Unique Stainless Steel Materials and Processing For High Strength
With Reduced Magnetic Performance
Tom Stuart, Delphi Corporation
Chris Schade, Hoeganaes Corporation
Francis Hanejko, Hoeganaes Corporation
Abstract
A new automotive part was designed having property requirements incorporating high
yield and tensile strengths with good ductility, while at the same time requiring reduced
magnetic performance. This combination of properties precludes the use of iron and low
alloy materials because of their intrinsic magnetic response. In an effort to satisfy both
requirements, type 200 stainless steel powders were prepared and processed into both
mechanical property test specimens and magnetic toroids. Processing of the compacted
samples was optimized to give the required strength characteristics with required
minimal magnetic response. This paper will detail the types of stainless materials
utilized and unique processing developed to accomplish these conflicting requirements.
The mechanical and magnetic properties will be presented.
Introduction
The driving force for this study was the development of a new application that had the
following design criteria:
¾ High yield and tensile strengths; ideally the yield strength should approach
100,000 psi (~700 MPa).
¾ Ductility should be as high as possible with a 2% minimum tensile elongation.
¾ Magnetic permeability response of the pressed and sintered component should
be nearly zero. This prevents flow restrictions when used in conjunction with a
magneto-rheological fluid.
¾ As-sintered net shape part without the need for sizing or secondary machining.
Initial modeling for this device utilized an aluminum alloy (type 7175) that was
extensively machined to produce the final component (Figure 1). Part complexity made
machining difficult; P/M processing offered significant cost reduction opportunities if all
desired design criteria were met.
Figure 1:
Schematic of application, parts are located top and bottom of drawing.
Initially, high strength low alloy irons were considered. Alloys containing 0.8% graphite
would provide the required strength; however, magnetic response of these materials was
still in excess of design requirements (permeability >200). P/M austenitic stainless
steels also were considered but these alloys lack the necessary yield strength and
tensile strength required in this application. Space restrictions within the final device
precluded the extensive redesign and larger part required to accommodate reduced
strength of the 316 alloy.
Wrought 200 series stainless steels have the high strength and austenitic stainless steel
magnetic response. [1] The 200 series stainless steels contain relatively low amounts of
nickel but also are alloyed with nitrogen for strengthening and as an austenite-forming
element. Currently, there are no P/M equivalents to the 200 series stainless steels;
prealloyed nitrogen dramatically reduces compressibility. A material and processing
scheme was needed that would simulate the magnetic and mechanical property
response of 200 series stainless steels while the advantages of using P/M. This paper
will outline the material and processing parameters evaluated to achieve the desired
design parameters.
Experimental Procedure
Materials investigated
The materials evaluated in this study are listed in Table 1. All alloys were prepared at
Hoeganaes Corporation’s pilot atomization facilities and were premixed with 0.75%
Acrawax C. Particle size analyses of these materials were comparable to commercially
available stainless steel alloys (-100 mesh with ~40% -325 mesh). Graphite was added
as Asbury 3203 natural graphite. Alloy “C” is a commercially available 316 austenitic
stainless alloy; it was incorporated into this study as a baseline for comparison. Alloy
“D” is a duplex stainless steel (ferritic / austenitic microstructure); wrought alloys of this
type possess high strength. [2]
Table 1
Alloys Investigated in this Study
Alloy ID
A
B
C
D
E
Cr, wt%
17.0
17.0
17.0
22.2
17.0
Alloy Composition
Ni, wt %
Si, wt%
Mo, wt%
4.0
0.8
0
4
0.8
1.0
9.0
0.8
2.0
5.5
1.0
3.4
4.0
0.8
1.0
Cb, wt%
0
0
0
0
0.50
Gr, wt%
0.35
0.35
0
0
0.35
Samples prepared
In order to satisfy the key design requirements, it was imperative to determine the yield
strength, tensile strength, and magnetic response of the various materials. The following
samples were prepared:
¾ TRS bars compacted at 40 tsi (550 MPa) and 50 tsi (690 MPa)
¾ “MPIF” Dog bone tensile bars compacted at 40 tsi (550 MPa) and 50 tsi (690
MPa)
¾ Magnetic toroids (2.0 inch (50 mm) OD x 1.8 inch (45 mm) ID x 0.25 inch tall
(6 mm)) were compacted at 40 tsi (550 MPa) and 50 tsi (690 MPa)
Compaction pressures of 40 tsi (550 MPa) and 50 tsi (690 MPa) were selected because
of the high strength / high-density requirements.
