MXPA99001790A - Method for hardfacing a metal surface - Google Patents

Abstract

A wear-resistant hardfacing and a method for applying such a hardfacing is taught herein. A finely powdered, wear-resistant alloy and a polyvinyl alcohol (PVA) solution slurry is coated onto the metal surface of a tool, implement, or similar item to be hardfaced. Alternatively, a binding coating of PVA solution may be applied to the metal surface followed by application of a layer of a powdered alloy. After the slurry or PVA binding coating has dried, leaving a dry coat of alloy in a PVA matrix, the metal surface is heated to the fusion temperature of the alloy in vacuum, in an inert gas atmosphere, or in hydrogen atmosphere. The metal item with the fused coating is heat treated to impart desired mechanical properties to the part substrate material. The method of the present invention gives a smooth, dense coating of the wear-resistant hardfacing without nonmetallic inclusions.

Description

METHOD FOR SHAPING A METAL SURFACE LASTING BACKGROUND OF THE INVENTION The present invention relates to a method for coating a metal surface, such as the metal surface of a tool or an agricultural implement with a hard and wear-resistant coating.

It is well known in the art of metallurgy to coat a surface of metal with another metal or with a metal alloy to improve appearance, for protection against corrosion or to improve wear resistance. The coating of tools, particularly the cutting edges of the tools, with a wear-resistant and hard alloy is a common industrial practice, especially in the art of manufacturing agricultural implements, and is often referred to as "hard coating" or as "form a hard surface". For example, see U.S. Patent Nos. Re. 27,852 issued to Alessi, U.S. Patent 5,027,878 to Revan ar, and U.S. Patent No. 5,443,916 to Brady et al., Number 4,682,987, and the patent. No. 5,456,323 issued to Hill.

Hard coating is often done by fusing a hard metal alloy powder on a metal surface. Typically, this method involves coating the metal surface with an aqueous solution or a homogeneous powder alloy, a powder flow or a binding agent and a suspending agent; drying the solution to form a solid layer; and heating the metal surface to a temperature high enough to melt the alloy on the surface. The flow is to protect the alloy from the reaction with the gases in the melting furnace atmosphere while the alloy is heating. The suspension agent promotes a uniform solution. The binder retains the alloy and flow powders in place until the alloy solution has dried on the metal surface.

A problem with this hard coating method is that the flow, binder and suspending agent additives in the solution remain in the fused coating as non-metallic and undesirable inclusions, and reduces the volume of effective wear-resistant coating. for a given coating thickness. These inclusions are discontinuous in the coating and increase the brittleness of the coating and therefore promote the removal of the coating material by fracture, rather than abrasive wear, resulting in premature wear and a shorter wear life of the coating.

Another problem with the art methods is the lack of uniformity of the coating thickness. There are two reasons for this problem. 1) The application of solution allows the solution to flow, when wet, on vertical and inclined surfaces thus forming an uneven distribution of the powder alloy. 2) The flow / binder mixture used in the coating solution melts in front of the coating powder, and the resulting liquid has to displace the powder particles on the vertical and inclined surfaces and distribute them in non-uniform form before the powder alloy begin to melt.

JP-A-60089503 describes a method of coating wear resistant material. A powder of an abrasive material, such as a cobalt-based or nickel-based alloy which includes less than 5 percent iron and an organic binder, such as a polyvinyl alcohol, are mixed to form a solution which is coated on the surface of the machine parts. The parts are heated in a non-oxidizing or vacuum atmosphere to form a sintered layer of a wear-resistant material which is bonded through a diffusion layer to the parts.

U.S. Patent No. 3,310,870 issued to Parkikh et al. Describes a process for producing a nickel-coated steel using a solution composition that includes a nickel powder in a binder., such as a solution of polyvinyl alcohol, which may contain a dispersion or deflocculating agent for purposes of assisting in the dispersion of the binder in the solution. The solution is coated on a metal substrate by spraying or roller coating, dried, sintered in a non-oxidizing atmosphere to the steel, compacted hot and cooled.

