technical trends
A new PM route
for machinable,
malleable iron
Research disclosed at the recent PowderMet 2012 show in
Nashville provides evidence that PM can indeed hold its
own against malleable iron, ductile iron, and compactedgraphite iron castings. Joe Capus, Metal Powder Report
consultant editor, reports.
T
he search for full density in
PM processing to replace
machined wrought steel
has engaged R & D group
efforts in metal powder companies for
a number of years. In most instances,
the achievement of higher density has
involved more complex and/or multiplestep processing, resulting in increased
cost while the maximum sintered density is currently about 7.45 g/cm³. At
the same time, there is a considerable
market, developed over many years, for
which moderate strength, fully dense
parts can be produced by malleable
iron/ductile iron castings. This is a market sector where PM has found it hard
to compete because of cost – until now.
Enter Rio Tinto Metal Powders
(QMP), which has developed a new
ferrous powder that is able to compete with malleable iron, ductile iron,
and compacted-graphite iron castings.
François Chagnon and colleagues
presented two papers at PowderMet
2012 in Nashville, Tennessee, USA, giving details of the new malleable iron
powder grade (MIP)*. The basis for
this development is the use of supersolidus liquid-phase sintering as applied
to the iron-graphite-silicon alloy system. The author noted: “Super-solidus
liquid-phase sintering is another way
to achieve high density. The process
requires pre-alloyed powders that –
when heated to a temperature intermediate between the solidus and liquidus
– nucleate a liquid within each particle.
The amount of liquid produced is a
function of the alloy content and the
sintering temperature. The individual
particles partially melt and promote
densification by capillary-induced
re-arrangement.”
Previous researchers have shown the
feasibility of densification of ferrous
PM materials by liquid-phase sintering, usually by sintering at higher temperatures than conventional sintering.
RTMP researchers have shown the feasibility of reaching full density through
super-solidus liquid-phase sintering at
conventional sintering temperatures by
using a new water-atomised Fe-2%C1%Si alloy powder. After a malleablising treatment, the annealed powder
particles are completely ferritic with
embedded graphite nodules, so that
there is no need for added graphite.
Carbon segregation is eliminated (see
Figures 1 and 2). Hence compressibility
and sinterability are each optimised.
The new powder requires careful
control of the sintering temperature as
well as the heating and cooling rates.
Liquid-phase sintering occurs between
Figure 1: SEM picture of MIP particles
­showing embedded graphite nodules.
(After Chagnon and Coscia.)
Figure 2: Unetched cross-section
­microstructure of MIP particles. (After
Chagnon and Coscia.)
Figure 3: Etched microstructure of sintered
MIP. (After Chagnon and Coscia.)
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MPR September/October 2012
0026-0657/12 ©2012 Elsevier Ltd. All rights reserved
Figure 4: Effect of sintering temperature on tensile properties of MIP. (After Chagnon and Coscia.)
1156°C and 1165°C, in which range
the alloy shrinks by about 2.5%, to
give a relative density of 99.0–99.7%.
Resulting microstructure is affected by
the sintering temperature and cooling
rate from 1166°C–1140°C. Sintering
at 1162°C resulted in a fine pearlitic
structure with nodular graphite particles showing some ferrite at the periphery (Figure 3). Tensile properties were
found to be surprisingly insensitive
to the sintering temperature between
1152°C and 1166°C (Figure 4). Thus,
tensile strength averaged 776 ±11 MPa,
yield strength was 523 ±20 MPa, with
elongation averaging 2.2 ±0.4%. These
values were said to be comparable with
pearlitic malleable cast iron class 800
02 and ductile iron grade 100-70-03.
TRS and hardness increased with sintering temperature, from 1500 to 1725
MPa and from 25 to 31 HRC, respectively. Dimensional change on sintering
is controlled mainly by the green density. Because of the large size change,
the authors emphasise “the need to
control the green density to minimize
the variation in the final dimensions for
MIP parts”.
In contrast to the tensile properties,
fatigue and impact values were found
to be “very sensitive to both the [part]
density and the shape of the graphite
particles.” Axial fatigue strength was
measured on machined round testpieces at 100 Hz and a load ratio of
R= -1. Results were correlated with
density in the fracture area and the
shape of the graphite particles, evaluated by image analysis. It was found
that the fatigue strength increased with
density (between 97.6 and 99.3%) if
Figure 5: Axial fatigue strength @ R= -1 for sintered MIP: Effect of density and number of
large graphite flakes (>100µm). (After Chagnon and Coscia.)
metal-powder.net
Previous researchers
have shown the
feasibility of
densification of
ferrous PM materials
by liquid-phase
sintering
the number of large (>100µ) graphite
flakes was small (about 1 per mm2 of
cross-section). However, the benefit
of increased density was wiped out if
the number of large flakes was above
4 per mm2. (Figure 5). On the other
hand, impact energy was not as sensitive to density and microstructure as
was reported for fatigue. With density
in the range of 99.2–99.4% theoretical, impact values of about 32J were
recorded.
