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Publisher’s version / Version de l'éditeur:

International Journal of Powder Metallurgy, 43, 4, pp. 39-49, 2007-01-01

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High-performance PM steels utilizing extra-fine nickel

Azzi, Lhoucine; Stephenson, Tom; Pelletier, Sylvain; St-Laurent, S.

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HIGH.PERFORMANCE

PM STEELS

UTILIZING

EXTRA'FINE

NICKET

Lhoucine

Azzi*,Tom Stephenson**,

Sylvain

Pelletier*** and

S.

St-Laurent****

INTRODUCTION

Nickel

is

an important alloying additive

in

PM steels.

It

increases

strength and allows for improved control of dimensional change (DCJ during sintering, particularly in PM steels that contain copper. At the

normal sintering temperature

of

1,120"C, the diffusion of nickel is

incomplete and this results in PM steels with nonuniform microstruc-tures, containing nickel-rich areas (NRAs). While these NRAs can be beneficial in relation to toughness and fatigue properties.I nickel must be diffused completely

in

the steel matrix for maximum hardenability and to maximize solution hardening. Higher sintering temperatures2

and/or longer sintering times promote nickel diffusion. However, these approaches are not used widely

in

the PM industry due to the

atten-dant cost. Recent studies3 have shown that the use of extra-fine nickel

powder (Db' 1.5 pm), instead of standard PM nickel powder (Duo 8 pm)

offers an attractive alternative to increasing the diffusion of nickel in

PM steels. In addition to improving the nickel distribution, the use of

extra-fine nickel

powder

in

nickel-copper-based PM steels also

improves the

distribution of

copper

by

increasing the interaction

belween these elements during sintering.a 5

Alloying additions can be admixed, diffusion-alloyed, or prealloyed in

steel powders. While prealloying leads to homogeneous microstructures and higher mechanical properties,

it

is usually detrimental to powder

compressibility. For this reason,

in

applications requiring high

densi-ties, alloying additions such as copper and nickel are usually admixed

with steel powders. However, this can lead to segregation

if

particles with different sizes and densities are mixed together. Admtxing can also

lead to flow and dusting problems in which case binding the alloying

additions to the surface of the base steel powder can be beneficial. Two types of bonding technique are used in PM, namely, partial alloying and

binder treatments.

In

partial alloying, the additives are partialiy dif-fused into the iron powders forming a strong metallurgical bond. In binder treatments, an organic binder, usually a polltner or wax, acts as

an adhesive. The bonding strength of binder-treated mixes

is

lower

than that of diffusion-alloyed mlxes. However, this approach is

relative-Disrnbulurn oJ the alloging a.ddiLtues in powder

meLallurgy (PM) sleels is a key element in achieuing

op Limutn sint ere d p r op ertie s. Segregatian musL be auotded in order Lo ensure consislen l. parl'to- part

propertie s. Re cenL studies indic ate tLnt e xtra.jne

rtickel powders haue a beneJicial impacL on Lhe

ouerall properties o!

nickel-copper-carbon PM sleels. ThereJore. the use oJ

extra-Jine ntckel powder in segregation-Jree PM mixe s

couldbe an eficienL wag to optimize properlies. To this end. Lhe effect of the size

and size distribution oJ nao ntckel powders on the phu sical and mechanical properties oJ Lwo binder-trealed steel powder

orem[xes processed. on a 'pt?ot

scale hrrs been assessed. The properties oJ these lwo mixes are compared with those oJ a

dffisbn-alloged mix oJ the

:s

.7te' corytp,osition.

'me

nital properties and,

dimensional change oJ the binder - Lrealed mtxe s are

:i:hoAn fu:be superior to

Lhose of the

diffitsion-allog'ed mix. Tl'rc phg sicat and sintered propertie s oJ the binder - tre ated m:xes

canbe Jurther improued bg using extra-jne nickel

powder (Dso

L5

ym) instead

oJ a standard stze (Dro 8 pm) nickel powden

*Research Assocutte, ***Group Leader lrtdustnal Matenals h:.stitute, NationaL Research Council Canada, BoucheruiLle, Qudbec: E-mail:

Ihoucine.ozzi@nrc.cct, **Technology & DeueLopment Manageq INCO Special Products, 2101 Hadwen Road. Mississauga. Ontario, Canaclct, L5K 213,

****R&D Manager

Quebec MetaL Powders Limited, 1655 Marte VictarIn, Tracg, Qubbeo Canadq, JSR 4R4

Volume 43, Issue 4,2OO7

(3)

ly inexpensive and has the advantage over

diffu-sion-alloying treatments that

it

can bind the lubri-cant and graphite. Binder-treated mixes usually exhibit acceptable flow properties, reduced

segre-gation, and, as a result, improved part-to-part

dimensional consistency.

