US3325259A - Ferrous base with nickel-iron coating - Google Patents

Description

June 13, 196 7 MAYER ETAL 3,325,259

' FERROUS BASE! WITH NICKELIRON comma Filed May 13, 1964 4 Sheets-Sheet 1 INVENTORS Edward H. Mayer Hi/fon N. Ra/m Jr. R/bhard M. Will/son June 13, 1967 MAYER ET AL FERROUS BASE WITH NICKELIRON COATING 4 Sheets-Sheet 2 Filed May 13, 1964 QQQ 33c v8 k mumkum S E EmQEm QQQ QN hm \MJWNNC 5 3 INVENTORS Edward H. Mayer Hilton /V. Rah/2 Jr.

Richard/14. Wi/l/san June 13, 1967 MAYER ET AL 3,325,259

FERROUS BASE WITH NICKEL-IRON COATING Filed May 13, 1964 4 Sheets-Sheet 5 Distance from surface in mills a a a g & Q Q

INVENTORS l w/1v Edward H. Mayer H/lfon IV. Rah/7 Jr. R/tharo M. W/l/ison June 13,1967 E. H. MAYER ETAL 3,325,259

FERROUS BASE WITH NICKEL-IRON COATING Filed May 13; 1964 INVENTORS Edward H. Mayer Hi/fon /V. Ra/m Jr. R/bhara' M. Will/S00 United States Patent 3,325,259 FERROUS BASE WITH NICKEL-IRON COATING Edward H. Mayer, Hilton N. Rahn, Jr., and Richard M.

Willison, Bethlehem, Pa., assignors, by mesne assignments, to Bethlehem Steel Corporation, a corporation of Delaware Filed May 13, 1964, Ser. No. 367,182 6 Claims. (Cl. 29-196.1)

This invention relates to a ferrous-nickel alloy coating for .steel articles, and to the formation of such coat- The principal object of this invention is to produce a corrosion resistant iron-nickel alloy coating on steel surfaces such as those of sheet and strip.

Another object is to produce a dense, impervious nickelcontaining coating for steel articles, which coating is subsequently electrocoated with a decorative metal.

A further object is to form a nickel-containing coating by the application of nickel powder to the steel surface.

Generally, heretofore, when nickel-containing protective coatings have been used on the surfaces of steel sheets, strips, bars, etc., the coating has taken the form of a substantially pure nickel layer on the steel or iron substrate. These coatings have been formed principally by electrodeposition, or by cladding with nickel strip.

We have found that by applying a uniform layer of finely divided nickel powder to the surface of a steel article, such as strip or sheet, under controlled conditions, a ditfused iron-nickel alloy coating is produced wherein the alloying of iron and nickel is complete, i.e. all of the nickel powder alloys with diffused iron. Furthermore, by varying the operating conditions, the iron-nickel ratio can be varied within certain limits.

In addition to having excellent adherence and corrosion resistance characteristics, which characteristics render the alloy useful as a protective coating, the alloy of our invention has a dense structure, which fact makes the alloy useful as an undercoat for certain porous decor-ative metals, such as electrodeposited chromium.

In accordance with this invention, a steel sheet or strip is coated with a thin film of liquid, the film acting as a temporary bonding agent for subsequently applied metal powder. A metal powder of nickel is next applied uniformly over the surface of the strip, the powder being held in place by the previously applied liquid film. The powder covered strip is then subjected to a rolling operation to compact the powder. In this step, the powder is rolled into a flat, compacted metallic layer in which adjacent grains of powder are bonded together; This metallic layer is in a semi-adherent conditionin relation 7 to the strip, the underside of the metallic layer having been mechanically bonded to the strip surface. The composite article of strip and compacted metal powder is sintered in a heat-treating furnace at a controlled temperature, and for a time period sufficient to form an adherent iron-nickel alloy on the surface of the strip. Sintering is performed in a protective atmosphere, preferably one which is slightly reducing.

In the accompanying drawings:

FIG. 1 is a reproduction of a photomicrograph of a,

transverse section of a steel strip coated with an ironnickel alloy which was sintered at 1725 F. for 3 hours.

FIG. 2 is a graph showing the effect of increased sintering time at 1725 F. on surface nickel content.

