CA1070529A - Master alloy for powders - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A master alloy powder is formulated for admixture to an iron-based powder to provide liquid phase sintering and production of a substantially homogeneous product having the characteristics of a wrought alloy product. The master alloy powder con-tains at least two elements selected from the group consisting of manganese, nickel, molybdenum, chromium, copper, carbon and iron. The master powder may contain additions of silicon up to 5% and rare earth metals up to 2%, either of which assist to speed up diffusion and create a more favourable liquidus-solidus relationship within the master alloy powder.

Description

~070529 The present invention is directed to alloys.
Consideration as to producing sufficiently homogeneous, hardenable low alloy powdered steel for processing as preforms for hot forming or as sintered shapes involves either or both of two procedures: pre-alloying or admixing. Pre-alloyed powders are currently in use as the basic material for low-alloy steel preforms or compacted shapes because of their homogeneity. However, pre-alloyed powders are relatively expensive compared to iron powder or conventionally produced iron and it is unlikely that parts producers will accept the limited number of alloyed compositions commercially available.
Accordingly, pre-alloyed powders properly represent only one of several means of providing a full range of alloy preforms which are substitutional for conventionally made wrought alloy compo~itions.
.

1~70~Z9 Mechanical mixture of powders, hereinafter referred to as admixtures, have ~een deemed capable of providing alloy-ing during sintering of the precompact, but exactly how to achieve adequate homogenization of the alloying ingred-ients is not known to the prior art. The prior art recognize$
that conceptually, admixtures seem to offer substantial economic advantages overpre-alloyed powders. Complete flexibility should result from ~lending a ~ase powder with a master alloy powder and thereby great reduction in manufac-turing costs. To arrive at this goal, there must be ; optimization of the master alloy powder and the total admixture must be designed to improve the kinetics of the sintering process.
A variety of mechanisms are at hand to produce the alloying condition by diffusion with degrees of success. ~or example, solid state particle diffusion can be used, aiffusion resultinq from gasification of one of the components to the admixture is feasible, or liquid phase sintering of the master alloy portion can be employed. Since diffusion in the solid state particle condition i8 limited by the number of the inner particle contacts, the hope of increasing the kinetics of complete alloying is limited. However, if the master alloy ingredient is converted to a gas or a liquid, there is an increase in the inner particle contact.
~ery few elements can ~e considered for the technique o~
gasification of one of the components and thus th~s aven~e is relati~ely narrow in application. ~herefore, there is a need for exploration and development o~ a master alloy , powder which will function by the liquid phase method of sintering.
The use of an iron-carbon eutectic as a base for a master alloy to behave much as copper in a standard production ~0705Z9 alloy during sintering was known more than 20 years ago.
Unlike nonferrous alloying additions, these master alloys were found to have much greater solubility. However, certain problems must be overcome if the advantageous solubility of master alloys is to be utilized. The ingredients of such master alloy powder must be selected with care so that each of the ingredients is compatible one with the other, and the melting range of the master alloy powder must be relatively narrow and as low as possible; the master alloy powder must have good fluidity and wetting characteristics to facilitate coating of the base ferrous powder with the alloy liquid for purposes of facilitating rapid and effective sintering and diffusion through a minimum di~tance.
In accordance with the present invention, there is provided a method of establishing alloying between solid and liquid phases of a powder mixture, comprising:
(a) prepare an iron-based powder substantially devoid of alloying ingredients, (b) prepare a pre-alloyed non-iron-based additive powder containing at least two element~
but up to all the elements selected from the group consisting essentially of manganese, molybdenum, nickel, chromium, copper and iron, molybdenum being in the range of ~ y weight when selected along with the absence of copper, the elements being selected and ~alanced to provide a span of meltin~ tem~eratures therefor of no greater than 350F and the additive powder having a liquidus temperature of between 1900-2250F, (c) uniformly blending l.S to 6% of the additive powder with the iron-based powder and with a predetermined amount of graphite to render 0.69~ car~on or less in the mixture ,~ .
~ ~ - 3 -1~705Z9 and compacting the blend to a d2nsity of at least 70%, and (d) heating the compacted blend to a temperature of up to 2250F for no greater than one hour, whereby the additive powder forms a liquid phase which readily diffuses along the particle boundaries and into the matrix of the iron powder thereby reducing the maximum diffusion distance to one particle radius or less.
It was observed in the course of the development of this invent~on that adding copper to a pre-alloyed base powder, containing some molybdenum and nickel, provided a substantial increase in impact strength of the hot formed powder. It was 1070~Zg theorized that copper, during the liquid phase sintering, coagulated the unreduced oxide films into globular or massive forms which are not detrimental to the physical properties of hot formed (forged) powder metal. The mechanical properties of the test samples containing admixed copper were equal to or superior to conventional steels of the same chemistry. The copper powder melted at 1981F (1083C) and was therefore li~uid at the sintering temperature; it di~fused quic~ly into the base powder increasing its hardena~ility (which is the critical aspect of preparing powder preforms).
After the benefits of admixing pure copper were dis-co~ered, a binary copper admixture containing 35~ manganese and S5~ copper was designed and investigated as a mixing agent for a ~ase steel powder; the binary alloy powder mix-ture melted at 1590F (868C). The diffusion occurred at a lower temperature and much more rapid pace than when pure copper along was admixed. From this it was theorized that ternary and quarternary powder alloy mixes of copper and manganese, alsng with nic~el and/or molybdenum could be prepared, the master alloy mix then being balanced in an amount to obtain a desired liquid fused precompact with steel or iron base powder. ~owe~er, with further experiment-ation it was found that copper in ~arger percentages was not compatible with molybden~m for purposes of liquid phase sintering, and presence of iron was required to lower the melting temperature when mo~y~denum and~or chromium was present. These refractory metals have a high melting point:
Mo-47~4F (2623CI and Cr-3389F (1863C). It was also found that it was important that the addition of the alloyin~
ingredients be critically controllPd so as to produce a narrow and relatively low sintering temperature range.