Sintering and post sintering heat treatment
Sintering was performed at the temperatures and atmosphere conditions listed in
Table 2. Time at temperature was ~40 minutes for each condition. Tempering of the
samples was investigated because the alloy content of these materials has a significant
potential of forming martensite during cooling. Similar to sinter-hardening alloys,
tempering was investigated as a means to improve the mechanical property response.
Table 2
Sintering and Tempering Conditions Evaluated
Sintering Temperature
2150 °F (1175 °C)
2300 °F (1260 °C)
2350 °F (1290 °C)
2350 °F (1290 °C)
Sintering Atmosphere
90 v/o nitrogen and
10 v/o hydrogen
90 v/o nitrogen and
10 v/o hydrogen
90 v/o nitrogen and
10 v/o hydrogen
50 v/o nitrogen and
50 v/o hydrogen
2350 °F (1290 °C)
100 v/o hydrogen
Tempering
None
None
None
None, 900 °F (480 °C),
1100 °F (595 °C)
None, 900 °F (480 °C),
1100 °F (595 °C)
Results
Mechanical property test results
Green densities for the two compaction pressures and five test materials are shown in
Figure 2. Compressibility of the 316 (austenitic stainless steel) is superior to the ferritic
grades. It is noteworthy that the compressibility of these materials is low in comparison
to traditional pure iron and low alloy irons. Thus, it will be imperative that sinter
densification occur to facilitate attainment of the mechanical properties required for this
application.
6.80
40 tsi (550 MPa)
50 tsi (690 MPa)
Green Density, g/cm³
6.60
6.40
6.20
6.00
5.80
5.60
5.40
A
B
C
D
E
Material ID
Figure 2:
Compressibility of the five alloys studied, all compacted at room
temperature with 0.75% admixed Acrawax C.
Sintered densities, dimensional change (DC), and tensile properties for the five materials
are presented as Tables 3 through 7. All data shown represent sintering times of ~40
minutes at temperature.
Table 3
As Sintered Properties of Alloy A
(17% Cr, 4% Ni, 0.8% Si, 0.35% Gr)
Sinter
Cond.
1175 °C,
90 / 10
1260 °C,
90 / 10
1290° C,
90 / 10
1290°C,
50 / 50
1290°C,
100
Compact Sinter
tsi (MPa) g/cm³
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
6.57
6.77
6.68
6.90
6.70
6.92
6.73
6.96
6.80
6.94
DC,
%
TRS, 10³
psi (MPa)
YS, 10³
psi (MPa)
UTS, 10³
psi (MPa)
El,%
-0.79
-0.71
-1.37
-1.35
-1.50
-1.47
-1.61
-1.65
-2.36
-1.90
151 (1040)
174 (1200)
160 (1605)
188 (1295)
144 (995)
210 (1450)
177 (1220)
187 (1290)
245 (1690)
278 (1915)
53.9 (370)
59.6 (410)
52.2 (360)
56.9 (390)
53.6 (370)
57.5 (395)
59.3 (415)
62.5 (430)
59.9 (415)
62.6 (431)
83.6 (575)
94.0 (650)
83.0 (570)
89.4 (620)
82.8 (570)
87.9 (605)
98.3 (675)
90.9 (625)
126.2 (870)
142.2 (980)
1.8
2.0
2.5
3.2
2.7
3.5
1.6
2.4
4.5
5.6
Table 4
As Sintered Properties of Alloy B
(17% Cr, 4% Ni, 0.8% Si, 1.0 % Mo, 0.35% Gr)
Sinter
Cond.
