ECONOMICS AND BENEFITS OF HIGH TEMPERATURE SINTERING OF HIGH
PERFORMANCE ALLOYS
John W. Schaberl
Hawk Precision Components Allegheny Clearfield Division, Falls Creek, PA
Michael C. Baran and Michael L. Marucci
Hoeganaes Corporation, Cinnaminson, NJ
Ronald A. Posteraro
Gasbarre Products, Inc. - Sinterite Furnace Division, St. Marys, PA
2
Presented at PM TEC 2002
International Conference on Powder Metallurgy & Particulate Materials
JUNE 16 – 21, 2002 ORLANDO, FLORIDA USA
ABSTRACT
Today’s global economy has produced an extremely competitive marketplace. Design
engineers constantly compare the economics and benefits of powder metallurgy (P/M) with
those of stamping, casting and wrought machining. While parts manufacturers have
traditionally exploited the near net shape cost savings of conventional P/M compositions, many
have turned to higher performance alloys and elevated temperature sintering in an effort to
optimize the metallurgical and mechanical properties of their products. Powder producers and
furnace manufacturers have continued to support these endeavors through new product and
process development.
This study will examine the metallurgical and mechanical enhancements achieved through the
combination of high performance alloy systems and high temperature sintering. In addition, it
will show how these benefits can be exploited to produce superior parts economically. High
o
o
performance silicon-containing P/M steels sintered at 1150 to 1343 C (2100 to 2450 F) will be
used to explore these property enhancements. These silicon-containing P/M steels yield
excellent properties when sintered above 1260 °C (2300 °F) and are capable of attaining high
apparent hardnesses .
INTRODUCTION
High temperature sintering (HTS) is certainly not a new concept. The idea of sintering at
temperatures above 1150 °C (2100 °F) has been common in industry for some time. For
instance, the typical metal injection molding (MIM) process employs sintering temperatures of
greater than 1250 °C (2280 °F) for ferrous materials [1]. By properly implementing these higher
sintering temperatures, the P/M parts manufacturer can realize measurable benefits over their
“conventional” process [2]. However, there are several obstacles to the realization of these
process improvements. The advantages of this technology as well as key parts manufacturer
concerns are shown in Table I.
1
TABLE I: Advantages and Concerns Associated with Choosing High Temperature Sintering
over Conventional Sintering Temperatures for the Production of P/M Parts
ADVANTAGES
KEY CONCERNS
•
•
•
•
•
•
Increased homogeneity
Elimination of some reducible oxides
Increased diffusion rates
Ability to properly process new high
performance materials / stainless
More pore rounding
Larger mean pore spacing
•
•
•
•
Equipment / consumable expenditures
Higher energy requirements
Furnace maintenance costs
Lower throughput*
•
•
•
Part distortion / concentricity issues
Slower cooling rates
Other processing issues
CAN TRANSLATE TO INCREASE IN:
•
•
•
•
•
•
Density
Mechanical properties
Axial / rotating bending fatigue strength
Rolling contact fatigue performance
Corrosion resistance
Physical properties
HIGHER PERFORMANCE!
*depends heavily on type of furnace chosen
It is illogical to expect a parts manufacturer to “turn up the sintering temperature” without
incurring some additional costs. Thus, prospective applications are carefully screened for
processing at higher sintering temperatures. Invariably, those selected for HTS fall into one or
more of the following categories (with examples of each):
•
Material being used requires HTS
• New silicon-containing materials
• Higher performance stainless steel
•
HTS is most efficient / only way to achieve required properties
• Meet specifications for an previously impossible application by adding HTS to a
double press / double sinter operation that also employs a heat treating step
•
HTS reduces processing steps / other equipment requirements
• Change from double press to single press operation
•
More efficient use of prealloyed / admixed alloying additions
• Reduce some oxides so that elements can fully contribute to hardenability and
increasing mechanical properties
• More homogenization of admixed additions such as nickel, ferro-manganese, ferrochromium, ferro-vanadium, ferro-boron, and copper
2
Nearly all P/M parts manufacturers currently have the ability to produce simple, “low tech”,
and/or low density parts. The competition for these parts is enormous, especially from overseas
markets. As higher densities and more complex part geometries become the norm, several key
technologies will help the P/M parts manufacturer thrive. Two of these are high temperature
sintering and lubricant/binder technology. In fact, these technologies are intimately connected.