FIG. 3 is a graph showing the effect of increased tem- FIG. 5 is a reproduction of the line scan of X-ray intensity for the coating of FIG. 1, as determined by electron microprobe.

FIG. 6 is a reproduction of a macrophotograph of a section of steel strip coated with an iron-nickel alloy, which has been subjected to a Swift cup test and etched with acid.

Referring to FIG. 1 it will be seen, from the welldefined line at the interface, that the alloy coating of our invention is a structure distinct and separate from the iron base metal. The iron base strip material used in the sample, from which the photomicrograph was obtained, had a carbon content of 0.003%, a very ductile and desirable type of iron where subsequent drawing operations are to be applied to the coated article. However, it is not essential that the carbon be as low as that shown for FIG. 1, Ductility requirements can be readily met with carbon contents as high as 0.10%. Higher carbon contents in the steel, those above about 0.10% to 0.15%, may lead to formation of porous coatings. Generally, if excellent ductility of the coating is desired, satisfactory results can be obtained with carbon in the base stock ranging up to 0.10%.

Our coating has several advantages over conventional nickel coating, including low production cost, and ease of producing the coating. Another advantage of the coating is its superior resistance to corrosion after the coated product has been subjected to moderate deformation.

In one preferred method, by which the alloy coating of the invention may be produced, a 20 gage strip of rimmed steel 5" wide, having a carbon content of 0.06% is unwound from a coil and passed through a roller coater bearing rubber applicator rolls, where a thin, uniform film of tridecyl alcohol is applied to both sides of the strip.

The strip is next passed through a fluidized bed of nickel powder. The nickel powder used in the fluidized bed is of a size less than 200 mesh, and is made from carbonyl nickel, analyzing 99.9% Ni. The alcohol film retains the powder layer on the strip in its passage to the next subsequent step, in which the strip is introduced into a set of 4 inch diameter hard finished rolls in a horizontal plane. In this operation, the powder is compacted with the base metal strip at a pressure sufficient to produce a strip elongation of about 10%. After leaving the rolls, the strip is wound on a take-up reel. The coil is unwound and recoiled with a spacer-wire inserted between adjacent laps to produce an open-coil effect. This spacing. of thelaps of the coil is necessary to prevent possible welding of adjacent laps in the subsequent sintering treatment, and to permit the sintering' gases to "circulate freely. The open-wound coil is placed in an annealing furnace, where it is heat treated at 1725 F. for 24 hours in a dry atmosphere of 18% hydrogen and 82% nitrogen on a volume basis. After sintering, the strip is brightened by cold rolling with polished rolls, to the extent of a 1% to 2% elongation of the strip.

In another example, exactly the same operating conditions are used as described above, except that a 36 inch wide strip is used as the base stock, and compacting is performed by means of 26 inch diameter rolls. The wider, compacted strip has an elongation of from 2% to 5%.

' While the foregoing examples relate to strip, it should be noted that the alloy coating can be i'applied to other flatware such as sheets and bars, and to rounds such as rods and wire. The same ,majorsteps of applying the powder, compacting and sintering are. applicable to any type of stock to which the coating'is applied, the method differing only in the particular manner of physically handling different types of base steel. Whatever the stock used, it is preferable, during sintering, to provide adequate spacing to prevent welding of adjacent parts of the article. When sheet or strip are coated, it is of course possible to coat one side, or both, as desired.

Generally, if the strip is soiled, it should be cleaned with a cleaning medium such as hydrocarbon solvent or an alkaline cleaner before coating.

The powder-retaining material, which is applied to the strip in the form of a thin film, and which acts temporarily as a powder-retaining medium, may be any liquid substance having the proper viscosity, volatility and tackiness characteristics, and which meets industrial safety requirements. The viscosity should, of course, be such that the film is maintained on the base stock until after the powder has been compacted. The liquid should be completely volatile at the sintering temperature so that no residue remains.

The metal powder can be applied to the steel backing member without a liquid substance having the above-stated characteristics first being applied, but the use of such film is preferred, for the film lends mechanical efficiency to the powder applying and compacting steps.

Liquids, other than tridecyl alcohol, which may be used as effective powder retaining films, are kerosene, transformer oil, strarw oil and certain naphthene base oils such as those having a Saybolt Universal viscosity of from 90 to 110 seconds at 100 F.

