A Metallographic Investigation Into the Effect of Sintering on an FC-0205 Premix
Thomas F. Murphy, George B. Fillari, Gerard J. Golin
Hoeganaes Corporation
1001 Taylors Lane
Cinnaminson, NJ 08077
Abstract
The properties of ferrous P/M materials are developed during the sintering process where
metallurgical bonds are formed at particle-to-particle contacts and alloying of mixed and bonded
additives occurs. Increasing either the sintering temperature or time can produce improvements
in the microstructure and, consequently, the ensuing properties. In this paper, an FC-0205
premix, sintered for various times at 1120 °C (2050 °F), will be used to study the microstructural
changes resulting from increases in sintering time. Features of interest include, changes to the
surface-to-volume ratios at the particle boundary and pore surface areas, diffusion of the copper
in the solid state and as a liquid component, and the homogenization of the microstructrue with
increasing sintering times. Stereological techniques, using light optical microscopy, will be
employed to examine the diffusion of the added alloying materials and to quantify the
improvement in the degree-of-sinter. Additionally, electron microscopy (SEM) will be used to
examine Charpy impact fracture surfaces from specimens sintered at the various times.
Introduction
Iron-copper-carbon materials, especially FC-0205 and FC-0208, have been the ‘workhorse’ of the
North American P/M industry for many years. They probably account for greater powder usage
than any other single alloy system. There are many reasons for the success of these compositions.
The liquid phase from melting of the copper enhances the sintering of the matrix iron and
acceptable properties can be developed with less than optimal conditions. Increasing the sintering
times can lead, however, to greater stability and consistency in parts production through the
development of a more homogeneous microstructure. In addition, from a property standpoint, the
carbon diffuses and forms pearlite and the alloyed copper strengthens the ferrite.
In the past, authors have addressed some aspects of the Fe-Cu and Fe-Cu-C systems. Pore
formation [1] and dimensional change [2] have been discussed for the Fe-Cu materials, while the
dimensional and microstructural characteristics [3] and the effect of the liquid phase have been
investigated for the iron-copper materials both with and without carbon additions [4]. Further,
the effect of sintering atmosphere on dimensional change [5] was determined for the FC-0208
material. In contrast to these offerings, this paper takes a metallographic approach to describe the
changes in microstructure as sintering progresses.
Experimental Procedure
The material used in this study was a mixture of iron (Ancorsteel® 1000) and 2 w/o
ACuPowder 8081 copper, 0.5 w/o Asbury 3203H graphite, and 0.75 w/o Lonza Acrawax C
lubricant. After sintering, this composition falls within the MPIF specification for an FC-0205
material. The test specimens utilized to record the changes in microstructure were unnotched
Charpy impact bars that were compacted at three pressures, 415, 550, and 690 MPa (30, 40, and
50 tsi). Sintering was performed in a pusher furnace with the temperature set at 1120 °C
(2050 °F) under a 90 v/o N2/10 v/o H2 atmosphere. A pusher furnace was used in order to vary
sintering time by pushing a tray loaded with the impact bars into the hot zone, starting a timer,
and pushing the tray into the cooling zone after the appropriate amount of time had elapsed.
Residence times in the hot zone ranged from five to a total of 50 minutes, in five-minute
intervals. In addition, one set of bars was sintered for a total of one minute to show the effect of
lubricant burn-off without the temperature reaching a point where alloying was initiated. In this
furnace, the time required for room temperature samples to reach the sintering temperature varied
between 10 and 15 minutes and was dependant on the load being sintered. Impact energy and
apparent hardness values were measured for each group of bars prior to microstructural
examination.
