ADVANCED SINTER-HARDENING MATERIALS AND PRACTICES
Francis Hanejko *, Alan Taylor **, & Arthur Rawlings *
*
Hoeganaes Corporation
Cinnaminson, NJ 08077
**
GKN Sinter Metals
Salem, IN
Presented at PM2TEC 2002
International Conference on Powder Metallurgy & Particulate Materials
JUNE 16 – 21, 2002 ORLANDO, FLORIDA USA
Abstract:
Sinter hardening is a well-established production technique utilized in the manufacture of
P/M components with hardness and tensile strengths that approach the values of
quench and tempered materials. The potential drawback of the sinter hardening process
is the uniform carbon content of the case and core. This uniformity of carbon content
does not promote a desirable compressive stress condition on the surface of the
component leading to less than optimum fatigue strength. Experimental work was
performed in which several sinter-hardening materials were produced with lower core
carbons and subsequently carburized after the sintering process to produce a carburized
case. Mechanical properties including tensile and fatigue of the non-carburized and
carburized material will be presented plus the effect of the carburizing cycle on the
carbon gradient of the new sinter hardening materials.
Introduction:
Sinter hardening is a process that combines sintering of the P/M part with furnace
accelerated cooling to produce as sintered microstructures containing greater than 75%
martensite [1,2]. As a general rule, the ferrous materials used in sinter hardening
contain high levels of alloying elements with graphite additions of approximately 0.8%
[3]. This combination of a highly alloyed premix with the high graphite level result in
reduced compressibility with corresponding low sintered densities. Concomitant with the
low density, the microstructure is a uniform carbon distribution from the case to the core.
The benefit of high hardness in the as sintered condition is often offset by less than
optimum mechanical properties that restricts opportunities for use of sinter hardening in
highly stressed applications.
Design engineers of highly stressed parts rely on carburizing or carbonitriding the
surface to give the often-opposing combination of high mechanical properties with good
surface wear and hardness. These heat treatments impart a wear resistance surface
layer plus apply a favorable compressive stress condition on the surface of the part to
further enhance the cyclic fatigue characteristics [4]. Gas carburizing or gas
carbonitriding is traditionally done on lean alloy systems and is followed by an oil quench
to develop the appropriate martensitic microstructure. Rolling contact fatigue resistance
of materials increases by approximately 20% [5]. Carburizing has been used extensively
in P/M components for the same purpose. P/M is unique in that the depth of carburizing
is a function of the time at temperature as well as the amount of porosity in the part.
One drawback of the oil quenching results in dimensional distortion that often negates
the net shape capability of the P/M process. What is needed is a processing technique
in which the benefits of sinter hardening can be merged with the advantages of surface
modification without the associated distortion inherent in oil quenching.
The objective of this paper is to summarize a series of materials that were either
conventionally sinter hardened or sintered and then vacuum carburized followed by a
rapid gas quench. Key variables that will be measured include the compressibility, the
dimensional change as sintered and as gas quenched, and the mechanical properties of
test samples prepared by both processing schemes. It is noted that vacuum carburizing
was chosen in this experimental work because it was a relatively small batch process
that could serve as the model for larger implementation to continuous gas carburizing.
Materials Evaluated and Testing
The goal of this investigation was to examine material and processing options that would
give enhanced fatigue properties in an automotive hydraulic pump application. When
first designed, the part was qualified using a FN-0205 material that was carbonitrided
and oil quenched for improved wear and mechanical properties. However, the
carbonitrided part required form grinding to meet the component’s final dimensional
specifications. To reduce cost, a sinter-hardening grade (FLNC-4408) was successfully
substituted, eliminating the need for secondary machining. One disadvantage of the
sinter hardening material was a reduction in the fatigue strength of the component [6].
With increasing pump pressures, this reduction in fatigue strength will soon be
unacceptable. A heat treatment design of experiments (DOE) demonstrated that the
effect of core vs. surface carbon in heat-treated steels results in a 10% improvement in
fatigue performance for the surface modified components. Consequently, unless higher
fatigue strength sinter-hardening options are developed, the mechanical property
requirements will necessitate that the pump component return to the carbonitrided FN0205 material system, thus adding cost to the P/M component.
