US3886637A

US3886637A - Method of producing heat treatable titanium carbide tool steel coatings on cylinders of internal combustion engines

Info

Publication number: US3886637A
Application number: US389212A
Authority: US
Inventors: John L Ellis; M Kumar Mal; Stuart E Tarkan
Original assignee: Chromalloy American Corp
Current assignee: Alloy Technology International Inc
Priority date: 1971-11-17
Filing date: 1973-08-17
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03

Abstract

A plasma-deposited coating of a heat-treatable titanium carbide tool steel composition is formed upon the surface of a ferrous metal core, the coating being thereafter transplanted onto a conforming surface of an aluminum substrate by pressure casting aluminum against said core to effect bonding of said aluminum to said coating. Because the coating is cooled rapidly on the core during plasma deposition from above the austenitizing temperature of the steel, a hardened coating is provided having a microstructure containing an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof. After pressure casting, the core is disengaged from the aluminum surface by differentially cooling the core relative to the casting so as to release the core, thus enabling the core to be removed from the casting, the coating being transferred to the aluminum substrate. The hardened structure is maintained in the coating even after it is transplanted onto the confirming aluminum surface. Preferably, the titanium carbide grains are rounded to assure a low friction surface.

Description

Unite States Patet Ellis et al. iune 3, 1975 [54] METHOD OF PRODUCING HEAT 3,539,192 11/1970 Prasse 117/93.1 PF TREATABLE TITANIUM CARBIDE TOOL 3,674,544 7/1972 Grosseau l17/93.l PF STEEL COATINGS 0N CYLINDERS 0F 3,705,818 12/1972 Grosseau 117/931 PF INTERNAL COMBUSTION ENGINES Primary Examiner c W Lanham Inventors: John L. Ellis, White Plains; Assistant Exa niner-Dgfl Crane Kumar p g y; Stuart Attorney, Agent, or Firm-Hopgood, Calimafde, Kalil E. Tarkan, Monsey, all of NY. [73] Assignee: Chromalloy American Corporation, 1 1 ABSTRACT New York, A plasma-deposited coating of a heat-treatable tita- [22] Filed: Aug 17 1973 nium carbide tool steel composition is formed upon the surface of a ferrous metal core, the coating being 1 1 pp 389,212 thereafter transplanted onto a conforming surface of Related Application Data an aluminum substrate by pressure casting aluminum [63] Continuation-in-part of Ser. No. 199 497 Nov. 17 agam'st Said-Core to effect bOndr-lg sald alumm-um 1971 Pat- No' 3 779 720. to send coat1ng. Because the coat ng 15 cooled rapidly on the core during plasma deposition from above the austenitizing temperature of the steel, a hardened [52] 29/156'4 29/527'3 3 8 coating is provided having a microstructure containing Int Cl Bzlk 3/00 an austenitic decomposition product selected from the [58] Fieid Z' 527 3 group consisting of martenslte, bainite andmixtures 29/l95"A 164/22 5 1 5 thereof. After pressure castlng, the core is disengaged 418/178 d 1231/1936 from the alumlnum surface by dlfferentlally cooling the core relative to the casting so as to release the [56] v References Cited core, thus enabling the core to be removed from the casting, the coating being transferred to the aluminum UNITED STATES PATENTS substrate. The hardened structure is maintained in the 3,083,424 4/1963 Bauer 29/527.3 oati g e en after it is transplanted onto the confirml Bauer aluminum urface Preferably the titanium carbide g grains are rounded to assure a low friction surface. auer r 3,368,882 2/1968 Ellis et a]. 29/195 5 Claims, 4 Drawing Figures PATENTEDJUHB B15 3,886; 637

1426 szezw L? METHOD OF PRODUCING HEAT TREATABLE TITANIUM CARBIDE TOOL STEEL COATINGS ON CYLINDERS OF INTERNAL COMBUSTION ENGINES This application is a continuation-in-part of Ser. No. 199,497, filed Nov. I7. 1971 now U.S. Pat. No. 3,779,720.

This invention relates to a method of producing an adherent, hard, wear resistant coating of a heat treatable titanium carbide tool steel on the inner wall of a cylinder of an internal combustion engine housing fabricated from aluminum, particularly the trochoidal housing of rotary engines, such as the Wankel engine. By employing plasma spraying. the titanium carbide tool steel composition is deposited as a quench cooled coating having a microstructure containing an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof. Because of the nature of the coating, it is capable of being further treated at temperatures below the melting point of aluminum.