Using extra-fine nickel powder

in

bonded PM mixes offers an efficient way to optimize the prop-erties of nickel-copper PM steels.

In this

study, the effect of two nickel powders

with

different

sizes and size distributions on the physical and

mechanical properties of two binder-treated nick-el-copper-molybdenum steel powder premixes was evaluated. The properties obtained utilizing the two binder-treated mixes processed on a pilot scale are compared

with

those

of

a

diffusion-alloyed mix of the same composition. Each of the

eight components of the study is presented as a stand-alone entity in terms of experimental

proce-dures, results/observations, and implications/

discussion. POWDER MIXES

Two base powders were used

in

the

study:

ATOMET 4001, a water-atomized prealloyed steel

powder containing 0.5

w/o

Mo and ATOMET DB

46, a diffusion-alloyed powder containing 0.5 w/o Mo, 1.75

w/o

Ni, and 1.5

w/o

Cu. Three 68 kg

pilot-size mixes (two binder-treated mixes and one

diffusion-alloyed mix) were prepared

in

a twin-shell V-Type blender/dryer.

All

three powder

mixes had the same composition: Fe-0.5 w/o

Mo-1.5

w/o

Cu-1.75

w/o

Ni-0.6

w/o

graphite (0.5

w/o sintered carbon).

The diffusion-alloyed

mix

was prepared by adding and dry mixing 0.6

w/o

natural graphite TABLE I, PARTICLE-SIZE DISTRIBUTION OF FINE AND EXTRA-FINE NICKEL POWDERS

Type Dro

(Um)

Dro

(um)

Deo (Fm)

(SW 1651) and 0.75 w/o lubricant (Acrawax C) to

the ATOMET DB46 diffusion-alloyed powder. The ATOMET DB46 powder was obtained by partially

diffusing

copper and

nickel to

ATOMET 4001

powder (a preailoyed powder containing 0.5 w/o Mo and 0.15

w/o

Mn). The regular mix (graphite

and lubricant not bonded) was identified as

AT-DB40A in this study.

The two binder-treated mixes of nominal

com-position FLN2C-4005 were prepared

in

the twin

shell V-Type blender/dryer by means of a patented

binder technologr.6 These mixes were prepared by

admixing

1.75

w/o Ni,

1.5

w/o

Cu

(Duo

-15 pm), 0.6 w/o natural graphite (SW 1651), and

0.65

w/o

wax

lubricant

(Acrawax C)

with

the

ATOME"| 4001 powder for 30 min. A solution

con-taining the binder at a concentration of 0.2 w/o of

the powder mix was then injected into the blender and rotated for an additional 10 min. The powders

were dried by vacuum extraction of the solvent. Two different carbonyl nickel powders, INCO Tl23 PM and INCO T1 10 D, and a fine commercial

cop-per powder (Db' -15-20 pm) were used

in

these

mixes. The binder-treated mixes contalning the

INCO T123 PM and T110 D nickel powders were

identified

in

this

study as F40A-123 and F40A-T110, respectively. The particle-size distribution of

these two nickel powders, as measured by laser

diffraction particle-size analysis, is given in Table L The characteristics of the 68 kg steel mixes used

in the study are summarized in Table II. PARTICLE BONDING AI{D DISPERSION

Figure 1 shows representative scanning electron

micrographs (SEM) of bonded powders

in

the binder-treated and regular diffusion-alloyed

pow-der mixes.

It

can be seen that,

in

the case of the

regular non-bonded mix, a significant proportion

of the graphite and lubricant particles are free and

not attached to the iron particles. Nevertheless,

dry-bonding of graphite

is

observed. This

is

due

primarily to the presence of the lubricant that acts as a binder. In the case of the binder-treated pow-tNc0123 tNCO T110 1.lc 0.5

I

20 4.7

TABLE II. MIX CHARACTERISTICS

Mix ldentification Type of Mix Prealloyed Additives Dif{usion Additives

(Mo)

(Mo) Admixed Additives (w/o) Cu Ni Cu Ni Mn Mo

40

l./3 AT.D84OA F40A-1 23 F40A-T110 Regula/Diff usion-Alloyed Brnder-Treated Binder-Treated 0.5 0.5 0.5 0.1 5 0.1 5 0.1 5 t.c 0,6 0.6 0.6 1.75 (T123

PM)

1.5 1.75 (T110

D)

1.5

Volume 43, Issue 4,2OO7 International Journal of Powder Metallurgy

(4)

Ni Kal

Figure 1. Representative SEM inages of powder nixes.

