ENHANCED PROCESSING OF SILICON-CONTAINING HIGH PERFORMANCE MATERIALS
Suresh O. Shah,
HazenTec, Hazen, Arkansas
Michael C. Baran, Sunil N. Patel, and Robert J. Causton
Hoeganaes Corporation, Cinnaminson, NJ
2
Presented at PM TEC 2002
International Conference on Powder Metallurgy & Particulate Materials
June 16 – 21, 2002 Orlando, Florida USA
ABSTRACT
In 2001, an extensive program was initiated to evaluate new silicon-containing materials
designed to compete with various grades of ductile and malleable cast irons. These bindertreated, press-ready premixes were compared to a standard FLN4-4405 in a production
environment on a complicated, high volume application. This year’s work investigates both
double pressed / double sintered and heat-treated performance of the new silicon-containing
materials. Mechanical properties and dimensional stability information are presented and
compared to several standard material candidates containing no silicon.
INTRODUCTION
Last year, a comprehensive program was undertaken to address several key challenges [1].
These challenges, made largely by part manufacturers and OEMs to ferrous powder producers,
involved the development of a family of materials that were capable of:
(1) Economically achieving high performance targets,
(2) Combining high strength with reasonable ductility and impact energy, and / or
(3) Offering the opportunity to reduce manufacturing operations. For example, convert a double
press / double sinter (DP/DS) operation to single press / single sinter (SP/SS) process.
Stage 1 of the program involved the single press / single sinter (SP/SS) processing of both
silicon-containing and silicon-free hybrid materials. Although the initial study was extremely
encouraging, further work was required to accurately mirror current production processes.
Hence, Stage 2 (this work) focused on double press / double sinter (DP/DS) processing as well
as DP / DS processing with an additional heat treatment step. When the entirety of this effort
was considered, a more complete understanding of material properties, processing, and part
consistency was gained. Despite processing differences, each test stage revolved around a
fairly complex, automotive application with an overall height of ~38 mm (~1.45 inches).
1
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 achieved through the processes listed In Figure 1. Two other materials,
FLN4-4405 and FLN6-4405, served as property benchmarks for the new silicon-containing
materials. Due to the differences in alloy contents, all materials were tested using the
sinterhardening process, but MDB and FLN4-4405 materials were investigated in the heattreated condition as well. Test and production specimens were produced at HazenTec from
production lots of 4,550 kg (10,000 lb).
Sinterhardening Process
Heat-Treat Process
ALL MATERIALS
MDB & FLN4-4405
Compact to
3
7.05 +/- 0.05 g/cm
Compact to
3
7.05 +/- 0.05 g/cm
Presinter @
760 °C (1400 °F)
Presinter @
760 °C (1400 °F)
Repress to
3
7.30 +/- 0.03 g/cm
Repress to
3
7.30 +/- 0.03 g/cm
High
Temperature
Sinter
High
Temperature
Sinter
Temper 1 hour @
205 °C (400 °F)
Heat Treat
Temper 1 hour @
205 °C (400 °F)
FIGURE 1: Processes Employed to Prepare Specimens and Production Parts of MDB, MDC,
FLN4-4405, and FLN6-4405 for the Demanding Automotive Application Considered
in this Study. Each Material is Tagged with Either “-SH” or “-HT” to Denote
Process Employed.
All test pieces were sintered at HazenTec in a production loaded, high temperature, pusher
furnace. The specifics on the sintering / tempering cycle are as follows:
Presinter Temperature:
Sintering Temperature:
Time in Hot Zone:
760 °C (1400 °F) in 80 v/o N2, 20 v/o H2
1205 – 1315 °C (2200 – 2400 °F) in 80 v/o N2, 20 v/o H2
45 minutes
2
Cooling:
Tempering:
Standard Water-Jacketed Section
205 °C (400 °F) for 1 hour in Air
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 completed at Westmoreland
Mechanical Testing and Research, Inc. using machined round tensile bars. Despite differences
between the two processing routes, all specimens were tested in the as-tempered condition.