European patent EP-A-0 459 637 describes a process for applying a coating consisting of a hard alloy to a metal or ceramic object. The hard alloy contains only a small percentage of iron. This is mixed with an organic binder, such as a vinyl polymer, and applied to the object by embedding, spraying, rolling or other techniques. In a first heating step the binder is decomposed and in a second heating step at a high temperature in conjunction with the application of a superatmospheric pressure the coating is consolidated.

Ikeno et al. In U.S. Patent 4,175,163 teaches a method for coating. a stainless steel product with a corrosion resistant surface layer. A metal powder, composed mainly of chromium and nickel is mixed with an organic solvent, such as an aqueous solution of polyvinyl alcohol. After spraying the mixture onto the surface of the product high-frequency heating is applied under a non-oxidizing atmosphere, such as nitrogen or argon, which causes the material to form an intersurface layer between the surface layer and the product. of steel.

It is an object of the present invention to provide a method for uniformly hard coating a metal surface with a wear resistant alloy without essentially non-metallic inclusions. A second object is to provide a wear resistant alloy solution for use in a hard coating.

SYNTHESIS OF THE INVENTION A first aspect of the present invention is a method for hard coating a metal surface with a wear resistant coating. A first incorporation of the method comprises the steps of: a) forming an essentially uniform aqueous solution of non-flowing polyvinyl alcohol and a meltable hard metal alloy of at least about 60 percent iron in the form of a finely divided powder; and one or more additives selected from the group consisting of dispersants, deflocculants, and plasticizers; b) coating the metal surface with an aqueous solution; c) drying the aqueous solution to leave a solid layer of the hard, fusible metal alloy in a polyvinyl alcohol matrix on the metal surface; d) heat the metal surface coated with the hard and fusible metal alloy layer in the polyvinyl alcohol matrix to the melt temperature of the alloy under a protective atmosphere at a pressure of between about 10 * torr and 2 pounds per square inch over the atmospheric pressure until the alloy has melted on the metal surface; Y e) Cool the metal surface with the hard melted coating at room temperature.

Steps b) and c) may be repeated one or more times to accumulate a thicker coating of the polyvinyl alcohol / alloy matrix.

A second embodiment of the method for hard coating a metal surface comprises the steps of: a) coating the metal surface with an aqueous polyvinyl alcohol solution; b) distributing an essentially uniform layer of a fusible hard metal alloy in the form of a finely divided powder onto the coating of the polyvinyl alcohol solution applied in step a) before the polyvinyl alcohol solution is dried; c) drying the aqueous polyvinyl alcohol solution coating to form a solid layer of the fusible hard metal alloy attached to the metal surface by coating the polyvinyl alcohol; d) heating the coated metal surface with the bonded hard metal alloy layer by coating the polyvinyl alcohol to the melting temperature of the alloy in a protective atmosphere at a pressure of between about 10 ~ 4 torr and pounds per square inch over the atmospheric pressure until the alloy has melted; Y e) cool the metal surface with the hard surface coating at room temperature.

Steps a), b) and c) may be repeated one or more times to accumulate layers of the alloy each attached to the layer below it by a coating of polyvinyl alcohol with the lower layer being attached directly to the metal surface.

A second aspect of the present invention is an aqueous solution of polyvinyl alcohol with flow and a fusible metal alloy in the form of a finely divided powder of at least about 60 percent of iron used in the first incorporation of the method. Preferably the average particle size of the alloy is about 200 mesh or finer.

The wear resistant coatings applied by the solution coating methods present for the hard coating are uniformly dense and essentially contain no inclusions unlike the solution coatings applied by the methods of the art. Therefore the coatings of the present invention are less brittle and are more durable than the coatings applied by the methods of the art.

DETAILED DESCRIPTION OF THE INVENTION A widely practiced method of hard coating metal surfaces, particularly in agricultural implements, is taught in U.S. Reissue Patent No. 27,851 of Alessi (incorporated herein by reference). This method comprises: a) preparing an aqueous solution of a hard alloy powder, a binder and a melt; b) coating the solution on the surface of a metal article to be coated in hard form; c) expelling the water from the solution with low heat to leave a deposit of dry alloy, binder and a melt on the metal surface; and d) heating the entire metal article to a temperature high enough to melt the alloy and form a hard coating tightly attached to the metal article. The method of the present invention is an improvement over Alessi and hard coating methods currently in use based on Alessi, for example, the process referred to as the "hard face" in U.S. Patent No. 5,456,323 .