STATE OF THE ART It is known to hard face metal substrates by using welding and brazing methods in which the metal substrate is simultaneously heated during the laying down of the hard facing material. Because of the general nature of the foregoing process, the metal substrates were limited to those having fairly high melting points, otherwise, the substrate would overheat and either melt or be adversely affected. 9

One attempt to enlarge the use of hard facing has been to employ flame spraying. This method comprises melting powder metal compositions in a heated Zone and propelling the molten particles to the surface of a metal substrate to form a coating thereon. This method had its limitations as to the type of materials that could be sprayed. For example, if refractory carbide particles are sprayed, generally a matrix metal powder is mixed with it, e.g., nickel, cobalt, etc., and the mixture sprayed to provide the means by which to anchor the carbide particles to the receiving surface. So long as no further heat treatment is required of the coating, certain types of hard coatings could be produced, although they tended to be porous.

In recent years, a special kind of hard titanium carbide tool steel has been developed which, besides having the intrinsic hardness of the titanium carbide, also is capable of being further hardened very much as tool steel is hardened. For example, a titanium carbide tool steel containing 33% by weight of TiC (about 45% by volume) and the balance a chromiummolybdenum steel (note U.S. Pat. Nos. 2,828,202 and 3,416,976) requires a relatively high temperature for heat treatment. Thus, to obtain a martensitic matrix, the titanium carbide tool steel composition is quenched from about l.750F in oil. However, the foregoing heat treating temperature is higher than the melting point of Cerium metal substrates such as aluminum.

In US. application Ser. No. l99.497. filed 7, I971. now U.S. Pat. No. 3,779.720, of whi this pp cation is a continuation-in-part. a method is provided for producing hard titanium carbide steel coatin on a metal substrate by plasma spraying. Because of the high temperatures used. the quenching effect of the metal substrate upon which the coating is deposited In such as to produce a hardened structure in situ characterized by the presence of martensite and/or bainite which does not require heating the coating to l,750F to produce a quench hardened coating. Thus, any further heat treatment. such as tempering, can be carried out at temperatures below the melting point of alumiln U.S. Pat. No. 3.083.424. a method is provided for producing coatings on die cast aluminum parts, for example. internal combustion engine housings by using a technique called the transplant process. Broadly. this method comprises forming a coating of a harder high melting point metal than aluminum on a smooth cylindrical core making up part of a wall of a die cavity by depositing the coating of said metal upon the core. the outer surface of said coating being rough and pitted. This coated core is assembled in the die casting mold which has a cavity corresponding to the housing to be produced. Molten aluminum which has a lower melting point than the coating material is injected into the mold under pressure (e.q., die casting) sufficient to force the molten metal into intimate and interlocking contact with the coating by virtue of the surface porosity of the coating. The aluminum solidifies in the mold and shrinks around the core. The cast assembly together with the core is removed from the mold. The aluminum housing is then either differentially heated relative to the core, whereby the expansion of the aluminum causes radial separation of the cavity of the housing from the core, the core being thereafter removed from the casting; or the core is differentially cooled relative to the casting and then removed from the casting.

The coatings used, e.g., ferritic stainless steel, however, do not have sufficient wear resistance for use in rotary engines. In some instances, chromium plating of the coating is resorted to in order to assure a hard surface. However, even this has not sufficed for rotary engines, such as the Wankel type.

It will be desirable to provide an improved transplant technique wherein hard wear resistant coatings can be provided in which the subsequent heat treatment, if any, can be carried out on the coated aluminum itself at temperatures below the melting point of aluminum.

OBJECTS OF THE INVENTION It is thus the object of the invention to provide a method of producing a hard, dense wear resistant coating of a heat treatable titanium carbide tool steel on the wall of an internal combustion engine cylinder made of aluminum.

Another object is to provide a method of producing a composite article of manufacture comprising an alu minum housing of a rotary engine having an adherent dense coating of a titanium carbide tool steel on the cylinder wall thereof comprised metallographically of primary grains of titanium carbide dispersed substantially uniformly through a steel matrix characterized by an austenitie decomposition product selected from the group consisting of martensite and bainite and mixtures thereof, and further characterized metallographically by rounded primary grains of titanium carbide.

' These and other objects will more clearly appear when taken into consideration with the following disclosure and the accompanying drawing. wherein:

FIG. 1 depicts schematically a device for the plasma flame spraying of metal powder onto a ferrous metal core;

FIG. 2 shows schematically a Wankel engine utilizing a heat treatable, titanium carbide steel as a hard-facing material on the inner end walls thereof: and

FIGS. 3 and 4 are a perspective and cross section. respectively, of a ferrous metal core used in producing the cylinder of an aluminum rotary engine housing.