1175 °C,
90 / 10
1260 °C,
90 / 10
1290 °C,
90 / 10
1290 °C,
50 / 50
1290 °C,
100
Compact,
tsi (MPa)
Sinter
g/cm³
DC,
%
TRS, 10³
psi (MPa)
YS, 10³
psi (MPa)
UTS, 10³
psi (MPa)
EL,%
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
6.53
6.77
6.67
6.92
6.68
6.93
6.70
6.98
6.86
7.01
-1.13
-1.02
-1.83
-1.75
-1.9
-1.88
-2.04
-2.04
-3.14
-2.62
157 (1080)
186 (1280)
186 (1280)
209 (1440)
203 (1400)
221 (1525)
210 (1450)
210 (1450)
239 (1650)
272 (1875)
56.9 (390)
61.7 (425)
59.3 (410)
60.7 (420)
60.7 (420)
63.0 (435)
64.4 (445)
65.9 (455)
54.7 (375)
60.3 (415)
88.5 (610)
98.7 (680)
93.2 (640)
93.4 (645)
93.8 (645)
95.2 (655)
104.7 (720)
104.7 (720)
125.8 (865)
138.8 (955)
2.0
2.4
2.3
3.0
2.4
3.1
2.7
3.0
6.6
7.0
Table 5
As Sintered Properties of Alloy C
(17% Cr, 9% Ni, 0.8% Si, 2% Mo)
Sinter
Cond.
1175 °C,
90 / 10
1260 °C,
90 / 10
1290 °C,
90 / 10
1290 °C,
50 / 50
1290 °C,
100
Compact,
tsi (MPa)
Sinter
g/cm³
DC,
%
TRS, 10³
psi (MPa)
YS, 10³
psi (MPa)
UTS, 10³
psi (MPa)
EL,
%
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
6.60
6.84
6.70
6.93
6.73
6.96
6.76
6.97
6.80
7.01
-0.43
-0.37
-1.01
-0.87
-1.17
-1.03
-1.31
-1.11
-1.68
-1.46
125 (860)
147 (1015)
142 (980)
177 (1220)
147 (1015)
161 (1110)
163 (1125)
177 (1220)
145 (1000)
157 (1080)
48.7 (335)
56.0 (385)
50.7 (350)
55.7 (385)
51.0 (350)
55.8 (385)
53.5 (370)
55.9 (385)
39.2 (270)
40.7 (280)
58.4 (405)
67.9 (370)
64.7 (445)
72.8 (500)
64.0 (440)
74.9 (515)
70.0 (485)
77.4 (535)
62.3 (430)
68.1 (470)
2.9
4.0
6.5
8.3
6.4
9.7
7.2
11.5
17.4
21.1
Table 6
As Sintered Properties of Alloy D
(%Cr, %Ni, %Si, %Gr)
Sinter
Cond.
1175 °C
90 / 10
1260 °C,
90 / 10
1290 ° C
90 / 10
1290 °C,
50 / 50
1290 °C,
100
Compact,
tsi (MPa)
Sinter
g/cm³
DC,
%
TRS, 10³
psi (MPa)
YS, 10³
psi (MPa)
UTS, 10³
psi (MPa)
EL,%
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
6.10
6.39
6.22
6.49
6.57
6.79
7.03
7.20
7.06
7.20
-0.48
-0.46
-1.17
-1.07
-3.15
-2.70
-5.36
-4.60
-6.21
-5.33
113 (780)
133 (915)
132 (915)
156 (1075)
192 (1325)
214 (1475)
194 (1340)
197 (1360)
200 (1380)
202 (1395)
44.8 (310)
52.8 (364)
47.3 (325)
54.4 (375)
61.5 (425)
66.8 (460)
64.4 (445)
61.0 (420)
61.2 (420)
61.9 (425)
58.7 (405)
70.0 (485)
65.8 (455)
75.7 (520)
88.4 (610)
95.8 (660)
81.4 (560)
80.9 (560)
81.7 (565)
84.1 (580)
1.1
1.4
2.1
2.6
4.7
5.3
5.5
7.6
13.3
10.8
Table 7
As Sintered Properties of Alloy E
(17% Cr, 4% Ni, 0.8% Si, 1.0% Mo, 0.50% Cb, 0.35% Gr)
Sinter
Cond.
