Atomized Low Apparent Density (AD) Iron Powder
For Advanced PM Applications
Peter Sokolowski and Francis Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077
ABSTRACT
A low apparent density atomized iron powder, Ancorsteel AMH, was developed to meet
market demands. The primary usages for this material are lower density applications where
green and sintered strengths are critical to the performance of the PM part. This paper will
focus on the green and sintered comparisons between Ancorsteel AMH and traditional
sponge iron in both straight graphite and copper graphite premixes. It will be demonstrated
that this material can be utilized as an alternative to traditional sponge iron powder with
increased micro-cleanliness and no loss in mechanical properties.

INTRODUCTION
The application of sponge iron in the marketplace has been successful for years, largely due
to its attractive features including favorable green strength at low part densities and high
specific surface area. These characteristics enabled sponge iron to be commonplace in
products ranging from shock absorbers to serving as catalysts for prolonged low
temperature heating in heat packs. In an effort to continually supply these areas of
application and potentially expand into new markets that require higher quality material at
reduced costs, an alternative ‘sponge like’ iron powder was developed, Ancorsteel AMH.
The capability to atomize an iron powder with characteristics approaching that of traditional
sponge has facilitated its incorporation into the PM industry as an alternate to sponge. While
for some applications the porous structure of sponge is ideal, the fine pore network inhibits
its ability to reach higher densities at reasonable compaction pressures, preventing it from
being useful in parts requiring improved sintered strengths. Therefore, this paper discusses
the relevant properties of a water atomized iron powder alternative for reduced sponge iron,
highlighting its usefulness and benefits.
The fabrication method used to produce traditional sponge iron versus that of water
atomized iron powder, as discussed here [1], form similar yet notably different particle
morphologies, which set them apart from one another. Table 1 lists a few key properties of a
conventional water atomized powder (steel), the newly developed low apparent density
powder (AMH), and that of a traditional direct reduced sponge powder (sponge). These
properties were measured on commercially available products. As seen in the table, the
apparent density (AD) of AMH at 2.55 g/cm3 lies between that of steel, 2.92 g/cm3, and the
sponge value, 2.50 g/cm3. The powder flow characteristics are similar among the powders
tested, suggesting comparable behavior during die fill.
As already mentioned, the surface area of sponge makes it ideal for applications sensitive to
chemical reactivity, such as controlled low temperature heat packs. The specific surface
areas were measured using the BET method with krypton gas over multiple partial pressures
as opposed to a single point analysis, providing a more accurate result. It is evident that
AMH has lower surface area than sponge, however it shows an increase over steel powders
at comparable particle size distributions and its already successful inception in heat pack
applications has proven it as a viable alternative to sponge.

Ancorsteel is a registered trademark of Hoeganaes Corporation
Table 1: Iron powder properties
ID
Material
AD
(g/cm3)
Flow
(s/50g)
BET surface area
(m2/g, ± 0.0002)
Steel
Ancorsteel 1000B
2.92
26
0.036
AMH
Ancorsteel AMH
2.55
28
0.039
Sponge
Traditional Sponge Iron
2.50
30
0.072
Shown in Figure 1 are representative SEM (top row) photomicrographs of a sponge particle
(A1), steel particle (B1), and an AMH particle (C1). The sponge and AMH particles share
similar surface irregularities greater than steel powder, where AMH is a hybrid of surface
characteristics between sponge and steel particles. Surface irregularities increase the total
surface area, Table 1, capable of mechanically locking with another particle surface during
compaction; thus promoting higher green strengths.
The light optical photos (bottom row) illustrate differences in porosity between the three
materials at a density of 6.8 g/cm3. In (A2), the sponge material exhibits widespread fine
internal porosity with few large pores. Steel particles (B2) typically have little or no internal
porosity and thus the porosity is largely between particle surfaces. As a result of the
proprietary processing method, AMH contains a greater amount of internal porosity over
steel powder and yet exhibits similar, larger inter-particle surface pores (C2). The reduced
amount of internal porosity is one characteristic that allows steel and AMH powders to
achieve higher green densities at lower compaction pressures.
Another distinction can be made concerning micro-cleanliness of the powder, which has a
strong affect on the sintered properties and consequently the structural performance. As an
integral processing step prior to water atomization, the liquid metal refining enables
increased control over cleanliness of the final product via slag formation. Typical sponge
production does not allow for similar processing methods to reduce impurities in the charged
material.