It was discovered that a successful multicomponent master alloy mixture (Designated No. 342) derived from metal melted under inert gas, gas atomized, and screened to a -200 mesh size and having the following chemical analysis provided an initially satisfactory li~uidus and melting range: nickel 28.20%, iron 10.52%, manganese 40.78%, molybdenum 5.37%, and chromium 15.15%. When this master alloy mixture was added into a base iron powder, the addition being 2~% by weight, together with natural graphite in four different proportions, and after being subjected to a con-~entional technique of precompacting, sintering in hydrogen atmosphere at 2250F~and hot forming at 1800OF (982C)' the resulting steels contained a final composition of 1.0 manganese, .03% copper" .82% nickel, .14~ molybdenum, .42 chromium, the remainder iron. The master alloy mixture had a liquidus of 2140~F (1171C) and a solidus of 1830F
~999C) during heating, producing a 310F (172C) melting range which i8 deemed useable for commercial applications.
Electron micropro~e analysis was performed on the hot formed preforms compacted to a density of 99+~ ~sing a 2~ master alloy powder in an iron based powder, the master alloy powders included, as candidates, the above descri~ed alloy powders No. 342 and 400 ~iven in Ta~le I.
~t was o~served that ~or the ingredients associated with the processing conditions used in the No. 342 experiment, the rela~i~e speed of diffusion was hi~hest for the manganese, while the di~fusion of moly~denum, nic~el and chromium was only approximately one third that of manganese. Man~anese ga~e a very narrow spread or deviation in the microcomposi-tion and is the most desirable element when using liquidphase powder alloying. It was also observed that the lo~er the melting temperature, the better the wetting action and fluidity of the master alloy and the better the homogeneity of the final product.
In search for an additional improvement to the wetting action, silicon and rare earth metals additions were made to several master alloy powders. The improvement of diffusion by an addition of only 1~% of silicon was sur-prising. Two heats of alloy powder No. 400~were made, one (No. 40~) without silicon and another (No. 400S) with 1 silicon. Both were made using the same melting method under inert gas and used inert gas atomizing. In a liquid diffusion test, the 400~ alloy powder exhibited twice as deep penetration into the iron powder as the alloy powder ~ithout silicon. A rare earth metal addition was bene-ficial to the liquidus-solidus relation, particularly ~n the presence of silicon. The mechanism of optimum improvement in diffusion i not ~nown but it might be due to 8ilicon reacting with residual oxide films present on the metal.
~ Certain ad~antageous multi-element alloys are summarized in Table I, Alloy No. 524 exhibiting the lowest liquidus and solidus - the respective values being 2065F
U169C) and 1730F (943QC), melting range being 335F
(1~6C~. Alloy powdar 524 had fi~e times deeper penetratio~
into the iron than the alloy powders No. 342 and No. 400 during the li~uid diffusion test run under the same conditions for all the alloy powders.
Following the multi-alloy success, as described further in alloy admixture examples, binary alloys of niekel-manganese (25% Ni, 75% ~n, Alloy No. 528) were tested and additions o~ silicon and rare earth metals were 10705.'Z9 also found beneficial. As nickel is a slow diffuser and forms "patches" of retained austenite at lower processing temperatures, copper was substituted for a portion of nickel. Copper was found to improve penetration and wetting action, but to a smaller extent than silicon. Thus in alloys without chromium and molybdenum, the composition 72~ Mn; 12.5% Ni; 12.5% Cu; 2% Si; 1% rare earth metals is advan~ageous.
Physical properties of powder metal steels for any heavy duty application, similar to conventional steels, depend upon good response to heat treatment and resultant microstructure, also clea~liness of material as regards non-metallic inclusions. Response of material to heat treat-ment is measured by hardena~ility. Hardenability of the resulting iron compact is expressed as Ideal Diameter (DI) which depends on the multiplying factors of alloying ingredients according to the formula:
I f x MfMo x Mf x Mf x Mf DI is the diameter of the bar which will harden in the center to 50% martensite. ~he most powerful elements contri-buting to hardenability are molybdenum, manganese, than chromium makes an intermediate contribution, nickel c~ntri-buting ~ery little at lower percentage level. Data regarding mu1tiplying factors vary co~siderably in literature, and these might not be fully applicable to powder metal steels, as silicon con~ent in powder metal usual~y is less than 0.02%. The molybdenum multiplying factor is typically cited as 1.8 at low carbon levels used in steels for car~uriz-ing, ~ut the same factor is 2.6 at high car~on levels, corresponding ~o the carbon in a car~urized case. Thus, depe~ding upon the particular application, the master al~oy 1070~Z9 steel powder has to be chosen to provide, for example in carburized steels, proper case hardness for the section involved and a tough low-carbon martensite core. Nickel, although not contributing much to hardenability such as at the 0.5% nickel level, does improve considerably the impact fatigue properties of gears and similar carburized parts.
With two groups of master alloy pow~ers available, one multi-alloy (Mo-Mn-Cr-Ni-Fe), the other binary ~Ni-Mn with copper substituted for some of the nickel), the master alloy powders can be made easily diffusible by small per-centage additions of silicon and rare earth, thus making it possible to provide a low alloy steel by liquid phase sintering responding to any hardenability requirement, either for quenched and drawn steel or for carburized parts.
Diffusion of molybdenum, even in a small amount, increases significantly the hardenability of the casé (E.g. 2~%
of alloy 524 results in .15% Mo and Mf = 1.37). Molybdenum i8 also known to overcome the difficulties associated with temper enbrittlement; upwards to 0.08~ Mo in the final product should be used as an al~oying addition for this purpos~.
Ta~le I below s~mmarizes nominal compositions of some master alloys pertinent to claims of this in~ention.