Segregation within a premix has been identified as a major cause of variation in the HTS of P/M
parts [2]. Bonding can reduce or eliminate such segregation and allow for more consistent
sintering behavior [3,4]. Additionally, warm compaction and some “room temperature”
binder/lubricant systems have been shown to reduce green density variations throughout a part,
resulting in a more consistent density distribution that minimizes part distortion and concentricity
issues [2,5].
BACKGROUND
The use of silicon in P/M materials can be highly beneficial. In addition to its relatively low cost
and tendency to increase hardenability, it can accelerate sintering in hybrid alloys and
substantially impact mechanical properties [6]. However, despite the inherent metallurgical
advantages of silicon, its use in P/M has been largely unsuccessful in the past. The difficulties
associated with conventional methods of silicon addition are numerous. For example, the
addition of ferro-silicon almost invariably leads to a high degree of shrinkage and distortion in a
P/M compact. Oxidation of silicon during sintering frequently results in an undesirable decline
in mechanical properties. Similar problems are experienced in prealloyed powders. Silicon
oxide pickup during atomization and/or annealing leads to lower compressibility and diminished
mechanical performance.
Through advancements in atomizing, annealing, and binder-treatment technology, new siliconbearing materials have been developed to provide exceptional combinations of properties
without the use of copper. Additionally, production experience has indicated that these
materials exhibit higher mechanical properties than diffusion-alloyed materials without the
production costs associated with diffusion alloying.
EXPERIMENTAL PROCEDURE
Two press ready, binder-treated materials, Ancorloy MDB and Ancorloy MDC (hereafter
referred to as MDB and MDC), were tested in a production environment in order to evaluate
mechanical properties and economics achieved through a single press / single sinter process.
The chemical compositions for these materials are listed in Table II.
TABLE II: Chemical Compositions, Apparent Density, and Hall Flow for Material Grades Tested
Grade
AD
3
(g/cm )
Flow
(s/50g)
Fe
(w/o)
Si
(w/o)
Cr
(w/o)
Mn
(w/o)
Ni
(w/o)
Mo
(w/o)
Gr
(w/o)
Ancorloy MDB
Ancorloy MDC
3.19
3.25
28
28
Bal.
Bal.
0.70
0.70
0.03
0.03
0.13
0.13
2.0
4.0
0.85
0.85
0.60
0.60
All test pieces were sintered at Hawk Precision Components – Falls Creek Plant in a 12” C.I.
Hayes / Sinterite pusher furnace. The sintering / tempering cycles used for specimens are
3
shown below. Additionally, the actual zone temperatures setting, gas flow rates, and measured
dew points are listed in Table III.
Various (from 1150 – 1343 °C or 2100 - 2450 °F)
Various (from 100 v/o H2 to 85 v/o N2 / 15 v/o H2)
33 minutes
Standard Water-Jacketed Section
205 °C (400 °F) for 1 hour in Air
Sintering Temperature:
Atmosphere:
Time in Hot Zone:
Cooling:
Tempering:
TABLE III: Pusher Furnace Temperature Settings, Gas Flow Rates, and Measured Dew Points
Test #
Atmosphere
Test Temp
Speed (IPM)
PreHeat N2
PreHeat H2
Shell N2
Shell H2
High Heat N2
High Heat H2
Coolers N2
Coolers H2
Curtain N2
Zone 1 Temp
Zone 2 Temp
Zone 3 Temp
Zone 4 Temp
Zone 5 Temp
Zone 6 Temp
Zone 5 Dew Point
1
100H2
2300
4.0
300
0
0
200
0
450
0
150
300
1400
1600
200
2300
2300
2300
-52.5
2
3
4
5
25N2/75H2 85N2/15H2 85N2/15H2 25N2/75H2
2300
2300
2450
2450
4.0
4.0
4.0
4.0
300
300
300
300
0
0
0
0
50
165
165
50
150
35
40
150
100
330
340
100
300
70
10
300
35
115
115
35
110
35
35
110
300
300
290
300
1400
1400
1400
1400
1600
1600
1600
1600
200
2000
2000
2000
2300
2300
2450
2450
2300
2300
2450
2450
2300
2300
2450
2450
-57.4
-59.6
-71.1
-53.3
6
7
100H2
2450
4.0
300
0
0
200
0
450
0
150
300
1400
1600
2000
2450
2450
2450
-48.5
100H2
2100
4.0
300
0
0
200
0
450
0
150
300
1400
1600
2000
2100
2100
2100
N/A
8
9
25N2/75H2 85N2/15H2
2100
2100
4.0
4.0
300
300
0
0
50
165
140
40
100
340
290
70
35
115
100
35
300
300
1400
1400
1600
1600
2000
2000
2100
2100
2100
2100
2100
2100
-74.5
N/A
Apparent hardness measurements were performed on the surface of the specimens using a
Rockwell hardness tester. All measurements were conducted on the Rockwell C scale (HRC).