To capitalize on the uniqueness of P/M, a program was designed that investigated two
high carbon materials for their sinter hardening response. Additionally, the same base
alloy compositions were prepared with lower graphite levels to determine the potential
for carburizing to improve the mechanical properties of the final component. Six premix
compositions were prepared; these are listed in Table 1. The material listed as FLC4905 (modified) is not a standard MPIF material designation; this material was chosen
because it utilized a higher hardenability base iron with the same premix additions of the
FLNC-4405. From these six premixes, MPIF standard flat tensile specimens were
compacted at 30, 40, and 50 tsi (415, 550, and 690 MPa). These samples were
sintered at temperatures of 2050 °F (1120 °C), 2150 °F (1177 °C), and 2300 °F (1260
°C) in a 90% nitrogen and 10% hydrogen atmosphere. Time at temperature was
approximately 20 minutes. The continuous belt sintering furnace was equipped with an
accelerated cooling unit; the cooling rate was ~1.0 °C per second in the temperature
range from 1650 °F (900 °C) to 930 °F (500 °C). After sintering all samples were
tempered at 400 °F (205 °C) for one hour.
In addition to the “dog-bone” tensile specimens, toroidal samples measuring 1.85 inch
OD x 1.00 inch ID x ~1 inch tall (46 mm x 25 mm x 25 mm) were compacted on a 150
ton mechanical compaction press at a rate of 10 parts per minute. These toroidal
samples were pressed to a nominal density of 7.1 g/cm³ and were sintered in a
continuous belt-sintering furnace utilizing accelerated cooling. The objectives of
pressing the toroidal samples were as follows:
• Determine the as sintered hardness of the alloys with varying the cooling
rate of the furnace
• Determine the variability of the dimensional change upon sintering with
accelerated cooling and compare this dimensional change with oil quenching
and tempering and vacuum carburizing and gas quenching.
• Provide samples to evaluate the depth of carburization for two vacuum
carburizing cycles.
Table 1
Summary of Materials and Processing
Material ID
FLNC-4408
FLNC-4405
FLC-4905 (modified)
Ancorloy MDC (modified)
Ancorloy MDC
Ancorloy MDCL
Alloy Content
0.85% Mo, 2% Ni, 1.5% Cu
0.85% Mo, 2% Ni, 1.5% Cu
1.5% Mo, 2% Ni, 1.5% Cu
0.85% Mo, 4% Ni, 0.70% Si
0.85% Mo, 4% Ni, 0.70% Si
0.85% Mo, 4% Ni, 0.35% Si
Graphite Level
0.75%
0.50%
0.50%
0.75%
0.50%
0.50%
Rotating bending fatigue samples were machined from compacted samples measuring
0.45 inch x 0.45 inch x 4 inch (11.5 mm x 11.5 mm x 101.6 mm) compacted at 40 tsi
(550 Mpa) and 50 tsi (690 MPa). Rough-machined rotating bending fatigue samples
were either rapid gas quenched or vacuum carburized. The vacuum carburizing cycle
was as follows: carburize at 1700 °F (925 °C) for 30 minutes followed by a 15 minute
diffuse cycle. Because of equipment limitations, the samples were slow cooled to room
temperature and transferred to a second vacuum furnace in which the samples were
reheated to 1700 °F (925 °C) in vacuum, given an additional 15 minutes diffuse cycle
and subsequently rapid gas quenched to room temperature. All samples were tempered
at 400 °F (205 °C) for 1 hour. Rotating bending fatigue testing was done in accordance
to ASTM procedure E466.
To measure the depth of the carburized layer, toroidal samples were vacuum carburized
at 1700 °F (926 °C) for 30 minutes and 60 minutes at temperature followed by a 15
minute diffuse cycle and then simply slow cooled to room temperature. The depth of
carburization was determined via machining 0.005-inch (0.12-mm) increments off the
ring samples and submitting for carbon content via Leco Carbon Analysis. Additional
samples were also reheated to 1700 °F (925 °C) and rapid gas quenched to measure
the dimensional change of the rings and measure the hardness of the case and core.