TABLE I O
Master O F
Alloy Chemical Com~osition, wt% F F ~lting Mix No. Mn Ni Cr Mo Fe Cu Si g.E. Liquidus Solidus Range 400 44 25 -11 ; 19 - - - 2200 213070 533 56 24 3 6 11 - - - 2115 18gO225 533S * 2.5 - 2130 1~20310 -533M * 2.5 1 2020 1850270 534S * - 2.5 - 2070 1870200 534M * - 2.5 1 2100 1860140 535S * , - - - 2100 1960140 535M * - - - 2145 1930215 527 74 12.5 - - - 12.5 1 - 1940 1700240 346, 38 23 18 6 15 - - - 2245 2000245 508 56 14 0 ~5 15 - - - 2300 2000300 ~09 56 14 lS 5 10 - - - 2170 2070100 510~ ~6 14 10 10 10 - - - 2240 2015225 Sll 59 11 15 5 10 - - - 228~ 2040240 513 56 14 22 8 - - - - lg2~ 2435SlS
S14 S0 20 15 5 10 - - - 209~ 2200110 515 46 24 lS 5 1~ 990 2220230 * The same a~ove percentages as immediately a~ove except reduced proportionately for the presence of silicon and/or rare earths~
_g_ ExAMæLEs A. Master Alloy No. 342 Master Alloy No. 342 was made using an inert gas atomizing technique and was screened to -200 mesh size. Its composition is given in Table I. Pure iron, water atomized powder (Atomet* 28, Quebec Metal Powders) was mixed with 2~%
addition of the prepared master alloy powder, four different le~els of natural graphite (~o. 1651), and 1% Acrawax* to provide die lubrication. The admixture was compacted into 3" diameter slugs and sintered in hydrogen atmosphere at 2250F ~1232C). The slugs were reheated by induction to 1800F (982C) in a protective nitrogen gas atmosphere and were hot formed into 4 diameter ~100 mm~ flat 1.1" (28 mm) thic~ cylinders, with a density close to 100~. Jominy hardenability bars and tensile and impact barR were prepared from these hot formed slugs.
The chemical composition o~ the bars was determined by X-Ray fluorescence and was 1.02% Mn; .14% Mo; .82% Ni, .42%
Cr, the remainder iron.
~ Hardenability of the alloy was calculated using a 50% martensite criterion; hardenability also was determined experimentally from standard Jominy 1" diameter ~25 mm) bars that were run using standard S~E procedure.
% Carbon Ideal Diameter Ideal DiameterPremix ~I Ca~cu~ated DT Experimental Al~oyin~
-Efficiency .20 1.57 1.~5 73~
.31 2.1~ }.B3 87%
.68 3.26 2.8 87 Premix alloying efficiency + DI Experimental DI Calculated x 100%
* Trademarks Mechanical test results of samples containing .31%
carbon and quenched and tempered to hardness of Rockwell C 26 were: Ultimate tensile strength - 119 k.s.i. (820 Mæa); Yield point 101 k.s.i. (696 MPa); Elongation - 24%; and Reduction of area 48% V-notch Charpy impact test" l0 mm square test bar, was 39 ft. lb~. (53 Joules) at -60F (651C), 34 ft. lbs.
~46 Joules) at OF (-18C) and 45 ft. lbs. (61 Joules) at 75P ~23C).
B. Master Alloy No. 400 Master Alloy No. 400 was atomized using inert gas method and screened to -200 mesh particle size. It was mixed with pure iron powder and the experimental procedure was identical to that described above for Alloy No. 342.
The chemical composition of the hot formed siugs was 1.09% manganese; .26% molybdenum; .73% nickel; and 0.04 chrom~um and 0.03% copper, the remainder iron.
Hardenability of the alloy was both calculated using a 50% martensite criterion and was determined experi-mentally using standard 1" diameter (25 mm) bars as per SAE
procedure.
% Carbon Ideal Diameter Ideal Diameter Premix DI Calculated ~I Experimental Alloyin~
Efficiencv .
.16 1.41 1.30 g3%
,2? 1.7~ 1.40 82%
.3~ 2.22 1.70 77 .6g 3.38' 2.7~ 8~%
Premix alloying efficiency = DI Experimental DI Calculated x ~0%
Mechanical test results of .31 car~on sample quenched and tempered to hardness 25 ~ockwell C were: Ultimate tensile.

