EFFECT OF COPPER ALLOY ADDITION METHOD
ON THE DIMENSIONAL RESPONSE OF SINTERED FE-CU-C STEELS
Michael L. Marucci and Francis G. Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077 - USA
Abstract
Fe-Cu-C is the most common alloy system used in press and sinter powder metallurgy. This system has
many advantages including excellent mechanical properties, sinterability, and competitive cost. However,
as end customers continue to require tighter dimensional control of finished parts this alloy is at a
disadvantage due to its inherent dimensional variability. Changing the method of copper addition influences
the dimensional stability of this system. This work studies the mechanical, dimensional, and microstructural
differences of sintered Fe-Cu-C steels with atomized copper, diffusion alloyed Fe-Cu, and chemically
bonded copper additions.
Introduction
Copper additions to iron powder were among the first additions to improve the strength of sintered steels.
Additions of graphite to Fe-Cu sintered steels are also desirable because the carbon promotes the
formation of a pearlitic microstructure, imparting additional strength and hardness to the steel.
Unfortunately, these admixed elements result in dimensional growth upon sintering which can result in
dimensional variation of the finished component. As dimensional precision requirements for PM
components continue to become more demanding, alternate alloying methods need to be considered to
reduce the inherent dimensional variation of Fe-Cu-C sintered steels.
Elemental copper is advantageous due to its melting point at 1083 °C (1981 °F), which promotes sintering
and enhances the strength of the steel. Despite the relatively low melting point of the copper, it does not
fully diffuse within the iron at conventional sintering times and temperatures. This results in a copper
gradient from the iron powder surface to the core. 1 Earlier work also has shown that the amount of carbon
present within a compact changes the rate at which copper alloys with the steel matrix due to the change
in the dihedrial angle of the molten phase. 2 Both effects result in dimensional growth after sintering. The
dimensional growth is dependent on the chemical composition of the compact and is particularly sensitive
at 2 w/o copper additions. 3 Unlike other PM material systems, FC-0208 type materials exhibit less
dimensional growth as the sintered carbon increases, as seen in figure 1. Utilizing a 1 w/o copper
addition reduces the sensitivity of sintered carbon on dimensional variation. However, at this reduced
copper addition level, the strength is below that of a standard FC-0208 material.
Figure 1: Dimensional change of Fe-Cu-C steels (chart from Lindsley, et-al, ref 3)
As a 2 w/o copper addition remains desirable for mechanical properties, every effort needs to be made to
reduce the local variation of copper and graphite within the compact. The current work examines the
effect of different copper addition methods on dimensional variation and also looks at the effects of
bonding the admixed ingredients. The use of ANCORBOND or Fe-20Cu Distaloy is of particular interest
because these methods limit the possibility of elemental powder segregation during premix handling,
which could induce the dimensional variation detailed in figure 1.
Experimental Procedure
Table I outlines the materials used for this study, all prepared as 225 kg (500 lb.) premixes. All materials
conform to FC-0208 4 . The base steel powder used was Hoeganaes’ Ancorsteel 1000B, which is an
unalloyed water atomized steel powder. All premixes were prepared with 0.80 w/o natural flake graphite.
Three different types of copper additions were investigated. The ‘Standard Atomized’ is water atomized
copper powder having a D90 of 84 μm and a D50 of 39 μm. The ‘Fine copper ’ is a reduced copper
powder having a D90 of 17 μm and a D50 of 10 μm. ‘FD-20Cu’ is steel powder that is diffusion alloyed
with 20 w/o copper powder. This alloy is produced by Hoeganaes. Figures 2-4 show SEM
photomicrographs of each type of copper evaluated. The photomicrographs clearly show the differences
in particle size and morphology.
Different lubricant systems were also evaluated. The standard premix used admixed EBS for the
lubricant. Hoeganaes’ ANCORBOND™ was also used in combination with the EBS to gauge the impact
of bonding the mix ingredients on dimensional stability.