In the art methods for the hard coating based on Alessi, the melt and the binder combination (melt / binder) used to prepare the coating solution melts in a liquid at a much lower temperature than the melting point of the content of Alloy powder solution. The melt or flow / binder continues to exist as a liquid, even at a higher melting temperature of the alloy powder. However, the binder / liquid flow can not rise to the surface of the melted alloy completely within the short melting time and before the metal solidifies. Therefore, the flux / binder is trapped as small non-metallic particles known as "inclusions" within the alloy coating. The inclusions are relatively soft and brittle, therefore weakening the alloy coating and reducing its wear resistance. Even if sufficient time is allowed for the binder / liquid flow to rise through the melted alloy layer, the flow / binder will not be removed from the coating but will form part of the topcoat layer.

In addition, because the melting point of the flux / binder is well below that of the coating alloy, the flux / binder becomes a low viscosity fluid before the melting temperature of the alloy is reached. Here the term "fusion" is taken to mean that the finely divided alloy becomes smooth and that the individual particles melt and agglomerate to form a continuous layer. The fluid flow / binder tends to flow easily on the non-horizontal surfaces, carrying with it some of the alloying powder before the melting of the alloy powder begins to occur. Therefore, the melting of the flow / binder results in a non-uniform thickness of the solidified coating caused by poor wear performance of the alloy coating.

In the first embodiment of the present method, an aqueous solution of polyvinyl alcohol (PVA) is used as the binder in an aqueous solution of an alloy without a flux. Polyvinyl alcohol when heated does not melt to a thermoplastic, but is decomposed by the loss of water from the two adjacent hydroxyl groups at a temperature above 150 degrees centigrade. When the polyvinyl alcohol / alloy coating is heated to the melting temperature of the alloy, the polyvinyl alcohol evaporates almost completely from the coating leaving behind an agglomerate of clean alloy-coating powder particles with sufficient cohesive strength that it melts into a clean and dense metallic coating without inclusions.

However, because the polyvinyl alcohol decomposes and escapes well below the melting temperature of a hard coating alloy powder, it does not protect the alloy as it reaches the melting temperature of reacting chemically with the gases of the atmosphere, for example, oxygen, nitrogen, and carbon dioxide. Such protection is a purpose of a flow material which is intentionally omitted in the present invention. Thus, a protective atmosphere is preferably provided during heating, melting and cooling wherein the alloy is sensitive to air at the elevated temperature.

In a laboratory and on a small scale, the fusion of the alloy can conveniently be carried out in a high vacuum furnace (about 10 ~ 4 torr or 0.1 μm), effectively eliminating atmospheric gases. It is also possible to operate the low pressure inert gas furnace (100-200 μm), for example argon or helium. At low pressures, nitrogen can also be used even when not as satisfactorily as argon or other inert gases. However, operations with low pressure inert gas and high vacuum in a vacuum furnace in a production environment are relatively costly and time consuming. Inert gases, for example argon and helium, just above atmospheric pressure, and reducing gases such as hydrogen, just above atmospheric pressure can be used as a protective atmosphere during fusion with alloy at a rate of production acceptable. Hydrogen, because it is less expensive than argon or helium, is preferred as a protective atmosphere in large-scale production. Ovens that use hydrogen as a protective atmosphere are known in the art of metallurgy and are commercially available.

A solution used in the present invention was prepared by thoroughly mixing a hard powder coating alloy with a polyvinyl alcohol binder solution to give the desired alloy the ratio by weight of binder solution. The solution compositions described herein are designated as an 8-digit code. For example, for a "0550/0750" solution, the first four digits "0550" indicate a weight ratio of 5.5 to 1 powder alloy to polyvinyl alcohol solution and the last four digits, "0750", indicate an aqueous solution of 7.5 percent (by weight) of polyvinyl alcohol as a binder. In this designation, the decimal point is presumed to occur in the middle of each group of four digits. Similarly, "1075/1025" means an alloy to polyvinyl alcohol ratio of 10.75 to 1, and the aqueous solution of polyvinyl alcohol is 10.25 percent polyvinyl alcohol, by weight, in water.

