A New Atomized Low Apparent Density “Sponge Like” Iron Powder
Francis Hanejko & Howard Rutz
Hoeganaes Corporation
Cinnaminson, NJ 08077
USA
Abstract:
The development of an atomized “sponge like” iron powder has been undertaken. The
goal of this program was to provide a cost effective atomized powder with high green
strength and similar apparent density to existing sponge iron grades. Additional design
criteria for the new powder were to match the dimensional change and strength found in
sponge powders. New atomizing techniques were utilized to provide a highly irregular
powder surface that allows the various design requirements to be met. The successful
results of this program will be documented.
Introduction:
The sponge iron powder process or more precisely the solid-state reduction was the
earliest technique to produce metallic iron. Dating back to 6000 BC, this methodology
was used primarily to produce weapons. [1] Sponge iron powder was the first
commercially mass produced iron powder for PM parts and was successfully used in
applications such as self-lubricating bearings, shock absorber pistons and guide rods,
friction materials, etc. Alternative uses of iron powder relied on its high specific surface
area; these applications included catalysts and hand warmers for prolonged low
temperature heating. The process can be summarized as the mining of iron ore,
grinding and beneficiating the ore to the proper purity and particle size, reduction via a
carbon source at temperatures below the melting point of iron, grinding the
agglomerated sticks, followed by annealing and shipment to the customer. Figure 1
presents a schematic illustration of the current sponge process. Typical time spent in
the kiln was on the order of 60 / 70 hours with a maximum temperature of ~1200 °C.
Despite the advantages of sponge iron (predominantly green strength at low part
densities), disadvantages of the process are as follows:
 Need for high purity iron ore with minimal impurities. Magnetite ore (Fe3O4) is the
most common raw material. Impurities such as silicon or titanium oxides
negatively affect the overall quality of the finished sponge iron for PM
applications.
 Relatively long processing time to produce the material. The average time in the
kiln approaches three days coupled with necessary preparatory and post
reduction processing. Internal studies conducted at Hoeganaes Corporation
showed an ~30% energy savings in the manufacture of atomized powders
compared with the traditional sponge powders. [2]
 An inability to achieve high density because of the porous nature of the powder
produced coupled with low apparent density that results in a greater die fill
height.
Figure 1:
Schematic of Sponge Iron Process
To offset these disadvantages, Hoeganaes Corporation initiated a laboratory /
production research program targeting the production of a “sponge-like” powder via the
melting and atomization process. Selected ferrous scrap is melted in an electric arc
furnace, in less than 90 minutes a liquid bath is created that is subsequently water
atomized into powder. Electric arc melting of iron enables refining of the liquid metal,
producing powder with reduced amounts of residual oxides. [3] Following atomization,
dewatering and drying, the powder is annealed, packaged and shipped; often in less
than 3 total days from the start of the process.
Key considerations in producing a “sponge-like” powder via atomization are the
atomization parameters and subsequent particle size of the as-atomized powder. Exact
processing details are proprietary; however, a finer as-atomized particle size combined
with agglomeration during annealing is a requirement to produce an atomized material
with reduced apparent density and improved green strength. Shown in Figure 2 are
representative SEM photomicrographs of traditional sponge iron powders, atomized
powders, and lastly atomized “sponge like” iron powders. Note the similar appearance
of the traditional sponge iron powder and the atomized sponge powder. Both possess
internal powder porosity and surface irregularity greater than conventionally water
atomized powder.
A: Traditional sponge
powder from iron ore
Figure 2:
B: Atomized iron powder
C: Atomized sponge iron
powder
SEM photomicrographs of traditional sponge iron, atomized material, and
atomized sponge powder
Key powder characteristics of traditional sponge iron powders and atomized low
apparent density powder are presented in Table 1. Note the lower hydrogen loss and
silicon content of the atomized powder. Apparent density and Hall flow rates of the two
materials are similar. The manganese content in the traditional sponge iron powders is
predominantly in the form of manganese oxide. Whereas, the manganese in the
atomized material acts as a beneficial alloying element.
Table 1
Powder Characteristics of Traditional Sponge vs. Atomized Low AD
Property
Apparent Density,
g/cm³
Flow, s/50 g
H2 Loss
Silicon Content, %
Sulfur, %
Manganese, %
Traditional Sponge
Iron
2.50
Rolled Sponge
Iron
2.60
Atomized “SpongeLike” Powder
2.60
30
0.26
0.20
0.01
0.10
30
0.26
0.20
0.01
0.10
28
0.15
0.02
0.015
0.15
Review of Selected Premixes Using Traditional Sponge vs. Atomized Low
Apparent Density Iron Powder
Six laboratory premixes were prepared using the newly developed atomized “sponge
like” iron powder and two traditional sponge iron powders. The premix compositions are
listed in Table 2. From these six premixes compressibility information, green strength,
sintered strength (via transverse rupture strength), and sintered hardness data were
developed. Compaction was performed without die heating and sintering was done at
1120 °C in a belt furnace for 20 minutes at temperature using a 90% nitrogen / 10%
hydrogen atmosphere with a cooling rate of ~0.6 °C per minute.