Experimental Results
A.) Tensile test results
The tensile properties of the six premixes are shown in Table 2 and Table 3. Table 2
presents the tensile properties of the two materials that are considered through
hardening; that is, a case and core carbon of approximately 0.8%. Table 3 presents the
tensile properties of the 0.5% graphite materials, which are characteristic of the core
properties of the carburized materials.
Table 2
Tensile Properties of High Carbon Sinter-Hardened Materials
Material
ID
FLNC-4408
Sintering
Temp.
2050 °F
(1120 °C)
2300 °F
(1260 °C)
Ancorloy
MDC Mod
2300 °F
(1260 °C)
MPIF
Reference
FLC-4608
•
Density,
g/cm³
YS, 10³
psi (MPa)
UTS, 10³
psi (MPa)
Elong.
%
Hdness,
HRA/ HRC*
6.78
79 (550)
92 (635)
1.2
58.3 / 16
6.98
7.08
90 (625)
98 (677)
103 (710)
114 (790)
1.2
1.3
63.4 / 25
64.3 / 28
6.77
107 (745)
124 (855)
1.4
65.5 / 30
6.99
7.09
122 (850)
127 (880)
141 (975)
150 (1035)
1.3
1.3
68.6 / 37
69.8 / 39
6.81
105 (730)
136 (940)
1.6
65.8 / 32
7.01
7.11
113 (785)
120 (835)
157 (1085)
176 (1215)
1.8
2.0
68.6 / 36
70.5 / 40
6.80
79 (545)
<1
63.5 / 26
7.00
7.20
100 (690)
120 (630)
<1
<1
66.0 / 31
69.0 / 38
Converted from HRA
The hardness values of the FLNC-4408 sintered at 2050 °F (1120 °C) were lower than
expected and below those values listed for the MPIF reference material. These lower
than expected hardness values were a result of slower cooling through the initial section
of the accelerated cooling section of the sintering furnace. Despite the lower hardness
values achieved at 2050 °F (1120 °C) the FLNC-4408 had tensile and yield strength
values exceeding the typical values listed in MPIF Standard 35. Sintering the FLNC4408 at 2300 °F (1260 °C) resulted in complete nickel diffusion, thus increasing the
hardenability and producing hardness values more consistent with the MPIF standard
and giving tensile properties exceeding the typical values listed in Standard 35. The
modified Ancorloy MDC material with its higher hardenability produced hardness values
in excess of the MPIF reference standard. Tensile properties of the modified Ancorloy
MDC material showed ultimate tensile strengths approaching 180,000 psi (1240 MPa) at
7.1g/cm³ density. In addition to the high strength, the silicon containing material
exhibited tensile elongation of approximately 2%. These results are superior to the
commercially available sinter-hardening materials.
Table 3
Tensile Properties of 0.5% Graphite Materials
Material
ID
FLNC4405
Sintering
Temp.
2050 °F
(1120 °C)
2300 °F
(1260 °C)
FLN-4905
(mod)
2050 °F
(1120 °C)
2300 °F
(1260 °C)
Ancorloy
MDC
2150 °F
(1177 °C)
2300 °F
(1260 °C)
Ancorloy
MDCL
2150 °F
(1177 °C)
2300 °F
(1260 °C)
Density,
g/cm³
YS, 10³
psi (MPa)
UTS, 10³
psi (MPa)
Elong.