To highlight the difference in cleanliness between sponge and AMH, representative powder
samples of each were sintered, forged, cross-sectioned, and analyzed using image analysis
software to determine the level of contamination. Figure 2 is an un-etched photomicrograph
that illustrates the non-metallic inclusion content inherent with reduced iron powder. The
dark areas are non-metallic particles and the light gray area is fully dense sponge with 0.6
w/o graphite. Owing to the source of raw material and nature of the processing route,
sponge typically contains a significant amount of non-metallic inclusions such as silicon or
titanium oxides. These inclusions, some quite large, can negatively impact properties and
lead to machining difficulties. In contrast, Figure 3 displays the cross-section view of fully
dense AMH with 0.6 w/o graphite. It is evident there are few inclusions present, providing a
greater degree of cleanliness.
A1
B1
C1
A2
B2
C2
Figure 1: Photomicrographs of (A1,2) top: sponge particle; bottom: cross-section sponge
powder at 6.8 g/cm3, (B1,2) top: steel particle; bottom: cross-section steel powder at 6.8
g/cm3, and (C1,2) top: AMH particle; bottom: cross-section of AMH powder at 6.8 g/cm3;
dark areas in bottom pictures are porosity
Figure 2: Cross-sectional view of forged
sponge with 0.6 w/o graphite; dark areas are
inclusions; light optical, as polished
Figure 3: Cross-sectional view of forged AMH
with 0.6 w/o graphite; dark areas are
inclusions; light optical, as polished
To quantify the non-metallic inclusion contents in each material, the powder-forged samples
were metallographically prepared and analyzed using an automated image analysis system.
The samples were evaluated twice, measuring first the number of inclusions without any
nearest neighbor joining and secondly using a smaller near neighbor distance of
approximately 6µm. The results are displayed in
Table 2. As a result of this analysis, the area percent of the detected inclusions for sponge is
2.80% compared with 0.03% for AMH. These frequencies are calculated as the number of
inclusions per 100 mm². Ancorsteel AMH has nearly 100 times fewer inclusions relative to
sponge.
Table 2: Non-metallic inclusion analysis results
Number of Inclusions
(No Joining)
Number of Inclusions
( ~6 µm Joined)
Inclusion Size
Sponge
AMH
Sponge
AMH
>30 µm
>75 µm
>100 µm
>150 µm
>200 µm
623
33
12
2
0.8
6
0
0
0
0
3235
135
34
6
1
13
0
0
0
0
ALLOY PREPARATION AND TESTING
Standard laboratory premixes using AMH, sponge, and rolled sponge powders were
prepared. These powders were mixed with Acrawax C and varying amounts of Asbury type
3203H graphite. Admixed copper was used to produce alloys with 1 w/o and 2 w/o Cu.
Green densities were measured on 25.4 mm diameter cylinders with 0.75 w/o Acrawax C,
following MPIF Standard 45. Green strength was measured using 12.7 mm thick bars in
accordance with MPIF Standard 15. [2]
Uniaxial compaction of transverse rupture strength (TRS) bars were pressed at 275 MPa,
415 MPa, and 550 MPa. Dogbone tensile bars were compacted at 415 MPa, 550 MPa, and
690 MPa. All samples were sintered concurrently in a mixed atmosphere of 90 v/o nitrogen
and 10 v/o hydrogen (90/10) at 1120 °C for 15 minutes in a continuous-belt furnace with a
cooling rate of 0.7 °C/s. Prior to mechanical testing, sintered density, dimensional change
(DC), and apparent hardness, were determined on the TRS samples. The densities of the
green and sintered steels were determined in accordance with MPIF Standard 42. Tensile
testing adhered to MPIF Standard 10 [2]. Sintered carbon values were measured using a
Leco 200 carbon-sulfur combustion gas analyzer with standards run before and after
samples. All designated graphite contents were within 0.03 w/o sintered carbon of each
other.
RESULTS AND DISCUSSION
Figure 4 displays the green strength versus green density of three iron powders in an FC0208 premix with 0.75 w/o Acrawax C lubricant. All other premix configurations reveal a
similar trend. The sponge material does exhibit higher green strength relative to AMH;
nevertheless AMH demonstrates equivalent strength as rolled sponge. There are
alternatives to further increase the green strength of AMH through judicious selection of
innovative lubricants or the use of warm die compaction; therefore potential issues during
part handling relating to lower green strength can be overcome.