Compact,
tsi (MPa)
Sinter,
g/cm³
DC,
%
TRS, 10³
psi (MPa)
YS, 10³ psi
(MPa)
UTS, 10³
psi (MPa)
EL,
%
2150F,
90 / 10
2300 F,
90 / 10
1290°C
90 / 10
1290°C
50 / 50
1290°C
100
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
40 (550)
50 (690)
6.27
6.53
6.39
6.67
6.45
6.72
6.53
6.77
6.82
7.00
-0.32
-0.28
-1.00
-0.97
-1.35
-1.27
-1.82
-1.61
-3.70
-3.05
100 (700)
123 (850)
125 (850)
147 (1000)
138 (950)
157 (1075)
141 (1000)
170 (1175)
222 (1550)
226 1550)
42.8 (295)
50.0 (345)
46.4 (320)
54.4 (375)
49.3 (340
55.3 (380)
53.1 (365)
57.3 (395)
57.9 (400)
65.5 (450)
53.6 (370)
63.8 (440)
63.2 (435)
74.4 (515)
64.9 (445)
73.6 (505)
70.3 (485)
78.6 (540)
102.3 (705)
104.3 (720)
1.2
1.3
3.1
3.8
4.4
5.1
4.7
8.0
5.7
5.7
As-sintered tensile properties show these 5 alloys develop the required 2% minimum
tensile ductility following sintering at 2300 °F (1260 °C) or 2350 °F (1290 °C). The
commercially available 316 have the highest ductility of all alloys; however, the yield
strength and tensile strength are significantly below the design objectives. Alloy A and
Alloy B possess the highest tensile strength but the yield strength at approximately
60,000 psi (415 MPa) is below the design parameter.
Another important aspect of the data present in Table 3 through Table 7 is the DC after
sintering. As depicted in Figure 1, the component has a complex geometry with critical
“cored” details, any sintering distortion will affect the in-service performance of this part.
Thus over the range of green densities studied, DC response should remain constant to
prevent component distortion; thus eliminating the need for a sizing operation. Alloys A,
B, C, and E show uniform DC at the two densities for the sintering conditions evaluated.
Alloy D (duplex phase) exhibits a large variation in DC with minor changes in green
densities. This implies that Alloy D has a greater tendency to distort during sintering,
thus this alloy was deleted from further evaluation.
One of the stated design criteria was a yield strength approaching 100,000 psi (690
MPa). The tensile data developed and presented in Table 3 through Table 7 fall short of
this objective. Consequently, tempering of Alloys A, B, and E was investigated what
effect it had on mechanical properties.
Presented in Table 8 through Table 10 are the tensile data for materials A, B & E as a
function of tempering temperatures, all tempering was done for 1 hour at temperature in
a 100% nitrogen atmosphere. As stated earlier, the tempering response in these alloys
occurs because of their high hardenability producing martensite during cooling.
Tempering acts to stress relieve the material, increasing the yield strength and tensile
strength. For brevity, the tensile data shown in the subsequent table represents the
50 tsi (690 MPa) compaction condition. Initially, a tempering temperature as low as
400 °F (205 °C) was investigated; however, no significant response was observed until a
temperature of 900 °F (480 °C) was utilized. This tempering temperature is consistent
with the tempering response of the wrought 200 series stainless steel.