SUMMARY OF THE INVENTION Stating it broadly, the method aspect of the invention for producing a wear resistant coating of a heat treatable titanium carbide tool steel on a metal substrate resides in selecting a powder composition consisting essentially of about l to 80% by weight of primary grains of titanium carbide and the balance essentially 90% to 20% by weight of steel-forming ingredients and quench-depositing said composition from the molten state onto the surface of a ferrous metal core by means of a plasma flame which heats the carbide steel-forming ingredients to substantially above the melting point, whereby a dense, coherent coating of the composition is produced on the metal substrate, the coating being thin relative to the metal substrate and preferably ranging up to about 0.025 inch in thickness, the surface of the coating being characterized by a microporous structure.

By employing the plasma flame for depositing the coating, rather high temperatures are obtained which melt the steel matrix of the composition at temperatures substantially above the melting point, such that thin coatings deposited on the metal core are drastically quenched by virtue of the cooling effect of the metal substrate whereby to produce a microstructure comprising grains of titanium carbide dispersed through a matrix formed of an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof. Thus, the carbide tool steel is deposited in the hardened state and does not require subsequent high temperature quenching to produce martensite.

As stated hereinabove, very high temperatures are obtainable with the plasma flame. However, for most spray applications, a plasma temperature of about 12,000 to 20,000F appears to be optimum. One of the advantages of the plasma flame is that it can be used with a controlled atmosphere. This is important to avoid decarburization of the steel matrix where carbon is essential to the heat treatment response of the titanium carbide tool steel. Thus, an inert gas or a chemically inactive gas can be employed for the flame medium.

The plasma flame is produced by striking an are between a cathode and an anode and passing a plasma gas through the arc. By confining the arc in a chamber under pressure, the arc temperature can be increased. By constructing the anode as a hollow nozzle and introducing the plasma gas into the arc chamber and forcing it through the nozzle, the gas dissociates and ionizes in the are stream and emerges from the nozzle as a plasma flame. A typical plasma gas is one comprised of 90% nitrogen and hydrogen. Argon or other gases can be used in place of nitrogen.

A schematic representation of a plasma flame device is given in the accompanying drawing which shows cathode l0 and anode ll electrically connected via power source 12 to produce an arc stream 13. Plasma gas 14, e.g., 71 nitrogen and 10% hydrogen is fed through pipe 15 which is converted to plasma 16 which exits through nozzle 17 at a very high temperature as free plasma. Spray powder is fed through pipe 19 into the nozzle where it is heated by the plasma flame and exits with the free plasma towards the workpiece or substrate to be coated. Plasma guns for metal powder spraying are readily available and. therefore. need not be discussed any further than the schematic described above.

One of the advantages of plasma spraying is that relatively thin coatings can be sprayed which are dense and substantially free of large pores. although the surface of the deposit will have micropores which are desirable in subsequent operational steps. By spraying thicknesses ranging up to about 0.025 inch, highly rapid quenching of the deposit is obtained generally comprised of martensite. Where the ultimate metal substrate to receive the coating is aluminum, very hard coatings are assured which can be further heat treated, e.g., tempered or aged, at temperatures below the melting point of aluminum.

The foregoing is particularly applicable to the transplant coating of aluminum during the die casting thereof in the production of aluminum rotary engine housings of the type shown in FIG. 2.

FIG. 2 shows schematically a Wankel engine comprising an aluminum housing 20 having a chamber 21 in which is mounted a triangularly shaped rotary piston 22 in sealing contact with the end wall 23 of the chamber at its apices 24 to 26. The rotary piston has an internal gear mounted thereon which is driven by gear 28 mounted on a shaft running perpendicular to the rotary piston. The hard-facing material is applied by the transplant technique to end wall 23 as shown by the heavy line to provide sufficient wear resistance to the material of the apices in rubbing contact with the end wall. The material of the apices may comprise spring mounted inserts 19 of the same titanium carbide tool steel maintained in continual sealing contact with the end wall via spring 30.

In operation, as the piston rotates, fuel and air are received at intake zone 31 through intake 32. The fuel-air mixture is then compressed and fired in compression zone 33 via spark plug 34 and the combusted gases at exhaust zone 35 exhausted through outlet 36. The temperature in the chamber rises to levels which may tend to temper certain heat treatable steels. However, by employing a hard-facing material described of the type containing about 10% Cr, 3% Mo, 1% C and the balance essentially iron, a temper resistant surface is provided capable of being heated up to about l,000F

without substantially diminishing in hardness and, if

anything, tends to increase in hardness slightly due to a secondary hardening effect engendered by the precipitation of secondary carbides containing chromium. By assuring the presence of rounded grains of titanium carbide in both the inserts of the seal and the end wall hard-facing material, low friction is assured during operation of the engine.