(a) F40A123, (b) F40A-T1 10, (c) AT-DB41A. C = graphite or

lubricant. BS = back-scattered electron imaoe.

der, the graphite and lubricant particles are

essen-tially all bonded to the iron particles. The size of

the nickel particles has a significant impact on

their bonding and dispersion. The extra-fine nickel

particles were more efficientJy bonded and distrib-uted on the surface of the iron particles than were

the standard-size nickel particles. This

is

attrib-uted to the increase of the adhesion forces as the

difference

in

size between the iron particles and the nickel particles increases.

The nickel distribution was also examined after

compaction. Green compacts were pressed

to

a

Figure 2. EDS mapping of nrckel in powder mixes. (a) F40A-123, (b) F40A-T110

density

of

7.0 g/cms and

partially

slntered at 600"C in argon for 30 min. Sintering at this

tem-perature provides sufficient green strength to

allow for polishing, while limiting diffusion of the

aiioying additives. The nickel

distribution

was

evaluated

by

energy dispersive spectrometry

(EDS) mapping, Figure 2.

It

can be seen that, in the green compacts, the extra-fine nickel particles

are more uniformly distributed than are the

stan-dard-size

nickel particles. This

is

attributed

directiy

to

the enhanced bonding efficiency and

dispersion

of the nickel particles during

the binder treatment.

DUSTING RESISTANCE

The bonding efficiency of

the

three bonded mixes was evaluated by measuring the level of retention of graphite, lubricant, nickel, and

cop-per after the powder mixes were subjected

to

a Volume 43, Issue 4,2OO7

(5)

100 90 80

e70

tr 'a -"

&40

ili

u""

10 0

AT-DB{OA F{OA-T23 F4OA-TlIO

Figure 3. Dusting reststance of powder nixes AT-DB41A, F40A-123, and F40A" T110. 100% = n0 dusting loss;0% = total loss of additive

strong flow of air. The dusting test consisted of pouring 25 g of powder into a 25 mm cylindrical tube and flowing an air stream into the tube at a

rate of 6

L/m|n for

5 min. The flow of

air

was strong enough to partially fluidize the powder. The powder was analyzed before and after the test and

the dusting resistance of a specific element deter-mined by means of the relation:

Dusting Resistance = [(w/o after test)

/

(w/o before test)l

x

100

(1) Figure 3 shows

tlie

dusting resistance factor for graphite, copper, and nickel of the powder mixes AT-DB40A, F40A-123 and F40A-T110.

As expected, the dusting resistance of copper and nickel in the diffusion-alloyed mix is high. The

mean dusting resistance of graphite

is

around 900/o in the binder-treated mixes. It could be

readi-ly

increased

to

>95%

by

seiecting appropriate binding parameters. The dusting resistance of

cop-per is between 450/o and 500/o

in

the two binder-treated mixes, which

is

typical of binder-treated mixes containing commercial copper powders.

It

should be noted that the dusting resistance

ofcop-per in reguiar non-bonded mixes is -200/o to 250/o.

The dusting resistance of graphite

in

the regular diffusion-alloyed mix is higher than that of copper

in the binder-treated mixes (60% vs, -500lo).

Fine natural graphite powders have large flat

surfaces with the abiiity to adhere to large parti-cles by van der Waals-type forces. The nickel dusting resistance was -600lo for mix F40A-123,

compared

with

-250/o

in

the regular

mixes. Essentially no nickel dusting loss was recorded

for the fine nickel mix. Several iron powder manu-facturers have independently confirmed this

ten-dency.l'7 However,

contrary

to

the study

by Nichols and Sawayama,T we conclude

that

the extra-fine nickel powder was not only more

effi-ciently bonded, but was also more uniformly dis-persed in the powder mix. These different findings

may be related to the different binders, base

pow-der and/or processing methods used to prepare

the binder-treated mixes. The use of extra-fine nickel in binder-treated mixes is an efficient way

to improve industriai hygiene in relation to nickel dusting

at

a reduced cost compared

with

diffu-sion-alloyed mixes.

POWDER FLOW AND GREEN PROPERTIES The flow rate, apparent density, green strength,

and compressibility

of the

powder mixes AT-DB40A, F40A-123. and F4OA-TIl0 were evaluat-ed in compliance with MPIF standards 03, 04, 41,

and 45. These properties are reported in Table III

and Figure 4.

