CA1130115A - Spray-and-fuse self-fluxing alloy powders - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A boron-containing nickel or cobalt spray-and-fuse self-fluxing alloy powder containing hard precipitates of chromium boride and, optionally, chromium carbide which are internally precipitated from a melt of said alloy is described. Said alloy powder is made by atomizing the melt at about the temperature at which the melt is viscous, and is disposed for application to metal articles to form improved coatings thereon, thereby providing for repair or build-up of warm, damaged or improperly machined parts as well as providing protection to new parts.

Description

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The present invention relates to spray-and-fuse self-fluxing alloy metal powders and more particularly to such powders having relatively large hard precipitates therein for improving the wear resistance of such powders.
Spray and fuse, self-fluxing metal powders are well known in the art and widely used. They can be deposited on a base metal by any available spray process including flame spray and plasma spray, for example, and the deposit fused simultaneously or subsequently. A dense coating on the base metal results with the powder particles metallurgically bonded to the base metal. The coating or overlay can impart wear resistance, corrosion resis-tance, o~idation resistance, high room temperature and hot hardness, and the like to the surface of the base metal to which the coating has been applied.
The alloy metal powders are used to repair or build up worn, damaged, or improperly machined parts as well as to provide protec~ion to new parts. The metal powders usually are nickel or cobalt based and descriptions of such alloy metal powders can be found in United States patents nos. 2,875,043;

2,936~229; and 3,305,326.
The present invention relates to a method of producing spray-and-fuse self-fluxing alloy powders which are very resistant to abrasive wear, to the alloy powders per se and to a metal article coated with the fused alloy powder.
A boron-contain m g nickel or cobalt spray and fuse, self-fluxing metal powder contains hard precipitates of at least internally precipitated chromium boride, chromium carbide, or mixtures thereof. Production of said powder comprises cooling down a liquid melt of a boron-containing nickel or cobalt spray-and-fuse, self-fluxing metal alloy to about the temperature at which the melt becomes thick and viscous or to a temperature about 50 to 100F higher than the viscous temperature. The viscous melt then is atomized " ' ~3~

at this YiSCous temperature (Tv) or about 50 to 100 F higher than the viscous temperature to produce the metal alloy powder containing hard precipitates of internally precipitated and grown chromium boride, chromium car~ide, or mix-tures thereo~. Advantageously~ at least a fraction of said precipîtates are larger than about 10 microns in average particle size, and preferably larger than about 15 microns. Desirably, the alloy powder contains by its volume at least about 5%~ and ad~antageously at least about 10%, of the internally precipitated hard precipitates.
According to the present invention, there is pro~ided a boron-con-taining nickel or cobalt spray-and-fuse, sel~fluxing alloy powder containing at least one hard precipitate selected from the group consistin~ of chromium boride and chromium carbide, said precipitate being internally precipitated from a YiSCoUS melt o~ said alloy, at least about 5% of said precipitate by Yolume of said powder havlng an average size of at least about 10 microns.
In another aspect, the invention proviaes a process for making a boron-containing nickel or cobalt spray-and-fuse, self-fluxing alloy powder which comprises:
a) forming a melt of boron- and chxomium-containing nickel or cobalt alloy;
b) cooling said melt so that a precipitate o~ chromium carbide and/or chromium ~oride results in the alloy po~der product, at least about 5% oP
said precipitate by volume of said powder ha~ing a particle size of at least about 10 microns;
d) finely dividing said melt into alloy powder.
Furthermore, the invention pro~ides a metal article coated with a fused coating of a boron-containing nickel or cobalt spray-and-~use~ self-fluxing alloy powder containine at least one hard precipitate selected from the group consistin~ of chr~mium boride and chxomium carbide, s~d precip-P\~ - 2 .