%
Hdness,
HRA / RC*
6.76
74 (510)
92 (635)
1.3
54 / 88Rb
7.00
7.10
82 (565)
89 (615)
107 (740)
115 (795)
1.5
1.6
58 / 93Rb
59 / 97Rb
6.80
95 (655)
109 (755)
1.3
59 / 97Rb
7.00
7.15
108 (745)
113 (780)
135 (933)
142 (980)
1.8
1.9
62 / 23
63 / 25
6.72
78 (540)
99 (645)
1.3
58 / 95Rb
6.96
7.11
96 (665)
105 (725)
125 (865)
133 (920)
1.6
1.5
62 / 23
63 / 25
6.75
97 (670)
110 (760)
1.3
59 / 97Rb
7.00
7.12
118 (815)
127 (880)
137 (950)
144 (995)
1.5
1.4
63 / 25
65 / 29
6.81
92 (635)
119 (825)
1.5
65 / 29
7.00
7.11
100 (690)
104 (720)
142 (980)
152 (1050)
1.8
1.9
67 / 32
69 / 37
6.80
104 (720)
141 (975)
1.7
66 / 31
6.99
7.09
112 (775)
118 (815)
163 (1125)
179 (1240)
1.9
2.2
68 / 35
70 / 39
6.86
92 (635)
117 (810)
1.5
61 / 21
7.07
7.17
105 (725)
111 (765)
142 (980)
153 (1060)
1.9
2.1
65 / 29
67 / 33
6.89
108 (750)
140 (970)
1.8
63 / 25
7.09
7.19
123 (850)
132 (915)
164 (1135)
181 (1250)
2.1
2.5
66 / 31
68 / 35
* Converted from HRA
The tensile properties of the 0.50% graphite materials are presented in Table 3.
Comparing the tensile properties listed in Table 3 with the properties listed in Table 2,
the tensile properties of the lower carbon materials are in some cases superior to the
tensile properties of the 0.80% graphite material. This seemingly contradiction in
mechanical property response is not uncommon. This reduction in tensile and TRS
properties of sinter-hardening materials with carbon levels above 0.7% was reported by
Baran and is a result of excessive amounts of retained austenite [7]. Figures 4 and 5
show the tempered microstucture of the low and high carbon materials. Note the greater
percentage of retained austenite in the high carbon material relative to the low carbon
steels.
Tensile properties of the Ancorloy MDC materials are superior to the FLNC-4405 and
FLNC-4905 materials evaluated. The presence of the silicon promotes high tensile
strength with good tensile elongation and impact energies not normally associated with
P/M materials at this strength level. The higher density of the Ancorloy MDC materials is
a result of the higher nickel content of these materials promoting greater shrinkage
during sintering. It is also interesting to note that the 0.50% sintered carbon content of
the Ancorloy MDC materials give equal or slightly superior ultimate tensile and yield
strengths relative to the 0.75% graphite addition. The reason for this behavior is the
greater amount of retained austenite in the higher carbon material. More importantly,
the strength is not compromised with the lower graphite addition relative to the high
carbon material.
B.) Carburizing Trials
The objective of the carburizing heat treatment was to develop a 0.030-inch (0.75-mm)
total case depth in the 7.1g/cm³ density components. Vacuum carburizing was selected
for this initial work because of process control and the capability of incorporating rapid
gas quenching to simulate the accelerated cooling used in traditional sinter-hardening
[8]. Figure 1 is a graph showing the carbon gradient developed after vacuum carburizing
for 30 minutes and 60 minutes at 1700 °F (925 °C). As expected, there is no significant
difference in the carburizing response of the FLNC-4405 and the FLNC-4905. The
longer carburizing time produced higher surface carbon and a less steep carbon
gradient.
1.40
FLN C-4405,
FLN C-4905,
FLN C-4405,
FLN C-4905,
Carbon Content, %
1.20
1.00
30
30
60
60
m in
m in
m in
m in
0.80
0.60
0.40
0.20
0.00
0
0.02
0.04
0.06
0.08
0.1
D epth from Surface, inches
Figure 1:
Carbon gradient resulting from vacuum carburizing at 1700 °F (926 °C)
for either 30 or 60 minutes.
From the data presented in Figure 1, the 30-minute carburizing cycle gave a 0.030-inch
(0.75-mm) total case depth. The samples carburized for 60 minutes showed excessive
sooting on the surface of the parts. Because of this sooting condition the carbon content
of the first 0.005-inch (0.12 mm) is not shown for either the 30-minute or 60 minute cycle
time. Metallographic analysis of the carburized samples showed that the 60-minute
carburizing cycle promoted greater retained austenite at the surface. Figure 2 through
Figure 5 show the microstructures of the two carburizing conditions for the FLNC-4405.
Figure 2: Photomicrograph of case of
FLNC-4405, 30 min carburize cycle
Original magnification 400X
Figure 3: Photomicrograph of case of
FLNC-4405, 60 min carburize cycle
Original magnification 400X.
