Hardenability Response of Lean Fe-Mo-Ni-C PM Alloys
Kylan McQuaig and Peter Sokolowski
Hoeganaes Corporation
Cinnaminson, NJ 08077
Abstract
The relationship between alloy content and part cross section is an important consideration for heat
treated PM parts. While increasing alloy content results in higher hardenability at a given cross section,
minimizing the alloying elements in a PM material can represent a significant cost savings, as long as the
desired mechanical properties are met. This study looks at optimization of hardenability and mechanical
properties in lean Fe-Mo-Ni-C heat treated parts, while minimizing alloy additions for a range of sample
sizes.
Introduction
Historically, PM parts producers have utilized additions of alloying elements such as molybdenum (Mo),
nickel (Ni), and copper (Cu) to increase the hardenability and mechanical properties of compacted parts
[1-4]. These types of alloying elements are especially effective in increasing hardenability when
prealloyed in the base iron, but the compressibility of the base powder can be drastically reduced. In
contrast, base iron compressibility is retained when these additions are admixed or diffusion bonded, but
the hardenability improvement is not nearly as great.
It has been found that using prealloyed Mo is ideal for increasing hardenability without losing the high
compressibility of the base iron [3,4]. Common prealloyed Mo additions to base iron are 0.3, 0.5, 0.85,
and 1.5 mass %, depending on the cross section and final application of the desired part. In general, it is
well-known that higher Mo content will raise hardenability of a material effectively, but at higher cost.
Faster cooling rates will also increase martensite content in a finished PM part, and can be achieved either
within the sintering furnace through an accelerated cooling system or using a post-sintering heat treating
operation. This study will focus on heat treated parts, where an increase in part hardness and strength is
achieved after sintering through austenization followed by a rapid oil quench. The martensitic
microstructure of the samples is then tempered to regain some ductility and improve strength.
Because of the price volatility observed in recent years, reducing or replacing excessive additions of
costly alloying elements has become an important directive in the PM industry. Completely replacing
alloying elements such as Mo and Ni can be difficult due to manufacturing and processing constraints
surrounding many of the alternative materials [2-4]. Therefore, limiting the amount of alloy content
based on part size and desired mechanical properties is vital. The aim of this study is to show that
depending on the final part cross section, alloy content and cost can be minimized while still achieving
the necessary mechanical properties for the desired application.
Experimental Procedure
Commercially available powders from Hoeganaes Corporation, Ancorsteel 30 HP (30 HP), Ancorsteel 50
HP (50 HP), Ancorsteel 85 HP (85 HP), and Ancorsteel 2000 were used as the base alloys for the leanalloy formulations in this study. The nominal compositions of the mixes are shown in Table 1 in mass %,
where the Mo additions are prealloyed in the base alloy, while the Ni and graphite additions are admixed
unless otherwise noted. Each Mo-containing base alloy (30 HP, 50 HP, and 85 HP) had 0.5 mass %
admixed Inco T123 Ni to create mixes 1-3. Mix 4 was a prealloy, designed to have a similar composition
to mix 2. Mix 5 contained no Mo and a nominal 2 mass % Ni addition with an unalloyed base iron. All
mixes were made with 0.75 mass % Acrawax C lubricant and 0.6 mass % Asbury type 3203H graphite.
Transverse rupture (TR) strength bars were compacted using pressures of 415, 550, and 690 MPa to
determine the static mechanical properties of each mix. Unnotched Charpy impact bars were also
compacted to 7.0 g/cm3 green density for impact energy determination.
Table 1: Nominal compositions of alloys studied (in mass %)
Mix
Alloy
Mo
Ni
Graphite
1
2
3
4
5
30 HP + 0.5 Ni
0.30
0.50
0.85
0.50
---
0.5
0.5
0.5
0.5*
2.0
0.6
0.6
0.6
0.6
0.6
50 HP + 0.5 Ni
85 HP + 0.5 Ni
FL-4205
FLN
FN-0205
* Prealloyed Ni addition
All test bars were sintered in an Abbott continuous-belt furnace at 1120 C (2050 F) for 15 minutes in a
mixed atmosphere of 90 volume % nitrogen and 10 volume % hydrogen. Heat treating was performed by
austenitizing at 900 °C (1650 F) for 45 minutes in synthetic dissociated ammonia, followed by
quenching in a 65 °C (150 F) circulating oil bath. All heat-treated samples were tempered at 205 °C
(400 F) for one hour prior to physical and mechanical measurements. The sintered density, dimensional
change, and apparent hardness were determined on TR bars following MPIF Standards 42, 43, and 44.
Impact energy and TR strength testing adhered to MPIF Standards 40 and 41 [5]. Sintered carbon values
were measured using a Leco 200 carbon-sulfur combustion gas analyzer with reference standards run
before and after test samples.