~3~15 itate being internally precipitated, at least about 5% of said precipitate by volume of said powder having an average size of at least about 10 mlcrons.
Pa~ticular embodiments of the present invention will now be de-scribed, by way of example only, ~i-th reference to the accompanying drawings, in which:
Figure 1 is a photomicrograph of a polished cross-section of a typical conventional, atomized spray-and-fuse nickel alloy powder, similar to Powders Mos. 1 and 2 of the E~ample hereinbelow;
Figure 2 is a photomicrograph of a polished cross-section of an atomi~ed spray-and-fuse nickel alloy powder of this invention, similar to Powder ~o. ~ of the Example herein below:
Figure 3 is a photomicrograph of a polished cross-section of a fused coating of the conventional powder of Figure 1~
Figure 4 is a photomicrograph of a polished cross-section of a fused coating of the inventive powder of Figure 2; and Figures 5-8 are sketches of Figure 1-4, respectively.
The alloy powders in the drawings are of composition and in prepara-tion, substantially similar to the powders of the Examples as noted above.
Referring to Figure 5 (and Figure 1), conventional nickel alloy po~der particles 11 are sho~n in cross-section and are held in block mount-ing material 12. The powder is substantially similar in composition to Powders ~os. 1 and 2 of the E~a~ples. Within powder particles 11 are a myriad ov very fine (about 1-4 micron average size) hard precipitates 13 o~
chromium boride and/or chromium carbide (chromium boride and chromium carbide are practically indistinguishable in the photomicrographs). Porosity 14 is --~ ~ .
typieal of most atomized metal powders.
In Figure 6 (and Figure 2), inventive nickel alloy powder particles 21 in cross-section are held in block moun~i~g material 12t The nickel alloy ., `~
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composition is substantially th~ same as conventional powder particles 11 of Figure 5. Chromium boride hard precipitates 22 (no chromium carbides are seen in this cross-section of powder par-ticles 21, but ~re contained in the powder) can be seen as irregular in shape. Hard precipitates 22 are rather large chromium boride particles of about 20-25 microns average particle si~e.
The difference i~ size of the chromium boride precipitates contained in con-ventionally atomized alloy po~ders and such precipitates in the instant alloy powder is dramatic and readily observable from the drawings.
Referring to Figure 7 (and Figure 3), fused nickel alloy matrix 33 is coated upon steel substrate 31. Application of the po~der and fusing operations were conventional. The alloy powder used to form this coating was a conventionally atomized nickel alloy powder like that powder described in Figure 5. Porosity 32 again is typical in these kinds of fused coatings.
Chromium boride/chromium carbide precipitates 34 can be seen scattered throughout nickel alloy matrix 33. Some particle size growth (e.g. about 2-6 microns) can be seen in comparing precipitates 34 in the fused coating to precipitates 13 in the conventional alloy powder shown in Figure 5.
Referring to Figure 8 ~and Figure 4), fused nickel alloy matrix 43 is coated on steel substrate 41 and comes from in~entive nickel alloy powder ~' like that shown in Figure 6. Porosity 42 again is seen. Contained within matrix 43 are a primary distribution of larger ~about 15-35 microns) chromium boriae precipitates 44 and a secondary distribution of smaller (about 3-10 microns) chromium boride precipitates 45. Again, particle si~e growth of the precipitates has occurred between the powder and the fused coating. This bimodal distribution of the hard precipitates is unique to the po~der of this invention and the difference be+.ween a ~used coating from conventional powders compared to the instant inventive powders is dramatic.

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The b~sic Rp~y~and-fuse~ selfrfluxing ~etRl po~der is con~entional `'' .. .. . .

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in composition such as those metal powders found, for example, in the follow-ing United States patents: 2,875,0~3; 2,936,229; and 3,305,326. Typically, a variety of other components are added to the basic nickel or cobalt matrix metal for providing a variety of special properties. Additions of silicon and boron are responsible for advantageous fluxing characteristics by form-ing low melting point glasses. Silicon and boron also lower the melting point of the alloy to facilitate spraying and fusing operations by forming lower melting point eutectic phases. Chromium is added to provide greater corrosion and oxidation resistance to the matrix. Chromium also combines with boron and carbon to form the hard precipitates responsible for wear and abrasion resistance. Copper and molybdenum can be added to the matrix metal for decreasing the fluidity during fusing of the applied metal powder and permit buildup of thicker coatings on the base metal to which the alloy pow-der is applied. On occasion, it can be advantageous to add aluminum to the matrix metal as a deoxidant and/or for obtaining a self-fusing alloy metal powder.
Some chemical compositions of commonly used nickel-based self-fluxing alloys suitable for use in the present invention are given in Table 1 for exemplary purposes.