Figure 4: Photomicrograph of core of
FLNC-4405 as tempered
Original magnification 400X
Figure 5: Photomicrograph of case of
FLNC-4408. Original magnification
400X
The photomicrographs shown in Figures 2 through 5 also show a difference in the
tempered martensite structure as a result of the carburizing. Figures 2 and 3 show an
accicular type microstructure that is developed in the case region with carburizing and
subsequent rapid cooling. Whereas, the sinter- hardening FLNC-4408 and the core of
the FLNC-4405 show a lathe type structure. The accicular type tempered martensite
gives better gear performance and is the preferred structure in high performance
wrought steel parts [9]. A similar accicular microstructure was developed in the
carburized Ancorloy MDCL materials. The higher sintering temperature utilized with the
Ancorloy MDCL materials showed a more uniform microstructure with no undiffused
nickel particles.
C.) Rotating Bending Fatigue Test Results
The rotating bending fatigue test results are shown in Table 4. The six sample
conditions tested compared sinter- hardening vs. carburizing after sintering with rapid
gas quenching. All samples were tempered for 1 hour at 400 °F (205 °C). The
carburized samples were rough machined to 0.193-inch +/- .002 inch (4.9-mm +/- 0.05
mm). Final machined and polished fatigue samples measured 0.187-inch +/- .001 inch
(4.85 +/- 0.02 mm) in the reduced area. From carburized to final machined samples,
approximately 0.0025 inch was removed from the radial dimension giving a carbon
profile in the rotating bending fatigue test bars similar to that shown in Figure 1. It was
noteworthy that there was minimal distortion of the pre-machined rotating bending
fatigue bars after the vacuum carburizing / quenching cycle.
Table 4
Rotating Bending Fatigue Properties
Material
FLNC-4408*
Ancorloy
MDC mod*
Sinter
Temp.
Density,
g/cm³
Hardness,
HRA / HRC
50% Fatigue
Limit, 10³ psi
(MPa)
90% Fatigue
Limit, 10³
psi (MPa)
2050 °F
(1120 °C)
7.07
69 / 37
45.7 (315)
44.4 (307)
7.18
70 / 38
47.8 (330)
46.5 (322)
7.07
71 / 41
60.7 (420)
58.4 (404)
7.16
71 / 41
64.7 (448)
62.5 (432)
7.20
70 / 40
54.7 (378)
53.3 (369)
7.20
71 / 42
75.1 (520)
68.1 (471)
2300 °F
(1260 °C)
FLNC-4405
2050 °F
Carburized** (1120 °C)
Ancorloy
2300 °F
MDCL
(1260 °C)
Carburized**
*
Based on 30 samples
**
Based on 20 samples
The fatigue results of the through-hardened materials (FLNC-4408 and Ancorloy MDCL
modified) show an increasing fatigue endurance limit with increasing density. The
fatigue ratio (fatigue limit / ultimate tensile strength) of the FLNC-4408 material is
approximately 38%. For the Ancorloy MDC modified, the fatigue ratio is approximately
36%. These ratios are higher than reported in the literature [11,12,13]. Some minor
discrepancies in the ultimate tensile strength may be the cause. The fatigue strength of
the FLNC-4408 is consistent with the data reported in MPIF Standard 35 [3]. The fatigue
properties of the Ancorloy MDC modified are significantly better than the FLNC-4405 at
equivalent densities. The values shown are superior to the fatigue values reported for
FLN-4205-175HT.
Rotating bending fatigue test results of the carburized FLNC-4405 or the Ancorloy MDCL
materials showed an approximate 15% increase in both the 50% and 90% fatigue
endurance limit compared to the through hardened materials. In wrought steels, these
higher fatigue limits are a result of the compressive surface stresses induced by the
carburized case delaying the onset of fatigue crack initiation at the surface. Unlike
wrought steels, in P/M the crack initiation stage is not limited to the surface of the part
because of the large number of pores within the part. Despite the higher number of
crack initiation sites within any P/M sample, the carburizing still shows a significant
improvement in properties. This improvement in rotating bending fatigue may result
from the nature of the testing itself. Specifically, the maximum stresses in a rotating
bending fatigue sample are on the surface and diminish with distance to the core. Thus,
the higher strength and compressive stresses at the surface of the part resulting from
the carburizing are still beneficial to the overall fatigue performance. It is unknown if
carburizing will result in a similar improvement in axial fatigue testing. However, many
gears and pump hydraulic pump components are subject to alternating stresses similar
to those imposed in rotating bending fatigue testing. It is interesting to note that the
FLNC-4405 material showed a tensile strength of ~120,000 psi, thus carburizing
increased the fatigue ratio to approximately 45% to 50% of the ultimate tensile strength.