The flow rates of the two binder-treated mixes

were higher than

that

of the regular

diffusion-alloyed mix, The flow rate of the regular diffusion-alloyed

mix

is

typical and

is

attributed to

the strong adhesion introduced by wax-type lubri-cants.8 Binding the wax

lubricant

reduces the

"stickiness" of the

powder

mix,

resulting in

TABLE III. FLOW AND GREEN PROPERTIES

Mixture ldentification

Apparent Density (g/cm3)

Flow

Rate

Green Strength

(s/50

g)

(7 g/cm3) MPa psi AT.DB4OA F40A-123 F40A-T1 1 0 3.1

I

3.1

I

3,1 4 1,270 1,360 1,420 8.7 9.4 9.8 35 27 27 7.25 '72 n br *'7 1 h d /.u) Et ! b9) 6.9 2"1" ,,1" ,,i,' .1r ^ AT-DB4OA . F40A-123 . F40A-T110

Figure 4. Compressibility of powder mixes AT-DB4lA, F40A-123, and F40A-T110

42

Volume 43, Issue 4,2007

(6)

improved flow behavior. The binder-treated mixes

and the regular mix exhibited similar low $reen

strengths. This is typical of green compacts

con-taining wax-[pe lubricants and is associated with the lack of metal-to-metal contact due

to

smear-ing of the lubricants during mixing. Partial

alloy-ing appears

to

affect the compressibility of the diffusion-alloyed

mix at

compacting pressures <550 MPa (40 tsi). However,

at

pressures >690 MPa (50 tsi) the diffusion-alloyed mix exhibited a

higher compressibility

than

the binder-treated

mixes. This is related to the lower level of organics

in the diffusion-alloyed mix.

TRANSVERSE RUFTURE AND TENSILE PROPERTIES

Transverse rupture strength (TRS) was evaluat-ed on sets of 10 samples compacted on a

double-acting

floating

die. Tensile properties

were

evaiuated on sets of 40 samples taken at random

during runs of 250 dog-bone specimens pressed

on an

industrial

i50

mt

mechanical press

at

a

stroke rate

of

10 parts per min.

All

the samples were sintered for 30 min at 1,120'C in a nitrogen-base atmosphere, containing 5

v/o

hydrogen uti-lizing a commercial mesh-belt furnace. The cooling

rate in the range 650"C-400"C was -0.9"C/s. One

half of

all

the test specimens were tempered at

205"C for

t

h in air before mechanical testing. Results of the transverse rupture (TRl tests are reported in Figure 5. The TRS of the binder-treated mixes was found to be higher than that of the

dif-fusion-alloyed mixes, especially

at

low sintered

densities. The TRS of the fine-nickel mix was found to be slightly higher (4%) than that of the

standard-nickel mix, and 5o/o

to

10% higher than that of the

diffusion-alloyed mix. The apparent hardness of

Figure 5. Sintered TRS as a functton of compactton pressure in as'sintered and tempered conditions

the binder-treated mixes was found to be slightly

higher than that of the diffusion-alloyed mlr. The results of the tensile tests are reported in Tables IV and V. As was the case

with

the TR

tests, the tensile properties of the binder-treated mixes were found to be higher than those of the diffusion-allo1red mixes. For specimens pressed to

7 .0 g/cm3, the as-sintered UTS and yield strength

of the fine-nickel mix were found to be -50lo higher

than those of the binder-treated mix containing ihe standard nickel powder, and -200/o higher than those of the diffusion-alloyed mix.

At

7.2 g/cms, the as-sintered UTS of the F40A-T1 10 mix was 70/o

higher than that of the F40A-123 mix and -13%

higher than that of the dilTusion-alioyed mlx. Tempering increased the yield stress of all three powder mixes at all the sintered densities evaluat-ed. This effect was more Dronounced

in

the

fine-220 215 210

=

o 205 A-o o ,o" '-- EF rso

I

'-- a 180 175 170 1422 (! L tttc

='"'-a x. 4f. a) 1272 .h '/ + AT-DB4OA o F40A-T1'10 (as-siffered) E F40A-123(as-slntefed) ... ... A AT-DB4oA(aS siffered)

TABLE IV, AS-SINTEREDTENSILE PROPERTIES-Mix ldentification Green Density (g/cm3) Yield at 0,2% Plastic Strain (MPa) Maximum Elongation (/"1 Apparent Hardness (HRA) UTS (MPa)