Those skilled in the art of metallurgy will appreciate that in order to obtain a wear-resistant and uniform coating, a metal surface to be coated in a hard manner must be a bare, clean metal that is free of rust. Preferably, before employing the hard coating methods taught herein, the metal surface to be hard coated has been prepared by cleaning the metal to leave it bare. Conveniently, a metal surface can be prepared for hard coating by brushing with a detergent material and then beating with grit. Preferably, the grit is of a mesh size of around 80 to about 120. If only a few articles are to be coated, the surface can be rust-ridden by rubbing it with a rag or fine abrasive paper, for example, a cloth or grit abrasive paper of 120. The grit material can be essentially any hard angular particle powder, for example of alumina "steel grit" and many other commercially available abrasives.

In the first embodiment of the method of the present invention, the preferred method for applying a solution to a metal surface to be coated depends on the shape and size of the metal article having the metal surface as well as the proportion of alloy and the concentration of the polyvinyl alcohol binder solution. Typically, the coating solution is poured, brushed or sprayed onto the metal surface to be protected, or the article having the metal surface to be protected can be imbibed into the solution. This method is useful for relatively thin coatings, for example, up to about 0.75 millimeters, but the uniformity of coating thicknesses is sometimes difficult to obtain and maintain. For this method, preferably the ratio of the alloy to the polyvinyl alcohol solution is in the range of about 4: 1 to about 8: 1 and the concentration of polyvinyl alcohol solution is about 1 percent to about 15 percent polyvinyl alcohol by weight. For example, 0500/0500, 0600/0150, 0700/0150, 0500/0750, 0600/0750 or similar solutions are suitable for this procedure.

Spray coating requires a solution which has a slow sedimentation rate of the alloy powder, therefore according to the Stokes law the terminal velocity (for example the velocity without acceleration), "Vt," of a particle of Dust through a fluid column is directly proportional to the square of the radius, "r", of the particle that is presumed to be spherical and inversely proportional to the viscosity of the fluid medium, "? For example, the smaller the mesh size of an alloy powder and the higher the viscosity of the binder, the slower the sedimentation rate of the alloy powder will be. because it is square, it has a stronger effect than the viscosity on the sedimentation rate, for example, the radius of the particles of 200 and 325 meshes are 75 μ and 45 μ respectively and the viscosities of 5 percent and 7.5 One hundred percent of the polyvinyl alcohol solutions are 15 mPa.s and 70 mPa.s The Vt value for the 325-mesh particle in the 7.5 percent polyvinyl alcohol binder will be 13 times lower than that of a 200-mesh particle in a 5.0 percent polyvinyl alcohol solution, the sedimentation rate can therefore be controlled by judiciously choosing combinations of binder concentration of powder particle size, for example, the settlement of pol Vo of alloy in a solution of 0500/0750 not stirred dust of less 200 meshes is negligible after 20 minutes.

A higher concentration of binder, for example, of 10 percent (binder viscosity of 250 mPa.s) will further reduce the settling rate, but the correspondingly large increase in solution viscosity will render the solution unsuitable for spraying. However, a high viscosity solution can be used for alternative application methods, for example, pastes and tapes, taught hereafter.

Thick solution compositions, for example, of a high proportion of alloy to polyvinyl alcohol solution, can be applied as a squeezable paste, they can be wound into tapes to be attached to the metal surface. Both of these procedures, however, usually require special additives to function as dispersants, deflocculants and plasticizers. For these processes, preferably the ratio of alloy to polyvinyl alcohol solution is in the range of about 8: 1 to about 15: 1 by weight and the concentration of polyvinyl alcohol solution is about 6 percent to about 15 percent polyvinyl alcohol by weight. Typical examples of thick solutions are 1000/1000, 1200/1500, and 1500/1200. Paste and tape methods can be used for thick coatings. However, these procedures are difficult to adapt to an environment of high speed production.