While not critical, it is desirable to control both the amount of liquid applied and the grain size of the metal powder. The alcohol, or other substance, used for retaining the metal powder, should be applied in a rather thin film of a thickness just sufiicient to cause adequate adherence of the powder particles. An excess of the liquid causes problems of slippage and inefiicient compacting during the rolling operation.

In respect to the nature of the powder which may be applied, electrolytic nickel powder, commerically pure nickel powder, nickel oxide, mixtures of nickel and nickel oxide powders, iron-nickel alloy, and mixtures of any of these with iron or iron oxide powder may be used to form the iron-nickel alloy coating. In general, elemental metal powders compact and sinter better than their oxides, or mixtures of metals and oxides. However, by psing a reducing sintering atmosphere, the nickel oxide can be completely reduced and the nickel alloyed with the steel base.

The compositions of these powders, as defined in the appended claims, may include minor amounts of inert materials, or other elements which do not adversely affect the process.

Powders have been used having a particle size ranging from a mixture 50% of 140 mesh to +325 mesh with 50% of 325 mesh, down to 100% of 325 mesh. Mesh numbers given herein correspond to the United States Standard Sieve Series. Generally, the finer powders appear to permit a wider variation in compacting and sintering practice, including the amount of elongation of the base steel required during compacting, and/ or time and temperature required in sintering. Coarser powders can be used to form the nickel alloy coating, but such powders tend to form porous coatings.

Very satisfactory compacting and sintering results when the weight of powder applied is approximately 20 grams per square foot of steel backing surface. Heavier or llghter applications of powder may be used, depending to some extent on the desired distribution of nickel in the alloy coating, and the desired thickness of the sintered coating. Satisfactory coatings have been made from powder in an amount of 8 grams per square foot, and from powder in an amount of 40 grams per square foot. Coatings formed from applications of powder greater than 20 grams per square foot will require correspondingly heavier applica tions of powder retaining fiuid.

Any relatively pure nickel powder, or nickel oxide powder, may be used to develop the alloy coating. Trace impurities such as cobalt, copper and iron have no noticeable effect on the resultant coating. It should be observed, however, that carbon content of the powder should be low, as earlier explained, if the coating is to be free of brittleness; satisfactory results have been obtained with powders containing up to 0.17% carbon.

Examples of nickel powder used to produce satisfactory coatings include: Inco Grade A, Sherritt-Gordon F Grade, Metals Disintegrating Co. No. 151 and Glidden Co. F110.

The powder is compacted to provide a uniform metal shell on the base metal where individual powder particles are welded together, and particles immediately adjacent the base metal are forced mechanically into the base metal surface. The compacted shell of powdered metal is porous in nature, and this characteristic is advantageous in the subsequent sintering step. During the heating-up period prior to sintering, the volatile oily material, originally applied to hold the powder to the strip, is vaporized and escapes through the pores of the compacted layer. If the compacted layer were not porous, the layer would be lifted off by the vapors during sintering.

The roll pressure to be applied in the compacting step will vary within wide limits, depending on the diameter of the rolls used and on powder particle size. For example, with a rolling mill having 4-inch diameter rolls, the elongation of the base steel may vary from between 5% and 50%, the finer powders yielding satisfactory coatings at all elongations, while with coarser powders, those having a particle size of from 30% to 50% of +325 mesh, the elongation should be at least 10% to produce a porefree coating after sintering. When rolls of 26 inches diameter were used in the compacting step, a base metal elongation of only 2% to 5% was satisfactory.

In the case of strip, the base metal may be either annealed cold rolled strip, or pickled hot rolled strip. Any gage which can be open coil annealed is satisfactory.

The temperature during sintering should range, preferably, between approximately 1550 F. and 1900 F. While 1400 F. is believed to be the lower limit at which satisfactory sintering can be performed, there is, in reality no upper limit, any practical working temperature above 1400 F. being satisfactory. The minimum time period required for proper sintering at 1400 F. is approximately 48 hours. For higher sintering temperatures, there is a decrease in the holding time requirement, until at 2100 F., or higher, satisfactory coatings can be made with a holding time of 15 minutes. Sintering times may vary somewhat, depending on the thickness and nickel alloy content desired in the coating. While high sintering temperatures have no adverse effect on the resultant alloy coating, there are, of course, obvious objections to operating at excessively high temperatures, temperatures above 2000 F.

