating between about 0.005 inch and about 0.040 inch thickness, heating the coated object to about 1400" F. and maintaining it at about that temperature for about five hours under an oxididing atmopshere, raising the temperature of the object to about 1600 F. and maintaining it at that temperature for about five hours, raising the temperature of said object to about 2000 F. and maintaining it at that temperature for about five hours, and cooling the object to room temperature.

13. The method of producing a metal article for use in high temperature metal working operations having a worksurface approaching diamond hardness and having a diiiinity for small metal particles which comprises the steps of metal spraying a ferrous metal object with an oxidation resistant alloy steel selected from the group consisting of nickel steels chromium steels and chromium-nickel steels and thereby coating said object, metal spraying the thus coated object with aluminum and thereby coating the object with aluminum and thereby producing on said object an aluminum coating between about 0.005 inch and about 0.040 inch thickness, heating the coated object to about 1400 F. maintaining it at that temperature to about five hours under an oxidizing atmosphere, then raising the temperature of said object to about 1600" F. and maintaining it at about that temperature for about five hours, then raising the temperature of said object to about 2000 F. and maintaining it at that temperature for about five hours, and cooling the object in airto room temperature.

14. The method of producing a metal article for use in high temperature metal working operations having a working surface approaching diamond hardness and having a diifinity for small metal particles which comprises the steps of roughening a surface portion of a ferrous metal object, metal spraying said surface portion with an oxidiation resistant alloy steel selected from the group consisting of nickel steels and chromium steels and nickelchromium steels and thereby coating said object, metal spraying the thus coated object with aluminum and thereby coating the object with aluminum and thereby producing on said object an aluminum coating between about 0.005 inch and about 0.040 inch thickness, heating the coated object to about 1400 F. and maintaining it at that temperature to about five hours under an oxidizing at mosphere, then raising the temperature of said object to about 1600" F. and maintaining it at about that temperature for about five hours, then raising the temperature of said object to about 2000 F. and maintaining it at that temperature for about five hours, and cooling the object in air to room temperature.

15. The method of producing a metal article for use in high temperature metal working operations having a working surface approaching diamond hardness and having a ditfinity for small metal particles which comprises the steps of heating a ferrous metal object to a temperature between about 200 F. and 500 F., metal spraying a surface portion of said heated object with an oxidation resistant alloy steel selected from the group consisting of nickel steels, chromium steels and chromium-nickel steels and thereby coating said object, metal spraying the thus coated object with aluminum and thereby coating the object with aluminum and thereby producing on said object an aluminum coating between about 0.005 inch and about 0.040 inch thickness, heating the coated object to about 1400" F. and maintaining it at that temperature to about five hours under an oxidizing atmosphere, then raising the temperature of said object to about 1600" F. and maintaining it at about that temperature for about five hours, then raising the temperature of said object to about 2000 F. and maintaining it at that temperature for about five hours, and cooling the object in air to room temperature.

16. The method of producing a metal article for use in high temperature metal Working operations having a working surface approaching diamond hardness and having a difiinity for small metal particles which comprises the steps of metal spraying with aluminum a metal object having a surface portion of an oxidation resistant alloy steel selected from the group consisting of nickel steels, chromium steels and nickel chromium steels and thereby producing on said surface portion an aluminum coating between about 0.005 inch and 0.040 inch thick, heating the coated object to about 1400" F. and maintaining it at that temperature to about five hours under an oxidizing atmosphere, then raising the temperature of said object to about 1600" F. and maintaining it at about that temperature for about five hours, then raising the temperature of said object to about 2000" F. and maintaining it at that temperature for about five hours, and cooling the object in air to room temperature.

17. The method of producing a metal article for use in high temperature metal Working operations having a working surface approaching diamond hardness and having a difiinity for small metal particles which comprises the steps of metal spraying a ferrous metal object with a chromium-nickel alloy and thereby producing a coat between about 0.005 inch and 0.025 inch thick on said object, metal spraying the thus coated object with aluminum and thereby producing a coat of aluminum between about 0.005 inch and 0.040 inch thick on said object, heating the coated object to about 1400 F. and maintaining it at that temperature to about five hours under an oxidizing atmosphere, then raising the temperature of said object to 10 about 1600" F. and maintaining it at about that temperature for about five hours, then raising the temperature of said object to about 2000" F. and maintaining it at that temperature for about five hours, and cooling the object in air to room temperature.

18. The method of making a corrosion. resistant metal article having a surface approaching diamond hardness and having a ditfinity for metal particles which comprises the steps of metal spraying with aluminum a metal ob ject and thereby producing on said object an aluminum coating between about 0.005 inch and about 0.040 inch thickness, heating the coated object for about five hours under an oxidizing atmosphere containing between about 15% and about 30% oxygen to between about 1400 F. and about 1800" F. and alloying the deposited aluminum with metal of said object, and cooling the object to room temperature.

19. The method of making a corrosion resistant metal article having a surface approaching diamond hardness and having a ditfinity for metal particles which comprises the steps of metal spraying With aluminum a ferrous metal object and thereby producing on said olbject an alumi num coating between about 0.005 inch and about 0.040 inch thickness, heating the coated object for about five hours under an oxidizing atmosphere containing between about 15% and about 30% oxygen to between about 1500" F. and about 1800 F., and furnace-cooling the object under said atmosphere to room temperature.

20. In combination, a furnace conveyor element having a work-supporting ferrous or alloy body, said body having a melting point above about 1400" R, an aluminum oxide surface covering the work-engaging portion of said body, said surface being in weld relation to the workengaging exterior portion of said body.

21. In combination, a furnace conveyor element having a ferrous or alloy body, a thin aluminum coating covering said body in bonded relation thereto and having a work-engaging surface consisting essentially of aluminum oxide, said body having a melting point above about 1400" F.

22. In combination, a furnace conveyor element having a ferrous or alloy body and a thin aluminum coating in weld relation thereto over the interface between the body and said coating, said body having a melting point above about 1400" F., said coating covering the work engaging exterior area of said body and having a depth greator than about .005 inch, said coating further having its work-engaging surface comprising aluminum oxide.

23. In combination, a furnace for heat treating metal work at 1400" F. or above, at least one conveyor element in said furnace to engage work passing through said furnace, said conveyor element having a work-supporting ferrous or alloy body, said metal body having a workengaging portion, said work-engaging portion having an aluminum oxide surface, whereby the formation of accretions is inhibited when work engages said aluminum oxide surface.

24. In combination, a furnace for heat treating metal work at 1400" F. or above, at least one conveyor element in said furnace to engage work passing through said furnace, said conveyor element having a work-supporting ferrous or alloy body, a thin aluminum coating in Weld relation to said body along the interface between said body and said coating, said coating having a Work-engaging surface of aluminum oxide, whereby the formation of accretions is inhibited when Work engages said aluminum oxide surface.

Gilson Mar. 24, 1914 Van Aller Oct. 5, 1915 (Other references on following page) '12 Whitfield et al. Aug. 1, 1939 Merritt Aug. 29, 1939 Axline Nov. 3, 1942 Simmons June 3, 1947 Robertson Dec. 6, 1949

Claims

20. IN COMBINATION, A FURNACE CONVEYOR ELEMENT HAVING A WORK-SUPPORTING FERROUS OR ALLOY BODY, SAID BODY HAVING A MELTING POINT ABOVE ABOUT 1400* F., AN ALUMINUM OXIDE SURFACE CONVERING THE WORK-ENGAGING PORTION OF SAID BODY, SAID SURFACE BEING IN WELD RELATION TO THE WORKENGAGING EXTERIOR PORTION OF SAID BODY.