Table I: Test alloy matrix
Designation
Base Steel
Std Cu – Mix
Std Cu – Bond
Fine Cu – Bond
FD-20Cu – Bond
Ancorsteel 1000B
Ancorsteel 1000B
Ancorsteel 1000B
Ancorsteel 1000B
Copper Addition
(w/o)
2.0 Standard Cu
2.0 Standard Cu
2.0 Fine Cu
10.0 Fe-20Cu Distaloy
Graphite
(w/o)
0.80
0.80
0.80
0.80
Lubricant Addition
(w/o)
0.75 EBS – Premix
0.75 EBS – ANCORBOND
0.75 EBS – ANCORBOND
0.75 EBS – ANCORBOND
Figure 2: SEM Photomicrograph
of the ‘Standard Cu’ powder, water
atomized, 1000x original
magnification.
Figure 3: SEM Photomicrograph
of the ‘Fine Cu’ powder, reduced,
1000x original magnification.
Figure 4: SEM Photomicrograph
of the ‘FD-20Cu’ powder, water
atomized, 800x original
magnification.
All premixes in Table I were compacted at room temperature. Green density and green strength were
measured using the green strength samples (MPIF Std 15). Mechanical properties were determined using
Transverse Rupture (MPIF Std 41), dog bone tensile (MPIF Std 09), and un-notched Charpy Impact
(MPIF Std 40) samples, which were compacted from each mix over a range of compaction pressures.
Sintering was conducted in a continuous belt furnace at 1120 °C (2050 °F) in an atmosphere of 90 v/o N2
+ 10 v/o H2. The test samples remained at sintering temperature for approximately 15 minutes.
Conventional cooling was used. Selected samples were tested in the heat-treated condition. Heattreatment consisted of austenitizing at 871 °C (1600 °F) in an atmosphere of 25 v/o N2 + 75 v/o H2 and
quenching in agitated oil heated to 66 °C (150 °F). The quenched samples were subsequently tempered at
204 °C (400 °F) in 100 v/o N2 for 1 hour.
Results and Discussion
The compressibility of the test alloys is shown in figure 5. The chart shows that the different copper
addition types have a small effect on the compressibility. The densities achieved are within 0.04 g/cm3
over the test range. The fine copper resulted in a slightly higher green density, most likely due to the
better packing of the fine particles.
Compaction Pressure (tsi)
20
30
40
50
60
7.30
Green Density (g/cm3)
7.20
7.10
7.00
6.90
Std Cu – Mix
Std Cu – Bond
Fine Cu – Bond
FD-20Cu – Bond
6.80
6.70
275
413
551
688
Compaction Pressure (MPa)
Figure 5: Compressibility of test alloys.
The as-sintered and heat-treated mechanical properties of the test alloys are summarized in Tables II and
III. At a given sintered density, the mechanical properties are largely within experimental error when
comparing the different copper addition methods along with different mixing techniques. All properties
meet or exceed MPIF Standard 35 for FC-0208.
