PROPERTIES OF A NEWLY DEVELOPED LOW APPARENT DENSITY ATOMIZED
POWDER
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 have 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

Ancorsteel is a registered trademark of Hoeganaes Corporation
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. Excessive handling of premixes can shift the AD higher, as is common for
most powders. The powder flow characteristics are similar among the powders tested, suggesting
comparable during die fill behavior.
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.
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, refer back to Table 1, capable of
mechanically locking with another particle surface during compaction; this promotes a higher green
strength in the part.
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 interparticle 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.
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
Another distinction can be made about the 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 within a given field. Figure 2 illustrates the amount of non-metallic inclusions
inherent with commercially supplied sponge iron. The dark areas are non-metallic particles and the light
gray area is fully dense sponge with 0.6 w/o graphite, unetched. Owing to the source of raw material and
nature of the processing route, sponge typically contains a generous amount of non-metallic inclusions
such as silicon or titanium oxides that are difficult to remove. These inclusions, some quite large, can
greatly impact (negatively) 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.
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.
Consequently, AMH shows to be nearly 100 times cleaner than 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 high temperature Abbott 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
Compressibility data for an FC-0208 version of the iron-based powders is shown in Figure 4. From these
trends, it is clear AMH has higher compressibility relative to sponge and rolled sponge. Of note, in order
for sponge to achieve a 6.8 g/cm3 green density, it requires over 100 MPa of applied pressure more than
AMH. This suggests that parts can be compacted at lower pressures on potentially smaller compaction
presses when using AMH in place of sponge. This higher compressibility can be attributed to a reduced
amount of internal porosity (which requires high pressures to close) and larger grain size upon annealing.
Figure 5 displays the green strength versus green density of three iron powders in an FC-0208 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: Compressibility of AMH, sponge, and
rolled sponge in an FC-0208 premix with 0.75 w/o
Acrawax C lubricant
Figure 5: 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 6 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. The sintered dimensional change versus green density for premixes F-0008 and FC-0208
based on AMH and sponge is given in Figure 7. The DC values for AMH exhibit more positive sintered
dimensional change over sponge for a given green density.
Figure 6: Sintered TR strength of AMH and sponge in a F-0008 and FC-0208 compositions
Figure 7: Sintered DC vs. Green Density in F-0008 and FC-0208 compositions
As shown in Figure 8, the sintered dimensional change for AMH is given versus graphite addition as a
function of admixed copper. When there is no copper present in the mix, adding graphite contributes to
increased expansion in 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]. These trends indicate that the dimensional change of parts can be tailored to meet
specifications.
Figure 8: 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
Composition
F-0005
FC-0205
F-0008
FC-0208
Compaction
SD
0.2% YS
UTS
Elongation Hardness
MPa
g/cm3
MPa
MPa
%
HRB
415
550
690
415
550
690
415
550
690
415
550
690
6.72
6.99
7.09
6.71
6.96
7.05
6.71
6.96
7.08
6.69
6.93
7.06
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.