Table 2
Premix Compositions of Mixes Evaluated
All with 0.75% Acrawax
Mix ID
1
2
3
4
5
6
Iron Powder
Atomized low AD
powder
Traditional Sponge
Rolled Sponge
Atomized low AD
powder
Traditional Sponge
Rolled Sponge
% Copper
1.5
% Graphite
0.7
1.5
1.5
1.5
0.7
0.7
0.3
1.5
1.5
0.3
0.3
Compressibility data for the 0.7% graphite containing premixes are shown in Figure 3
(compressibility data developed for the 0.3% graphite containing premixes demonstrated
a trend similar to the data shown in Figure 3). The atomized “sponge like” iron material
has higher compressibility relative to the two traditional sponge iron materials. This
higher compressibility results from the liquid metal refining minimizing impurities thus
producing a more compressible material. Significance of this data is that parts can be
compacted at lower compacting pressures on potentially smaller compaction presses (at
potentially faster press rates).
Figure 3:
Compressibility Data of 0.7% Graphite Containing Premixes
Shown in Figure 4 is the green strength comparison of the three premixes with 0.7%
added graphite. The traditional sponge material does exhibit higher green strength
relative to the atomized grade. Alternatives to increasing the green strength of the
atomized material include the use of alternative lubricants. Laboratory testing of an irongraphite-lubricant premix at 6.1 g/cm³ density using both the atomized low AD material
and traditional sponge showed green strengths of 12.0 MPa and 12.8 MPa, respectively.
The atomized sponge used a proprietary high green strength lubricant and the traditional
sponge utilized Kenolube as the powder lubricant. Apparent density and powder flow
rates were unaffected and the advantage of a lower compaction pressure was
maintained. Thus through innovative lubricants, potential issues relating to lower green
strength can be overcome.
Figure 4:
Green Strength of 0.7% Graphite Containing Premixes
A: DC vs. Green Density for 0.7% added
graphite containing premixes
Figure 5:
B: DC vs. Green Density for 0.3% graphite
containing premixes
Plots of DC. Vs. Green Density for Two Graphite Levels
Shown in Figure 5 are the sintered dimensional change data developed for the six
premixes. For higher carbon materials (0.7% added graphite), the atomized material
has identical dimensional change to the two traditional sponge grades evaluated. At
lower added graphite contents, the atomized grade exhibits more positive sintered
dimensional change. Additions of a prealloyed carbon steel powder to the atomized
sponge lowers the dimensional change to levels comparable with traditional sponge with
minimal loss in compressibility and no increase in apparent density. [4]
Figure 6 presents the sintered TRS of the six premixes evaluated. Figure 6A shows the
TRS for premixes containing 0.7% added graphite and Figure 6B shows the TRS for
premixes containing 0.3% added graphite. For both graphite conditions evaluated, the
atomized material has equivalent sintered strength at equivalent densities compared
with the traditional sponge products. Although not shown, sintered hardness values for
the atomized sponge iron are equivalent to the traditional sponge materials.
A: Sintered TRS for 0.7% added graphite
containing premixes
Figure 6:
B: Sintered TRS for 0.3% graphite
containing premixes
Sintered TRS of the six materials evaluated
Conclusions:
An atomized “sponge like” powder has been developed possessing low apparent density
and higher compressibility compared with traditional sponge materials. Sintered TR
strength of the atomized low apparent density material is equivalent to the sintered TRS
of traditional sponge materials. Sintered dimensional change at higher graphite
additions (>0.5% graphite) for the atomized “sponge like” powder is equivalent to the
direct reduced iron ore. At lower graphite additions (~0.3% graphite), the atomized
“sponge like” exhibits more positive sintered dimensional change. A potential
disadvantage of these grades is the reduced green strength; however, utilizing nontraditional powder lubricants overcome this shortcoming and produce green strengths
equivalent to traditional sponge materials. These materials have successfully replaced
direct reduced iron in many applications.
References:
1. Iron Powder Metallurgy Handbook, published by Hoganas-Billesholms AB, Hoganas,
Sweden, Vol. 1, 1957.
2. J. Wilson, Internal Hoeganaes Corporation report
3. ASM Handbook, Volume 7, published by ASM International, Metals Park Ohio, 1998,
p.36.
4. Hoeganaes Corporation Technical Literature at www.hoeganaes.com, product,
unalloyed steel.