Note: Descriptions are shown in the official language in which they were submitted.
~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
--2--
~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.
--4--
1070529
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
--5--
10705Z9
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
-6-
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
--7--
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.
10705Z9
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
342 40 30 155 10 - - - 2140 1830310
400 44 25 -11 ; 19 - - - 2200 213070
524 55 18 3 8 14 - 2 - 2065 173033S
533 56 24 3 6 11 - - - 2115 18gO225
533S * 2.5 - 2130 1~20310 -
533M * 2.5 1 2020 1850270
534 52 22 8 6 12 - - - 2110 207040
534S * - 2.5 - 2070 1870200
534M * - 2.5 1 2100 1860140
535 47 20 13 6 13 - - - 2210 213080
535S * , - - - 2100 1960140
535M * - - - 2145 1930215
528 75 25 - - - - - - 1930 1800130
527 74 12.5 - - - 12.5 1 - 1940 1700240
344 36 30 18 6 10 - - - - 2205 2005200
345 41 25 18 6 10 - - - 2220 1970250
346, 38 23 18 6 15 - - - 2245 2000245
506 64 16 0 10 10 - - - 2250 1955290
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
512 53 17 15 S 10 - - - 2000 2220220
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_
10705Z9
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
1070529
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.
--11--
10705Z9
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.
-12-
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.
-13
10705Z9
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
531 72 14 - - - 14 2 - 1910 1770 140
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.
.~
10705Z9
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~
- 16 -
1 070529
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
A - 22 -
~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 -
