Single Press / Single Sinter Solutions to High Density
Francis Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077
USA
Abstract:
Powder metallurgy main asset is its ability to produce structural components that meet
functional requirements in a cost efficient manner. Many PM parts are produced at
densities lower than 7.1 g/cm³ because achieving higher densities often require
additional processing steps, which result in increased part cost. Although warm powder
warm die processing is not new, new methods that employ die heating only have been
introduced that enable high green density via single press / single sinter. This simplified
processing will potentially lead to greater market acceptance of high density PM parts.
However, drawbacks of die heat only are part size restrictions and higher compaction
loads. Both processes are useful in the production of high density PM components.
This paper will detail the advantages and disadvantages of both processes. Practical
part production will be discussed with mechanical properties achievable via high density
processing.
Introduction:
The PM community must continue to advance its parts making potential, mechanical
performance of it components, and dimensional precision so as to continue to improve
and solidify its competitive position within the various parts making disciplines. Longestablished lower density / lower performance parts potentially will be lost to competitive
processes or to emerging countries with lower labor costs. Higher densities in
conjunction with more exacting tolerances and higher mechanical properties are key
strategies toward improving PM’s competitiveness.
Methods to increase density via single press single sinter have followed two paths; one
utilizes heated powder and heated dies, so called warm compaction. This process uses
tooling and powder temperatures of ~130 °C to ~150 °C. [1] Incorporating both powder
and die heating results in reduced compaction pressures to achieve higher densities.
Parts produced via this technique including turbine hubs for automotive automatic
transmissions, gearing for industrial power tools, engine timing sprockets, etc. [2] Size
restrictions on parts were limited to press capacity coupled with an adequate supply of
heated powder through the powder heating and delivery system. The disadvantage of
this processing was the tight temperature control (+ / - 5 °C) of powder temperature
required to achieve steady state production. If this condition was not satisfied then
excessive green scrap could result, thus increasing overall part cost.
A second strategy for increasing green density utilized die heating without any powder
preheating. This process referred to as warm die compaction has advantages of
reduced capital cost, no restriction on through put of a powder delivery system, plus
simplified press set up and part production. Die temperatures of 50 °C to 100 °C are
most common. [3] Successful applications of this processing system include: speed
sensors, gears for industrial power tools, recreational vehicle applications, etc.
Development of proprietary lubricants and premixing technologies enabled reduced
amounts of powder lubricant thus facilitating higher green densities with reduced
lubricant burn-off. A key restriction of this process is a size limitation of parts that can be
made: components weighing in excess of 0.75 kilogram with wall thickness greater than
20 mm have proven difficult. Components of this size do not achieve uniform powder
temperature within the die during compaction, resulting in increased compaction
pressure to achieve the specified part density.
Effects of Die Compaction Temperature on Final Part Density
Figure 1 presents the effects of die temperature on the green density attained at various
compaction pressures. The material evaluated in Figure 1 was a 0.3% molybdenum
prealloyed steel powder with 0.4% added graphite and 0.4% proprietary warm die
compaction lubricant, this binder treated material had a pore free density of ~7.58 g/cm³.
Green density increased with increasing die temperature, the amount of increase in
density was approximately 0.08 / 0.10 g/cm³ for each 60 °C increase in die temperature.
Although the data shown in Figure 1 was generated using a laboratory hydraulic
compaction press, comparable results were achieved in production compaction
equipment at pressing rates up to 12 parts per minute. One concern with larger parts is
the need to de-aerate the powder mass during compaction. Assuming an apparent
density of powder of ~3.2 g/cm³ and a final part density of 7.2 g/cm³, the part will have
approximately 125% of its final compacted volume of air that needs to escape during
compaction. Unless consideration is given to this effect green cracking may occur.
Introducing a cycle delay during compaction and / or incorporating top punch hold down
after compaction facilitates removal of the entrapped air.
Figure 1:
Effects of die preheat temperature on part density
Coincident with the increase in green density, increasing die temperature also increase
the green strength of the part as shown in Figure 2. This increase in green strength
results from the greater deformation of the powder particles and viscoplastic flow of the
lubricant. An additional benefit to reduced lubricant content is also the reduced green
expansion after compaction. The resulting higher green strength and lower green
expansion proposes a potential for reduced green part damage and corresponding
reduced rejection rates.
Figure 2:
Effect of compaction pressure on green strength
The trend in Figure 1 verified that increasing die temperature (hence part temperature)
increased green density at any given compaction pressure. A logical extension of this
data is to evaluate the effects of still higher compaction temperatures on green density.