strength - 119 k.s.i. 'C820 MPa~; Yield point - 104 k.s.i.
(717 MPa~; Elongation - 26%; and Re2uction of Area - 53%.
V-notch Charpy impact test on 10 mm square bar was 23 ft. lbs.
(31 Joules) at -60F (-51C); 48 ft. lbs. (65 Joules) at OF
(-18C); and 50'ft. lbs. (68 Joules) at 75F (23C3.
Both premixes, using 2.5~ of either master alloy #342 or #400,~ exhibited good diffusion of the alloying elements into the pure iron powder. Hardenability was equal or superior to that of the now popular MO~-4600 low alloy pre-alloyed steel powder,. While alloy #400 exhibited complete dissolution in the matrix as observed in its microstructure, the premix with alloy #342 has shown some very small areas of undissolved residual master alloy.
Hardenability as iudged by DI using S~% martensite criterion for both alloys is 70-90% of that calculated for conventional, prealloyed steels of the same chemical compos-ition; this i8 considered very satisfactory. There is, however, a drop-off of hardness at the beginning of 3Ominy curves and DI using 90% martensite criterion is much lower for premix with alloy #342 than #400. Thus, alloy #400 appears to be superior to ~342, as its DI value for 9 martensite is only somewhat inferior to the value for ~0~
martensite. A narrower melting ranqe for alloy ~400 will result in better liquidity and diffusion; thus sintering at temperatures higher than 2250F will result in still higher hardenabili~y due to better dissolution of alloying e~ements.
~oth premixes have shown mechanical properties, impact stren~th and ductility close to that of M~0-4S~
hot fo~me~ powder metal prealloyed steel sin~ered in hydroge,n at 22S0F. These properties are useable for many heavy duty engineering applications.

107~5Z9 The properties outlined in the above two examples also compare favorably with conventional steels and are considered as entirely satisfactory for many engineering applications.

SUPPLEMENTARY DISCLOSURE
The principal disclosure defines low melting master admixtures of alloying ingredients to be added to an iron based powder for use in methods of making sintered alloy steel parts by the compaction and sintering of an admixed powder to obtain alloying.
The admixture consists essentially of at least two elements selected from the group consisting of manganese, nickel, molybdenum, chromium, copper and iron, with molybdenum being in the range of 5 to 15~ by weight of the admixture when selected along with the substantial absence of copper, and iron being less than 20% by weight of the admixture when selected. The selected elements are blended and balanced to provide in the admixture a liquidus temperature residing between 1800 and 2250~F and-a melting range for all ingredients of no greater than 350F. I
The following additional master alloys are provided in accordance with this supplementary disclosure:
TABLE I (Contd) Master ~F
Alloy ~ical Gx~osition, wt% F F Melting Mix No. Mn Ni Cr Mb Fe Cu Si R.E. Liquidus Solidus Range S32 72 14 - - - 14 - 1 2020 17~0 230 ~n the principal disclosure, si~icon and rare earth metals are described as wetting agents and diffusion promoters ~or the alloy powders. In accordance w~th this supplementary disclosure, yttrium (an element which acts li~e a rare earth for the purposes of this invention) also may be used as a wetting agent and di~fusion promoter.