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~L3tDi~5 A representative chemical composition of a cobalt-based sel~-~lux-ing alloy suitable ~or use in the present invention is given below:

TABLE II
Component Wei~ht-~

Co Balance ; ~i 18.0~21.0 Cr 18.0-20.0 Fe 1.0-4.0 Si 3.2-3.8 B 2.8-3.2 C 0.5~0.8 5 .o-8.o Rockwell Hardness (Rc) 55-61 Other representative nickel and cob~lt alloy powder compositions can be ~ound in the reference patents cited above and/or are well-known in the art.
The microstructure of the alloy powder consists basically o~ a matrix of nickel-rich solid solution and eutectic with dispersed hard par-ticles therein. ~he eutectic is actually a complex mi~ture o~ low melting eutectic phases. During fu~ing operations, the eutectic lique~ies for a short period of ti~e, thus closing the porosity of the deposit.
The alloy powder generally should not have a ~esh size above about 100 mesh (~yler Standard Sieves Series~ with the exact size depending upon the particular equipment used ~or ~praying and the particular fuel gas. For exsmple, when intended for spraying with a plasma Mame, the particles should be of a size of bet~een about 100 mesh to about 8 microns, and pre~erably between 270 mesh to 15 microns. For use with acetylene, the particles should all be below about 115 mesh with not more than about 15% be-low 325 mesh.
~hen ~ntended fo~ a~n~ with h~dragen ~ the ~uel ~as~ the lo~e~ limit is about 5 microns and all particles may be below 325 mesh.
Conventionally, the superheat temperatures used in melting and pouring the alloys for atomization normally are higher than those used in standard casting foundry practice. In the atomi~ation process, small orifices and 10~J pouring rates are involved relative to casting techniques. The high er temperatures, therefore, are necessary to prevent the melt from thickening or free~ing before the liquid metal stre~l is disintegrated. In general, melt temperatures of about 150 to 200F. above the melting point are used in conventional atomQzation processes, and often higher temperatures are used if the fluidity o~themelt is low. In con~ention~l spray and fuse, self-fluxing alloy metal powders, the pouriDg temperature during atomization is kept at a minimum of 2600 F. and more often at a higher temperatre than this.
(The liquidus temperatures that are reported usually for these alloys range from about lôOO F. to 2200 F. depending upon their composition.) In the present process, the alloy melt is cooled to a temperature whereat the fluiaity of the molten metal drops abruptly and it becomes thick and viscous.
The actual upper temperature limit where this viscous condition begins or ` ends depends somewhat upon the heating rate and cooling r~te used and cer-tainly upon the chemistry of the melt. Typically, the viscous condition occurs at a temperature somewhere below about 2500 F. The viscous condition of the melt is readily observed in actual practice. For present purposes, the term "alloy melt" means the melt of the components which is formed for producing the novel alloy powder of this invention.
In the present process, the melt is poured for commencement of atomlzation when the molten alloy is in this viscous state or at a tempera-ture not substantially higher than about 100 F. above the temperature at which the viscous state commences and preferably not substantially above about 5Q F1 higher than the ~iscous te~perature, Of cour~e~ it is recognized - 8 _ ~''`' `