Dimensional change was determined per ASTM B610 by measuring the major outer diameter
and axial height of the production part.
RESULTS AND DISCUSSIONS
Powder Properties and Chemistries
The powder properties of interest were apparent density, flow, and nominal chemical
composition. These properties are presented in Table I. As can be seen, the flows and
apparent densities of MDB and MDC do not represent a significant departure from similar “high
performance” materials.
Table I: Nominal Chemical Compositions, Apparent Density, and Hall Flow for Materials 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)
C
(w/o)
FLN4-4405
FLN6-4405
MDB
MDC
3.30
3.25
3.19
3.25
28
28
28
28
Bal.
Bal.
Bal.
Bal.
--0.70
0.70
0.03
0.03
0.03
0.03
0.13
0.13
0.13
0.13
4.0
6.0
2.0
4.0
0.85
0.85
0.85
0.85
0.60
0.60
0.60
0.60
Mechanical Properties and Apparent Hardness
Tensile strength, ductility, gear tooth strength, and apparent hardness are among those
properties specified for automotive applications and the part that is considered in this work is no
exception. Table II lists the properties obtained by the sinterhardening process described in
Figure 1, while Table III list those for the heat-treat process. Another important characteristic,
impact energy, is presented in the following section.
Although small differences were noted between MDB –SH and FLN4-4405 –SH in Table II, the
materials behaved similarly in tensile tests and apparent hardness measurements. In fact, the
only statistically significant difference was a slight ultimate tensile strength (UTS) advantage for
MDB. However, properties such as impact energy and dimensional consistency clearly favor
MDB and are examined extensively in a later section.
The analysis of Table II data for MDC –SH and FLN6-4405 –SH was much more challenging.
Inexplicably, the UTS of MDC –SH declined as sintering temperature increased. In fact, the
reduction of UTS was quite dramatic when a sintering temperature of 2400 °F was employed.
The decrease may be attributable to cracking that occurred during machining, microstructural
transformation caused by mechanical working during machining, and/or “embrittlement” caused
by high apparent hardness (not unlike that seen when highly hardenable materials are rapidly
quenched). Further confusion resulted from the absence of such a declining trend in tooth
strength measurements on actual parts. In an effort to further investigate this issue,
3
metallographic examination and dog-bone tensile testing was ordered. Unfortunately, neither of
these items was completed prior to publication. However, partial results from dogbone testing
confirmed the declining trend in UTS for MDC -SH. Further work is required to fully explain this
phenomenon and formulate processing steps to avoid such a decline in ultimate strength.
Once again, no such effect was noted in the tooth strength measurements on production parts.
When MDB and FLN4-4405 were heat-treated, an appreciable effect was noted in both
materials. Tensile strength increased by as much as 30% from the “sinterhardened” condition,
while yield strength improved by 60 – 80%. Higher apparent hardness and tooth strength
values were also obtained. Not surprisingly, the ductility of the materials decreased upon heattreating. However, the ductilities obtained were extremely good when one considered the high
strength and hardness values obtained. While MDB was slightly superior in yield strength and
apparent hardness, the FLN4-4405 material exhibited higher ultimate strength and ductility.
Nevertheless, the work of James, et al. suggested that the use of an optimum tempering
temperature (475 – 575 °F) led to higher ultimate strength and ductility in silicon-containing
materials such as MDB [2]. Further investigation of this tempering effect is warranted.
Table II: Properties Attained by Double Press / Double Sinter Processing of Materials per the
“Sinterhardening Process” listed in Figure 1 (Machined Rounds)
Grade
MDB
-SH
FLN4-4405
-SH
MDC
-SH
FLN6-4405
-SH
Sinter
Temp.