Rotary engine assemblies can be produced by die casting as described in U.S. Pat No. 3,359,615. As will be noted from this patent, a die core adapted to produce the trochoidal housing is coated with a metal layer of a ferrous or cuprous alloy which ultimately is transferred by the transplant technique to the rotary engine housing (the cylinder) during die casting ofthe part. As pointed out in column 2 of the patent, a chrome plate is applied to the transplanted coating to provide the desired surface characteristics.

Thus, according to the patent. the coated core piece, of the type 40 shown in FIGS. 3 and 4 herein is introduced into a die casting die and aluminum cast around it under die casting pressure to effect interlocking of the cast aluminum to the surface of the coating on the core. The casting is thereafter stripped from the core, with the coating adhering to the inner surface of the housing. In view of the fact that die casting techniques are old as evidenced by U.S. Pat. No. 3,359,615, such details need not be repeated here.

Thus, in its broad aspects, the invention resides in a method of producing a hard, wear resistant coating of a heat treatable titanium carbide tool steel on the wall of a cylinder of an internal combustion engine housing cast from aluminum, the method comprising, providing a ferrous metal core having a smooth surface corresponding in configuration to the wall of the cylinder to be cast, selecting a powder composition containing about to 80% by weight of primary grains of titanium carbide and the balance essentially about 90% to by weight of steel-forming ingredients which, when melted to form a steel and quenched from an austenitizing temperature, tends to form martensite and then quench-depositing said composition upon the smooth surface of said core by means of a plasma flame, whereby an adherent coating of said titanium carbide tool steel composition is produced on said ferrous metal surface characterized by primary grains of titanium carbide uniformly dispersed through a matrix containing an austenitic decomposition product se lected from the group consisting of martensitc. bainite and mixtures thereof and further characterized in that said coating is capable of being heat treated at a temperature below the melting point of aluminum. The method additionally comprises inserting said coated core in a mold for casting said engine housing, causing aluminum to flow into said mold under pressure, surround said core in Contact with said coating and solidify therein and then separating said core from said casting whereby the wall of the cylinder of said housing has transferred to it said hard carbide coating.

DETAILS OF THE INVENTION As illustrative of titanium carbide tool steel compositions that can be plasma sprayed onto the ferrous metal core, the following examples are given.

Example 1 Broadly, the titanium carbide tool steel consists essentially of about 10 to 80% by weight of primary grains of titanium carbide dispersed through a steel matrix making up about 90% to 20% of the balance. The steel may be low and high carbon steel, medium alloy steel or high alloy steel containing at least 50% iron which, when cooled substantially rapidly from above the melting point, provides metallographically a matrix containing an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof. In this connection, reference is made to US. Pat. No. 2,828,202. Examples of such matrix steels are SAE 1010 to SAE 1080 steels, and including the following illustrative composition, to wit: 0.8% Cr, 0.2% Mo, 0.3% C and iron substantially the balance; 5% Cr. 1.4% Mo, 1.4% W 0.45% V, 0.35% C and iron substantially the balance; 8% Mo, 4% Cr. 2% V, 0.8% C and iron substantially the balance; 18% W, 4% Cr, 1% V, 0.75% C and iron substantially the balance; 20% W, 12% Co, 4% Cr, 2% V, 0.8% C and iron substantially the balance.

A preferred composition is one containing about 1% to 6% Cr, about 0.3% to 6% Mo, about 0.3 to 0.8% C and the balance essentially iron.

The core 40 to be coated is shown in FIGSv 3 and 4, the surface coated being designated generally by the numeral 41. The core is provided with an extension 42 which is adapted to fit in a socket of a die casting machine. The sprayed coating 43 is shown in H6. 4. Example 2 A particular titanium carbide tool steel is one containing 10% to by weight of TiC and the balance essentially a high chromium high carbon steel containing about 6 or 7% to 12% chromium, 0.6 to 12% carbon, 0.5 to 5% molybdenum, up to about 5% tungsten, up to about 2% vanadium, up to about 3% nickel, up to about 5% cobalt, and the balance essentially iron. A preferred composition of the foregoing high chromium, high carbon steel is one containing about 10% chromium, 1% carbon, 3% molybdenum, 1% vanadium, and the balance essentially iron. This steel is characterized in that it forms martensite when applied from a plasma spray onto a relatively cold metal substrate. By double tempering at 1,000F for one hour each after the transplant is made (below the melting point of aluminum), the hardness is further augmented by secondary hardening.