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that the upper viscous temperature limit may be affected by the composition of the alloy so the above temperature limitations are eiven 8,S a guide in practicing the present invention. Some preliminary testing of the alloy ~elt is reco~mended in order to establish the requisite viscous temperature of each particular alloy powder as a function of its composition. The same is tr~eregarding the lower temperature limit of the viscous state of the alloy melt. Of course, the melt also must be at, a suf~icient temperature for atomizin~ the melt.
Typically, the molten stream of metal exiting the orifice is sub-jected to the action of Jets of water ~hich are directed upon such stream to assist in the atomization. Of course, gas-assisted atomization or other conventional techniques may be employed. On occasion it may be desirable to conduct a shotting operation to produce rather large particles of the metal alloy and then sub~ect such particles to size reduction by conventional attrition techniques in order to produce the desired sized alloy po~der and also have a rather narrow size distribution of such powder. Such rather large particles cool at a slower rate than atomized powder. This slower cooling rate allows an extra amount of time for the formation of large hard precipitates. Thus, shotting tends to produce a greater proportion of large precipitates than atomizing the same composition from the same temperature.
Shotting temperatures can thus be adJusted slightly higher than atomizing temperatures and the required proportion of large precipitates will be main-tained. This can be a preferred technique in some instances where difficulty is encountered in atomizing from a viscous melt. Further, cutting finer and coarser (oversized) alloy particles from the atomizing operation is a recom-mended procedure regardless of the particular type of atomization procedure used in amking the alloy metal powder of this invention.
~ ~he resultant po~de~ from the atomization process ~ith or without :: - 9 - :

' ~3~5 subsequent size reduction) is unique because of the internally precipitated chromium boride and/or chromium carbide precipitates coDtained therein. Such hard precipitates generally are larger in size than the chromium boride par-ticles typical of conventionally atomized alloy powders which atomize the me~t at hi~her temperatures whereat such melt is not in the viscous state. It may be desirable to conduct the present invention in such fashion that a distribu-tion of intermediate precipitates results~ e.g. precipitates of about ~ to 10 microns in size or even somewhat larger. Such sized precipitates in the alloy powder would provide an improved powder over con~entionally atomized powders ; 10 of substantially similar composition. On oth~r accasions, it may be desirable to conduct the present process in such fashion to provide rather large hard precipitates in the powder, e.g. precipitates of about 10 microns to 50 microns and even larger. A much improved alloy powder would result from such very large hard precipitates.
A presently preferred embodiment of the present invention results when the resultant powder contains very large precipitates of chromium boride as well as a smaller amount of very large chromium carbides. The microstructure of the resulting spray-and-fuse powder produced in accordance with this embodi-ment exhibits very large chromium boride particles, typically ranging up to about 20 to 25 microns and frequently up to 50 microns in size and larger.
: There is observed also a smaller number of chromium carbides, some of which are even larger in particle si~e than the chromoum boride particl0s. Additionally, a secondary distribution of finer chromium boride particles typically ranging in size from about two to ten microns in diameter normally is seen distributed in the matrix. It is to be noted that the distribution of these hard precip-` itates can be fairly uniform throughout the alloy powder since the hard pre-cipitates are precip:itated and grown internally in the resulting alloy powder, but such uniformity in distribution of the hard precipitates is not restrictive :: ~ : ~ :, . . . .