When a thick coating is desired, a reliable and economical alternative to pulp and tape is a multiple coating procedure which produces uniformly thick solution coatings even on large surfaces. The required thicknesses can be constructed by repeated spraying with intervening drying cycles. Drying can be done at around 80- to about 120 degrees centigrade in a forced air oven. A solution of 0500/0750 is particularly suitable for this method even when other formulas can be used.

The method of the present invention is particularly useful for hard coating surfaces of steel articles subjected to high impact, corrosion and abrasive wear including, but not limited to tools (especially tool cutting edges), bearings, pistons, shafts crank, gears, machine parts, firearms, agricultural implements, and surgical instruments. The method can be used for ductile hardcoated iron and gray iron, often used in setting articles such as motor blocks and assembly boxes. An alloy can be cast on the surface of an iron article set at a temperature just below the melting point of the iron article. In addition, the methods of the present invention can be used to coat non-ferrous metals and alloys as long as the hard coating alloy is compatible with the metal surface being coated and the melting temperature of the hard coating alloy is significantly below. from the melting point of the metal that is being hard coated.

Alternatively, using the second embodiment of the present invention the metal surface to be protected can be coated with an aqueous polyvinyl alcohol solution (from about 1 percent to about 15 percent polyvinyl alcohol by weight) to form a coating binder followed by the distribution of dry powder alloy over the coating of polyvinyl alcohol binder solution while it is still wet, preferably with a powder sprayer and more preferably with an air spray. Preferably, both the aqueous polyvinyl alcohol solution and the alloy powder are sprayed onto the metal surface. The polyvinyl alcohol binder solution is then dried to give a solid layer of alloy powder bound to the surface by a coating of polyvinyl alcohol. The multiple layers of the alloying powder can be obtained by applying successive coatings of polyvinyl alcohol solution and layers of the alloying powder and drying each successive polyvinyl alcohol solution coating by binding an alloy layer before adding another polyvinyl alcohol coating. This method eliminates the problems of sedimentation of dust in a solution and the flow of solution in thick coatings. In addition, this incorporation is very suitable for high-speed production.

The heat treatment of the metal to modify or improve its properties is well known and widely practiced in the art of metallurgy as for example, see the heat treatment manual, International, Metals Park, Ohio (1991). The process of heat treating essentially involves heating the metal uniformly to its austenitizing temperature (cooling) and then rapidly cooling, for example, by cooling, in a cooling medium, such as water, oil cooler, or a cooling polymer, or even air. A metal article having a hard surface coating by the method of the present invention can be heat treated by removing the article from the furnace after melting the alloy, cooling slowly to the cooling temperature of the metal, and then immersing it in the metal. an adequate cooling medium. Alternatively, a metal article having a previously coated and hard surface can be heat treated by heating it to its cooling temperature and then cooling it.

A polyvinyl alcohol binder, unlike the binders / flux shown in the art does not melt to form a liquid before or during the coating melt process and thus does not provide an opportunity for the coating powder to "travel" before let the dust begin to melt. This property of the polyvinyl alcohol ensures that the thickness of the final molten coating corresponds to the thickness of the starting solution coating at each location in the coating. Solutions up to 0.040 inches thick cast on a vertical steel surface did not show a displacement of the powder metal, before or during melting. A coating of up to 1.5 millimeters thick on a 60 degree sloped surface also showed no metal flow. Therefore, polyvinyl alcohol is a binder that minimizes the problem of non-uniformity of the coating found in the hard coating processes of the art.

Revankar et al. In U.S. Patent No. 5,027,878 employs polyvinyl alcohol, in setting with an evaporative pattern or EPC process, as a means for holding ceramic particles, such as metal carbide particles, in a place on a polymer pattern which is then placed in a sand mold within which the melted iron is being set. However, US Pat. No. 5,027,878 teaches that the ceramic particles are impregnated in the iron and do not melt on a metal surface like the alloy particles in the method of the present invention. Furthermore, said patent 5,027,878 teaches a ceramic particle size preferably of about 30 meshes, more preferably of about 100 meshes, while the alloy particles of the present invention are preferably about 200 meshes or thinner.