The performance of the new MIP
powder could be significantly improved
by admixing a small fractional amount
of ferro-phosphorus. In a second
paper, presented by Maryam Moravej,
mechanical properties and machinability results were reported for a comparison of MIP pressed and sintered
with and without 0.2%P (samples
MIP-A and MIP-B, respectively). The
addition of ferro-phosphorus reduces
the temperature range for sintering,
with accompanying benefits from the
resulting structure. Sintering of these
materials each produced nearly fully
dense structures when sintered in
September/October 2012 MPR
31
Figure 6: Machinability test results: tangential force versus cutting speed for sintered MIP-A (plain MIP) and MIP-B (MIP+0.2%P)
­compared to ductile iron and powder-forged FC-0205PF+MnS at two
lead angles; semi-roughing operation. (After Moravej et al.)
the respective sintering windows of
1158°C–1166°C (for plain MIP) and
1118°C–1124°C (MIP + 0.2%P).
The sintered densities of 7.55 and
7.52 g/cm³ represent 99.0 and 99.4%
of full density because of the low
density of the graphite content. The
addition of phosphorus to the mix not
only lowered the sintering temperature
but also increased the number of graphite particles formed on cooling by a factor of almost three. Thus it significantly
increased the proportion of spherical
or nodular graphite and reduced the
number of large graphite flakes.
As summarized in Table 1, tensile, yield strength and elongation
were all higher for the MPI + 0.2%
phosphorus composition, while hardness remained the same. Axial fatigue
Figure 7: Machinability test results: feed force versus cutting speed
for sintered MIP-A (plain MIP) and MIP-B (MIP+0.2%P) compared to
ductile iron and powder-forged FC-0205PF+MnS at two lead angles;
semi-roughing operation. (After Moravej et al.)
strength was improved by around 20%
with ­phosphorus. On the other hand,
impact energy was reduced from 33J
to 22J.
The machinability of sintered MIP
materials was compared with that of
ductile iron grade DI 80-55-06 as well
as powder-forged FC-0205PF +0.3%
manganese sulphide. Face-turning tests
were made on ring-shaped samples
measuring 50.8 mm.OD, 25.4 mm.ID,
and 22.86 mm thick. Two types of
cutting operations were performed: a
semi-roughing operation at two
cutting speeds (183 and 274 m/min)
with feed rate of 0.254 mm/rev and
1.52 mm depth of cut; the second test
was a finishing operation using a cutting speed of 137 m/min, feed rate
of 0.102 mm/rev and depth of cut of
Table 1: Properties of MIP-A (plain MIP) sintered at either 1158°C or 1166°C,
and MIP-B (MIP +0.2% phosphorus) sintered at either 1118°C or 1124°C. (After
Moravej et al.)
MIP-A
MIP-B
Temperature, °C
1158
1166
1118
1124
Sintered density, g/cm³
7.53
7.56
7.51
7.53
Tensile strength, MPa
780
795
884
868
Yield strength, MPa
552
572
586
592
Elongation, %
1.82
1.78
2.6
2.5
Hardness, HRC
28
29
28
29
Axial fatigue strength,
241
279
312
333
MPa (R= -1; 50% surv.)
Temperature, °C
1162
1121
Sintered density, g/cm³
7.56
7.52
Impact energy, J
33
22
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MPR September/October 2012
1.02 mm cutting-tool lead angles of
+15° and -5° were employed. Two cutting forces were measured during the
tests: feed force and tangential forces.
Results obtained for the four materials are shown in Figures 6 and 7. With
the same roughing operation, both feed
force and tangential force values for
MIP with phosphorus (MIP-B) were
lower than for MIP alone (MIP-A)
and were comparable with the ductile
iron and powder-forged steel containing MnS. The same was more or less
found for the finishing cuts. The lower
machinability of MIP without phosphorus was attributed to its lower content
of graphite clusters.
According to Rio Tinto Metal
Powders, the MIP powder is now
in production and expected to find
applications in the replacement of castings. It will be interesting to see what
the industry makes of this intriguing
development.
*The papers, “Development and
properties of a new malleable iron powder grade,” by F.Chagnon and C.Coscia,
and “Characterization of mechanical
properties and machining performance of new malleable iron powder
grades,” by M.Moravej, F.Chagnon and
J.Campbell-Tremblay, will be published
in the proceedings of PowderMet 2012,
Advances in Powder Metallurgy &
Particulate Materials – 2012, by MPIF,
105 College Road East, Princeton, NJ
08540-6692, USA. metal-powder.net