.~

Further, in accordance with this supplementary disclosure, the operative ranges of the wetting agents and diffusion promoters are silicon l to 5~, rare earths 0.2 to 1.5~ and yttrium 0.05 to 0.20~, each by weight of the alloy powders. Usually, the total quantity o~ such additives does not exceed ~.0~ by weight of the alloy powder.
In establishing alloying between solid and liquid phases of a powder mixture by heating a heated compacted mixture of iron-based powder and the powder admixture described in the principal disclosure, the iron based powder may be prealloyed with molybdenum in the range of 0.08 to 0.4~ by weight.
In the making of a sintered metallic compact ~y heating a compacted blend of iron-based powder, graphite, and an alloy powder to liquify the alloy powder to a liquid phase while the iron powder remains in a solid phase to effect diffusion of the liquid phase into or onto all particles of the solid phase and subsequently cooling the compact, the alloy powder may constitute 0.20 to 6% of the blend.
In the last-mentioned procedure, the alloy powder may ~e one consisting essentially of at least t:wo elements selected from a first group consisting of manganese, molybdenum, nickel, chromium, copper and iron, and up to two elements selected in a total ~uantity of no greater than 5.0% ~y weight from the group consisting of silicon and rare earth e~ements.
Alternatively, the alloy powder used in the last-mentioned procedure may contain at least three elements selected from the group consisting of manganese, moly~denum, nic~el and chromium, the manganese and nic~el each constituting at least 30~ and ~ respectively o~ the alloy powder.
In the accompanying drawings, ~igures 1 to 3 graphically ~7052~

represent the variatîon of hardenability with carbon variation for respectively a 1.6 to 2% master alloy powder admixture with pure iron powder, a 2.5~ master alloy powder admixture with pure iron powder and a 1.5% master alloy powder combined with a pre-alloyed iron powder containing 0.3% molybdenum.
The invention is illustrated by the following further Examples:
C. Master Alloy No. 524 ~ulti-element master alloy No. 524 was atomized, using the inert gas method, and screened to -200 mesh particle size. It was mixed with pure iron powder and graphite, the experimental procedure was identical to that described above for alloy No. 342.
The chemical composition of the master alloy was

2.7% chromium, 7.79% molybdenum, 56.48% manganese, 14.29~ iron, 18.10% nickel and 2% silicon. Two and one-half percent of thi~ master 524 alloy was admixed with a pure iron powder to produce a final composition in the powder metallurgy sintered steel as follows: 1.41~ manganese, 0.45~ nic~el, 0.07%
chromium, 0.19~ molybdenum.
Hardenability of the alloy was calculated using both 50% and 90% martensite criterion and was determined experimentally using standard 1" diameter ~25 mm3 bars as per SAE procedure.
Adm~re Car~on Ideal Diame ~ Actual Ideal Act ~ I~ Alloying B Ca~culate~ Diameter 50~ Diameter ~0% Efficiency S~% M~nsite ~tensite Martensite ~5~% Mart .23 2.17 l.B8 1.56 87%
.29 2.45 2.55 2.13 104 .39 3.08 2.S5 ~.96 ~3 .81 4.15 4.10 2.88 gg~

The maximum scatter of hardness readings from the mean Jominy curve was + 2.5 Rockwell "C" poin~s.
In common with master alloys 342 and 400, the premix of master alloy 524 exhibited good diffusion of the alloyed elements into the pure iron powder. Hardenability was equal or superior to that of the now popular MOD-4600 low alloy prealloyed steel powder. The hardena~ility as judged by DI using 50% martenite criterion was even higher for alloy 524 than the 7~ to 90% observed for alloys 342 and 400.
Also in common with master alloys 342 and 400, the master alloy 524 exhibited mechanical properties, impact strength and ductility close to that of modified 4600 hot formed powder metal prealloyed steel sintered in hydrogen at 22S0F. These pr~perties ~re usable ~or many heavy duty engineering applications.
The properties of this alloy also compare favourably with con~entional steels and are considered entirely satisfactory for many engineering applications.
D. In~luence of Sillcon_and Rare Earth ~e'ta'l' Ad'ditlons,to Mhte Mla(st/r)Allo,y Powders on the' Harden'ab'i'l'i'tv o~-Powder Master alloys o~ very similar chemical compos~tion ;were made wlth and without the add1tions o~ silicon and rare ,earth metals. Two and one-hal~ percent of master alloys were premixed with pure ~ron powder and graphite, sintered at ~,22~0F (1232C) and hot ~ormed. 3Om~ny bars were tested ~or ; ;hardenabil~ty as per SAE procedure. Pavorable in~luence o~
s~licon and rare earth metal add~tions on liquid phase sintering and di~usion of master alloys are reflected in a '30 very signi~icant improvement o~ hardena~i~ity at about ~.2 carbon level as shown ~elow:

1~70529 Group Master Addition of Carbon We1ght Ideal Dlameter Al~oy Silicon or Percent ~0% 90~
No. Rare Earth ' Martensite ~artensite 1 527** None 0.22 1.45 1.12 531 Silicon 0.22* 1.67 1.21-532 Rare Earth 0.22 2.30 1.90 2 400 None 0.22 1.~0 1.20 400S Sil~con 0.22 1.88 1.40

3 342 None 0.21 1.15 0.72 530 Rare Earth 0.21 1.40 1.23 * Hardenabil~ty corrected ~o the indicated carbon level.