(°°F)
Final
Density
3
(g/cm )
UTS
(MPa /
3
10 psi)
0.2% YS
3
(MPa / 10
psi)
2200
2300
2400
2200
2300
2400
2200
2300
2400
2200
2300
2400
7.36
7.36
7.36
7.37
7.38
7.40
7.38
7.39
7.38
7.41
7.43
7.45
1025 / 149
1035 / 150
1000 / 145
960 / 139
970 / 141
925 / 134
1350 / 196
1270 / 184
995 / 144
1125 / 163
1170 / 170
1205 / 175
675 / 98
675 / 98
685 / 99
640 / 93
660 / 96
650 / 94
750 / 109
820 / 119
860 / 125
710 / 103
770 / 112
835 / 121
Elong
(%)
Part
Tooth
Strength
(lbs.)
Apparent
Hardness
(HRC)
3.5
5.0
5.3
5.0
5.5
5.5
3.0
2.2
1.7
5.5
5.3
4.0
17,360
17,230
17,060
17,570
17,210
16,590
25,180
25,210
25,010
21,360
23,170
23,140
29
28
27
24
26
27
39
41
43
30
34
35
Table III: Properties Attained by Double Press / Double Sinter Processing of Materials per the
“Heat-Treat Process” listed in Figure 1 (Machined Rounds)
Grade
MDB
-HT
FLN4-4405
-HT
Sinter
Temp.
(°°F)
Final
Density
3
(g/cm )
UTS
(MPa /
3
10 psi)
0.2% YS
3
(MPa / 10
psi)
Elong
(%)
Part
Tooth
Strength
(lbs.)
Apparent
Hardness
(HRC)
2200
2300
2400
2200
2300
2400
7.33
7.33
7.34
7.34
7.37
7.39
1350 / 196
1330 / 193
1305 / 189
1570 / 228
1595 / 231
1580 / 229
1195 / 173
1220 / 177
1170 / 170
1055 / 153
1105 / 160
1095 / 159
1.0
1.3
1.8
2.6
2.2
3.0
19,260
22,750
23,190
26,830
27,890
27,030
48
48
47
44
45
45
4
Impact Properties
Although impact energies are often collectively lumped into the “mechanical properties”
category, the performance of MDB and MDC demanded a separate section. Given the
capabilities of these materials, they may represent a fitting alternative to wrought steels and
castings in impact loaded automotive and industrial applications. Figure 2 presents the impact
data for the materials tested over a wide range of sintering temperatures.
When the impact data in Figure 2 was considered, several key observations were made. First,
the impact properties of MDB and MDC processed by the “sinterhardening process” were
exceptionally high when compared to the other materials in this study. In these cases, the
addition of silicon raised the overall impact energy for these materials. Furthermore, the
presence of silicon in the matrix allowed for high toughness and high apparent hardnesses to
be achieved simultaneously. A general trend line of impact energy versus apparent hardness
for many common P/M grades is shown in Figure 3 [3]. Secondly, although heat-treated MDB
exhibited good impact properties, its performance was the lowest of the group studied.
However, given the exceptionally high yield strengths seen in heat-treated MDB, these impact
energies were expected. Nonetheless, the pairing of 47 HRC and 19 ft.lbf (2300 °F sintered
and heat-treated MDB) is still superior to those combinations along the trendline in Figure 3.
Finally, it was determined that increasing nickel content from 4 w/o to 6 w/o in the FLN-4405
materials led to only a slight increase in impact properties. Hence, if an FLN4-4405 application
required more toughness, additional nickel may not provide sufficient improvement.
Dimensional Control
As in any high temperature sintering process, a paramount issue, especially in high nickel
premixes, is dimensional consistency. Poor dimensional control is capable of casting an
overwhelming shadow on otherwise good materials. However, in this case, the siliconcontaining grades were found to dominate the two standard materials. The radial and axial
shrinkages from repressed dimensions are shown in Figures 4 and 5, respectively.