Table II: As-Sintered Mechanical Properties
Sintered at 1120 °C (2050 °F) – 90 v/o N2 + 10 v/o H2
Compaction
Transverse
Sintered
Yield Strength
Pressure
Density Rupture Strength
3
3
MPa
MPa
tsi
MPa
psi x 103
psi x 10
g/cm
2.0 w/o Std Cu – Mix
30
414
6.75
134
924
55
376
40
552
6.97
157
1082
62
427
50
689
7.08
168
1160
64
442
2.0 w/o Std Cu – Bond
30
414
6.75
132
911
55
377
40
552
6.95
154
1065
60
415
50
689
7.06
166
1144
63
433
2.0 w/o Fine Cu – Bond
30
414
6.74
130
897
55
378
40
552
6.95
147
1012
62
430
50
689
7.04
151
1044
63
434
10.0 w/o FD-20Cu – Bond
30
414
6.74
130
899
53
367
40
552
6.95
151
1041
63
432
50
689
7.06
162
1116
68
468
Ultimate Tensile
Strength
MPa
psi x 103
Elongation
%
Impact
Energy
ft.lbf
J
Apparent
Hardness
HRA
66
77
80
453
530
552
1.3
1.3
1.5
8
10
12
11
13
17
45
49
50
65
74
79
450
511
543
1.2
1.3
1.3
7
10
12
10
13
17
45
49
51
66
76
77
454
524
530
1.2
1.4
1.2
7
9
10
10
12
14
46
50
51
65
77
84
451
530
579
1.3
1.4
1.5
7
10
12
10
13
16
47
50
51
Table III: Heat-Treated Mechanical Properties
Sintered at 1120 °C (2050 °F) – 90 v/o N2 + 10 v/o H2
Austentized at 871 °C (1600 °F), Tempered at 204 °C (400 °F)
Compaction
Transverse
Sintered
Yield Strength
Pressure
Density Rupture Strength
3
3
tsi
MPa
MPa
MPa
g/cm
psi x 103
psi x 10
2.0 w/o Std Cu – Mix
30
414
6.75
143
988
60
412
40
552
6.95
157
1079
67
464
50
689
7.05
170
1171
79
547
2.0 w/o Std Cu – Bond
30
414
6.75
132
907
66
455
40
552
6.95
159
1096
77
533
50
689
7.05
174
1196
68
471
2.0 w/o Fine Cu – Bond
30
414
6.75
135
931
69
474
40
552
6.94
166
1147
74
509
50
689
7.04
171
1177
86
591
10.0 w/o FD-20Cu – Bond
30
414
6.75
134
924
60
411
40
552
6.95
168
1155
69
479
50
689
7.05
169
1164
73
504
10.0 w/o FD-20Cu - HD
30
414
6.83
73
504
25
173
40
552
7.05
82
568
23
162
50
689
7.16
92
632
29
198
60
827
7.21
90
621
29
197
Ultimate Tensile
Strength
MPa
psi x 103
Elongation
%
Impact
Energy
ft.lbf
J
Apparent
Hardness
HRA
82
94
101
567
647
693
0.6
0.6
0.7
5
7
8
7
9
11
67
71
71
83
98
99
572
673
683
0.6
0.6
0.5
5
6
7
7
9
10
67
69
71
88
98
100
605
673
689
0.6
0.6
0.5
6
7
8
8
9
11
67
69
71
80
99
101
550
680
697
0.5
0.5
0.5
5
6
8
7
8
11
67
70
71
45
46
56
60
313
316
384
415
0.2
0.2
0.2
0.2
3
3
3
3
4
5
4
5
69
71
73
74
Figure 6 shows the tensile strength of both the as-sintered and heat-treated condition for the bonded test
mixes. The copper addition type does not influence the ultimate tensile strength. The quench and temper
heat-treatment results in a 140 MPa (20,000 psi) increase in tensile strength over the range of densities
evaluated.
120
759
Heat-Treated
100
690
90
621
80
552
As-Sintered
70
483
60
414
50
345
6.70
6.80
6.90
7.00
7.10
UTS (MPa)
UTS (psi x 1000)
110
828
Std Cu – Bond
Fine Cu – Bond
FD-20Cu – Bond
Std Cu – Mix
7.20
3
Sintered Density (g/cm )
Figure 6: Ultimate tensile strength comparison.
The as-sintered axial fatigue behavior of the different copper addition types is detailed in figure 7. The
fatigue response is very similar for all copper addition types. Both the standard Cu and the FD-20Cu had
a fatigue endurance limit (FEL) of 131 MPa (19.0 psi x 1000). The fine Cu version had a slightly lower
FEL of 124 MPa (18.0 psi x 1000). This indicates that all copper addition types would be acceptable for
use in the as-sintered state. Testing of heat-treated materials was not completed due to difficulty in
gripping the dog-bone type samples.