One bar from each sintered set was selected for metallographic testing. Transverse cross-sections
parallel to the pressing direction were removed and mounted with the cut surface exposed for
examination. Each mount was ground through 600 grit (U.S. Standard) SiC abrasive paper and
polished using a vibratory polisher with successively finer Al2O3 particle sizes, beginning with
1 µm, and continuing through 0.05 µm. The prepared mounts were then outgassed in a vacuum
chamber to remove the entrapped moisture, vacuum impregnated with warmed epoxy to fill the
void spaces and, hopefully, retain any loosely sintered particles. The impregnated mounts were
then reground and polished, again using the vibratory polisher with the various sizes of Al2O3
polish. All samples were examined in both the unetched and etched conditions.
Unetched specimens were used for general microstructural examination and stereological testing,
while etched mounts were used to show the variation in alloying due to diffusion of the added
alloying materials. Both specimen conditions were documented using light optical microscopy.
The stereological test methods were designed to measure changes in the surface-to-volume ratio
(SV) of the particle boundaries as the sintering time was increased. However, pressed and
sintered materials present problems in stereological sampling. The inherent directionality in the
microstructure resulting from compaction prevents uniform sampling on a single plane using
parallel, straight-line sample probes. To remedy this, ‘vertical sections’ (cross-sections
containing the pressing direction) were prepared and an array of test probes in the form of
oriented cycloids was overlaid on the magnified, unetched specimen surfaces. The cycloid shape
was shown, first by Baddeley, et. al. [6], and then by others [7,8,9], to sample oriented structures
uniformly when coupled with prepared vertical sections. The four cycloid orientations were
joined into a sine-like wave pattern to simplify counting. A counter was incremented as the
overlaid cycloid wave pattern intersected the individual particle boundaries. The relationship for
determining SV is shown in Equation 1:
SV = 2PL
(1)
where SV is the surface-to-volume ratio of the microstructural feature being measured [10,11] and
PL is the number of incidents of the cycloid wave crossing the feature boundaries per unit cycloid
line length. In this case, the microstructural features of interest were the particle boundaries.
Fracture surfaces from samples with selected sintering times were examined in an effort to find
trends in the failure characteristics as the sintering time was extended and the diffusion of the
alloying elements became more homogeneous. Secondary electron imaging was used to
document these features.
Results
As was described in the Introduction, the present study uses a metallographic approach to
describe the response of the Fe-Cu-C material to various sintering times. James [12] has
described the three factors governing the properties of P/M materials as the following:
•
•
•
Density – Porosity (volume fraction, size & shape distributions)
Chemical Composition & Alloying Method (distribution of the added alloying materials)
Resultant Microstructure
All three factors are affected, either directly or indirectly, as the sintering time is varied with
development of the final microstructure occurring upon cooling from the sintering temperature.
The major effect on the microstructure is from the distribution of the alloying additives during the
sintering process, with diffusion increasing with increased sintering time. In many cases, this
causes the development of a nonuniform distribution of some of the additives. This leads to
variations in local hardenability, and consequently a variety of transformation products are
developed as the material is cooled. With the copper containing materials, the copper becomes
liquid and diffuses along pore and particle boundaries. With short sintering times the distribution
is heterogeneous. The distribution becomes more homogeneous, however, as the sintering time is
increased.
The process of sintering, from pre-heat to cooling, controls the constituents in the microstructure.
Sintering can be separated into three basic areas where each has associated effects on the
development of the microstructure. Each section is detailed briefly below:
•
•
•
Pre-heat
Ö Lubricant is burned out
Time at Sintering Temperature
Ö Carbon diffuses
Ö Copper becomes liquid and diffuses
Ö Sintering occurs
Ö Other added alloying elements diffuse and homogenize
Cooling
Ö Transformation products are formed
Cross-sectional Analysis (Light Optical Microscopy)
The change in microstructure can be seen in Figures 1 through 6 where the samples compacted at
550 MPa resided in the furnace hot zone for increasing periods of time. Figure 1 was taken of the
sample sintered for one minute. The boundaries between the particles are evident as are the
unmelted copper particles. A faint ferritic grain structure can be seen within the iron particles and
no graphite has diffused to form pearlite.
2 v/o nital + 4 w/o picral
Figure 1. Photomicrograph of a sample residing for one minute in the hot zone.