Example 3 As illustrative of another titanium carbide tool steel composition that can be plasma sprayed onto a metal substrate and which can be further heat treated at a temperature below the melting point of the substrate metal is a heat treatable, low carbon nickel-containing titanium carbide tool steel (note US. Pat. No. 3,369,891). As in the foregoing examples, the titanium carbide ranges from about 10% to 80% by weight and the steel matrix from about to 20% by weight. The matrix composition contains by weight about 10 to 30% nickel, 0.2 to 9% of titanium, and up to about 5% aluminum, the sum of the titanium and aluminum not exceeding about 9%, up to about 25% cobalt, up to about 10% molybdenum, and substantially the balance of the matrix at least about 50%iron; the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of the aluminum andd titanium is less than 1.5%, the cobalt and molybdenum contents are each at least about 2%; and such that when the nickel content ranges from about 18% to 30% and the molybdenum content is less than 2%, the sum of the aluminum and titanium exceeds 1.5%.

When the foregoing titanium carbide tool steel is deposited from the plasma flame and rapidly quenched, the metallographic structure is essentially soft martensite. In this condition, after the coating has been transplanted to the aluminum substrate, it can be agehardened by heating it to a temperature of about 500F to below the melting point of aluminum, e.g., 1.000F, (260C to 594C) for about 3 hours. A typical agehardening temperature is 900F (483C). As will be noted. the age-hardening temperature is below the melting point of aluminum. Normally, the solution temperature for obtaining soft martensite in the matrix ranges from about 1,400F to 2.150F (760C to 1,165C). As will be observed, the foregoing temperature range is above the melting point of aluminum. However. a solution treatment is not necessary for the coating after the transplant is made since the steel is solution-quenched due to the rapid cooling following plasma spraying of the composition on the ferrous metal core.

A typical composition is one containing about 35% by weight of TiC and the 65% remainder a steel matrix containing 21.7% Ni, 8.49% Co. 3.42% Mo. 0.37% Ti and the balance essentially iron. The alloy upon aging at 900F (483C) for 3 hours exhibited a hardness of about 60 R It is preferred when working with compositions of the type illustrated in Examples 1, 2 and 3 to work with a prealloyed titanium carbide tool steel. This assures the presence of rounded grains of titanium carbide which not only provides resistance to wear but also imparts very low coefficient of friction. This is important in applications involving the continuous rubbing of parts, such as occurs in rotary combustion engines where the apices of the rotary piston are in continuous contact with the inner wall of the housing. A transplanted face coating of a titanium carbide tool steel of the type illustrated in Example 2 provides adequate resistance to wear on the aluminum housing and, in addition, low coefficient of friction by virtue of the rounded grains of TiC.

In assuring the rounded grain structure of titanium carbide, the titanium carbide tool steel is prealloyed by liquid phase sintering a powder metallurgical composition of the carbide steel. The prealloyed carbide steel is then ground into a particle size passing through 200 mesh for use in plasma spraying.

In producing a prealloyed steel composition with rounded grains of TiC, the following method is employed.

A titanium carbide tool steel composition containing 33% by weight of TiC (45% by volume) and substantially the balance a steel matrix, such as a chromiummolybdenum steel composition, is produced by mixing 500 grams TiC (of about 5 to 7 microns in size) with 1,000 grams of steel-forming ingredients in a mill half filled with stainless steel balls. To the powder mix is added 1 gram of paraffin wax for 100 grams of mix. The milling is conducted for about 40 hours using hexane as a vehicle. A specific steel-forming composition for the matrix is one containing 0.5% C, about 3% Cr, about 3% Mo and the remainder substantially iron. It is preferred to use carbonyl iron powder in producing the mixture. A carbidic tool steel of the foregoing type is disclosed in US. Pat. No. 3.416.976.

Following completion of the milling, the mix is removed and dried and compacts of the desired shape pressed at about tsi and the compacts then subjected to liquid phase sintering in vacuum at a temperature of about 2,640F (1,450C) for about one-half hour at a vacuum corresponding to microns or less. After completion of the sintering. the compacts are cooled and then removed from the furnace. The primary titanium carbide grains which are angular before sintering, assume a rounded configuration as a result of liquid phase sintering. By liquid phase sintering is meant heating the compact to above the melting point of the steel matrix but below the melting point for titanium carbide. for example. up to about 180F 100C) above the melting point of the steel matrix.

Following the production of the sintered compact. the sintered compact may be converted into chips by machining and the chips milled in a ball mill to a size passing through 200 mesh (e.g.. 1 to 5 microns). The powder is cleaned and dried for use for plasma flame spraying. As stated above. rounded titanium carbide grains are preferred in the ultimate coating since this configuration imparts low friction characteristics to the transplanted coating, the rounded grains being advantageous in wear applications.

In plasma spraying the ferrous metal core. the core is first provided with a smooth polished surface so as to assure a nice smooth surface on the transplanted coating. The hard-facing material in the finely powdered form (-250+325 mesh) is fed into the stream of a superheated plasma gas. The particles are melted and are carried by the gas at high velocity to the surface being plated. A coating can be built up to the desired thickness by forming multiple layers. It is preferred that the coating be thin and preferably range up to about 0.025 inch and, more preferably. up to about 0.015 inch, to minimize cracking due to cooling stresses.