Dry hydrogen, dry 18% hydrogen with 82% nitrogen, and dry 4% hydrogen with 96% nitrogen atmospheres have been used as annealing (sintering) gas with equal success. For best results, the annealing atmosphere should be. reducing to both nickel and the base steel; however, high purity with regard to oxygen, water vapor or carbon contaminants is not necessary for good alloy formation.

In lieu of the type of sintering atmosphere just described, a partial vacuum is quite satisfactory.

The choice of the type of base steel used is dependent on the properties desired in the finished alloy-coated product. This, as has been previously remarked, the carbon should be no higher than 0.1%, if freedom from porosity is required. There is another consideration, involving possible loss of carbon due to the nature of the sintering atmosphere. When producing our alloy coating on decarburized base stock, i.e., 0.01% or lower, there is no problem in retaining base stock properties. If a higher carbon base stock, such as rimmed steel, is used, that is, 0.03% carbon or higher, there may be some loss of carbon in the base metal during sintering. Decarburization can occur during sintering if the sintering atmosphere contains a high percentage of hydrogen. Hence, when employing a base stock having, say, from 0.03% to 0.05% carbon, where it is desired to retain strength properties and fine grain characteristics, the sintering atmosphere should be low in hydrogen and impurities which favor decarburization, such as water, oxygen and carbon dioxide. One example of a low hydrogen sintering atmosphere already given is that having 18% hydrogen and 82% nitrogen. With sufficient sintering time and temperature, successful sintering may be performed with a hydrogen-nitrogen atmosphere wherein the hydrogen is as low as 4%. e

X-ray analysis of the sintered coating indicated that the entire coating consists of an iron-nickel alloy. e

Referring to the drawings, FIGURE 1 represents a typical nickel alloy coating produced by our method. The sample was etched in cyanide-persulfate and the photomicrograph was made at 250 diameters. The average thickness of this coating is 0.8 mil, as measured by metallographic inspection from the outside surface to the interface between alloy coating and metal substrate. Compacted samples sintered at other temperatures and times, produced coatings which varied in thickness from 0.5 to 2 mils. Depth of the coating is influenced by the type of nickel powder used, Weight of powder applied, percent elongation in compacting, and sintering temperature and time.

The coating shown in FIGURE 1 possesses surface nickel equal to 59 weight percent of the alloy coating. Other coatings varied from 28 to 90% nickel at the outside surface of the coating. This surface nickel can be varied by changing the sintering temperature and time. FIGURE 2 shows the effect of variation in time at constant temperature. Increased time promotes diffusion between nickel and iron resulting in lowered surface nickel content. FIGURE 3 shows the effect of variation in temperature with constant sintering time. Just as with increased time, increased temperature also favors diffusion, again lowering surface nickel.

The curve in FIGURE 4 is developed from points representing actual nickel content at different positions on the line scan of FIGURE 5. The scan was performed on the coating shown in the photomicrograph in FIG- URE 1. This coating was sintered for a relatively short time (3 hours), therefore nickel concentration is fairly high at the surface, dropping off quite sharply as the coating-steel interface is approached. Higher sintering temperatures and/or longer times act to lower surface nickel, while decreasing the steepness of the nickel graclient in the alloy coating.

High surface nickel content, produced by the lower temperature and/ or shorter time cycles is somewhat beneficial to corrosion resistance. In one test, in which sintering was performed at 1400" F. for 48 hours, the nickel at the outside coating surface analyzed 90%. The longer time-higher temperature treatments, which lower surface nickel content, are effective in reducing voids to a minimum throughout the coating. This is helpful wherever coatings must undergo extensive deformation in forming. Any voids within the coating could act as severe stress raisers, causing the coating to break apart.

To be truly pore-free and corrosion resistant to mineral acids, the nickel content near the surface should be at least 28%.