The polyvinyl alcohol, the binder used in the present invention, is an environmentally safe and inexpensive polymer. In the absence of acids or bases, an aqueous solution of polyvinyl alcohol is stable even after several months of storage at room temperature. The stability of polyvinyl alcohol solutions is an advantage for production applications. When an alloying powder solution with polyvinyl alcohol is heated as a binder at the melting temperature of the alloying powder in a protective atmosphere such as cotton or helium or in a reducing atmosphere such as hydrogen, the polyvinyl alcohol appears to be evaporates completely, resulting in a dense coating of the alloy without inclusions.

An alloy useful in the present invention is essentially harder and more resistant to wear than the steel typically used for tools, gears, engine parts and agricultural implements, for example steel class 1045. Preferably, the alloy has a value of Knoop hardness in the range of about 800 to about 1300. The alloy has a melting temperature of about 1100 degrees Celsius or less, for example, which is lower than the melting point of the metal to be coated . Preferably the alloy powder has a particle size which is sufficiently small to form a uniform solution and a uniform hard coating. Preferably, the alloy is of a single phase and preferably has a melting temperature of between about 900 degrees centigrade and about 1200 degrees centigrade. This is in the form of a finely divided powder having particles typically varying in size from about 90 meshes to about 400 meshes. Preferably, the average particle size is finer than about 200 meshes and more preferably, finer than about 325 meshes.

The alloys useful in the present invention are preferably at least 60 percent of a transition metal of group 8 of the Periodic Table, such as iron, cobalt, or nickel, for example, these are based on iron, cobalt or nickel, but can be based on other metals as long as the alloys have the physical properties stated above. Minor components (from about 0.1 to about 20 percent) are typically boron, carbon, chromium, iron (in cobalt and nickel based alloys), manganese, nickel (in cobalt-based alloys and iron), silicon, tungsten, or combinations thereof, see Alessi. Elements in trace amounts (less than about 0.1 percent) such as sulfur, may be present as minimal contaminants. Even though it may be possible to prepare an alloy containing radioactive, highly toxic or precious elements that fill the required physical and chemical properties mentioned above, such an alloy may be of limited or no value due to health, safety and / or safety considerations. or economic.

The methods for preparing the finely divided powder alloys are well known in the art of metallurgy. The information and background on powdered alloys useful for the present invention can be found in standard textbooks teaching art such as that of Hausner, HH and Mal, MK, Metallurgy in Powder Text, second edition (especially beginning in page 22) of Chemical Publishing Company, Inc. (1982). Powdered alloys useful in the present invention are available from commercial suppliers, such as Wall Colmonoy Corporation, Madison Heights, Michigan and SCM Metal Products, Inc., of Research Triangle Park, North Carolina.

The following examples are presented to further illustrate the present invention and should not be considered as limitations of this.

EXAMPLES Example 1. Alloy Alloys useful in the methods of the present invention include but are not limited to those described in Table 1.

Table Elemental Composition (percent by weight) of Selected Alloys Useful for Coating Metal Surfaces Example 2 Application of a Wear Resistant Coating to a Low Argon Sweep Polyvinyl alcohol (PVA) (75-15 Elvanol (brand) supplied by DuPont) is mixed with enough water to make a 7.5 percent by weight solution of polyvinyl alcohol. Alloy powder # 3 (see Table 1, Example 1) averaging about 200 meshes, supplied by SCM Metal Products, Inc., was added to the polyvinyl alcohol solution in the proportion by weight of 5.0 parts of alloy # 3 to a part of polyvinyl alcohol solution to make a solution of type 0500/0750.

A tracker is scrubbed with a hot detergent solution, and the area to be coated is beaten with grit to a non-glossy finish with a grit of 100 mesh. A 2-millimeter thick layer of the polyvinyl alcohol / alloy solution was sprayed over the area of the tracker to be coated, and the tracer was heated in a forced circulation oven at about 120 degrees centigrade for 30/60 minutes until the solution had dried to form a deposit of polyvinyl alcohol / alloy.