As illustrative of a preferred embodiment of the invention, the following example is given.

Example 4 In this example. a prealloyed titanium carbide tool steel produced by liquid phase sintering is used as the plasma spray powder, the mesh size of the powder being in the range of about 170 to +325, the composition consisting essentially of about 33% by weight of TiC and 67% by weight of a steel matrix having the following composition; 3% Cr. 3% M0, 0.5% C and the balance essentially iron.

A ferrous metal core (note FIG. 3) comprising a steel containing 5% Cr, 1.5% M0, 0.4% V, 0.35% C and the balance essentially iron is employed, the core having a polished surface. The plasma gas used comprised nitrogen and 10% hydrogen. A coating of about 0.015 inch is produced.

The plasma spraying is conducted using a plasma spray gun referred to in the trade as Metco plasma flame system" consisting of a specially constructed torch-type gun in which powdered coating material suspended in a suitable carrier gas (N is fed into a chamber in which plasma gas is excited to high temperatures by an electric are. This produces a coating having a uniform microstructure.

Microhardness measurements of thin coatings produced in this manner indicate hardness across the thickness of coating ranging from about 650 to 770 VHN gram load) which corresponds to a Rockwell C hardness of about 52 to 60. This hardness is characteristic of the presence of an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof. The foregoing steel composition in the annealed state (pearlitic microstructure or spheroidized carbon) normally has a hardness in the neighborhood of 45 R Thus, the invention enables the production of transplanted coating of a hardened titanium carbide steel composition without the necessity ofquench hardening the coating from an austcnitizing temperature of about I.750F which is above the melting point of aluminum. The quenching effect achieved during the deposition of the coating provides the desirable austenitic decomposition product which contains martensite. Example A hard-facing composition particularly resistant to softening at elevated temperatures ranging up to 1,000F is one comprising about 35% by weight of TiC and the balance 65% of a steel matrix comprising essentially about Cr. 3% Mo, 0.8% C and the bal ance essentially iron. As in Example 4, this steel composition is provided essentially in the prealloyed condition to assure the presence of rounded primary grains of TiC dispersed through the steel matrix.

A powder of the foregoing titanium carbide tool steel of 200+325 mesh is plasma sprayed onto a mild steel substrate of about one-fourth inch thick to produce a coating thickness of about 0.01 inch in order to determine the hardness of the coating after being quench deposited. Because of the mass of the steel substrate, the coating is rapidly cooled to provide a martensiticbearing metallographic structure. The hardness of this coating will generally be in the range of about 50 to 55 Rockwell 'C and will generally transplant at this hardness since die castings cool fairly rapidly. However, this steel composition can be further hardened after the coating is transplanted by utilizing its secondary hardening characteristic by the formation of secondary carbides by heating the coated metal substrate and the coating to about 1,000F and holding at the temperature for about 1 /2 hours. Thus, heating to 1,000F will have a two-fold function: (1) to utilize the secondary hardening characteristics of the titanium carbide steel composition, and (2) to minimize the effect of any residual stresses in the coating arising from the rapid quenching of the coating during deposition from the plasma flame.

Example 6 This hard-facing titanium carbide steel composition is advantageous in that the coating deposited on the steel core and then transplanted by die casting to an aluminum substrate, can be hardened after transplant by heat treatment at a temperature below the melting point of aluminum. A particular composition is one containing 30% by weight of titanium carbide and the balance of 70% essentially a steel matrix containing about 21.5% Ni, 9.5% Co, 3.4% M0, 0.4% Ti and the balance essentially iron.

The foregoing steel in the prealloyed condition and as a powder of l50+325 mesh size is plasma sprayed onto the steel core as described in Example 4. By using a steel matrix very low in carbon. e.g., below 015%, soft martensite is obtained in the coating. After the coating has been transplanted to an aluminum substrate. the coating can be heat treated to a temperature of about 900F to age harden the steel to the desired hardness.