Average Standard Average Standard

Deviation

Devialion Average Standard Deviation Average AT-DB4OA F40A-1 23 F40A-T110 1

.8

0.13

1.8

0]2 1

.7

0.13

1.6

0.32

1.7

0.11

1.7

0.12 7.00 7.20 7,00 7.20 7.00 7.20 50 54 50 55 54 56 420 485 490 520 c tu 560 650 660 690 690 740 28

I

to 41 23 IJ 4 9.6 l.) 8.7 6.7 'Average of 20 samples

43

(7)

TABLE V. TENSILE PROPERTIES-Mix ldentification Green Density (g/cm3) Yield at 0.2% Plastic Strain (MPa) Maximum Elongation (%) Apparent Hardness (HRA) UTS (MPa) Average Standard Deviation Average Standard Deviation Average Standard Deviation Average 7.00 7.20 7.00 7.20 7,00 7.20 t.v 1.8 t.u 1.9 1.9 1.9 0.11 0.13 0.1

I

0.31 0.17 0.1 9 50 54 cl 55 53 55

462

I

505

8

511

10

544

I

551

6.3

586

10

589

20

643

25

665

26

711

22

723

10

770

19 AT-DB4OA F40A-123 F40A-T 1 1 0 *Average of 20 samples

nickel mix (70lo increase vs. 4.5o/o increase for the

other mixes) except for specimens pressed

to

7.0

g/cms where an increase of 9% was recorded in

the

diffusion-alloyed

mix.

The

elongation remained essentially unchanged

in

all the mixes,

which is typical for formulations with -0.50 w/o sintered carbon. UTS was also increased by tem-pering the fine-nickel mix at all sintered densities. However, this gain

in

strength was more modest

than the gain in yield strength (4% vs. 7o/o). The effect of tempering on UTS for the other mixes was marginal. Apparent hardness was also

basi-cally unchanged by tempering for all three mixes.

Similar behavior has been reported in other

stud-iesg and was explained in terms of a reduction in the residual stresses during tempering. The fact that tempering affected the tensile properties of the fine-nickel powder mix more positively and

more consistently is an indication of a more uni-form microstructure containing more martensite compared

with

the coarser-nickel powder mix. Dimensional change (DC) after tempering of the

TR specimens also supporls this claim. DIMENSIONAL CHANGE AT"TER SINTERING TR Specimens

Figure 6 shows that DC from die size was

posi-tive

in

all the powder mixes. DC from green size

was negative, showing

that

shrinkage actually occurred during sintering. The net positive DC was due to spring-back after ejection of the parts.

DC

in

the binder-treated mixes (from green size)

was more negative than

in

the diffusion-alloyed mix at

all

compacting pressures. The fine-nickel mix exhibited a more negative DC than mix

F40A-123.

In

addition, the difference

in

DC between

mix F40A-Tll0 and the other two powder mixes

was more pronounced as the compaction

pres-sure increased. The more negative DC (from green size)

in

the F40A-T1 10 mix compared

with

mix F40A-123 is associated with enhanced nickel dif-fusion

in

the iron matrix. As density increases,

this

shrinkage effect also increases. The more positive DC recorded

in

the diffusion-alloyed mix might be due

to

slight differences

in

the copper

powder size and/or distribution, due to

agglomer-ation of copper during annealing.

The partial diffusion of nickel and copper in the

iron matrix that occurs during annealing of the iron-copper-nickel mixture is also a contributing

factor. The diffusion-bonded (DB) material and the

binder-treated mix utilizing the 123 nickel powder

contain the same nickel type (particle size). The

shape of the DC curves (Figure 6) of these two

mixes is similar but offset. In contrast, the shape of the DC curve of the mix containing 110 nickel

is

different and

close

to the

123

material.

Therefore,

it

is

concluded that size of the nickel powder controls the DC response (shape of the curves

in

Figure 6), while the bonding type

(par-tial alloying vs. binder) controls the curve offset.

Evidence of enhanced nickel diffusion

in

the fine-nickel mix can be seen in Figure 7 where the

density variation after sintering

is

reported. The samples containing fine nickel powder resulted in

a higher density; this is attributed to the swelling

effect of

the

copper negated by

the

increased shrinkage due

to

the extra-fine nickel powder.