It was immediately evident that both FLN4-4405 and FLN6-4405 were much more sensitive to
differences in sintering temperature. Hence, in practice, temperature variations within a
production sintering furnace posed a much greater threat to the dimensional stability of the
FLN4-4405 and FLN6-4405 materials. Incredibly, the dimensional stability of MDB was 4 times
better than FLN4-4405 radially and 2 times as stable axially. Similarly, MDC was 6 times better
than FLN6-4405 radially and showed no variation with sintering temperature axially. These
results were similar to those reported previously for specimens prepared by SP/SS [1]. This
superior dimensional capability was a consequence of the proprietary method of silicon
addition. Obviously, the high shrinkage and dimensional distortion inherent to previous
methods of incorporating silicon were avoided [4].
Comparison of DP/DS to SP/SS
As mentioned earlier, this investigation represented a follow-up to work presented in 2001 [1].
In 2001, the sole focus was a thorough understanding of SP/SS processing of MDB, MDC, and
FLN4-4405 materials. In an effort to more accurately mirror actual production conditions, this
year’s study incorporated DP/DS processing and some heat treating data. Additionally, a more
costly FLN6-4405 composition was included for benchmarking purposes.
The properties for SP/SS and DP/DS –SH materials are shown in Table IV. Since heat-treating
was not employed in the SP/SS investigation, no direct comparison of materials could be made.
However, the DP/DS –HT materials shown in Table III exhibited the highest ultimate tensile
strengths, yield strengths, and apparent hardness values seen in either investigation.
5
(68)
50
(61)
45
Impact Energy, ft.lbf (Joules)
MDB -SH
(54) 40
MDC -SH
(47) 35
FLN6-4405 -SH
(41)
30
(34)
25
(27)
20
(20)
15
(14)
10
2150
FLN4-4406 -SH
FLN4-4405 -HT
MDB -HT
2200
2250
2300
2350
2400
2450
Final Sintering Temperature (deg F)
Figure 2: Impact Energies of Unnotched Charpy Impact Bars for All Materials and Variation in
Impact Properties with Sintering Temperature. * SH and HT Denote Process
Employed (see Figure 1).
45
FN-0208HT
40
FN-0405HT
MDB -SH
(2300 F)
Apparent Hardness (HRC)
FLC-4608
35
MDC -SH
(2300 F)
30
25
20
15
10
FN-0208
FD-0205
5
FN-0405
FL-4605
0
0
5
(7)
10
(14)
15
20
25
30
35
40
45
(20)
(27)
(34)
(41)
(47)
(54)
(61)
Impact Energy, ft.lbf (Joules)
Figure 3: A Comparison of the Impact Energy / Apparent Hardness Combinations Possible
with Common P/M Grades (from MPIF Standard 35 [3]) as well as Silicon-Containing
MDB and MDC. * SH and HT Denote Process Employed (see Figure 1)
6
0.85
FLN6-4405 -SH
0.75
Radial Shrinkage (%)
FLN4-4405 -HT
0.65
FLN4-4405 -SH
0.55
0.45
MDC -SH
0.35
MDB -HT
0.25
2150
MDB -SH
2200
2250
2300
2350
2400
2450
Final Sintering Temperature (deg F)
Figure 4: Radial Shrinkage From Repressing Dimensions for All Materials Over a Wide Range
of Sintering Temperatures. * SH and HT Denote Process Employed (see Figure 1)
0.95
FLN6-4405 -SH
0.90
Axial Shrinkage (%)
0.85
0.80
FLN4-4405 -HT
0.75
0.70
MDB -SH
FLN4-4405 -SH
0.65
MDC -SH
0.60
0.55
MDB -HT
0.50
2150
2200
2250
2300
2350
2400
2450
Final Sintering Temperature (deg F)
Figure 5: Axial Shrinkage From Repressing Dimensions for All Materials Over a Wide Range
of Sintering Temperatures. * SH and HT Denote Process Employed (see Figure 1)
7
Table IV: Comparison of the SP/SS* and DP/DS Properties of Several Materials Utilized in this
Study and the 2001 Investigation (Sinterhardening Process Shown for DP/DS)
Sinter
Temp
(°°F)
Grade
SP/SS
2300
2400
DP/DS
2300
2400
SP/SS
2300
2400
FLN4-4405
DP/DS
2300
2400
SP/SS
2300
2400
MDC
DP/DS
2300
2400
2300
FLN6-4405 DP/DS
2400
*SP/SS data from Reference 1
MDB
Final
Dens.