The development of advanced lubricants enabled compaction die temperatures up to
175 °C (at this die it recommended that powder preheating be utilized). Shown in
Figure 3 is the effect of various die temperatures (and powder preheat as noted) on the
green density of an MPIF FLN2-4400 material with 0.35% graphite. Compaction
conditions are detailed in Table 1.
Differences in final green densities at either 700 or 830 MPa are explained by the
differences in pore free density for each of the materials, Table 1. The standard premix
material has the lowest overall compressibility. Increasing both the powder temperature
and die temperature improves the green density at any compaction pressure.
Implications of the data shown in Figure 3 are as follows:
 Higher densities can be achieved at lower compaction tonnage with increasing
die temperature and / or powder temperature.
 Increasing powder or die temperature will lower the required compaction
pressure to achieve a specified density.
 Larger parts benefit from powder preheating because of reduced temperature
gradients within the part.
The cross over of the warm die material vs. the warm compaction 1 material at ~7.3
g/cm³ green density is a result of differing lubricant amounts within the two materials.
The data shown in Figure 3 indicates that a green density of 7.45 g/cm³ may represent
the practical upper limit for an engineered PM premix utilizing internal lubricant.
Development of new lubricants that can operate at reduced levels and / or advances in
die wall lubrication are required for further increases in green density.
Figure 3:
Effect of compaction conditions on green density of an MPIF FLN2-4405
Table 1
Compaction Conditions for Data Shown in Figure 3
Material
Std Premix
AncorMax 200™
ANCORDENSE™
ANCORDENSE
450™
Lubricant
type
Acrawax
Proprietary
warm die
Proprietary
warm powder
Proprietary
warm powder
Lubricant
Amount,
%
0.75
0.40
Die
Temp.
°C
25
93
Powder
Temp.
°C
NA
NA
Pore Free
Density,
g/cm³
7.41
7.58
0.60
140
130
7.48
0.40
175
160
7.58
Mechanical properties of a PM material (FLN2-4405) compacted and sintered to high
density were compared to wrought AISI 8620 steel (0.8% Mn, 0.25% Si, 0.55% Ni, 0.5%
Cr, 0.2% Mo, 0.2% C). Table 2 presents the mechanical property comparison. Both
materials were austenitized at 925 °C (1700 °F) for 1 hour in a 75 % hydrogen / 25%
nitrogen atmosphere, oil quenched in 70 °C oil, and subsequently tempered at 205 °C.
As expected, the wrought steel has higher tensile elongation and impact energy
compared with the PM steel. However, the wrought steel shows a marked directionally
of properties from the longitudinal direction (direction of primary working) to the direction
perpendicular to the primary working direction (transverse). For the wrought steel, the
notched impact energy is reduced from 37 Joules to about 14 joules. Additionally, the
50% fatigue endurance limit (as measured by rotating bending fatigue) is reduced from
490 MPa to 370 MPa. The PM steel has comparable yield and tensile strengths to
wrought steel. Comparing the fatigue response, the PM material has equivalent fatigue
properties to the properties in the transverse direction of the wrought steel. PM has little
if any directionality of mechanical properties. Rolling contact fatigue of PM components
can be enhanced via surface densification to levels equal to wrought steel. Thus
achieving a minimum density of 7.4 g/cm³ is key to obtaining this parity in both tensile
strength and fatigue properties. [3,4]
Table 2
Mechanical Properties of PM vs. AISI 8620 Steel
Property
Yield St. MPa (103 psi)
Yield St. MPa (103 psi)
Tensile Elongation, %
Hardness, HRA
Notched Impact Energy,
Joules (ft.lbf)
Un-notched Impact Energy,
Joules (ft.lbf)
50% Fatigue Endurance
Limit, MPa (103 psi)
AISI 8620 Steel
Longitudinal
Transverse
1075 (156)
1070 (155)
1355 (197)
1330 (193)
8.0
6.1
71
70
PM at 7.4
g/cm³
1240 (180)
1445 (210)
1.0
81
37 (27)
14 (10)
NA
312 (230)
308(227)
18 (13)
490 (71)
370 (54)
405 (59)
Effects of Binder Treatment on Dimensional Stability of Gears
A study was conducted comparing an MPIF FC-0208 with a prealloyed 0.3%
molybdenum steel with 0.8% added graphite. Two types of premixes were prepared for
both materials, one was a standard premix with 0.75% acrawax and the second was a
warm die compaction grade with 0.4% lubricant. Compressibility information of the four
materials is present in Figure 4. As was shown earlier in Figure 1, increasing the die
temperature from ambient conditions to 93 °C resulted in a ~0.08 to 0.10 g/cm³ increase
in green density.