Figure 8 shows that dimensional change is

affect-ed by tempering. DC was slightly more negative after tempering in ail the powder mixes. The effect

of tempering on DC has been monitored

in

other studies and was related to the levels of martensite

and carbon in the alloys,to

44

Volume 43. Issue 4.2OO7

(8)

0.300 0.250 0.200 ^ 0.150 E o.loo ,c 0.050 ?i 0.000 ? 9 -0-050 i5 -0.100 -0.1 50 -0.200 + t-40A-T1 l0 + F40A-123 +AT-DB4OA 55 60 65 70 75 80 Compaction Pr€sstrre (MPr)

Figure 6. Mean dimensional change as a function of compaction pressure

t) -0 05 'i -0 t5 u -0.35 -0.45 -0.5

Compaction Pressure (MPa)

Figure 7. Density variation after sintering as a function 0f c)mpaction pressure

Figure 8. DC associated with tenpering (205"Chh) for specimens pressed to 7.0 a/cms

Dimensional Change-Ring Specimens

In

order to compare part-to-parl consistency of the two binder-treated mixes, DC and distortion

were measured on ring specimens (OD 51 mm, ID

43 mm, height 6.35 mm) compacted

to

a green

density

of

7.2 g/cmu on

a

150

mt

mechanical press. Limited runs of 250 parls were carried out for each powder. DC and distortion after sintering of

the

rings were measured on

30

specimens

selected at random during the mns. The sintering

conditions were identical to those used for the TR and tensile specimens. Measurements were carried

out on the green and as-sintered samples utilizing the coordinate measuring machine (CMM) tech-nique. CMMs are mechanical systems designed to move a measuring probe to determine coordinates of points on the surface of the workpiece. By

meas-uring the coordinates of these points before and after sintering, DC and distortion can be evaluated.

The wall thickness (OD

-

ID) of the rings was

measured before and after sintering at 40", 80',

160", 200", 280", and 320" from a reference point marked on the rings. For each ring an average

wall thickness was calculated as the mean of the measured (OD

-

ID) values obtained at each of the

six angles. This procedure was applied

to

each

ring

in

the green and as-sintered states. DC on

the wall thickness was calculated as the

differ-ence of the mean values in the green and

as-sin-tered states. Mean DC values for mix F40A-123

and mix F40A-T110 were calculated as the mean DC from 30 different samples. The standard

devi-ation from the 30 samples was used a measure of

DC consistency.

For the determination of out-of-roundness, the ODs of the green rings were measured at 40", 80",

160". 200". 280o. and 320" from the reference

mark and the difference between the maximum OD and the minimum OD calculated. The same procedure was used for the sintered rings. Values

reported in Table VI refer to the difference in out-of-roundness before and after sintering (i.e.,

dis-tortion due to sintering).

The out-of-roundness variation was marginal and similar for the two powder mixes evaluated.

However, the results show

that,

as

for

the TR

specimens, more shrinkage occurred

in

powder

TABLE VI. DC OUT-OF.ROUNDNESS VARIATION IN

RINGS-Powder Mix

WallThickness

Out-of-Roundness

(AD/Do)

Average Standard Average Standard

f/")

Deviation

fk)

Deviation F40A-1 23 F40A-T1 1 0 -0.002 -0.036

0.14

0.00056 0.0004

0.05

0.00057 0.0004 -0.018 E -o.oro '!'o.ota o o I -o.or CI < -0.008 ! -o.ooo s .5 -o.ool € -o.ooz 0

F4OA-123 F4OA-T110 AT-DB4OA

*Green

density 7 .2 glcn3, D = wall thicknes = (OD - lD)

45

Volume 43. Issue 4.2OO7

(9)

mix F4OA-TI. 10 containing the extra-fine nickel.

In

addition to being more negative, the DC was

more consistent for the binder-treated mix

con-taining the extra-fine nickel. The use of the

extra-fine nickel powder, instead of the standard nickel powder, reduced the standard deviation

in

the wall thickness variation by a factor of 3.

MICROSTRUCTURE

The phases constituting the microstructure of the three powder mixes under study were similar.

A

representative

optical micrograph

(OM) is shown

in

Figure 9[a) where areas of fine pearlite

(300 FrV 10 g0, divorced pearlite (180 HV 10 g0,

martensite (500

to

740 HV 10 90, and retained austenite (150

to

200 HV 10 gfl are evident. A high-magnification scanning electron micrograph

(SEM) of the fine pearlite (backscattered image (BSI))

in

mtx F40A-123

is

shown

in

Figure 9(b).

Figure

10 shows representative OMs

of

three

sintered mixes

confirmin{ that

the fine-nickel

Figure 9. (a) Representative microstructure showing fine pearlite

(FP) bordered by divorced pealrte (DP), martensite (M), and

nickel-rich retained austenite. OM (b) high magnrfication of black area

tFP). Mix F40A-123 pressed to 7.0 g,cn3. SEMBS

powder mlx contained more martensite and less

retained austenite than the other two mlxes. The

microstructure of the standard-nickel mix was

Iess homogeneous

than

that

of

the

DB

mix (Figures 10-13) but the proportions of the

differ-ent phases were similar in the two mires.