Table 8
Tempering Response for Material A
(17% Cr, 4% Ni, 0.8% Si, 0.35% Gr)
Density,
g/cm³
6.77
6.90
6.92
6.96
6.96
6.96
6.94
6.94
6.94
Sinter
Cond, ° C
1175
1260
1290
1290
1290
1290
1290
1290
1290
Temper
°C
None
None
None
None
480
595
None
480
595
YS, 10³ psi
(MPa)
TS, 10³ psi
(MPa)
59.6 (415)
56.9 (415)
57.5 (400)
62.5 (430)
70.5 (490)
62.0 (430)
62.6 (435)
89.4 (620)
76.5 (530)
94.0 (650)
89.4 (620)
87.6 (605)
90.9 (625)
87.4 (600)
94.0 (650)
142.2 (980)
134.8 (931)
117.6 (813)
El, %
2.0
3.2
3.5
2.4
1.3
3.1
5.6
2.7
5.9
Hardness,
HRA
59
55
55
52
51
50
60
62
54
Table 9
Tempering Response for Material B
(17% Cr, 4% Ni, 0.8% Si, 1.0% Mo, 0.35% Gr)
Density,
g/cm³
6.77
6.92
6.93
6.98
6.98
6.98
7.01
7.01
7.01
Sinter
Cond, ° C
1175
1260
1290
1290
1290
1290
1290
1290
1290
Temper
°C
None
None
None
None
480
595
None
480
595
YS, 10³ psi
(MPa)
61.7 (425)
60.7 (420)
63.0 (435)
65.9 (455)
73.9 (510)
69.5 (480)
60.3 (415)
84.5 (585)
76.0 (525)
TS, 10³ psi
(MPa)
98.7 (685)
93.4 (650)
95.2 (655)
104.7 (725)
104.3 (715)
105.4 (725)
138.8 (960)
142.0 (980)
121.6 (840)
El, %
2.4
3.0
3.1
3.0
2.3
4.0
7.0
6.8
6.8
Hardness,
HRA
59
56
56
55
56
55
59
61
56
Table 10
Tempering Response for Material B
(17% Cr, 4% Ni, 0.8% Si, 1.0% Mo, 0.50% Cb, 0.35% Gr)
Density,
g/cm³
6.53
6.67
6.72
6.77
6.77
6.77
7.00
7.00
7.00
Sinter
Cond, ° C
1175
1260
1290
1290
1290
1290
1290
1290
1290
Temper
°C
None
None
None
None
480
595
None
480
595
YS, 10³ psi
(MPa)
50.0 (345)
54.4 (370)
55.3 (380)
57.3 (395)
57.4 (395)
58.6 (405)
65.6 (450)
86.6 (600)
89.3 (615)
TS, 10³ psi
(MPa)
63.8 (440)
74.4 (510)
73.6 (510)
78.6 (545)
78.8 (545)
78.7 (545)
104.3 (715)
115.9 (800)
110.3 (760)
El, %
1.3
3.8
5.1
8.0
9.1
8.4
5.7
5.3
5.4
Hardness,
HRA
52
49
48
47
48
48
55
57
58
Tempering of Alloy A, Alloy B, and Alloy D developed yield strengths greater than 70,000
psi (485 a) approaching the 100,000-psi (690 MPa) design criteria. The highest
mechanical properties were achieved with sintering at 2350 °F (1290 °C) in 100%
hydrogen. However, as will be discussed in the subsequent section, this sintering
conditions results in a magnetic permeability greater than 200 precluding its use in this
application. Alloy A and Alloy B show minor improvements in the yield strength up to ~
70,000 psi (480 MPa) with tempering at 900 °F (480 °C) subsequent to sintering in a 90
v/o nitrogen and 10 v/o hydrogen or 50 v/o nitrogen and 50 v/o hydrogen atmosphere.
Magnetic Properties
The magnetic data of the various materials are summarized in Table 11. The 316
stainless (Alloy C) has no magnetic response, Alloy D was not tested because of its DC
response. For brevity, only data at compaction conditions of 50 tsi are presented.
Testing was done at 15 Oersteds. The key in this testing was to determine the magnetic
response and determine which material and processing produced nearly zero magnetic
response.
Table 11
Magnetic Response at 15 Oersteds of Various Alloys
Material
A
A
A
B
B
B
E
E
Sintering
Temperature, °C
1175
1260
1290
1175
1260
1290
1175
1290
Sintering Atmos
%N2/%H
90 / 10
90 / 10
50 / 50
90 / 10
90 / 10
50 / 50
90 / 10
50 / 50
Permeability
5
2
30
4
3
83
3
45
Bs at 15 Oe,
Gauss
92
55
850
77
70
1010
48
1400
From the data in Table 11, sintering in a 90/10 atmosphere gave almost no magnetic
response (approaching paramagnetic conditions). [3] Increasing the amount of hydrogen
in the atmosphere to 50% and then to 100%, increased the permeability to
approximately 50 and then to >200, respectively. [4] Thus from this part of the study, it
was determined that sintering in the 90 v/o nitrogen and 10 v/o hydrogen at either 2300
°F (1260 °C) or 2350 °F (1290 °C) was necessary to give the required magnetic
response and mechanical properties. Additionally, the necessity of sintering in 90 v/o
nitrogen and 10 v/o hydrogen eliminates Alloy E because its sintered density and
mechanical properties are lower than Alloy A or Alloy B under these processing
conditions. Alloy B is the preferred material system because it has the highest strength.