Figure 7: As-Sintered axial fatigue comparison (R = -1).
Apparent hardness is highlighted in figure 8. Again, the copper addition type does not impact the
hardness level achieved. Heat-treating results in a substantial increase in apparent hardness. For all
materials at 6.95 g/cm3, the hardness goes from 50 HRA (81 HRB) to 68 HRA (35 HRC).
Apparent Hardness (HRA)
80
75
Heat-Treated
70
65
60
55
As-Sintered
50
45
Std Cu – Bond
Fine Cu – Bond
FD-20Cu – Bond
Std Cu – Mix
40
35
30
6.70
6.80
6.90
7.00
7.10
7.20
3
Sintered Density (g/cm )
Figure 8: Apparent hardness comparison
Unlike the mechanical properties, the dimensional change is impacted by the copper addition technique.
Figure 9a shows the dimensional change in the as-sintered condition. For all materials, the dimensional
growth increases as density increases. Typically, dimensional change as close to 0.0% is desired for
dimensional stability. The standard copper and the FD-20Cu had similar values and the fine copper
addition produces the highest dimensional growth. It is hypothesized that the higher growth is caused by
larger number of Fe-Cu interfaces within the compact. Upon melting, the copper diffuses along the grain
boundaries causing swelling; the finer copper has more particles resulting in more interfaces and added
swelling. The higher growth caused by the fine copper makes this powder type an undesirable copper
addition method for the FC-0208 system.
As-Sintered
a.)
Heat-Treated
b.)
0.70
Dimensional Change (%)
Dimensional Change (%)
0.70
0.60
0.50
0.40
0.30
Std Cu – Mix
0.20
Std Cu – Bond
Fine Cu – Bond
0.10
FD-20Cu – Bond
0.00
6.70
0.60
0.50
0.40
0.30
Std Cu – Mix
Std Cu – Bond
Fine Cu – Bond
FD-20Cu – Bond
0.20
0.10
0.00
6.80
6.90
7.00
7.10
7.20
3
Sintered Density (g/cm )
Figure 9: As-sintered and heat-treated dimensional change.
6.70
6.80
6.90
7.00
7.10
3
Sintered Density (g/cm )
7.20
Heat-treatment results in a similar trend in dimensional response (figure 9b), however, the final
dimensional growth is lower than in the as-sintered state. Typically, the transformation to a martensitic
microstructure results in a positive size change. However, additional shrinkage occurs during the
austenization/tempering of the steel, offsetting this effect. Again, the standard copper and the FD-20Cu
have the lowest dimensional growth.
Data from a separate study 5 show that when the carbon and copper content of the test alloys are moved
over a range of values, the dimensional change varies, as shown in figure 10. Unlike the behavior shown
in figure 1, the rate of change as a function of chemistry is almost the same for each alloying method and
carbon content, but the relative dimensional growth is similar to what is found in the present study. This
shows that the FD-20Cu could be used as an alternative to the standard Cu.
2.0 w/o Cu
a.)
0.80
0.70
Dimensional Change (%)
0.80
Dimensional Change (%)
0.8 w/o Graphite
b.)
Std Cu
Fine Cu
FD-20Cu
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.70
Std Cu
Fine Cu
FD-20Cu
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.5
0.6
0.7
0.8
0.9
Graphite Addition (w/o)
1.0
1.0
1.5
2.0
2.5
3.0
Cu Addition (w/o)
Figure 10: As-sintered dimensional change as a function of carbon and copper addition. All samples
compacted to 6.90 g/cm3.
The as-sintered and heat-treated microstructures for the standard Cu and FD-20Cu are compared in
figures 11 and 12. As expected, both copper addition types produced a pearlitic as-sintered
microstructure. The heat-treated microstructure consists of lath martensite with some areas of bainite.
The FD-20Cu sample has a larger amount of bainite; this is most likely due to incomplete diffusion of
copper in the iron particles. Moving from a pure copper particle to an Fe-Cu Distaloy has minimal effect
on the finished microstructure.
a.)
b.)