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3~3L3~ 5 of the performance of the present alloy powder. Such dual or bi~odal distribu-tion of the hard precipitates in the alloy powder is unique and provides a superior powder over conventionally atomized alloy powders. It should be understood that so~e preformed hard precipitates ~chromium carbide, chromium boride, tungstun carbide, e~c.) may be added to the melt prior to atomization in order to augment the internally precipitated hard pre~ipitates formed by the present process, but generally such a~dded hard particles should be relatively few in number and in amount compared to the amount of hard precipit-ates formed by the present process. Generally, the internally precipitated hard precipitates range from about 5% to 354 and higher by volume of the alloy powder.
It should also be understood that powders of the present invention can comprise blends, preferably with conventionally atomized powders. This can be done by producing a powder according to the present invention with a high proportion of internally formed large precipitates and blending it with conventional powder to form a blend having about the same proportion of large precipitates as powders previously described in the present invention. If blending with conventionally atomized powder is to be practiced, a powder can be produced having such a high proportion of internally formed precipitates that alone it would not be a suitable spray-and-~use self-1uxing alloy, how-ever, when blended, the blend would comprise a suitable alloy.
In most other respects, the powder appears and exhibits character-istics akin to commercially available atomized self-fluxing alloy powders. The bulk hardness of the coating produced from the present powder is about the same as in conventionally atomized powders and the other micros~ructural features also appear to be similar to those found in conventional atomized powders.
However, the size and distribution of the hard precipitates, especially chro-mium boride, formed in powders atomized by the instant process are unique and make such powders well equipp~d to provide m~lch better resistance to abrasive wear and other properties than heretofore is provided by commercially atomized powders.
The following Examples show hGIY the instant invention can be prac-ti~ed, but should not be construed as limiting the invention. In this applica~
tion, all parts are parts by weight, all mesh sizes are Tyler Standard Sieves Series, all percentages are weight percentages, and all temperatures are in degrees Fahrenheit, unless otherwise expressly indicated.
E.YAMPLES
Comparative measurements of abrasion resistance of coatings obtained from powders made according to the present invention and from standard commer-cially available powders were conducted. The following powders were used:
Powder No. 1: Code 74~M-60, a standard commercial grade of atomized nickel-based powder manufactured by Glidden Metals, SCM Corporation, Cleveland, Ohio.
Powder No. 2: Code 74-W-60, a standard grade of atomized nickel-based powder manufactured by Glidden Metals, SCM
Corporation, Cleveland, Ohio.
Powder No. 3: COLMONOY No. 6, a crushed nickel-based powder with separately produced and added CrB particles manufac-tured by Wall Colmonoy Company; Detroit, Michigan, COLMONOY being a registered trademark.
; Powder No. 4: Nickel-based powder made according to the instant invention with an atomization pouring temperature of 2300F (melt in viscous state).
Powder No. 5: Nickel-based powder made according to the instant invention with an atomiæation pouring temperature of 2400F (melt at temperature just above viscous state temperature.) - 12 _ ~: :

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Powders Nos. 4 and 5 were each made by for~ing a 12-pound melt of the components and cooling down the mol*en melt to the indicated pouring temperature. Atomization was conducted using a 3/8" orifice with a pouring rate of about 40-50 lbs. per minute with water jets directed upon the stream to assist in the atomization. Nominal par~icle size of all powders ranged from about 150 to 325 mesh with finer and oversized particles being cut from the alloy powder product.
Each of the five powders has nearly the same chemical composition and each forms a fused coating with about the same bulk hardness. The micro-structures of each powder with respect to the size and distribution of the hard particles, however, vary greatly. In powders Nos. 1 and 2, the hard particles ~chromium boride and chromium carbide) ranged in particle size from about 1 to 4 microns in diameter and accupied about 21% by volume of the alloy powder. In powder No. 3, the separately produced and added hard par-ticles ~chromium boride) were of about 5 to 10 microns in particle size and were measured at about 13% by volume of this powder. In powder No. 4 made in accordance with the present invention, the hard precipitates ~chromium boride and chromium carbide) ranged in size from about 8 to 20 microns. The proportion of such precipitates was about 11% by volume of the alloy powder.
~ 2Q In powder No. 5 made in accordance with the present invention~ there was a ~` primary distribution of hard precipitates ranging from about 9 to 25 microns in a proportion of about 3% to 5% by volume. Also, there was a secondary distribution of smaller precipitates ranging from about 2 to 7 microns. The total of these smaller precipitates was about 33% by volume of the powder. ;
The fused coatings of these powders also were subjec~ed to -,- analysis in the same fashion as were the powders. Por ~he fused coatings , ~rom powders Nos. 1 ,and 2 the hard particles ranged in particle size ~rom ,! about 2 to 6 microns and were present at about 21~ by volume of the fused ~ 13 -:-. .
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~:~L3~15 coating. For powder No. 3, the chromium boride particles (separately formed and added during the powder formation process) still ranged from about 5 to 10 microns and wera present at about 6% to 7% by volume of the fused coating.
For powder No. 4, the hard precipitates ranged in size from about 15 to 35 microns and were present at about 8% to 9% by volume. For powder No. 5, the primary distribution of larger hard precipitates ranged from about 15 to 35 microns and the secondary distribution of smaller hard precipitates ranged from about 2 to 7 microns. The total of all hard precipitates was about 20% by volume with the primary distribution being estimated at about 3%
10 to S% by volume.
Table III below summarizes and compares the above-reported measure-ments of the hard precipitates contained in the powdeTs and fused coatings of Powders Nos. 1-5.
TABLE III