Examples of titanium carbide steel coatings that can be produced by plasma spraying are as follows:

Titanium Carbide Steel Coating l. TiC and 80% steel matrix containing 3% Cr, 3% M0, 0.5% C and the balance essentially iron 2. 30% TiC and 70% steel matrix containing 8% Mo, 4% Cr, 2% V. 0.85% C and the balance essentially iron 3. 35% TiC and 65% steel matrix containing 5% Cr,

1.4% Mo, 1.4% W, 0.45% V. 0.35% C and the balance essentially iron 4. 40% TiC and 60% steel matrix containing 8% Cr. 3% Mo, 1% V, 0.9% C and the balance essentially iron 5. 35% TiC and steel matrix containing 20% Ni,

1.75% Ti, 0.8% Al, 0.15% C, 0.5%- Mn, 0.2% Si and the balance essentially iron 6. 60% TiC and 40% steel matrix containing 10% Cr;

2% Mo, 2% W, 1% C and the balance essentially iron 7 As stated hereinbefore, the coating material should preferably be prealloyed before spraying so as to assure the presence of rounded grains of primary titanium carbide dispersed through martensitic-containing steel matrix. All of the coatings in the foregoing table by virtue of plasma spraying are characterized by the presence of martensite in the matrix. This assures a hard matrix in situ or a hard matrix by tempering at temperatures below the melting point of aluminum. If the matrix is too soft in the aluminum housing of rotary combustion engines, then preferential wearing of the matrix will result in dislodgement of titanium carbide and the gradual fretting away of the coating material.

Present developments in the Wankel engine contemplate the use of an aluminum housing. The rotating piston which has a generally triangular shape is in contact with the end walls of the housing by means of the apices thereof which require the use of a seal material as a seal-off between the spaces defined between the apices. The seal must have wear resistance. However, the aluminum in the housing is generally soft compared to most materials of construction and has poor wear resistance. By transplanting a hard carbide coating to the inner walls of the aluminum housing, the life of the housing can be prolonged. An aluminum housing is desirable as it is capable of being air cooled easily in view of its high thermal conductivity. The heat treatable hard-facing material should preferably be temper resistant so as to resist softening due to the generation of heat in the fuel combustion chamber.

A coating material which appears promising as a coating transplant for aluminum housings is one containing by weight about 10 to TiC and the balance to 20% of a steel matrix containing 6 or 7 to 12% Cr, 0.5 to 5% M0, 0.6 to 1.2% C and the balance essentially iron. The matrix composition may contain by weight up to about 5% W, up to about 3% Ni, up to about 5% Co, up to about 2% V, amounts of Mn and Si usually found in steel and the balance essentially iron. A specific composition is one containing 10% Cr, 3% Mo, 1% C, and the balance essentially iron. This steel resists tempering at temperatures as high as 1,000F and in fact tends to undergo secondary hardening at the latter temperature.

The term aluminum employed herein is meant to cover aluminum and aluminum alloys used commer cially.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. In a method of producing a hard, wear resistant metal coating on the wall of a piston-receiving chamber of a housing of an internal combustion engine cast from aluminum, wherein said coating metal is first deposited by a plasma flame upon a smooth surface of a core corresponding in configuration to the wall of said aluminum housing, the core being then inserted in a mold for casting said engine housing by causing molten aluminum to flow into said mold under pressure and surround said core in contact with said coating and solidify therein and wherein said core is thereafter removed from said cast housing with the coating transferred to the wall of said cast housing, the improvement which comprises,

selecting a powder composition containing about to 80% by weight of primary grains of titanium carbide and the balance essentially about 90% to by weight of steel-forming ingredients which when melted to form a steel and quenched from an austenitizing temperature tends to form martensite, and quench-depositing said composition upon the smooth surface of said core by means of a plasma flame to form a coherent coating therein, such that when said coherent coating of said titanium carbide tool steel composition is transferred to the wall of said housing by casting aluminum about said core and the core thereafter removed, the transferred coating is characterized by primary grains of titanium carbide uniformly dispered through a matrix containing an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof and further characterized in that said coating is capable of being heat treated at a temperature below the melting point of said aluminum housing. 2. The method of claim 1, wherein the core is removed from the housing cast around it by differentially cooling said core relative to the cast housing.

3. The method of claim 1, wherein the powder composition is a prealloyed composition characterized in that the titanium carbide grains are rounded and dispersed through a steel matrix and wherein the coating applied ranges in thickness up to about 0.025 inch.

4. The method of claim 1, characterized in that the steel matrix is selected from the group consisting of A) about 1% to 6% Cr. about 0.3% to 6% Mo, about 0.3% to 0.8% C and the balance essentially iron; (B) about 6% to 12% Cr, about 0.5% to 5% Mo, about 0.6% to 1.2% C. up to about 5% W, up to about 2% V, up to about 3% Ni, up to about 5% Co and the balance substantially iron; and (C) a high nickel alloy steel containing about 10% to 30% Ni, about 0.2% to 9% Ti. up to about 5% Al, the sum of the Ti and Al content not exceeding about 9%, up to about 25% Co, up to about 10% Mo, substantially the balance of the matrix being at least about 50% iron, the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of Al and Ti is less than about 1.5%, the molybdenum and cobalt contents are each at least about 2%, and such that when the nickel content ranges from about 18% to 30% and the molybdenum content is less than 2%, the sum of Al and Ti exceeds 1.5%; said matrix produced from compositions (A), (B) and (C) being characterized metallographically by the presence of an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof.