No sintering has taken place. All particle boundaries are visible.
Figure 2 shows the structure after five minutes in the hot zone. The start of carbon diffusion is
apparent with formation of the lamellar pearlite; the copper particles, however, remain unmelted.
The apparent lack of uniformity in the distribution of carbon is shown also.
In Figure 3, there is evidence of a large copper particle that has melted without complete
distribution of the liquid copper. The liquid metal has moved to the edge of the surrounding
matrix material and the molten copper has been pulled into the surrounding pores by capillary
action. The time above the melting point of copper (1083 °C, 1981 °F) was probably insufficient
for the entire volume of the copper particle to distribute completely. It should also be noted, that
the area surrounding the melted Cu particle is free of particle boundaries; although some angular
features are still apparent along the pore edges. This image is from near the top surface of a bar
compacted at 415 MPa and sintered for 5 total minutes. Only a small amount of the cross-section
exhibited the initiation of the copper melting.
2 v/o nital + 4 w/o picral
Figure 2. Five-minute sintered sample showing diffusion of the graphite leading
to the formation of pearlite, but the unmelted copper particles are still visible.
Unetched
Figure 3. Melted Cu particle before the completion of copper distribution through
the local porosity. The liquid copper has moved into the surrounding region through
capillary action. The image is from the upper surface of a 415 MPa compacted bar
sintered for 5 total minutes.
As the copper becomes liquid, the molten material travels along the particle boundaries and
diffuses at both pore and grain boundaries. This can be seen in Figure 4, where a sample sintered
for ten minutes exhibits partial diffusion of the copper and small areas of free copper along the
prior particle and grain boundaries. The brown staining indicates regions with a higher copper
content in solution with the iron. This will become less evident as sintering proceeds and the
copper is more evenly distributed.
2 v/o nital + 4 w/o picral
Figure 4. A ten-minute sintered sample showing diffusion of the copper along
particle and grain boundaries. The brownish staining indicates regions where
copper has a higher concentration than the grain interiors.
One consequence of liquid phase sintering is the presence of the spaces vacated by the particles
after melting. This is shown in Figure 5, in a sample sintered for a total of 30 minutes. The total
sintering time equates to a time at temperature of approximately 15 minutes. The large pores are
prior sites of the larger copper particles, and it is apparent that, at this sintering time, the carbon is
more uniformly distributed.
2 v/o nital + 4 w/o picral
Figure 5. Low magnification photomicrograph showing the distribution of the
carbon and the pores produced from the melting of the large copper particles
in a sample sintered for a total time of 30 minutes.
2 v/o nital + 4 w/o picral
Figure 6. Also taken of the 30 minute sintered sample. The carbon and copper
distributions appear more uniform. No obvious particle boundaries remain.
In Figure 6, the microstructure has become more uniform with no obvious free copper or particle
boundaries. Increasing the sintering time will, however, improve the distribution of the alloying
materials and give a more stable material.
Stereological Testing
The samples used in the previous discussion were part of the larger population used to evaluate
the evolution of the microstructure through stereological testing; although for the latter, the
samples were in the unetched condition. The evaluation procedure required IUR (isotropic,
uniform, and random) sampling, which presented difficulties with pressed and sintered P/M
materials. During compaction, the deformation of the particles, caused by the high compaction
pressures, elongates the particles and pores in a plane 90° to the pressing direction. This
directionality precludes the use of parallel, straight-line probes as the sampling mechanism. An
alternative sampling method was found that used of an array of cycloid-shaped probes. The
oriented cycloids have been shown to sample directional structures randomly when combined
with sections containing the orientation.
In the present study, the cross-sections were taken in an orientation that included the pressing
direction. Consequently, it was permissible to use the oriented cycloid array as the sampling
means. Image analysis, using automation both with and without operator interaction, was used to
determine the SV of the particle boundaries and the pore surfaces. Figure 7 shows an example of
an area sampled using the cycloid array.