Transverse rupture strength was measured according to ASTM B 528. Impact energies were
determined according to ASTM E 23. Tensile testing was performed on a 267 kN Tinius Olsen
universal testing machine at a crosshead speed of 0.635 millimeters/minute. Elongation values
were determined by utilizing an extensometer with a range of 0 to 20%. The extensometer was
left on until failure. All specimens were tested in the as-tempered condition. Dimensional
change was determined per ASTM B 610. Tensile tests were conducted using standard dogbone tensile specimens.
RESULTS AND DISCUSSIONS
General Points of Interest
3
• The green density of specimens was relatively consistent at 6.93 – 6.94 g/cm for
3
compaction at 550 MPa (40 tsi) and 7.00 g/cm at 620 MPa (45 tsi). Hence, green
density is not included in the presented data.
•
Some incipient melting was noted on specimens sintered at 2450 °F. For this reason,
this data was used sparingly and should be viewed as suspect.
Tensile Properties
It is often the combination of ultimate strength, yield strength, ductility, impact energy, and
hardness that differentiate good materials from workhorses in the P/M industry. However, the
4
strength and ductility of a material are often the initial considerations during the design process.
As can be seen in Tables IV and V, both materials show promise in these categories.
TABLE IV: Tensile Properties and Sintered Densities for Dog-bone Tensile Specimens Sintered
at 1150 °C (2100 °F) in Different Sintering Atmospheres
Comp
Sintered
UTS
Yield
Atmosphere
Elongation
Material
Pressure
Density
(MPa /
(MPa /
(v/o)
(%)
3
3
3
(MPa / tsi)
(g/cm )
10 psi)
10 psi)
550 / 40
6.97
627 / 91
455 / 66
1.7
100 H2
620 / 45
7.04
655 / 95
496 / 72
1.8
550 / 40
6.95
655 / 95
483 / 70
1.8
MDB
75 H2 / 25 N2
620 / 45
7.03
690 / 100
510 / 74
1.8
550 / 40
6.96
676 / 98
510 / 74
1.9
15 H2 / 85 N2
620 / 45
7.03
676 / 98
510 / 73
1.7
550 / 40
7.05
841 / 122
593 / 86
2.2
100 H2
620 / 45
7.10
849 / 124
572 / 83
1.8
550 / 40
7.04
814 / 118
614 / 89
1.6
MDC
75 H2 / 25 N2
620 / 45
7.09
848 / 123
607 / 88
1.3
550 / 40
7.05
889 / 129
648 / 94
1.7
15 H2 / 85 N2
620 / 45
7.11
896 / 130
627 / 91
1.8
TABLE V: Tensile Properties and Sintered Densities for Dog-bone Tensile Specimens Sintered
at 1260 °C (2300 °F) in Different Sintering Atmospheres
Comp
Sintered
UTS
Yield
Atmosphere
Elongation
Material
Pressure
Density
(MPa /
(MPa /
(v/o)
(%)
3
3
3
(MPa / tsi)
(g/cm )
10 psi)
10 psi)
550 / 40
7.03
752 / 109
517 / 75
3.0
100 H2
620 / 45
7.11
827 / 120
579 / 84
2.2
550 / 40
7.01
745 / 108
517 / 75
2.7
MDB
75 H2 / 25 N2
620 / 45
7.08
793 / 115
538 / 78
2.9
550 / 40
6.95
772 / 112
538 / 78
2.8
15 H2 / 85 N2
620 / 45
7.00
793 / 115
545 / 79
2.7
550 / 40
7.16
1041 / 151 703 / 102
2.2
100 H2
620 / 45
7.19
1172 / 170 758 / 110
2.7
550 / 40
7.16
1062 / 154 717 / 104
2.6
MDC
75 H2 / 25 N2
620 / 45
7.19
1179 / 171 724 / 105
2.5
550 / 40
7.07
1055 / 153 765 / 111
2.0