When sintering temperatures are about 1650 F., or above, the iron-nickel coating alloy, and the base steel, will have an austenitic structure (face centered cubic) at the sintering temperature. Because of the high nickel content of the alloy, this structure will be retained through the cooling cycle, down to room temperature.

One of the outstanding characteristics of the alloy coating is the relative freedom from porosity. It is believed that the dense structure obtained is due, at least in part, to the method of alloy formation. During the heat-treating, or sintering, operation, nickel atoms diffuse inwardly to the base. At the same time, iron atoms diffuse outwardly from the base into the compacted nickel powder. This diffusion first causes bonds, or bridges, of continuous 'metal between powder particles, and between powder par- 5 ticles and the base steel. This action produces voids between particles. As ditfusion progresses, it reduces the surface area of the voids to a point where the voids are 'very small, or do not exist.

Another characteristic of the coating is its ductility. 1O Coated sheet products are able to withstand bending of 180 over a radius of twice the sheet thickness, without harming the integrity of the coating.

Corrosion resistance of the alloy coating is excellent, as indicated by accelerated tests. For example, specimens of our alloy coated product, which had been rolled to a bright finish and which were immersed in tap water for a period of six months at room temperature, exhibited no signs of rusting or discoloration.

As another example of corrosion resistance, a specimen 4 inches by 4 inches of our alloy coated product was immersed in 6.; weight percent sulfuric acid at 180 F. for one hour. Similiar tests were made on the base stock and on the base stock electroplated with one mil of nickel. The weight loss results in grams/sq. ft. of surface area/ hour, are shown in the table below:

Weight loss in A further example of the corrosion resistance of this material is shown by performance of a standard salt fog test, made in accordance with A.S.T.M. Specification B117-61. Four inch by six inch specimens, buffed to a highly polished surface finish, showed no signs of corrosion after 24 hours.

Furthermore, a coated article made by our process can be reduced in cross-section by as much as 80% and still withstand immersion in tap water for 60 days without display of any porosity.

As an example of the ductility of coatings made by our method, a 20 gage strip of rimmed steel, having a carbon content of 0.06% was filmed with alcohol, coated in a fluidized bed with nickel powder of a particle size-Jess than 325,mesh, andthe powder compacted "on the'strip at a pressure equivalent to a strip elongation of about 10%. After compacting, the strip was recoiled with spacer wire to produce an open coil, and annealed in an annealing furnace at 1725 F. for 24 hours in a dry atmosphere of 18% hydrogen and 82% nitrogen. After" sintering,'the

strip was cold rolled with polished rolls to a reduction in cross-sectional area of 50%. The rolled strip was then continuously annealed at 1250 F. in an atmosphere of 4% hydrogen and 96% nitrogen.

The annealed coated strip had a thickness of 0.018 inch,

or a gage of approximately 26. Coated strip of this gage can be made only by reduction of cross-sectional area after the sintering step, as the sintering step is limited to about 24 gage strip. Strip below this gage would collapse in the sintering furnace.

After reduction of the coated strip, in the range of from 5 to 80 percent, the ductility will be reduced to the point where use of the product will be limited to those applications requiring a minimumbf deformation. If additional formability is required for the intended application, the product should be annealed after cold rolling to restore its ductility.

The coating of the cold rolled, annealed product of the previous example was continuous and pore-free.

A pore-free coating, as the term is used herein, refers to that type of coating which has no discontinuities, or

holes, extending from the coating surface to the base steel,

and is not meant to exclude small openings in the body of the coating, which do not penetrate the entire coating thickness.

Both the ductility characteristic, and the corrosion resistance of the product made by our invention, is shown graphically in FIG. 6.

The macrophotograph, made at 2 magnifications, represents the cup resulting from a Swift cup test. A round test specimen, 4 inches in diameter was prepared from a decarburized rimmed steel base metal having 0.003% carbon. The alloy coating, containing a minimum of 30% nickel at the coating surface, has a thickness of approxi mately 1 ml. The sample was formed into cup by the conventional Swift cup test procedure, and the formed cup was immersed in 6.5 weight percent sulfuric acid for 8 hours at 180 F., suflicient time to permit the steel substrate to be removed to a depth of 1.5 millimeters at the edge of the cup. The ductile iron-nickel alloy coating shows no degradation from the acid treatment.