One of the objectives of this study was to develop a material with better fatigue
properties relative to the FLNC-4408 material. From this part of the study, two options
exist. First, a change in the material to the Ancorloy MDC modified produces an
approximate 30% improvement in fatigue strength. Alternately, carburizing a FLNC4405 will produce a 15% improvement in fatigue performance. If optimum fatigue
performance is desired, carburizing the Ancorloy MDCL gives a 50% improvement in
fatigue life relative to the standard sinter-hardening material evaluated in this study.
D.) Dimensional Analysis of Sintered, Hardened and Carburized Samples
Shown in Table 5 is the dimensional change data measured on pressed and sinteredhardened toroids of the FLNC-4408 and Ancorloy MDC modified (the sinter hardening
material options evaluated in this study). Although the magnitude of the DC is important
for tool design, a key manufacturing issue is the variability of the dimensional change
from part to part. Throughout this discussion, the variability is defined as one standard
deviation (1σ) of the measured dimensions (a minimum of 10 parts were checked). The
data in Table 5 shows that the variability of the sinter hardening materials ranges from
0.02% to 0.080% (a 6σ of 0.001 to 0.004 inches per inch) on both the OD and the ID.
This dimensional change is within the range reported by MPIF and is consistent with the
results reported by Haberberger, etal [13].
In conjunction with the dimensional change data shown in Table 5, additional toroids
(1.75in x 1in x 1in) from materials 2 and 3 were pressed at both 40 and 50 tsi (550 and
690 MPa). These toroids were sintered at 2050 °F (1120 °C) in a 90% nitrogen and 10%
hydrogen atmosphere and subsequently slowed cooled. After sintering the rings were
then precision machined (+/- 0.0005 inch) on the OD and ID. The objective of this
experiment was to simulate a sintered and sized P/M part which would then be
carburized / gas quenched to increase surface hardness and mechanical properties.
Two sets of toroids were prepared and subjected to two distinct heat treatments; these
were as follows:
• Austenitize at 1600 °F (870 °C) for 30 minutes followed by oil quenching and
tempering at 400 °F (205 °C) for one hour.
• Vacuum carburize at 1700 °F (925 °C) for 30 minutes followed by rapid gas
quenching, and tempering at 400 °F (205 °C) for one hour.
The dimensional change of the OD and ID was then determined and the data in the
quenched and tempered condition is presented in Table 6.
Table 5
Dimensional Change Data for Through Hardening Materials
Material
FLNC
4408
Sintering
Temp.
2050 °F
(1120 °C)
2300 °F
(1260 °C)
Ancorloy
MDC mod
2300 °F
(1260 °C)
HT
Condition
As
sintered
Hardened
Tempered
As
sintered
Hardened
Tempered
Sintered &
Tempered
%DC,
OD
OD DC
(1σ
σ)
%DC,
ID
DC ID
(1σ
σ)
-0.043%
0.0138%
-0.214%
0.0425%
-0.005%
0.012%
0.022%
0.015%
-0.322%
-0.187%
0.028%
0.0768%
-0.045%
0.005%
0.095%
0.036%
0.120%
-0.090%
0.02%
0.018%
0.522%
0.010%
0.03%
0.044%
0.587%
0.01%
0.467%
0.04%
The data presented in Table 6 indicates that the variability (1σ) of the dimensional
change from part to part of the vacuum carburized, rapid gas quenched and tempered
samples is at the minimum 30% lower than the samples that were austenitized, oil
quenched and tempered. This reduction in dimensional variability is significant because
it does demonstrate that the equivalent hardness can be achieved with rapid gas
quenching with part to part variability equivalent to the conventionally sintered-hardened
materials.