15 H2 / 85 N2
620 / 45
7.12
1165 / 169 745 / 108
2.3
As evidenced by the combination of strength and ductility at both 1150 °C (2100 °F) and
1260 °C (2300 °F), the silicon did not impart its maximum benefit at the standard sintering
temperature. In fact, previous unpublished work suggested that a minimum sintering
temperature of 1230 °C (2250 °F) was necessary to facilitate proper sintering of these siliconcontaining materials. Additionally, it should be noted that although the more alloyed MDC
sintered at 1150 ° C (2100 °F) was capable of meeting / exceeding the strength of less alloyed
MDB sintered at 1260 °C (2300 °F), the lower sintering temperature could not provide the same
ductility. The hydrogen / nitrogen composition of the sintering atmosphere was found to have a
5
marked effect on densification at the higher sintering temperature. As hydrogen content
increased, sintered density was generally seen to increase. This trend was thought to arise
from enhanced reduction with higher hydrogen gas mixtures. Recall that all specimens (at like
compaction pressures) started at the same green density.
TABLE VI: Apparent Hardnesses and Impact Energies for Specimens Sintered at 1150 °C
(2100 °F) in Different Sintering Atmospheres
Comp
Apparent
Impact
Atmosphere
Material
Pressure
Hardness
Energy
(v/o)
(MPa / tsi)
(HRC)
(J / ft.lbf)
550 / 40
13
18 / 13
100 H2
620 / 45
16
15 / 11
550 / 40
18
11 / 8
MDB
75 H2 / 25 N2
620 / 45
20
12 / 9
550 / 40
18
14 / 10
15 H2 / 85 N2
620 / 45
20
15 / 11
550 / 40
27
19 / 14
100 H2
620 / 45
27
22 / 16
550 / 40
28
19 / 14
MDC
75 H2 / 25 N2
620 / 45
30
23 / 17
550 / 40
26
19 / 14
15 H2 / 85 N2
620 / 45
28
22 / 16
TABLE VII: Apparent Hardnesses and Impact Energies for Specimens Sintered at 1260 °C
(2300 °F) in Different Sintering Atmospheres
Comp
Apparent
Impact
Atmosphere
Material
Pressure
Hardness
Energy
(v/o)
(MPa / tsi)
(HRC)
(J / ft.lbf)
550 / 40
21
20 / 15
100 H2
620 / 45
26
22 / 16
550 / 40
23
20 / 15
MDB
75 H2 / 25 N2
620 / 45
24
23 / 17
550 / 40
22
22 / 16
15 H2 / 85 N2
620 / 45
26
27 / 20
550 / 40
36
33 / 24
100 H2
620 / 45
37
37 / 27
550 / 40
36
31 / 23
MDC
75 H2 / 25 N2
620 / 45
37
35 / 26
550 / 40
33
33 / 24
15 H2 / 85 N2
620 / 45
34
35 / 26
Apparent Hardness / Impact Energy
The impact energies and apparent hardness values for test specimens are listed in Table VI
and VII. Again, it can be seen that the higher sintering temperature was imperative for
achieving the unique combination of hardness and impact properties. Material MDC was most
impressive with impact energies as high as 37 J (27 ft.lbf) coupled with sinter-hardening
capabilities. It should be noted that impact energies in excess of 27 J (20 ft.lbf) are all but
unheard of in the P/M industry (and can even be difficult to obtain in wrought steels) at the
6
hardness levels listed. The conventional relationship between impact energy and apparent
hardness in common P/M grades is illustrated in Figure 1 [7].