** Premix with 2.5% of alloy No. 527 without any silicon or rare earth exhibited a considerable scatter of hardness I from the mean average Jominv hardenabllit~ curve.

P/M alloy steels made by premixing of master alloys showed a less smooth 3Ominy cur~e than a corresponding prealloyed 8teel due ~o the changes in the micro-composition of the mat~lx. It was observed that the add~tions of silicon, and to a smaller extent addltions of rare earth metals decrease 2~ the extent of the scatter, which is an lndlcation o~ im~roved dl~fusion E. Exam~ es o~ Subst~tuta~ o~ P~M S'teels 'tau~ht herein ~or Conventional S~eels on the Bas'~'s of ~ard'enab~litY
. ~ ,.

. Substitution o~ P~M ~nalloYed' Powder-4dmixtures for SAE 4000~ and ~600~ Steels .
It was demonstrated t~at the master al~oy ~owders w~th additions of silicon and rare earth metals can ach~eve approximately a 90% alloylng e~ficiency (~.e. the P/M al~oy l~ a~ter sintering and hot forming ha~ing hardenability, as 30 1l expressed by DI, e~ual to 90% o~ the hardenability o~ a pre-, alloyed steel o~ equivalent chemistry), sinterlng ~ein~

;
; ~ _ lQ _ 107~529 performed for 0.5 hrs. at 2250F (1232C~ in an atmosphere low in oxygen potential. Sintering could be shorter with a higher sintering temperature. ~igure l shows the actual hardena~ility zones for several 4000H and 4600H SAE series steels and shows calculated hardenability curves C for 1. 6 and 2.0% master powder alloy ~owder No. 5~4 (see ~able T) when mixed with a pure iron base powder. The coordinates of the graph of Figures 1-3 are as follows: the ordinate axis l represents hardenabllity as expressed by ideal diameter (DI) ¦ in inches and the abscissa represents the car~on content. The hardenabiltty of conventional steels ts represented by rectangles (zones ~), the vertical l~nes of the rectangle lim~tlng the carbon of the SAE specification and the horizontal lines limlting the calculated minima and maxtma of the ideal dtameters for these steels. One can say that whenever the ~scatterband o~ the hardenability of premixes crosses both vertlcal sides o~ the rectangle the P/M steel will ~e fully equlvalent to the conventional steel with regard to harden-labllity. For simpllcity, calculated lines of hardenability l~alues (DI) at the above-mentloned percentages of premix were plotted ~or different car~on levels, The hardenabtlity of prem1xes can ~e more closely controlled than that of the con-ventional steels by ~arying the amount of the master a~oy ~powder. For example, a premix containing 1,6% of master alloy powder ~o~ 53~ ls satisfactory as a su~s~itute for the SAE
~OOOH series since the curve crosses ~oth sides of each zone.
Approximately 2% of the same master alloy pow~er ts required ~when su~stituting for SAE 4620H or modtfied 4600 (see Icalculated cur~e D)-prealloyed P/M steel in or~er to o~ain 3~ lan e~uivalent hardenabilt~y ~oth of the case and of the core.

~0705Z9 II. Substitution of P~ Unallo~e~Powder Admixtures for the PoPular SA~ OOH
Series of Steels.
Figure 2 represents the actual hardenability of SAE 8600H series of steel zones E and the calculated harden-ability of a 2.5~ admixture of powder alloy No. 534 and pure iron powder (curve F) assuming 90% alloying eff~clency a~ter 0.5 hrs. of sintering at 2250F (1232C) in a low ~oxygen potentlal atmosphere, It can be seen that this proportlon admlxture (2.5% of 534) has a significantly higher hardenabllity than the now popular modlfied 4600 P/M steel (see curve ~) and results in a good substitution for the 8630 and 8640H steels. While the core hardenability is in the middle of the SAE 8617 and 8620H rectangles J the hardena~ilitv of the case for these steels is slightly below the hardenab11itty f the 8600H series of the steels. This is due to the fact jthat the conventional steel contains 0.20 to 0.35% Si while ¦the ~/M steel co~talns only residual silicon. Silicon contrlbutes signlficantly to hardenabillty at a high carbon ! content and increases the hardenability of the case of con-ventlonal steels by 15-25%. The slightly infer~or value of the case hardenability for a 2.5% premix addition is not ~ considere~ to be of significance for smaller parts, as the ,I ma~orlty of the new EX- series of low alloy steels as a substitute for the SAE 8~o~H series (which are now ~inding wtde acceptance) have a DI hardenabiltt~ o~ the case on the a~erage of 0.4 inches below that of the SAE 860~ series.
Except ~or larger components, this is of no consequence. The SAE steels 8~50H and 8660H reau~re slightly more master alloy:
, 2.7~ of alloy No. ~34 (see curve G) will be a sat~s~actory substitution; it will also give for 8617 a~d ~62~H steels a - 2~ -~705Z9 case hardenability within the range of the 8600H series.