Hard Hard Precipitates in Precipitates in Powder Fused Coating ~ Size Size - Powder No. ~ Volume-% ~ Volume-%
1 and 2 1-4 21% 2-6 21%
3 5-10 13% 5-10 6-7%
.

4 8-20 11% 15-35 8-9%

Primary Distribution 9-25 3-5% 15-35 3-5%

Secondary ~istribution 2-7 33% 2-7 15-17%
:
The increase in particle size of the hard particles in the fused coatings is typical o these types of powders. Note should be taken, though, that the fused coat;ngs from the inventive powders Nos. 4 and 5, do contain much larger haTd precipitates than the comparative powders. The variations in .:, , . .
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~3(~ 5 volume percentages between the powders and fused coatings are most likely functions of the following factors. Typically in atomized powders, some pow~
der particles contain little or no hard precipitates while other powder par-ticles are quite rich in the hard precipitates. The foregoing measurements were taken fTom powder particles rich in the hard precipitates and, thus, the reported values would tend to be higher than the actual values for a given batch of powder particles containing a mixture of powder particles more or less rich in the hard precipitates. Also, the measurements were taken from cross-sectional cuts of the powder particles whereat the concentration of the chrom-ium borides may be greater than in the outer areas of the powder particles.
The cross-sectional cuts of the fused coatings, however, should be quite accurate and representative of the entire coatings. Conventional grid measure-ments using the point count method on photomicrographs like those of ~he drawings were used in order to determine the Yolume of the hard precipitates ~ and their size was determined by scaled measurements of the photomicrographs.
Typical chemical composition of each powder in weigh*-percent is given below:
Ni balance Cr 13.5%
Fe 4.7%
Si 4.3%
B 3.0%
C Q.6%
- Also, each powder had a bulk Rockwell hardness (C scale) of 55-61.
Each powder was applied onto steel rods ~3 inches long by 0.5 in diameter~ by a standard oxygen-acetylene flame spray procedure followed by torch fusing. Each powder was screened for and sprayed with the manufacturer's recom~ended equipment and procedures.

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s The relative abrasion resistance of each coating was determined by grinding each coated rod to a uniform diameter of about 0.58 inches (corres-ponding to a coating thickness of about 0.04 inch~s~ on a centerless grinding machina with a silicon carbide grinding wheel.
The uniformly ground bars then each were fed in repeatedly and ground 0.005 inches per pass. The number of passes required to begin dulling the silicon carbide wheel was recorded as was the number of passes required to fully dull the wheel to an extent requiring re-dressing of the wheel. The results recorded appear below in Table IV.
TABLE IV

NU~BER OF PASSES ~O:
_ POWDER NO. _START DULLING _ FULLY DULL
1 8*

; 3 4 5 _ _ _ _ . . . _ _ . . . _ *Entire coating ground off after 8 passes at which time the test was stopped.
The foregoing results show that Powder No. 3 (separately produced and added CrB crushed powder) is better in abrasion resistance to conventionally atomized powders Nos. 1 and 2. However, powders Nos. 4 and 5 of this invention also are not only superior in abrasion resistance to powders Nos. 1 and 2, but powder No. S is roughly as good as powder No. 3 and Powder No. 4 is even more abrasion-resistant than is Powder No~ 3.