45
FN-0208HT
FN-0405HT
40
MDC 2300°F
MDC
Apparent Hardness (HRC)
FLC-4608
35
MDC (2100°F)
30
25
MDB (2300°F)
MDB
20
MDB (2100°F)
15
10
FN-0208
FD-0205
5
FN-0405
FL-4605
0
0
5
10
15
20
25
30
35
40
45
Impact Energy (ft.lbf)
FIGURE 1: The Relationship between Impact Energies and Apparent Hardness Values for
Some Common P/M Grades (as listed in MPIF Standard 35) Compared to MDB and MDC
Sintered at 1260 °C (2300 °F)
Metallography
In the interest of brevity, the only photomicrographs shown in this effort are those used to
illustrate the microstructural constituents of each material and/or any microstructural differences
between sintering temperatures.
Firstly, the effect of sintering temperature on final
microstructure is evident in Figures 2 and 3 for MDB and Figure 4 and 5 for MDC. At the higher
sintering temperature, the nickel distribution and overall microstructure in the field of view were
more homogenous. This improved nickel uniformity caused an increase in hardenability,
apparent hardness, and strength. Furthermore, densification and pore rounding assisted in
increasing apparent hardness, strength, and ductility.
7
2% Nital / 4%
FIGURE 2: Photomicrograph of MDB Sintered at 1150 ° C (2100 °F) in 25 v/o N2 – 75 v/o H2
Containing Pearlitic, Bainitic, Nickel Rich, and Martensitic Regions (Originally 200X)
2% Nital / 4% Picral
FIGURE 3: Photomicrograph of MDB Sintered at 1260 ° C (2300 °F) in 25 v/o N2 – 75 v/o H2
Containing Bainitic, Pearlitic, and Martensitic Regions (Originally 200X)
8
2% Nital / 4% Picral
FIGURE 4: Photomicrograph of MDC Sintered at 1150 ° C (2100 °F) in 25 v/o N2 – 75 v/o H2
Containing Martensitic, Bainitic, Nickel Rich, and some Pearlitic Regions (Originally 200X)
2% Nital / 4% Picral
FIGURE 5: Photomicrograph of MDC Sintered at 1260 ° C (2300 °F) in 25 v/o N2 – 75 v/o H2
Containing Martensitic and Bainitic Regions (Originally 200X)
9
Economics
High temperature sintering was required to properly process both materials utilized in this study.
At lower sintering temperatures, these materials offer good mechanical and physical properties,
but do not exhibit their full potential. By fully exploiting these materials, high apparent
hardnesses coupled with exceptional impact energies and tensile properties were possible.
These properties came at a price. This price was largely due to the increased costs associated
with the high temperature sintering process. At the time of this effort, very little work existed to
characterize the relative cost of high temperature sintering versus conventional “continuous
belt” sintering.
Cost categories analyzed for both a 24” continuous belt furnace and a 12” high temperature
pusher were capital, utilities (electricity and atmosphere), and labor. Based upon conditions
observed during operation, the following items were concluded (Appendix A lists the assumed
costs and method of calculation of the economic data.):
•
•
•
•
Labor was deemed the same for loading / unloading of each furnace
Atmosphere for pusher was ~20% less costly due to difference in gas flows
Electricity for pusher was ~35% less costly due to differences in size and heat
retention
The pusher furnace was approximately two and a quarter times the cost of the
continuous belt, depending on size / options / special requests for each furnace
When all of the above items were considered, the use of a high temperature pusher furnace
cost 10 - 15% more per pound than a continuous belt furnace when material throughput was
identical. As furnace loading and belt / pusher speeds vary throughout the industry, the cost
differential can fluctuate likewise. P/M parts manufacturers are urged to discuss their specific
needs with a sintering furnace manufacturer.
Pusher furnace processing generally improves mechanical properties. Therefore, parts
produced in a pusher furnace should command a higher price in the marketplace. Thus, part
manufacturers must selectively process materials in the pusher to maximize profitability and
provide the highest return on the investment.
Additional Considerations
When this work and its data are reviewed, several important items must be considered. Firstly,
as is the case in most high temperature furnaces, the cooling rate seen by the materials was
slow. At faster cooling rates, one would expect strength / apparent hardness to increase
slightly and impact energy / ductility to decrease slightly. Secondly, despite incipient melting of
specimens at 1343 °C (2450 °F) and the omission of said data, some promise was seen in
sintering at temperatures higher than 1260 °C (2300 °F). Finally, dimensional change was
largely overlooked in this effort, but previous work suggested that the dimensional consistency
of these materials was superior to a standard FLN4-4405 composition [9]. The reader is
referred to this previous work for detailed dimensional change information.