F. Prealloyed Base Powder - Master Allov Powder Combination.
As determined and outlined in previous paragraphs, manganese ls the fastest diffusing element while nickel, chromium and molybdenum, in the conditions examlned, were only about one-third ~ fast as manganese. It is economicall~
advantageous to make alloys of the hlghest hardenabilitv in the I following way: Use a base powder (identified No. 133) con-¦ tainlng a prealloyed 0.3~ molybdenum content only and no other ! alloying elements. Such a powder is easy and economical to manufacture as molybdenum is more noble than iron with regard to oxidation and any molybdenum oxides will be reduced during the powder annealing operation after water atomization. To this ~ase powder one can admix any hi~h manganese master alloy powder containln~ also some nickel and/or cop~er with wettlng and di~fusion promoting a~ents such as silicon, rare earth or yttrlum ~ut wlthout molybdenum and chromium. Even alloy No. 527, which did not contain any of the above-mentioned wetting or diffusion agents, and which was added in the proportion of 1.5% to a prealloyed base iron powder No 133>
gave an alloying efficiency close to 10~% as shown in the table below an~ in ~igure 3, even though the Jo~l1ny curves have shown some undesirab~e scatter This scatter coul~ be minimized by the addit~on o~ silicon, rare earth meta~s and yttr~um to this master alloy. The graphical re~resentat~on o~ hardenab~ity ~n F~gure 3 demonstrates the advantages o~
us~ng a prealloy-premix combinat~on to adapt the hardenab~lity for a particular eng~neering applicat~on. Molybdenum is an I important alloying element which has a considerably higher mult~p~ying factor at high carbon content than at low carbon ,~
~ - 21 -1~705Z9 level. Thus molybdenum ls an important element in the car-burizing grade of steels. Iron base powders, water atomized by the nature of the P/M process, cannot contaln any silicon, as sllicon during water atomization will be preferentially oxidized and creates irreducible silicon oxide ~ilms which prevent sinterlng and degrade the properties o~ hot ~ormed P/M steels. As explained in ~xample ~, silicon contributes s~gnif~cantly to the case hardenab~lity during carburizing;
molybdenum is another element which has similar properties ln this respect. Thus in the absence of silicon, to obtain a hlgh core and case hardenabilit~, molybdenum is the most desirable element to em~loy in the base iron powder.
In Fi3ure 3, calculated hardenability curve J was for a 1.5% o~ powder No. 527 admixed with graphite into the iron jbase powder (No. 133) containln~ 0.30% molybdenum only. The ~resultant chemlcal compositlon for the résult~n~ P/~ steel was 1.30%manganese, 0.165%nickel, 0.164% copper and 0.30%
molybdenum. Jominy bars were pre~ared and tested using the procedure described in example A and the results were as 20outllned below:

Hardenability - Ideal Diameter, Inches Alloying Experimental Experimental Calculated Efficienc~
50% 90% 50% 50%
% Car~on Martensite Martensite Martensite Martensite 0.175 1.60 1.48 ~.68 g5 0.25~ ~.25 2.03 2~22 101~
0.34 2.6~ 2 22 2.75 94%
0.78 4.79 4.27 4.30* g~%*

~i * 90% martensite criterion.

1 ~he above ~igures show that very high allov~ng efficiency approaching 100% 1s achleved using as a base ~realloyed powder with ~lolybdenum as the only alloyin~ element and a ~070s29 manganese-r~ch master alloy. It can be seen from ~igure 3 that this alloying combination in ~he proportions used was equlvalent to the SAE 8600H series of steels. Figure 3 shows both calculated (see L) and experimental (zones K) values of hardenability as expressed bv Ideal Diameter.
The master alloy powder premix of this in~ention is partlcularly helpful when working with molybdenum which requlres delicate control to get good response. Molybdenum has a large atomic radius and thus is difficult to diffuse readily between lron atoms unless precise controls are employed. The absence of copper facilitates the molybdenum diffusion as well as the carbon control.

!~ - 23 -

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of establishing alloying between solid and liquid phases of a powder mixture, comprising:
(a) prepare an iron-based powder substantially devoid of alloying ingredients, (b) prepare a pre-alloyed non-iron-based additive powder containing at least two elements but up to all the elements selected from the group consisting essentially of manganese, molybdenum, nickel, chromium, copper and iron, molybdenum being in the range of 5-15%
by weight when selected along with the absence of copper, said elements being selected and balanced to provide a span of melting temperatures therefor of no greater than 350°F and the additive powder having a liquidus tempera-ture of between 1900°-2250°F, (c) uniformly blending 1.5 to 6% of said additive powder with said iron-based powder and with a predetermined amount of graphite to render 0.69% carbon or less in the mixture and compacting the blend to a density of at least 70%, and (d) heating said compacted blend to a tempera-ture of up to 2250°F for no greater than one hour, whereby the additive powder forms a liquid phase which readily diffuses along the particle boundaries and into the matrix of said iron powder thereby reducing the maximum diffusion distance to one particle radius or less.