Figure 11: Standard copper, a.) as-sintered and b.) heat-treated, 2% nital/4% picral etch
a.)
b.)
Figure 12: FD-20Cu, a.) as-sintered and b.) heat-treated, 2% nital/4% picral etch
To determine the relative dimensional stability of the test materials a prototype spur gear was produced
and a measurement over wires (MOW) technique was used to determine dimensional variation. The
geometry of the prototype gear is shown in Figure 13. 500 samples per material were compacted with a
target density of 6.9 g/cm3 on a 140 t Dorst press. All samples were sintered/heat-treated under the same
conditions as listed above. MOW was conducted on 20 random gears from each test condition.
Major OD
Minor OD
Pitch Diameter
Pressure angle
ID
# Teeth
Module
Figure 13: Prototype gear produced to evaluate part-to-part consistency
1.10 in (28.3 mm)
0.85 in (21.6 mm)
0.96 in (24.5 mm)
20°
0.38 in (9.5 mm)
16
1.66
Table IV summarizes the MOW results. The data show, as predicted above, that gears in the as-sintered
and heat-treated condition produced with fine Cu resulted in slightly larger dimensions. The scatter in the
dimensional measurements for all materials falls within a tight range. However, the bonded version with
the standard Cu results in a lower measured scatter than the premixed version. The standard Cu compared
to the FD-20Cu shows mixed results and indicate that the dimensional scatter is similar under these test
conditions. The fine Cu version resulted in the largest scatter of the group.
Table IV: Measurement Over Wires Evaluation – As-Sintered
As-Sintered
Average (mm)
Standard Deviation
Heat-Treated
Average (mm)
Standard Deviation
Std Cu
Premix
Std Cu
Bond
Fine Cu
Bond
FD-20Cu
Bond
30.70
0.021
30.69
0.015
30.72
0.024
30.69
0.019
30.72
0.019
30.71
0.017
30.74
0.020
30.71
0.015
Conclusions
•
•
•
•
•
Different types of copper additions are viable in FC-0208 alloys. The resulting mechanical
properties in the as-sintered and heat-treated states are within measurement error for fine copper
and FD-20Cu addition when compared to the standard Cu.
The finished microstructure is minimally impacted by changing the copper addition type.
The use of fine Cu in this alloy induces higher dimensional growth than other copper addition
types. Standard Cu and FD-20Cu additions produce a similar dimensional response.
ANCORBOND processed premixes show a reduction in dimensional scatter when compared to a
standard premix
The use of bonded FD-20Cu in FC-0208 is equivalent to bonded Standard Cu under the
conditions evaluated. The use of FD-20Cu in place of standard Cu should be considered where
there the possibility of copper segregation due to powder handling exists.
References
1
T. Murphy and M. Baran, “An Investigation into the effect of Copper and Graphite Additions to SinterHardening Steels”, Advances in Powder Metallurgy & Particulate Materials – 2004, Metal Powder
Industries Federation, Princeton, NJ, part 10, pp. 266-274
2
R. Lawcock and T. Davis, “Effect of carbon on dimensional and microstructural characteristics of Fe-Cu
compacts during sintering”, Powder Metallurgy, Vol. 33, No. 2, 1990, p 147-150, Elsevier
3
B. Lindsley, G. Fillari and T. Murphy, “Effect of composition and cooling rate on physical properties
and microstructure of prealloyed P/M steels”, Advances in Powder Metallurgy & Particulate Materials,
compiled by C. Ruas and T. A. Tomlin, Metal Powder Industries Federation, Princeton, NJ, 2005, part 10,
p. 10-353.
4
MPIF Standard 35, Material Standards for PM Structural Parts, 2009 ed. MPIF, Princeton, NJ - USA
5
B. Lindsley, 2009, Internal Hoeganaes Study on the effects of premix chemistry on dimensional change.