10
CONCLUSIONS
High temperature sintering of two silicon-containing materials, Ancorloy MDB and Ancorloy
MDC, was explored as an avenue to higher P/M performance. While sintering at 1150 ° C
(2100 °F) was capable of achieving good mechanical and physical properties, raising the
temperature to 1260 °C (2300 °F) yielded exceptional combinations of strength, ductility,
apparent hardness, and impact energies. In fact, Ancorloy MDC exhibited impact energies
superior to any other P/M grade at apparent hardness values of 35 – 40 HRC. Since high
temperature sintering incurred additional costs, a cursory cost analysis was also presented.
ACKNOWLEDGEMENTS
The authors would like to thank Gerald Golin of Hoeganaes for his metallographic work and
George Fillari and David Southwick of Hoeganaes for their assistance in specimen testing. In
addition, John Lougee, Jason Forster, and Jodie Smith of Hawk for their help in running the
sintering runs and obtaining furnace cost data.
REFERENCES
1. German, Randall M., “Powder Injection Molding,” in ASM Handbook, Vol. 7, ASM, Metals
Park, Ohio, 1998, pp.356 - 364.
2. Sanderow, Howard I., “High Temperature Sintering in Ferrous Powder Metallurgy
Components,” in ASM Handbook, Vol. 7, ASM, Metals Park, Ohio, 1998, pp.828 - 833.
3. Luk, S.H. and Hamill, J.A., “Dust and Segregation-Free Powders For Flexible P/M
Processing”, Advances in Powder Metallurgy & Particulate Materials - 1993, Vol. 1, pp 153,
Metal Powder Industries Federation, Princeton, NJ.
4. Semel, F.J., Luk, S.H., “Continuing Improvements in Binder Treatment Technology”,
Advances in Powder Metallurgy & Particulate Materials - 1996, Vol. 4, pp 353, Metal Powder
Industries Federation, Princeton, NJ.
5. Hanejko, Francis G., “Warm Compaction,” in ASM Handbook, Vol. 7, ASM, Metals Park,
Ohio, 1998, pp.376 - 381.
6. Salak, A., Ferrous Powder Metallurgy, Cambridge International Science Publishing,
Cambridge, England, 1995, p.235.
7. MPIF Standard 35 – Materials Standards for P/M Structural Parts, Metal Powder Industries
Federation, Princeton, NJ, 1997, p.32-35.
8. Shah, Suresh O., Baran, M.C., Patel, S.N., and Causton, R.J., “High Performance Materials
– Ancorloy MD Series”, Advances in Powder Metallurgy & Particulate Materials–2001, Vol.1,
p.227, Metal Powder Industries Federation, Princeton, NJ.
11
Appendix A: Sintering Furnace Economic Analysis
Cost
CAPITAL COST
Furnace Cost
Life
Utilization
Shifts/Day
Days/Week
Weeks/Year
Hours/Year
Hours/Life
Depr Cost/Hour
UTILITIES
Electricity
$/kw-hr
Full Load kw
Electric Cost/Hour
24" Belt Furnace 12" Pusher Furnace
$200,000
7
85%
3
5
50
5,100
35,700
$5.60
$450,000
7
85%
3
5
50
5,100
35,700
$12.61
$0.0598
145.2
$8.68
$0.0598
95.0
$5.68
N2 ($/cfh)
H2 ($/cfh)
v/o N2
v/o H2
Total Flow
Atmosphere Cost
$0.25
$0.90
90%
10%
1,800
$5.67
$0.25
$0.90
90%
10%
1,450
$4.57
Cost/hr
Furnace/Operator
Labor Cost/Hour
$15.00
3
$5.00
$15.00
3
$5.00
$5.60
8.68
5.67
5.00
$24.96
$0.055
$12.61
5.68
4.57
5.00
$27.85
$0.062
Atmosphere
LABOR
DIRECT COST SUMMARY
Depr Cost/Hour
Electric Cost/Hour
Atmosphere Cost
Labor Cost/Hour
TOTAL COST/HOUR
Cost/Pound
ANNUAL CAPACITY
Pounds/Year
2,295,000
12
2,295,000