The method of performing the Swift cup test is described in an article by O. H. Kemmis in Sheet Metal Industries at vol. 34, 1957, pages 203 and 251.

Another example of coating ductility is shown in the ability of our coating to undergo deformation by the Olsen cup test, without any decrease in its ability to protect the base steel from corrosion. Samples deformed to a cup height of 0.30 inch, and immersed in tap water at room temperature, showed no rusting after 90 days exposure. By contrast, samples of electroplated nickel coattings of comparable thickness (about 1 mil) showed loss of corrosion resistance with deformations as low as 0.10 in. in Olsen cup height after one hour exposure in tap water.

The procedure for the Olsen test is described in Making, Shaping and Treating of Steel, 7th Edition, 1957, pages 923-924.

The dense structure of our alloy, free from pinholes,

makes this alloy admirably suitable as an undercoat for a chromium electroplated finish. Because of the porosity of chromium plate, it is necessary to form an undercoat of a metal such as nickel before the chromium is applied. Our alloy can perform the function of a chromium undercoat in the same manner as electrodeposited nickel, and 'with equally satisfactory results.

An added advantage of an alloy coating, as an undercoat for an article having an outer surface of electrodeposited chromium, is that the article can be deformed after electroplating without loss of corrosion resistance. In a tap water test, samples of our nickel alloy coating, superimposed with an electrodeposited chromium layer of a thickness of 10 millionths of an inch, were deformed to an Olsen cup height of 0.3 inch, and immersed in the water for 24 hours. At the end of the immersion period,

the deformed samples showed no loss of corrosion resistance. By contrast, a sample having a similar coating of chromium, over a 1 mil thick electrodeposited nickel coating, and deformed to a 0.1 inch Olsen cup height, failed after a 1 hour immersion in tap water.

The nickel alloy coating may be buffed to a bright finish, when a shiny, pleasing appearance is desired. The buffed coating is especially advantageous when the nickel alloy coating is used as an undercoat for chromium.

In addition to chromium, our alloy coating may be electroplated with other metals, for example, nickel.

We claim:

1. An article comprising a ferrous base having a ductile, continuous, pore-free coating, said coating consisting of an austenitic alloy of iron and nickel wherein the nickel content at the surface ranges from between and 28 weight percent of the alloy.

2. An article comprising a ferrous base, a continuous, pore-free coating on said base, said coating consisting of an austenitic alloy of iron and nickel, and a metallic chromium coating completely enveloping said alloy coating.

3. An article comprising a ferrous base and a continuous, pore-free coating on said base, said coating comprising a ductile austenitic alloy of iron and nickel.

4. An article comprising a ferrous base containing not more than 0.1% carbon and having a continuous, porefree coating, said coating comprising an austenitic alloy of iron and nickel wherein the nickel content at the surface ranges between 28 and 90 weight percent of the alloy.

5. An article comprising a ferrous base, a continuous, pore-free coating on said base, said coating consisting of an austenitic alloy of iron and nickel, and a metallic coating of a member of the group consisting of chromium and nickel completely enveloping said alloy coating.

6. An article comprising a ferrous base having a ductile, continuous, pore-free coating of an austenitic alloy of iron and nickel, said coating containing iron diffused from said base and having a nickel content at the surface ranging from 90 to 28 weight percent of the alloy and a decreasing nickel concentration gradient to the coating interface.

References Cited UNITED STATES PATENTS 2,292,694 8/1942 Jerabek 29182.3 X 2,350,179 5/1944 Marvin 29-182.3 X 2,872,311 2/1959 Marshall et al. 29-182 3,094,415 1/1963 Gallatin 29-1823 X 3,166,796 l/l965 Wehinger 29-196.1 X

HYLAND BIZOT, Primary Examiner.

Claims (1)

1. 4. AN ARTICLE COMPRISING A FERROUS BASE CONTAINING NOT MORE THAN 0.1% CARBON AND HAVING A CONTINUOUS, POREFREE COATING, SAID COATING COMPRISING AN AUSTENITIC ALLOY OF IRON AND NICKEL WHEREIN THE NICKEL CONTENT AT THE SURFACE RANGES BETWEEN 28 AND 90 WEIGHT PRECENT OF THE ALLOY.

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