The tracker is then transferred to a vacuum oven operating with a partial argon pressure of 100-500 microns. The tracer is heated to approximately 1100 degrees centigrade and maintained at that temperature until the fusion of the coating to the surface of the tracer is complete (about 2 to 10 minutes). The tracker is then slowly and uniformly cooled while maintaining the argon atmosphere until the temperature reaches about 300 degrees centigrade or is lower at which time the tracer is removed from the oven and allowed to cool to room temperature. (As used herein, "room temperature" is synonymous with "room temperature", for example about 15 degrees centigrade to about 35 degrees centigrade).

Example 3. Apply Wear Resistant Coating to a Low Hydrogen Tracker A wear-resistant coating was applied to a tracer as in Example 2 except that it was heated in a vacuum oven under nitrogen to a slightly positive pressure (about 1 to about 2 pounds per square inch above atmospheric pressure).

Example 4. Heat Treatment ** '"• ** Metal Substrate A wear-resistant coating was applied to a tracer as in Example 2. The tracer is then reheated to the austenitizing (cooled) temperature of the substrate steel (e.g. 845 degrees centigrade for steel 1045) and then cooled in a commercially available oil cooler. The tracer is then reheated to about 275 degrees centigrade at 300 degrees centigrade to temper the martensite formed by cooling, and allowed to cool to room temperature in the air.

Example 5. Apply a Wear Resistant Coating to a Scraper Bar of a Grain Combined A wear-resistant coating was applied to a scraper bar surface by spraying on the cleaned surface a solution of alloy # 2 (Table 1, Example 1), for example the ratio by weight of the PVA solution to the weight of alloy is 6.0: 1, and the aqueous polyvinyl alcohol solution is 5.0 percent polyvinyl alcohol to form a solution type of 0600/0500. After drying the solution on the scraping bar in a manner similar to the procedure of Example 2, the alloy was fused onto the scraping bar in a band-type furnace under a positive pressure hydrogen atmosphere at about 1100 degrees centigrade. . The coated scrap bar is then cooled to the cooling temperature which is selected according to the kind of substrate steel as mentioned in Example 4 above and then cooled in a commercially available polymer or oil cooler depending on the the steel class. The cooled scraping bar can then be further treated with heat as in Example 4.

Example 6. Application of Wear-Resistant Coating to the Edge of a Meadow Harvester Blade A meadow mower blade is coated with a wear resistant coating according to the procedure of Example 2 except that alloy # was used 1 (Table 1, Example 1) instead of alloy # 3. This is then treated with heat as in Example 4.

Example 7. Application of a Wear-Resistant Coating to a Crate of Agricultural Combined Supply Box Catch Made of Ductile Iron The surface of the retainer box was prepared to receive a wear-resistant coating as in Example 2. The part to be coated in hard form is then sprayed with 10 percent aqueous polyvinyl alcohol solution. Immediately, the area covered with the polyvinyl alcohol solution was sprayed with an alloy # 4 (Table 1, Example 1) and the box was heated in an air oven forced circulation to about 120 degrees until the alcohol binder coating poly was dried to form a polyvinyl alcohol / alloy deposit. The area of the part that is not to be hard coated was filed free of the polyvinyl alcohol binder and alloy. Note that this second embodiment of the method of the present invention is not necessary to form a solution prior to the application of the alloy powder.

The box is then heated to a temperature of about 1100 degrees centigrade to melt the coating. The heating is done in a band-type conveyor furnace at a positive pressure (about 1 to 2 pounds per square inch over atmospheric pressure) of hydrogen, and the retainer box is maintained at about 1065 degrees centigrade to about 1075 degrees centigrade for about 2-5 minutes. The box is then transferred to a bath of austemplate salt heated to about 275 degrees centigrade to about 325 degrees centigrade and kept in the bath for 4 to 6 hours at this temperature until the transformation of the material structure was completed . This was then removed from the bath and cooled in the air to room temperature.