The effect of sample size on final martensite content was studied for each material on cylindrical slugs
pressed to a density of 7.0 g/cm3, a height of approximately 2.5 cm (1 inch), and increasing diameters of
1.3, 2.5, and 3.8 cm (0.5, 1.0, and 1.5 inches). The relative sizes of the cylindrical slugs are shown in
Figure 1. The samples were then sintered and heat treated using the methods described previously.
Following heat treatment, all slugs were sectioned and Rockwell A apparent hardness measurements were
taken along the diameter of each sample as shown in the schematic in Figure 2 below. The hardness
measurements along the diameter were repeated three times and averaged in order to ensure accuracy in
the event that there were density variations or imperfections in the samples. These samples were then
ground and polished using standard metallographic preparation, and the microstructures were observed at
the center of each specimen.
Figure 1: Relative sizes of the final sintered slugs with diameters of 1.3 cm (a), 2.5 cm (b), and 3.8 cm (c)
Figure 2: Hardness measurement locations on heat treated cylindrical specimens
Results
Mechanical Properties
The mechanical properties of the heat treated materials were determined and are summarized in Table 2.
TR data were collected at three different compaction pressures, and Charpy impact results were obtained
from samples pressed to a 7.0 g/cm3 green density. The parts had similar sintered density values and the
sintered carbon content was found to be within 0.54 ± 0.02 mass % for all five materials. All the samples
used to measure mechanical properties were largely martensitic with similar carbon contents, resulting in
relatively minor differences in the observed mechanical properties between the premixes being compared.
The predominance of martensite in each of these samples was verified by the high apparent hardness
values in Table 2 as well as the final microstructures.
Table 2: Heat-treated and tempered mechanical properties
Material
30 HP + 0.5 Ni
50 HP + 0.5 Ni
85 HP + 0.5 Ni
FL-4205
FN-0205
GD
SD
g/cm3
6.85
7.08
7.17
6.80
7.04
7.16
6.81
7.05
7.16
6.71
6.97
7.11
6.81
7.02
7.11
TRS
g/cm3
DC
%
MPa (psi x 103)
6.81
7.03
7.16
6.77
7.01
7.15
6.78
7.02
7.15
6.68
6.94
7.09
6.80
6.99
7.07
+ 0.04
+ 0.12
+ 0.17
- 0.01
+ 0.05
+ 0.10
+ 0.07
+ 0.11
+ 0.15
- 0.01
+ 0.07
+ 0.08
- 0.13
- 0.03
- 0.01
1153 (167)
1407 (204)
1489 (216)
1119 (162)
1443 (209)
1678 (243)
1085 (157)
1458 (211)
1661 (241)
1062 (154)
1281 (186)
1503 (218)
1205 (175)
1367 (198)
1475 (214)
Apparent
Hardness
HRA (HRC)
62 (24)
68 (36)
71 (41)
63 (24)
64 (27)
69 (37)
66 (31)
68 (36)
70 (40)
61 (22)
67 (34)
68 (36)
60 (21)
66 (31)
66 (31)
Impact
Sintered
3
at 7.0 g/cm
Joules (ft-lbf)
Carbon
%
12 (9)
0.54
14 (10)
0.53
11 (8)
0.55
11 (8)
0.52
14 (10)
0.56
Figure 3 shows the compressibility curves for the all five materials pressed using compaction pressures of
415, 550, and 690 MPa. As expected, of the four materials with 0.5 mass % Ni, the prealloyed material
(FL-4205) had the lowest compressibility at all three compaction pressures tested in this study. The three
prealloyed base alloys with admixed Ni showed little effect of Mo content on compressibility. While the
prealloyed Mo had little effect on compressibility, Figure 3 shows the detrimental effect that prealloyed
Ni can have on green density at a given compaction pressure. Admixing Ni had little effect, as displayed
by the FN-0205 material, even though a higher mass % was added.
Figure 3: Compressibility curves for the materials tested
Figure 4 shows the dimensional change vs. green density for all materials studied. All materials
displayed an increasing dimensional change value with increasing density, and the material with the high
Ni content shrank from die size upon sintering. Figure 5 shows the TR strength vs. sintered density for
the five materials in the heat treated and tempered state. The Mo content, Ni content, and alloying
method were not found to have a dramatic influence on static mechanical properties in this study. For
these materials in the heat treated condition, mechanical properties were governed largely by density and
sintered carbon content. At a given sintered density, little difference was observed in strength regardless
of the material’s chemical composition and initial processing. Similar consistency was observed for the
impact energy for heat treated test bars pressed to a density of 7.0 g/cm3. The impact energy was between
11-14 joules for the all materials tested in this study.