Table 6
Dimensional Change Data for Oil Quenched and Tempered &
Vacuum Carburized, Gas Quenched, and Tempered
Material
FLNC
4405
FLNC
4905
Density
Thermal
Treatment
DC of OD
DC of OD,
(1σ
σ)
DC of ID
DC of ID,
(1σ
σ)
7.17
Oil Quench
-0.12%
0.08%
-0.11%
0.09%
7.18
7.25
7.26
Carburized
Oil Quench
Carburized
-0.25%
-0.12%
-0.24%
0.03%
0.06%
0.04%
-0.18%
-0.06%
-0.17%
0.03%
0.10%
0.04%
7.15
Oil Quench
-0.09%
0.07%
+0.01%
0.08%
7.16
7.26
7.27
Carburized
Oil Quench
Carburized
-0.27%
-0.10%
-0.25%
0.02%
0.05%
0.02%
-0.19%
+0.01%
-0.17%
0.05%
0.07%
0.05%
When this program was originally designed, it was anticipated that rapid gas quenching
from the carburizing temperature would give reduced dimensional change variability
relative to oil quenching. The data presented in Table 6 also indicates that the absolute
magnitude of the dimensional change is quite stable for the two densities evaluated.
Specifically, the dimensional change of the OD was approximately -0.25% for all the two
materials and two densities while the dimensional change of the ID were approximately 0.18% for the four conditions. The nearly identical dimensional change response
indicates that the carburization was uniform but as importantly, the dimensional change
response is predictable and consistent despite potential density variations within the part
or part to part. This suggests that this processing can improve the overall part
tolerances of P/M components over the range of densities examined.
The reduced variability of the DC inherent with vacuum carburizing indicates that a
pressed, sintered, and sized component will give very predictable dimensional response.
With the increasing demands for dimensional precision of both the automotive and nonautomotive sectors it is important for the P/M industry to continue to explore new
opportunities to improve the net shape capabilities of the P/M process. To this end,
many P/M parts are sized to improve dimensional precision. Once sized, any
subsequent heat treatment must not degrade that improved dimensional tolerance. The
process described here gives the P/M parts producer the option of heat treating to
increase the mechanical properties while preserving the inherent dimensional precision
achieved with sizing a sintered P/M component.
Discussion
One key objective of this investigation was to develop material and processing options
that would provide improved mechanical properties in an automotive hydraulic pump
application. The reference material for this study was a sinter-hardened FLNC-4408
material. Results presented in this report show that two options exist to achieve
improved properties. The first option is a both a material and processing option;
specifically utilizing a modified Ancorloy MDC (0.75% graphite) produced approximately
a 30% improvement in rotating bending fatigue performance. Along with the improved
fatigue performance, the ultimate tensile strength of this material approached
180,000 psi (1250 MPa) with approximately 2% tensile elongation. The potential
drawback to this option is the necessity to sinter the Ancorly MDC modified at 2300 °F
(1260 °C). As reported by Baran [14], the modified Ancorloy MDC material has
inherently a high hardenability, which implies slower cooling will produce the desired
martensitic microstructure. Also, larger section sizes can be effectively through
hardened with standard sinter hardening furnaces. Some loss in compressibility is
observed with the Ancorloy MDC material but the enhanced properties offset any
lowering of density.
A second option to improved performance is the incorporation of a secondary
carburizing cycle after sizing or sintering. This experimental work utilized vacuum
sintering to achieve the desired results but it is anticipated that conventional furnace
carburizing followed with an accelerated gas cooling will produce the same properties
and dimensional precision. The advantages of this second process are as follows:
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•
•
•
•
Leaner alloy systems can be utilized promoting higher part densities with the
potential to use a smaller compaction press for the same size part
Sizing of the part for improved dimensional precision
Eliminating oil quenching and the associated oil entrapment problems and
the greater variability in dimensions associated with oil quenching
The development of a carburized case and the resulting compressive
surface stresses with the corresponding improvement in mechanical
properties (notably up to a 50% improvement in fatigue endurance limits)
Development of a favorable accicular tempered martensitic microstructure in
the case region that is the preferred microstructure of high performance
wrought components.