EDS mapping revealed obvious differences in the distribution of the NRAs in the microstructure

of

the

three

powder

mixes (Figures

11-13).

Figure 10. Representative microstructures of sinteretl mixes. (a)

F40A-T1 10, (b) F40A-123, and (c) AT-DB41A. 0M

46

Volume 43, Issue 4,2OO7

(10)

smailer

in

size, indicating that more nickel went into solution

in

the iron matrix. The nickel-rich

areas

in

mix F40A-123 were frequently found to

Figure 12. Dispersion of copper and nickel rn powder mix F40A-123

after compaction to 7.2 g/cms and sintering at 1,120"C for 30 min'

(a) back-scattered electron image, (b) EDS map for copper, and

(c) EDS map for nickel. Agglomerates ol copper and ntckel evident near plres (white circles), ln dense areas, agglomerated nickel does

not associate with copper

Figures 11 and 12 show that the NRAs are more

evenly distributed

in

the microstructure of the

fine-nickel powder mix. NRAs with a high nickel content are observed in the microstructure of mix F40A-123.

In

the fine-nickel powder mix, NRAs

were

usually

more

uniformly distributed

and

Figure 11. Drspersion of copper and nrckel in powder mix F40A-T110 after compactron to 7.2 g/cms and sintering at 1,120"C for 30 min.

(a) back-scattered electran rmage, (b) EDS map for copper, and (c) EDS map for nickel

Volume 43, Issue 4,2OO7

(11)

per. This result is explained by the fact that when

copper melts,

it

leaves behind pores

that

corre-spond to the initial particle size of the copper. The enhanced mechanical properties, and the reduced

swelling of the fine-nickel powder mix, are attrib-uted primarily to: (i) the more-uniform dispersion of both NRAs and copper, (ii) more solution hard-ening due to the increased amount of nickel dif-fused

in

the

iron

matrix, and (iii) the

higher

percentage of martensite.

NRAs

in

the Ciffusion-alloyed mix were also

found

to

be concentrated preferentially

in

the nickel compared with the NRAs

in

the fine-nickel

mix

(Figures 11-13). Qualitative

line

scans

showed

that

the concentration of nickel

in

the

NRAs of the DB and the standard material is simi-lar. However, contrary to mix F40A-123, copper

agglomeration was not obvious

in

concentrated NRAs. The copper

distribution

was

virtually

immune

to

the nickel distribution

in

the

diffu-sion-alloyed mix. This result suggests that partial alloying of copper occurred during annealing of

the iron-nickel-copper mixture. This phenomenon

might also explain the more positive DC exhibited by the DB material.

The reasons why the mechanical properties of the DB materiai are lower than those of the

stan-dard-nickel material are not obvious. This

behav-ior

has been observed

in

premixes

of

similar composition of diffusion-alloyed grades of similar materials.lo The fact

that

nickel and copper do

not interact

in

a similar

fashion

in

these two

mixes could be a factor. The difference

in

the nature of the organics in the two mixes could also have an impact on the final mechanical properties

of the two mixes. Indeed, even

if

the same lubri-cant were used for both the DB and the binder-treated mixes, the presence

of

a binder might change

the

properties. While

the

effect of the extra-fine nickel powder in the fine-nickel mix can

explain the higher performances of this material compared

with

the DB material, further studies

are needed to explain the differences betw-een the DB and the standard-nickel materials.

CONCLUSIONS

o The use of extra-fine nickel powders signifi-cantly reduced nickel dusting in

binder-treat-ed mixes to levels as low as the nickel dustinp

levels in diffusion-alloyed mixes.

o TRS and tensile properties of the two binder-treated mixes were superior to those of their Figure 13. Dispersion of copper and nrckel in powder nix AT-DB41A

after compaction to 7.2 g/cm3 and sintering at 1,120"C for 30 min. (a) back-scattered electron image, (b) EDS nap for copper, and

(c) EDS map lor nrckeL Agglomeratron of nickel does not result in agglomeration of copper

be associated with copper-rich areas. As a result, the copper distribution was less uniform in these mixes. NRAs located around large pores contained

a

high

level

of

copper,

while

NRAs located

in

dense (pore-free) areas contained almost no

cop-48

Volume 43. Issue 4. 2OO7

(12)

diffusion-alloyed counterparts. The binder-treated mix containing extra-fine nickel

pow-der showed the largest improvements. TRS

was 50lo

to

10% higher than that of the diffu-sion-alloyed mix while UTS and yield stress

were l5olo to 200/o higher.