Figure 4: Dimensional change vs. green density for all heat treated samples
Figure 5: TR strength vs. sintered density for the heat treated samples
Evaluation of Alloy Hardenability
Figure 6 shows the apparent hardness of each material in a heat-treated 1.3 cm diameter slug. The slugs
were sectioned following heat treatment and apparent hardness was measured across the diameter in order
to show the hardenability of each material. Because of the small cross section of these slugs, all samples
were found to be almost completely martensitic following heat treatment. As a result, a nearly constant
apparent hardness was observed from the edge of each sample through the center in the heat treated
condition. A small dip in apparent hardness can be seen in the center of the sample with the lowest
prealloyed Mo content (30 HP + 0.5 Ni). The FN-0205 material had the lowest apparent hardness by a
large margin, due to the low hardenability of the unalloyed base iron and the presence of relatively soft
Ni-rich regions. Because of the amount of Ni admixed in the premix (2 mass %), diffusion was not
complete and many of these regions were visible in the final microstructure.
Figure 6: Hardness vs. distance across 1.3 cm diameter heat-treated slug
Different trends were observed when looking at the apparent hardness vs. distance across the diameter in
Figure 7 for 2.5 cm diameter slugs. The hybrid material with the 85 HP base had the highest apparent
hardness, and this hardness value was consistent along the entire cross section of the sample. The
FL-4205 prealloy and 50 HP hybrid material showed similar hardness curves. The accompanying
microstructures supported these observations. The 30 HP-based hybrid shows lower hardenability, while
the FN-0205 showed the lowest hardenability of all the materials studied.
The apparent hardness profiles are shown in Figure 8 for the 3.8 cm diameter heat treated slugs. Once
again, the hybrid 85 HP material and FL-4205 material were found to have the highest hardenability and
were largely martensitic. This time, the 50 HP material showed a small reduction in apparent hardness at
the center of the sample, where the cooling rate was not high enough to achieve a fully martensitic
microstructure at this alloy content. The 30 HP material and FN-0205 had the lowest hardenability
values, and showed a large reduction in apparent hardness only a short distance from the edges of the
sample. The overall apparent hardness values are lower than the other fully hardened materials possibly
due to the difference in sintered density, presence of softer Ni-rich regions, or retained austenite. The
shape of the curves, though, shows a constant apparent hardness value from the edge of each sample
through the center, characteristic of a complete martensite transformation.
Figure 7: Hardness vs. distance across 2.5 cm diameter heat-treated slug
Figure 8: Hardness vs. distance across 3.8 cm diameter heat-treated slug
The microstructures for all the materials are shown in Figure 9. As described previously, the centers of
all five of the 1.3 cm diameter samples show a full transformation to martensite. The samples with
admixed Ni also show several Ni-rich regions, but no pearlite or bainite was observed in samples of this
size. As sample diameter was increased, reduced amounts of martensitic transformation were observed in
several samples. While the 85 HP sample was still completely martensitic in the 2.5 cm diameter sample,
the 50 HP sample showed some small regions of bainite and pearlite and the 30 HP sample had an even
higher number regions that had not transformed to martensite.
At the largest sample diameter, the fully prealloyed material and the material with the 0.85 mass %
prealloyed Mo base alloy still showed fully martensitic microstructures at the center. The 50 HP sample
had a lower hardenability than these materials, with many regions of bainite and pearlite throughout the
center of the sample. The 30 HP and FN-0205 materials had little hardening at this large of a sample size.
In both materials, less than 50 percent of the observed microstructure was martensitic.
1.3 cm
2.5 cm
3.8 cm
30 HP + 0.5 Ni
50 HP + 0.5 Ni
85 HP + 0.5 Ni
FL-4205
FN-0205
Figure 9: Microstructures at the center of the five heat treated samples (micron bars all have length of
100 µm), increasing in slug diameter from left to right
Conclusions
As shown in the results, the amount of prealloyed Mo in the base alloy had only a minimal effect on the
mechanical properties of the heat treated samples as long as a martensitic microstructure was achieved.
The importance of the Mo content is its ability to increase the hardenability of the final part. As
prealloyed Mo content increases, the hardenability of the part during heat treatment also increases. It was
found in this study that the Mo addition in the 30HP was effective for small parts, while 50HP and 85HP
were necessary for larger parts. Determining the amount of Mo needed for a given part geometry can
then eliminate excess cost and alloy content.
It was shown, furthermore, that by controlling alloying method, the properties of the powder such as
compressibility, strength, and ductility can be optimized. While prealloying Mo is ideal for hardenability
improvement, prealloyed Ni provided little benefit and is more efficient as an admixed addition to a mix.
Admixing prevents the negative impact prealloyed Ni has on compressibility, while promoting an
increase in final material properties such as strength and ductility. As was the case with Mo additions, Ni
content can be managed to maximize the effect on mechanical properties. More work still needs to be
done in order to determine the optimum Ni content for these premixes.
Acknowledgements
The authors wish to thank Ron Fitzpatrick for collecting the data, as well as Barry Diamond and Jerry
Golin for contributing the photomicrographs presented within the manuscript.
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