Data presented in this report show improved fatigue life with the carburized case.
Although no tensile testing was performed on carburized samples, no significant
improvement in tensile properties is expected [12]. What this process offers is both
mechanical properties plus manufacturability. Although special furnaces may be
required, these are not so unique that they can not be easily produced and maintained.
Additionally, one disadvantage of sinter hardening is the inability to machine the part
after the sinter stage. This process offers the ability to machine after sizing to
incorporate counter bores, cross-holes, etc.
Future Work:
This work has demonstrated that gas carburizing followed by a rapid gas quench can
develop a desirable compressive stress on the surface of a P/M part which results in
higher rotating bending fatigue properties when compared to a standard sinter hardening
material. Additionally, the dimensional distortion associated with this processing is
significantly less than oil quenching and equivalent to a sinter hardening material. All the
carburizing trials done in this paper were performed on simple rings using vacuum
carburizing for a 30 minute carburizing cycle with relatively small furnace loads. To
verify the applicability of this process, it would be useful to extend this work to an actual
P/M part and process larger volumes of parts to monitor the dimensional stability of the
process. To this end, future work to perform in conjunction with this effort is as follows:
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Work with actual P/M part that is currently in production to evaluate the
process capability and resulting mechanical properties
Use ANCORDENSE processing to increase initial part density to produce
higher core mechanical properties
Evaluate lower core carbon contents with the objective of producing higher
core densities and promoting greater ease of coining.
Work with gearing to determine applicability of process to complex gear
geometry.
Acknowledgements:
The authors would like to acknowledge Tom Christian, Craig Gamble, William Bentcliff,
and Ron Fitzpatrick of Hoeganaes Corporation for their assistance in performing the
experimental work reported in this paper. Additionally, we acknowledge the CI Hayes
heat treating division and in particular Richard Houghton for his work in performing the
vacuum carburizing trials.
References:
1.) W. J. James, “ What is sinter hardening?”, Hoeganaes Corporation publication,
www.hoeganaes.com, Technical Library, paper #62.
2.) ASM Metals Handbook Vol. 7, Powder Metallurgy Technologies and Applications,
Published by the American Society for Metals, Copyright 1998, p.652.
3.) Material Standards for P/M Structural Parts, 2000 Edition, MPIF Standard 35,
Published by Metal Powders Industry Federation.
4.) Gear Design, Manufacturing and Inspection Manual, Published by the Society of
Automotive Engineers, Warrendale, PA, 19096, 1990, p.51.
5.) H. Sanderow, CPMT Status Report-RCF Test Program, April/May 1999.
6.) Alan Taylor, 2002, GKN Corporation, private communication.
7.) M. Baran, T. Murphy, “Metallographic Testing to Determine the Influence of Carbon
and Copper on the Retained Austenite Content in a Sinter-Hardening Material”, P/M
Science and Technology Briefs, Vol. 1, No. 3, pp. 22-26.
8.) R. Shivanath, R. Peters, P. Jones, E. El-Sawaf, “Vacuum Carburization of HighPerformance Automotive PM Parts:, Industrial Heating, May 2001, pp. 37-39.
9.) W. Jandeska, 2002, General Motors Corporation, private communication.
10.)
R. C. O’Brien, “Impact and Fatigue Characterization of Selected Ferrous P/M
Materials”, Progress in Powder Metallurgy, Vol. 43, 1987, p.749, published by MPIF,
Princeton, NJ.
11.)
W. B. James, “Ferrous Powders-How Alloying Method Influences Thermal
Processing and Properties”, Industrial Heating, June, 1992, Pp. 34-40.
12.)
S. Saritas, W. James, A. Lawley, “Fatigue Properties of Sintered Steels: A
Critical Review”, Presented at the European Powder Metallurgy Association, October
2001.
13.)
T. Haberberger, F. Hanejko, M. Baran, “Advanced Processing of SinterHardening Materials”, Hoeganaes Corporation publication, www.hoeganaes.com,
Technical library, paper #90.
14.)
M. Baran, “High Performance Alloys – Ancorloy MD Series”,
www.hoeganaes.com, Technical library, paper #93.