2. The method of claim 1 wherein said additive powder also contains up to two elements selected in a quantity no greater than 3.5% by weight from the group consisting of silicon and rare earth elements.

3. The method of claim 2 wherein, when selected, the following elements have the following proportions:
Ni 20-30%, Mn 40-54%, Mo 5-11%, Fe 10-20%, Cr 0.05-16% to result, after cooling the heated blend in a shape having mechanical strength properties equal to or better than a wrought steel.

4. The method of claim 3 wherein said powder contains 0.3% carbon and the mechanical properties are characterized by an ultimate tensile strength of at least 115 k.s.i. and a charpy V-notch value at -60°F
of about 23 and at +75°F of about 45, at a hardness of 25 Rc.

5. The method of claim 2 wherein iron must be selected for said additive powder in the range of 10 to 20% when effective amounts of molybdenum and/or chromium are present therein.

6. A method of producing a sintered metallic compact wherein alloying between solid and liquid phases of a powder mixture is established, comprising:
(a) preparing an iron-based powder devoid of alloying ingredients, (b) preparing a pre-alloyed non-iron-based addtive powder mixture containing at least three elements but up to all the elements selected from the group consisting of manganese, molybdenum, nickel, and chromium, said manganese and nickel each constituting at least 30%
and 5% respectively of said additive powder, said molybdenum being in the range of 5-15% by weight when selected along with the absence of copper, said elements being selected and balanced to provide a span of melting temperatures therefor of no greater than 350°F

and the additive powder having a liquidus temperature of between 1900°-2250°F, (c) uniformly blending 1.5 to 6% of said additive powder with said iron-based powder and with a predetermined amount of graphite to render 0.69% carbon or less in the blend, (d) compacting said blend into a shape having a theoretical density of the order of 80%, (e) heating such shape in the environment of a reducing atmosphere to a temperature of up to 2250°F
for no greater than one hour, whereby the additive powder forms a liquid phase which readily diffuses along the particle boundaries and into the matrix of said iron powder thereby reducing the maximum diffusion distance to one particle radius or less, and (f) allowing said shape to cool.

7. The method of claim 6, wherein said additive powder further contains at least 1-40% iron in addition to said three elements.

8. The method of claim 7, wherein said iron is increased above 5% and said manganese is added to con-stitute more than 50% of said additive powder.

9. The method of claim 7, wherein said chromium constitutes at least 12% of said additive powder.

10. The method of claim 7, wherein said additive powder comprises about 30% nickel, 40% manganese, 5%
molybdenum, 15% chromium and about 10% iron, the admixture having a liquidus of about 2140°F and a solidus of 1830°F.

11. The method of claim 7, wherein said additive powder is comprised of about 27% nickel, 45% manganese, 10% molybdenum and about 18% iron.

12. The method of claim 7, wherein said additive powder consists of about 14% nickel, about 56% manganese, about 15% chromium, about 5% molybdenum, and about 10%
iron, the admixture having a liquidus of about 2170°F, a solidus of about 2070°F and a melting range of 100°F.

13. The method of claim 7, wherein said additive powder consists of 22% nickel, 52% manganese, 8%
chromium, 6% molybdenum and 12% iron, the admixture having a liquidus of about 2100°F, a solidus of about 1860°F, and a melting range of 240°F, to the above powder 2.5% silicon and 1% rare earth metals are added.

14. The method of claim 6, wherein said shape is hot formed at 2050 to 2250°F or lower prior to cooling.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

15. A method of establishing alloying between solid and liquid phases of a powder mixture, comprising:
(a) prepare an iron-based powder substantially devoid of alloying ingredients, (b) prepare a pre-alloyed non-iron-based additive powder containing at least two elements but up to all the elements selected from the group consisting essentially of manganese, molybdenum, nickel, chromium, copper and iron, molybdenum being in the range of 5-15% by weight when selected along with the absence of copper, said elements being selected and balanced to provide a span of melting temperatures therefor of no greater than 350°F and the additive powder having a liquidus tempera-ture of between 1900°-2250°F, (c) uniformly blending 0.2 to 6.0% of said additive powder with said iron-based powder and with a predetermined amount of graphite to render 0.81% carbon or less in the mixture and compacting the blend, (d) heating said compacted blend to a tempera-ture of up to 2250°F for no greater than one hour, whereby the additive powder forms a liquid phase which readily diffuses along the particle boundaries and into the matrix of said iron powder thereby reducing the maximum diffusion distance to one particle radius or less.

16. The method of claim 15 wherein said additive powder consists of about 72% manganese, about 14% nickel and about 14% copper.

17. The method of claim 15 wherein said additive powder further contains an auxiliary wetting agent selected from the group consisting of silicon in the range of 1 to 5% rare earths in the range of 0.2 to 1.5%
and yttrium in the range of 0.05 to 0.20%.

18. The method of claim 15 wherein said iron-based powder is pre-alloyed with molybdenum in the range of 0.08 to 0.4% by weight.