Discussion
This investigation focused on material and processing selection to produce a part that
possesses good mechanical properties but almost zero magnetic response. Of the
alloys chosen, Alloy B (17% Cr, 4 % Ni, 0.8 % Si, 1.0 % Mo) has the highest level of
mechanical properties, consistent DC, and low magnetic response if sintered in a 90 v/o
nitrogen and 10 v/o hydrogen atmosphere. Increasing the amount of hydrogen in the
sintering atmosphere improves the mechanical properties but results in increased
permeability. Shown in Figure 3 is test data developed on components relative to the
machined aluminum model material. The ideal response is that represented by the top
three curves: one is the aluminum standard and two P/M components sintered at 2350F
in 90 v/o nitrogen and 10 v/o hydrogen. Note the nearly identical response of developed
force vs. velocity. The lower response curve in Figure 3 represents sintering at 2350F in
a 50/50 atmosphere. Note the degradation in response. This lower response is
because of the increased permeability affecting the magneto-rheological fluid.
Figure 3:
Test data illustrating effect of magnetic permeability on component
response
Sintering in high nitrogen atmospheres results in nitrogen pick up of the components as
shown in Table 12. The nitrogen acts both to strengthen the stainless steel and pins the
magnetic domains, thus giving the desired overall response. Mechanical property test
results and FEA showed the reduced yield strength necessitated thickening of the
central region of the component. However, the increase in thickness dimension was
less than required if a 316 austenitic steel was specified.
Table 12
Nitrogen Pick-up with Sintering Temperature and Atmosphere
Sintering Temperature,
°C
1175
1260
1290
1290
1290
Sintering Atmosphere
(v/o N2 / v/o H2)
90 / 10
90 / 10
90 / 10
50 / 50
0 / 100
Nitrogen Content, %
0.63
0.63
0.71
0.22
0.03
Conclusions
From this study, the following conclusions were reached
¾ It was possible to produce a 200 series stainless steel that produced yield
strengths of ~ 70, 000 psi (480 MPa) with the magnetic permeability response
of less than 5, as required for the new part design. The optimum alloy had
the following composition: (17 % Cr, 4 % Ni, 0.80% Si, 1.0 % Mo)
¾ A minimum sintering temperature of 2300 °F (1260 °C) was required to
produce the sinter densification to give the required mechanical properties.
¾ Sintering atmosphere was critical, a 90 v/o nitrogen and 10 v/o hydrogen
atmosphere is optimal to promote the nitrogen pick-up to give the increase in
mechanical strength plus the reduced magnetic response.
¾ FEA modeling of the reduced strength showed that an increase in thickness
of the central region of the part was necessary.
¾ Actual component performance demonstrated that the P/M parts had identical
service performance to machined wrought aluminum parts.
¾ Dimensional stability of the parts is critical to insure proper function and
eliminate the need for a sizing step after sintering.
References:
1. Making, Shaping and Treating of Steel, edited by Harold McGannon, United
States Steel, Ninth Edition, 1970, p 1164 – 1184.
2. The International Molybdenum Association, “Practical Guideline for the
Fabrication of Duplex Stainless Steels”, Edited by Technical Marketing
Resources, Inc. Pittsburgh, PA 2001.
3. Richard Bozorth, “Ferromagnetism”, D. Van Nostrand Company,
Princeton,NJ,1951.
4. H. Kopech, H. Rutz, P. dePoutiloff, “Effects of Powder Processing on Soft
Magnetic Performance of 400-Series Stainless Steel Parts”