3
(g/cm )
UTS
3
(MPa / 10
psi)
0.2% YS
3
(MPa / 10
psi)
Elong
(%)
App
Hard
(HRC)
Impact
Energy
(J /
ft.lbf)
7.07
7.10
7.36
7.36
7.21
7.26
7.38
7.40
7.14
7.14
7.39
7.38
7.43
7.45
760 / 110
760 / 110
1035 / 150
1000 / 145
860 / 125
860 / 125
970 / 141
925 / 134
1070 / 155
1170 / 170
1270 / 184
995 / 144
1170 / 170
1205 / 175
550 / 80
550 / 80
675 / 98
685 / 99
620 / 90
620 / 90
660 / 96
650 / 94
795 / 115
860 / 125
820 / 119
860 / 125
770 / 112
835 / 121
1.7
2.2
5.0
5.3
1.9
1.9
5.5
5.5
1.8
2.0
2.2
1.7
5.3
4.0
93 HRB
94 HRB
28
27
97 HRB
97 HRB
26
27
34
36
41
43
34
35
23 / 17
30 / 22
52 / 38
62 / 46
19 / 14
20 / 15
37 / 27
39 / 29
30 / 22
34 / 25
52 / 38
58 / 43
41 / 30
42 / 31
When the values in Table IV were compared, it was immediately evident that SP/SS MDC was
an extremely efficient material. It was capable of closely matching or exceeding the tensile
strength, apparent hardness, and impact energy of DP/DS FLN4-4405. SP/SS MDC achieved
3
this goal with fewer processing steps, less production cost, and a 0.34 – 0.36 g/cm lower
density. Additionally, when compared to the highly alloyed DP/DS FLN6-4405, SP/SS MDC
was extremely competitive despite 2 w/o less nickel content. The incorporation of silicon into
the nickel-molybdenum-carbon system was seen to provide a potent increase in properties and
offered the possibility of lower processing costs.
Metallography
Microstructures from actual parts are shown in Figures 6a & 6b (FLN4-4405), 7a & 7b (MDB),
8a & 8b (FLN6-4405), 9a & 9b (MDC), 10a & 10b (Heat Treated FLN4-4405), and 11a & 11b
(Heat Treated MDB). While the “a” designation indicates a photomicrograph with an original
magnification of 200X, the “b” designation refers to a 500X original magnification. The
microstructural constituents present in each material are as follows:
•
•
•
•
•
•
FLN4-4405: Divorced pearlite, unresolved pearlite, nickel-rich areas, martensitic nickel-rich
areas, and bainite
MDB: Divorced pearlite, martensitic nickel-rich areas, nickel-rich areas, and bainite
FLN6-4405: Martensite, divorced pearlite, unresolved pearlite, nickel-rich areas, and bainite
MDC: Martensite, divorced pearlite, unresolved pearlite, bainite, nickel-rich areas, and
martensitic nickel-rich areas
Heat Treated FLN4-4405: Martensite, nickel-rich areas, and martensitic nickel-rich areas
Heat Treated MDB: Martensite with martensitic nickel-rich areas
8
2% Nital / 4% Picral
50 µm
Figure 6a: Photomicrograph of Actual Production Part Produced Using FLN4-4405 Material
Sintered at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 200X.
20 µm
2% Nital / 4% Picral
Figure 6b: Photomicrograph of Actual Production Part Produced Using FLN4-4405 Material
Sintered at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 500X.
9
50 µm
2% Nital / 4% Picral
Figure 7a: Photomicrograph of Actual Production Part Produced Using MDB Material Sintered
at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 200X.
20 µm
2% Nital / 4% Picral
Figure 7b: Photomicrograph of Actual Production Part Produced Using MDB Material Sintered
at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 500X.