Figure 7. Example of the cycloid array (the red sine-like waves) overlaid on
a sample sintered for a total of five minutes. Using the automated image analysis
system, the pores have been colored green in this example. Red arrows point to
the approximate locations of coincidence of the particle boundaries and the
cycloids. A PL count of 12 would be made for this field.
In practice, as the array is overlaid on the field, the locations of cycloid and particle boundary
coincidence are counted and totaled in a multi-field analysis. These counts are then entered into
the expression previously shown as Equation 1, along with the total line length of the cycloid
array. The resulting calculation produces the surface-to-volume ratio of the particle boundaries
within the total material volume. Figure 8 is a graph showing the reduction of the SV in samples
processed from 1 to 10 total minutes.
40
FC-0205
Particle Boundary SV (mm²/mm³)
35
30
Compaction Pressure
(MPa)
25
690
550
415
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
11
Total Sintering Time (min)
Figure 8. Graph showing the reduction in SV of the particle boundaries as the sintering time was
increased for the three compaction pressures. The one-minute sintered impact bars compacted at
690 MPa suffered considerable damage during testing and were not fit for SV measurement.
From the graph, it appears that the higher compaction pressures result in an increase in the
amount of particle boundaries, regardless of the sintering time. This is due to the higher densities
and the corresponding increase in the amount of particle contact as the compacting pressure is
raised. The differences become less, possibly insignificant, as the sintering time reaches ten total
minutes. The measurable difference in sintering at this point is significant because, at ten minutes
total sintering time, the samples may not have reached the actual set-point sintering temperature.
Consequently, the sintering may have taken place at temperatures between the melting point of
copper (1083 °C) and 1120 °C. This helps to explain the relatively forgiving sintering response
of the Fe-Cu-C materials.
Fractography
Fracture surfaces from tested impact bars were examined in an SEM to find possible trends in the
failure mechanism as the sintering time was extended. In this case, the group of bars compacted
at 690 MPa was examined. Secondary electron imaging (SEI) was used as the signal source for
this portion of the sintering evaluation. Other than simple removal of dust from the surfaces, the
samples required no additional preparation.
The surface of a one-minute sintered sample is shown in Figure 9. No apparent sintering has
taken place. The particle surfaces appear as failed ‘green’ materials.
Figure 9. Fracture surface of a one-minute sintered bar. No sintering has taken
place and separations between adjacent particles are clearly seen.
At the five-minute time, sintering has apparently started as evidenced by the small ductile regions
at the sintered particle necks (Figure 10, a and b). The location of the image on the right is
outlined in white within the image on the left.
a
b
Figure 10, a and b. Five-minute sintered sample showing the start of sintering.
At ten minutes, the effects of increased sintering are clearly visible. Figure 11, a and b, show the
presence of, not only an increase in the amount of ductility, but also the start of transgranular
cleavage. Additionally, the progress of sintering, as indicated by the amount of material fracture,
has increased considerably.
a
b
Figure 11, a and b. Surface from a ten-minute sintered sample showing a combination of ductile
dimples and brittle transgranular fracture.
At 30 minutes total furnace time, the greater amount of metallurgical bonding that has occurred
results in more evidence of fractured surfaces. Ductile rupture, transgranular cleavage, and
fracture through pearlite are all seen in significant proportions in Figure 12.
TC
D
P
a
b
Figure 12, a and b. An area showing the fracture from a 30-minute sintered sample. The b image
contains transgranular cleavage, labeled TC, pearlite, labeled P, and ductile rupture, labeled D.
The samples from the 50-minute total sintering time were also examined. An increase in the
amount of transgranular cleavage fracture was apparent as seen in Figure 13, a and b. Significant
amounts of ductile rupture and fractured pearlite also are present.
a
b
Figure 13, a and b. A fracture surface from one of the 50-minute sintered samples. An increase
in the amount of brittle fracture, in the form of transgranular cleavage is apparent.