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Claims (25)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A boron-containing nickel or cobalt spray-and-fuse, self-fluxing alloy powder containing at least one hard precipitate selected from the group consisting of chromium boride and chromium carbide, said precipitate being internally precipitated from a viscous melt of said alloy, at least about 5%
of said precipitate by volume of said powder having an average size of at least about 10 microns.

2. The alloy powder of claim 1 wherein said precipitates are at least about 15 microns in average particle size.

3. The alloy powder of claim 1 wherein said precipitates range from about 10 microns to about 50 microns.

4. The alloy powder of claim 1 which contains a primary distribution of at least about 5% by volume of said precipitate of about 20 to 25 microns average particle size and a secondary distribution of said precipitate of about 5 to 10 microns in average particle size.

5. The alloy powder of claim 4 wherein said primary and said secondary distribution of said precipitate together ranges between about 5% and about 35% by volume of said alloy powder.

6. The alloy powder of claim 4 wherein the volume ratio of said primary distribution to said secondary distribution ranges between about 0.16 and 6.

7. The alloy powder of claim 1 which also contains internally pre-cipitated hard precipitates of chromium carbide and of chromium boride.

8. The alloy powder of claim 5 which also contains internally pre-cipitated hard precipitates of chromium carbide and of chromium boride,

9. A process for making a boron-containing nickel or cobalt spray-and-fuse, self-fluxing alloy powder which comprises:
a) forming a melt of boron- and chromium-containing nickel or cobalt alloy b) cooling said melt so that a precipitate of chromium carbide and/or chromium boride results in the alloy powder product, at least about 5% of said precipitate by volume of said powder having a particle size of at least about 10 microns;
c) finely dividing said melt into alloy powder.

10. The process of Claim 9 wherein step by comprises cooling said melt to within about 100°F. above the temperature at which said alloy melt becomes viscous.

11. The process of Claim 10 wherein step c) is atomizing said alloy melt to form alloy powder.

12. The process of Claim 9 wherein step c) is shotting said melt and attrition milling said shot to form alloy powder.

13. The process of Claim 9 further comprising blending said alloy powder with conventionally atomized powder, at least about 5% of said precip-itate by volume of said blend having a particle size of at least about 10 microns.

14. The process of Claim 10 wherein at least a fraction of said precip-itate are larger in particle size than an alloy powder atomized at a tempera-ture significantly greater than about 100°F. above the temperature at which said alloy melt becomes viscous.

15. The process of Claim 9 wherein step b) comprises cooling said melt to within about 50°F. above the temperature at which said alloy melt becomes viscous.

16. The process of claim 9 wherein said precipitate ranges from about 10 microns to about 50 microns.

17. A metal article coated with a fused coating of a boron-containing nickel or cobalt spray-and-fuse, self-fluxing alloy powder containing at least one hard precipitate selected from the group consisting of chromium boride and chromium carbide, said precipitate being internally precipitated, at least about 5% of said precipitate by volume of said powder having an average size of at least about 10 microns.

18. The metal article of claim 17 wherein said precipitate is at least about 15 microns in average particle size.

19. The metal article of claim 17 wherein said precipitate ranges from about 10 microns to about 50 microns.

20. The metal article of claim 17 wherein said precipitate contains a primary distribution of at least 5% by volume of said precipitate of about 20 to 25 microns average particle size and a secondary distribution of said precipitate of about 5 to 10 microns in average particle size.

21. The metal article of claim 20 wherein said primary and said secondary distributions of said precipitate together ranges between about 5% and about 55% by volume of said powder.

22. The metal article of claim 20 wherein the volume ratio of said primary distribution to said secondary distribution ranges between about 0.16 and 6.

23. The metal article of claim 17 wherein said hard precipitate is of chromium carbide and of chromium boride.

24. The metal article of claim 21 wherein said precipitate comprises internally precipitated hard precipitates of chromium carbide and of chromium boride.

25. The metal article of claim 17 wherein said alloy powder is a blend of alloy powder containing hard precipitates of chromium boride and/or chromium carbide which are internally precipitated and conventionally atomized powder, at least about 5% of said precipitate by volume of said blend having an average particle size of at least 10 microns.