10
50 µm
2% Nital / 4% Picral
Figure 8a: Photomicrograph of Actual Production Part Produced Using FLN6-4405 Material
Sintered at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 200X.
20 µm
2% Nital / 4% Picral
Figure 8b: Photomicrograph of Actual Production Part Produced Using FLN6-4405 Material
Sintered at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 500X.
11
50 µm
2% Nital / 4% Picral
Figure 9a: Photomicrograph of Actual Production Part Produced Using MDC Material Sintered
at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 200X.
20 µm
2% Nital / 4% Picral
Figure 9b: Photomicrograph of Actual Production Part Produced Using MDC Material Sintered
at 1260 °C (2300 °F) and Tempered at 205 °C (400 °F). Originally 500X.
12
50 µm
2% Nital / 4% Picral
Figure 10a: Photomicrograph of Actual Production Part Produced Using FLN4-4405 Material
Sintered at 1205 °C (2200 °F), Heat Treated, and Tempered at 205 °C (400 °F).
Originally 200X.
20 µm
2% Nital / 4% Picral
Figure 10b: Photomicrograph of Actual Production Part Produced Using FLN4-4405 Material
Sintered at 1205 °C (2200 °F), Heat Treated, and Tempered at 205 °C (400 °F).
Originally 500X.
13
50 µm
2% Nital / 4% Picral
Figure 11a: Photomicrograph of Actual Production Part Produced Using MDB Material
Sintered at 1205 °C (2200 °F), Heat Treated, and Tempered at 205 °C (400 °F).
Originally 200X.
20 µm
2% Nital / 4% Picral
Figure 11b: Photomicrograph of Actual Production Part Produced Using MDB Material
Sintered at 1205 °C (2200 °F), Heat Treated, and Tempered at 205 °C (400 °F).
Originally 500X.
14
CONCLUSIONS
When the data from SP/SS and DP/DS -SH processing of silicon-containing grades MDB and
MDC were compared to similarly processed FLN4-4405 and FLN6-4405 materials, the following
items were noted:
• MDC processed by SP/SS was capable of closely matching or exceeding the tensile
strength, apparent hardness, and impact energy of DP/DS FLN4-4405 with 40% less
processing,
• MDC processed by SP/SS was competitive with DP/DS FLN6-4405 despite containing less
nickel and requiring a reduced amount of processing,
• Significant dimensional control benefits were realized when the MDB and MDC were
substituted for FLN4-4405 and FLN6-4405 materials,
• MDB and MDC exhibited unmatched impact energies of 33 - 46 ft.lbf and 37 - 43 ft.lbf,
respectively, and
• MDC demonstrated a potent sinter-hardening response and was capable of higher
apparent hardnesses than any other (non heat-treated) material.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the support of HazenTec and Hoeganaes Corporation. They
would like to thank the staff of HazenTec and, in particular, Amanda Castleberry, Enola Byrd,
Joe Berry for their assistance in producing and processing the test specimens and also for
mechanical testing and data collection. Additionally, the authors acknowledge the contribution
of Steven Kolwicz at Hoeganaes in preparing samples, collecting photomicrographs, and
providing metallographic expertise.
REFERENCES
1. Shah, S.O., Baran, M.C, Patel, S.N., and Causton, R.J., “High Performance Materials –
Ancorloy MD Series,” Advances in Powder Metallurgy & Particulate Materials, Metal Powder
Industries Federation, Princeton, NJ, 2001.
2. James, W.B., Baran, M.C., Causton, R.J., and Narasimhan, K.S., “New High Performance
P/M Alloy Substitutes for Malleable and Ductile Cast Irons”, Proceedings of the Powder
Metallurgy World Congress & Exhibition, Kyoto, Japan, November 12-16, 2000, p.959.
3. MPIF Standard 35 – Materials Standards for P/M Structural Parts, Metal Powder Industries
Federation, Princeton, NJ, 2000 edition.
4. Salak, A., Ferrous Powder Metallurgy, Cambridge International Science Publishing,
Cambridge, England, 1995, p.235.
15