.

Increased

densification occurred

in

the binder-treated mix containing extra-fine nick-el powder. As density increased, this

shrink-age effect also increased and the density after

sintering of the binder-treated mix containing extra-fine nickel powder was higher than of

the

diffusion-alloyed

mix

and the binder-treated mix containing the standard nickei

powder.

.

DC measurements indicate that,

in

addition

to being more negative, DC is more consistent

in the fine-nickel mix.

.

NRAs were more evenly distributed and less

concentrated

in

nickel

in

the microstructure of the fine-nickel powder mix compared with

the

diffusion-alloyed mix. As a

result,

the copper

distribution

was improved

in

the microstructure of the fine-nickel powder mix.

This study has shown

that

the properties of binder-treated mixes can be improved over those

of equivalent diffusion-alloyed mixes. The use of

extra-fine nickel powders, instead of standard-size

PM

nickel

powders, can

further

extend these

improvements. In binder-treated mixes containing nickel, copper, carbon, and molybdenum, these

improvements are associated with (i) a more uni-form dispersion of NRAs and the presence of

cop-per in the steel microstructure, and (ii) enhanced

solution hardening due to increased nickel diffu-sion. The

results

suggest

that

the

beneficial impact of

the

extra-fine nickel powder on the

properties of PM parts increases with density.

REFERENCES

I. T.F. Stephenson, T. Singh and S.T. Campbe11, "Performance Benefits in Sintered Steels with Exlra-Fine Nickel Powder", Euro PM 2004, edited by H. Danninger

and R. Ratzi, European Powder MetallurS/ Association,

Shrewsbury, UK, 2004, vo1. 7, pp. 105-115.

2. J.A. Hamill, R.J. Causton and S.O. Shah, "High

Performance PM Materials Utilizing High Temperature Sintering", Aduances in Pouder Metallurgg & PorticuLate

Materials, complled by J.M. Capus and R.M. German, Metal Powder Industries Federation, Princeton, NJ, 1992,

vol. 5, pp. 193-214.

3. S.T. Campbell, T. Singh and T.F. Stephenson, "Improved

Hardenabllity of PM Steels Using Extra-Fine Nickel

Powder", Aduances in Powder MetoLLurgy & ParticuLote Materlals, compiled by R.A. Chernenkoff and WB. James,

Metal Powder Industries Federation, Princeton, NJ, 2004,

part 7, pp. 105-l 15.

4. T. Singh. T.F. Stephenson and S,T. Campbell,

"Nickel-Copper Interactlons in PM Steels". lbid. reference no. 3,

pp. 93-104.

5. T.F. Stephenson, T. Singh and S.T. Campbell, "Influence of Extra-Fine Ni Powder on PM Steel Properties", Aduances in Powder Metallurgg & Partbulote Materio"Ls, compiled by R.

Lawcock and M. Wright, Metal Powder Industries

Federation, Pdnceton, NJ, 2003, part 7, pp. 111-121. 6. F. Gosselin, "Segregation-Free Metallurgical Powder

Blends using Pol1wrny1 Pyrrolidone Binder", U.S. Patent No. 5,069,714, December 3, 1991.

7. B. Nichols and T. Sawayama, "lnvestigation of Powder Properties ln a FN-0208 Binder-Treated Powder", ibid.

ref-erence no. 1, pp. 1-9.

8. S. Uenosono,Y. Ozaki and H. Sugihara, "Development of

a High Flowable Segregation-Free Iron Based Powder Mix

with Wax Lubricant", Journal oJ the Japon Societg of

Pouder and Powder MetaLLurgg, vol. 48, no. 4, 2001, pp. 305-3 10.

9. S. St-Laurent and F. Chagnon, "Dynamic Properties of

Sintered Molybdenum Steels", Aduances in Powder

MetaLLurgg & Particulote MotertoLs, compiled by V.

Arnhold, C-L Chu, W.F. Jandeska, Jr. and H.l. Sarderow,

Metal Powder Industries Federatlon, Princeton, NJ, 2002,

part 5, pp. 121-r33.

10. F,J. Semel, "Ancorloy Premhes: Binder-Treated Analogs

of the Diffusion Alloyed Steels", Aduantces tn Powder

MetalLurgy & PartiruLate McLtertab, compiled by C.L. Rose

and M.H. Thibodeau, Metal Powder Industries Federatlon, Princeton, NJ, 1999, part 7, pp. 93-115.;:*

Volume 43, Issue 4,2OO7

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