5. The method of claim 4, wherein the following transfer of the metal coating on the wall of the aluminum housing, the coating is heat treated by heating it to an elevated temperature below the melting point of the aluminum housing.

Claims

1. IN A METHOD OF PRODUCING A HARD, WEAR RESISTANT METAL COATING ON THE WALL OF A PISTON-RECEIVING CHAMBER OF A HOUSING OF AN INTERNAL COMBUSTION ENGINE CAST FROM ALUMINUM, WHEREIN SAID COATING METAL IS FIRST DEPOSISTED BY A PLASMA FLAME UPON A SMOOTH SURFACE OF A CORE CORRESPONDING IN CONFIGURATION TO THE WALL OF SAID ALUMINUM HOUSING, THE CORE BEING THEN INSERTED IN A MOLD FOR CASTING SAID ENGINE HOUSING BY CAUSING MOLTEN ALUMINUM TO FLOW INTO SAID MOLD UNDER PRESSURE AND SURROUND SAID CORE IN CONTACT WITH SAID COATING AND SOLIDIFYING THEREIN AND WHEREIN SAID CORE IS THEREAFTER REMOVED FROM SAID CAST HOUSING WITH THE COATING TRANFERRED TO THE WALL OF SAID CAST HOUSING, THE IMPROVEMENT WHICH COMPRISES, SELECTING A POWDER COMPOSITION CONTAINING ABOUT 10% TO 80% BY WEIGHT OF THE BALANCE ESSENTIALLY ABOUT 90% TO 20% BY EIGHT OF STEEL-FORMING INGREDIENTS WHICH WHEN MELTED TO FORM A STEEL AND QUENCHED FROM AN AUSTENITIZING TEMPERATURE TENDS TO FORM MARTENSITE, AND QUENCH-DEPOSITING SAID COMPOSITION UPON THE SMOOTH SURFACE OF SAID CORE BY MEANS OF A PLASMA FLAME TO FORM A COHERENT COATING THEREIN,

1. In a method of producing a hard, wear resistant metal coating on the wall of a piston-receiving chamber of a housing of an internal combustion engine cast from aluminum, wherein said coating metal is first deposited by a plasma flame upon a smooth surface of a core corresponding in configuration to the wall of said aluminum housing, the core being then inserted in a mold for casting said engine housing by causing molten aluminum to flow into said mold under pressure and surround said core in contact with said coating and solidify therein and wherein said core is thereafter removed from said cast housing with the coating transferred to the wall of said cast housing, the improvement which comprises, selecting a powder composition containing about 10% to 80% by weight of primary grains of titanium carbide and the balance essentially about 90% to 20% by weight of steel-forming ingredients which when melted to form a steel and quenched from an austenitizing temperature tends to form martensite, and quench-depositing said composition upon the smooth surface of said core by means of a plasma flame to form a coherent coating therein, such that when said coherent coating of said titanium carbide tool steel composition is transferred to the wall of said housing by casting aluminum about said core and the core thereafter removed, the transferred coating is characterized by primary grains of titanium carbide uniformly dispered through a matrix containing an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof and further characterized in that said coating is capable of being heat treated at a temperature below the melting point of said aluminum housing.

2. The method of claim 1, wherein the core is removed from the housing cast around it by differentially cooling said core relative to the cast housing.

4. The method of claim 1, characterized in that the steel matrix is selected from the group consisting of (A) about 1% to 6% Cr, about 0.3% to 6% Mo, about 0.3% to 0.8% C and the balance essentially iron; (B) about 6% to 12% Cr, about 0.5% to 5% Mo, about 0.6% to 1.2% C, up to about 5% W, up to about 2% V, up to about 3% Ni, up to about 5% Co and the balance substantially iron; and (C) a high nickel alloy steel containing about 10% to 30% Ni, about 0.2% to 9% Ti, up to about 5% Al, the sum of the Ti and Al content not exceeding about 9%, up to about 25% Co, up to about 10% Mo, substantially the balance of the matrix being at least about 50% iron, the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of Al and Ti is less than about 1.5%, the molybdenum and cobalt contents are each at least about 2%, and such that when the nickel content ranges from about 18% to 30% and the molybdenum content is less than 2%, the sum of Al and Ti exceeds 1.5%; said matrix produced from compositions (A), (B) and (C) being characterized metallographically by the presence of an austenitic decomposition product selected from the group consisting of martensite, bainite and mixtures thereof.