Prototype gears were produced using the four materials shown in Figure 4. The gears
were compacted on a Dorst 140 metric ton mechanical press at a rate of 10 parts per
minute. The gears were sintered in a continuous belt sintering furnace at 1120 °C (2050
°F) in a 90% nitrogen / 10% hydrogen atmosphere with a time at temperature of ~20
minutes. After sintering approximately 50 gears from each material grouping were heattreated by quenching and tempering utilizing a commercial heat-treat cycle for FC0208.
After quenching, the gears were then tempered at 205 ° C (400 °F). The gears were
checked for dimensional variation using 3.162 mm (0.1245-inch) diameter pins.
Measurements were taken at four locations around the gear starting with the front of the
gear (as compacted) and then measuring three additional locations around the outside
diameter. The results of the measurement over wires (MOW) are presented in Table 3.
Figure 4:
Compressibility information of MPIF FC-0208 and a prealloyed 0.30%
molybdenum steel with 0.8% graphite.
Table 3
Dimensional Variations of Heat Treated Gears
Material
FC-0208 (regular premix)
0.3% Mo with 0.8% gr
(regular premix)
FC-0208 (bonded premix
0.3% Mo with 0.8% gr
(bonded premix)
Av. MOW
Inches (mm)
1.2096 (30.72)
Max – Min
Inches (mm)
0.0043 (0.11)
St Dev MOW
Inches (mm)
0.0013 (0.03)
1.2090 (30.71)
0.0015 (0.04)
0.0003 (0.01)
1.2099 (30.73)
0.0036 (0.09)
0.0008 (0.02)
1.2084 (30.69)
0.0014 (0.04)
0.0003 (0.01)
This data demonstrates the reduced scatter realized with the prealloyed 0.3%
molybdenum steel relative to a standard FC-0208 material. However, the data also
shows that a bonded premix reduced the scatter in MOW for the FC-0208 material by
~30%. Thus, the binder treatment results in reduced part-to-part scatter even for the
inherently variable copper-containing steel. No measurable difference in dimensional
variation was observed for the binder treated 0.3% molybdenum steel.
Conclusions:
1. The use of proprietary lubricants enabled warm die compaction with die
temperatures up to 110°C. In addition to increased green density, the green
strength also increases with increasing part / die temperature.
2. Warm die compaction is not suitable for all parts, parts possessing weights in
excess of 0.75 kilograms or having wall thickness greater than 20 mm thwart
uniform powder temperatures in the die increasing the compaction pressure or
lowering the green density.
3. For larger parts, heating of both powder and die are recommended. Recent
lubricant developments enable die temperatures up to 175 °C. This gives higher
green density and more uniform part densities.
4. Using a “binder treated” powder showed ~ a 30% reduced variability in
measurement over wires for heat treated FC-0208. Although not seen with the
0.30% molybdenum prealloyed steel, similar trends are expected for other
materials.
5. Mechanical properties of high density PM materials have equivalent yield and
tensile strengths to wrought steels. However, the PM steel has reduced impact
and tensile ductility. The PM steel has equivalent fatigue to the wrought steel in
the transverse direction.
References:
1. H. G. Rutz, F. G. Hanejko, “High Density Processing of High Performance
Ferrous Materials”, Advances in Powder Metallurgy and Particulate Materials –
1994, Vol. 5, Metal Powders Industries Federation, 1994, pp 117-133.
2. T. Miller, F. Hanejko, “Development of a Warm Compacted Automatic
Transmission Torque Converter Hub”, Paper 970428, Society of Automotive
Engineers.
3. G. Poszmik, S. Luk, “Binder Treated Products for Higher Density and Better
Precision”, Advances in Powder Metallurgy and Particulate Materials – 2003,
Metal Powder Industries Federation, 1993, p. 3-33 to 3-44.
4. MPIF, Std 35 2007 edition, Metal Powder Industries Federation, p. 63.
5. G. Hoffmann, C. Landgraf, J. Mandel, “Effect of Pore and Porosity on Rolling
Contact Fatigue of Sinter Hardened P/M Steel”, Advances in Powder Metallurgy
and Particulate Materials-2003, Part 7, pp. 7-229 – 7-313, Metal Powders
Industry Federation, Princeton, NJ, 2003.