Discussion & Conclusions
Sintering of the FC-0205 material appears to begin well before the typical sintering temperature
of 1120 °C (2050 °F) is reached. Evidence for this is presented in Figures 4, 10, and 11; where
the sample time in the hot zone was below the 10-15 minute pre-heat time. Diffusion of the
carbon and copper is illustrated in Figure 4, while ductile and brittle fractures are evident in both
Figures 10 and 11. In addition, the loss of particle identity through the loss of particle boundaries
occurs during heating to the sintering temperature as may be observed in Figure 8.
The fracture surface examinations appeared to show an increase in the amount of brittle,
transgranular fracture as the sintering time was increased to a total of 50 minutes. This may be
due to homogenization of the copper distribution and consequent hardening of the ferrite.
The impact energy and apparent hardness measurements from these sintered materials were
compared with the MPIF Standard 35 typical values for FC-0205. The results are shown in
Figures 14 and 15. The apparent hardness results in Figure 14 indicate the diffusion of carbon
and at least partial copper diffusion, at 10 to 20 minutes total time, produces values approaching
and in excess of the standard values. In reality, these samples may barely have reached sintering
temperature, or, were at temperature for less than 10 minutes. The vertical gray lines on both
figures indicate the probable time to sintering temperature.
Impact results appeared to exhibit a similar trend. At a density of 6.6 g/cm³, the values either
nearly met or exceeded the MPIF Std. 35 typical values for a density of 6.7 g/cm³ at all times of
10 minutes or longer in the hot zone. The 6.8 g/cm³ samples exceeded the 6.7 g/cm³ MPIF Std.
35 typical values at 10 minutes and longer sintering times. In addition, the 6.8 g/cm³ samples
nearly met the value of the 7.1 g/cm³ MPIF Std. 35 typical value at 20 minutes and surpassed
these values at 25 minutes and longer total sintering time. The 7.0 g/cm³ samples exceeded the
7.1 g/cm³ MPIF Std. 35 typical values at all times of 20 minutes and longer. As this material
reaches the sintering temperature, there appears to be a region of inconsistency in the mechanical
properties as exhibited by the impact results at the 10 and 15 minute total sintering time.
However, with an increase in the time-at-temperature, the impact energy appears to stabilize.
90
Standard 35
Typical Values
3
7.1 g/cm
80
70
3
6.7 g/cm
Hardness (HRB)
60
50
40
Impact Bar
Sintered Density
(g/cm³)
30
20
7.0
6.8
6.6
10
0
0
5
10
15
20
25
30
35
40
45
50
55
Total Sintering Time (min)
Figure 14. Rockwell B apparent hardness values for the three density levels and the variety of
sintering times.
18
16
Impact Energy (joules)
14
Standard 35
Typical Values
12
3
7.1 g/cm
10
8
3
6.7 g/cm
6
Impact Bar
Sintered Density
(g/cm³)
4
7.0
2
6.8
6.6
0
0
5
10
15
20
25
30
35
40
45
50
55
Total Sintering Time (min)
Figure 15. Unnotched Charpy impact test results for each set of sintered materials.
The microstructure of this material undergoes an evolution as the time in the hot zone is
increased. At one minute in the hot zone, the maximum temperature reached is probably high
enough to burn-out the lubricant but not to start diffusion of the graphite or copper. Extending
the time in the hot zone to 5 minutes, the graphite starts to diffuse and the sintering at particle
contacts commences. Nevertheless, in most cases, the copper remains unmelted, although the
temperature seen by the samples at this time is probably very close to the melting point of copper.
At 10 minutes total time in the hot zone, the graphite has diffused, the copper has melted, and the
majority of particle boundaries have been sintered. Apparently, the copper has diffused into the
matrix and strengthened some of the ferrite, resulting in the beginning of brittle transgranular
failure and fracture through pearlite colonies. Sintering for longer times homogenizes the
chemical composition and improves the sintered particle bonds. As sintering continues, the
fracture surfaces exhibit progressively greater amounts of ductile rupture and brittle transgranular
failure.
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