Claims (13)

R E I V I N D I C A C I O N S

1. A method for hard coating a metal surface with a wear resistant coating comprising the steps of: a) forming an essentially uniform aqueous solution of polyvinyl alcohol without flow and a meltable hard metal alloy of at least about 60 percent iron in the form of a finely divided powder; and one or more additives selected from the group consisting of dispersants, deflocculants and plasticizers; b) coating the metal surface with the aqueous solution; c) drying the aqueous solution to form a solid layer of the fusible hard metal alloy in a polyvinyl alcohol matrix on the metal surface; d) heat the metal surface coated with the hard and fusible metal alloy layer in the polyvinyl alcohol matrix to the melting temperature of the alloy in a protective atmosphere at a pressure of between about 10"4 torr and 2 pounds per square inch over atmospheric pressure until the alloy has fused to the metal surface; e) Cool the metal surface with the hard molten coating at room temperature.

2. The method as claimed in clause 1 characterized in that steps b) and c) are repeated at least once.

3. A method for hard coating a metal surface with a wear resistant coating comprising the steps of: a) coating the metal surface with an aqueous polyvinyl alcohol solution; b) distributing an essentially uniform layer of a fusible hard metal alloy in the form of a finely divided powder onto the coating of the polyvinyl alcohol solution applied in step a) before the polyvinyl alcohol solution is dried; c) drying the aqueous polyvinyl alcohol solution coating to form a solid layer of the fusible hard metal alloy attached to the metal surface by coating the polyvinyl alcohol; d) heating the coated metal surface with the bonded hard metal alloy layer by coating the polyvinyl alcohol to the melting temperature of the alloy in a protective atmosphere at a pressure of between about 10 ~ 4 torr and pounds per square inch over atmospheric pressure until the alloy has merged; Y e) Cool the metal surface with the hard molten coating at room temperature.

4. The method as claimed in clause 3 characterized in that steps a), b) and c) are repeated at least once.

5. The method as claimed in clauses 3 or 4 characterized in that the alloy is at least about 60 percent iron.

6. The method as claimed in one of clauses 3 to 5 characterized in that the hard metal alloy in the form of a finely divided powder is distributed by a powder sprayer.

7. The method as claimed in one of clauses 1 to 6 characterized in that the alloy consists essentially of one or more elements selected from iron, nickel, and cobalt, and two or more elements selected from boron, carbon, chromium, milibdene, manganese, tungsten and silicon.

8. The method as claimed in one of clauses 1 to 7 characterized in that the metal surface is on an agricultural implement.

9. The method as claimed in one of clauses 1 to 8 characterized in that the alloy is heated to the melting temperature under an argon atmosphere.

10. The method as claimed in one of clauses 1 to 9 characterized in that the alloy is heated to the melting temperature under a hydrogen atmosphere.

11. A solution for hard coating a metal surface comprising: a fusible hard metal alloy in the form of a finely divided powder of at least about 60 percent iron in an aqueous polyvinyl alcohol solution without flow.

12. The solution as claimed in clause 11 characterized in that the alloy is composed of boron, carbon, chromium, iron, manganese, nickel and silicon.

13. The solution as claimed in clauses 11 or 12 characterized in that the average particle size of the alloy is about 200 meshes or finer. SUMMARY A wear resistant hard coating and a method for applying such a hard coating are taught herein. A finely powdered wear-resistant alloy and a solution of polyvinyl alcohol (PVA) is coated on the metal surface of a tool, implement or similar article that is to be hard-coated. Alternatively, a binder coating of the polyvinyl alcohol solution can be applied to the metal surface followed by the application of a layer of a powder alloy. After the polyvinyl alcohol binder or solution coating has dried, leaving a dry layer of alloy in the polyvinyl alcohol matrix, the metal surface is heated to the melting temperature of the alloy in a vacuum, in an atmosphere of inert gas, or in a hydrogen atmosphere. The metal article with the fused coating is then heat treated to impart the desired mechanical properties to the part substrate material. The method of the present invention gives a dense and smooth coating of the hard wear-resistant coating without non-metallic inclusions.