Figure 4: Green strength vs. Green Density of AMH, sponge, and rolled sponge in an
FC-0208 premix with 0.75 w/o Acrawax C lubricant
Figure 5 shows the transverse strength trends in both a F-0008 and FC-0208 premix for
AMH and sponge. The sintered strength of AMH is comparable to sponge at a given density.
If comparing sintered strength values for a given compaction tonnage, the atomized product
actually out-performs sponge due to the increased compressibility. These results indicate
that a low AD powder with attractive green strength can provide improved sintered strengths
for applications with more demanding structural requirements.
Figure 5: Sintered TR strength of AMH and sponge in a F-0008 and FC-0208 composition
The sintered dimensional change versus green density for premixes F-0008 and FC-0208
based on AMH and sponge is given in Figure 6. The DC values for AMH exhibit more
positive sintered dimensional change over sponge for a given green density.
Figure 6: Sintered DC vs. Green Density in F-0008 and FC-0208 compositions
Figure 7 presents the sintered dimensional change for AMH versus graphite addition as a
function of admixed copper. When no copper is present, increasing the graphite results in
increased growth of the sintered part. At 1 w/o copper, diffusion of copper into the iron
creates growth, over-riding the effect of carbon in solution; yet dimensional precision of AMH
is fairly stable at 0.30% over the range in graphite additions studied. In mixes containing 2
w/o Cu, there is significant growth at low graphite, however the interaction between Cu and
graphite at higher graphite content leads to less growth. This phenomenon is common in
ferrous PM alloys and has been studied in numerous alloy systems [3].
Figure 7: Sintered DC vs. graphite content for AMH with 0, 1, and 2 w/o Cu
Table 3 presents the tensile properties of AMH with 0.6 w/o and 0.8 w/o graphite with or
without 2.0 w/o copper. The compositions without copper reveal a lower strength, yet greater
ductility when compared to the compositions containing copper. These results show that
AMH can be tailored to meet specific mechanical properties while maintaining improved
compressibility, sintered properties, and comparable green strength to that of traditional
sponge iron.
Table 3: Tensile properties of AMH
Compaction Graphite Copper
SD
3
MPa
w/o
w/o
g/cm
415
550
690
415
550
690
415
550
690
415
550
690
0.6
0.6
0.6
0.6
0.6
0.6
0.8
0.8
0.8
0.8
0.8
0.8
2
2
2
2
2
2
6.72
6.99
7.09
6.71
6.96
7.05
6.71
6.96
7.08
6.69
6.93
7.06
0.2% YS
UTS
Elongation Hardness
MPa
MPa
%
HRB
177
206
219
307
340
367
197
230
246
362
411
441
247
295
313
402
478
509
296
347
377
459
546
581
3.6
4.0
4.1
2.1
3.0
2.8
2.7
2.6
2.8
1.7
2.2
2.2
27
43
52
63
72
77
45
59
63
74
81
84
CONCLUSION
An atomized iron powder, Ancorsteel AMH, with characteristics approaching that of
traditional direct reduced iron sponge has been successfully developed. This atomized
“sponge like” powder with low apparent density and higher compressibility compared with
sponge makes it attractive for many applications. The micro-cleanliness has been shown to
be higher in AMH than in sponge as a result of the difference in processing methods
employed for each. The green strength, while lower than traditional sponge, is equivalent to
rolled sponge and can be further tailored to meet specific needs. The sintered dimensional
change, while generally exhibiting more positive growth over sponge, can be modified to
demonstrate similar growth as sponge thus making it possible to prevent re-tooling. As a
result of increased compressibility, comparable and even higher sintered strengths can be
attained without having to increase compaction tonnage. The benefits of AMH provide a
promising substitute to sponge in applications demanding similar powder qualities with
improved performance.
ACKNOWLEDGEMENTS
The authors wish to thank Cynthia VanDuser and Thomas Murphy for their contributions in
collecting the data presented in the manuscript.
REFERENCES
1. F. Hanejko and H. Rutz, “A New Atomized Low Apparent Density “Sponge Like” Iron
Powder”, in EURO PM2009 Proceedings of the International Powder Metallurgy
Congress & Exhibition, Vol. 2, Copenhagen, Denmark, 12-14 October 2009, EPMA, pp.
163-168.
2. MPIF Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2010.
3. B. Lindsley, T. Murphy, “Dimensional Control in Cu-Ni Containing Ferrous PM Alloys”,
Advances in Powder Metallurgy & Particulate Materials, compiled by W. Gasbarre, and
J. von Arx, MPIF, Princeton, NJ, 2006, part 10, p. 140-152.