STAINLESS STEEL AISI GRADES FOR PM
APPLICATIONS
Chris Schade
Hoeganaes Corporation
Cinnaminson, NJ 08077
John Schaberl
Ancor Specialties
Ridgway, PA 15853
Alan Lawley
Drexel University
Philadelphia, PA 19104
ABSTRACT
Applications requiring stainless steels are growing at a rate of about 5% annually.
Opportunities for using powder metallurgy (PM) exist, but additional grades not covered
by MPIF Standard 35 are required. The American Iron and Steel Institute (AISI) has
standards for a broad range of stainless steels that can be used in many applications, but
the compositions of these grades must be modified to be conducive to manufacture by
conventional PM techniques. Several of these grades have been produced as standard
press and sinter powders. The physical properties, mechanical properties and
microstructures of these various grades are reviewed to serve as a guideline for PM parts
manufacturers and potential applications of these grades are addressed.
INTRODUCTION
MPIF Standard 351 lists the most common grades of stainless steel used by PM parts
manufacturers. These include austenitic grades such as 303L, 304L and 316L, and ferritic
grades such as 409L, 410L, 430L and 434L. However, with the continued growth of
stainless steel there exists many opportunities for specialized stainless steel grades not
covered by MPIF Standard 35. These include applications requiring enhanced physical
properties, corrosion resistance, weldability and machinability. There are additional
grades covered by the American Iron and Steel Institute (AISI) that can be manufactured
by conventional press and sinter powder metallurgy (PM), Figure 1. The AISI
designation for these alloys is well known with the number series 200 and 300 referring
to austenitic stainless steels and the 400 series covering the ferritic and martensitic
stainless steels. Letter designations attached to the end of the number series indicate
modifications to the composition.2 Many societies such as the Society for Automotive
Engineers (SAE) and the American Society for Testing and Materials (ASTM) use the
AISI specification with the latter adding physical property specifications.3 SAE and
ASTM have worked together to create the unified numbering system (UNS) for metals
and alloys, which is recognized globally and can be used as a cross-reference
internationally. 4 Other references covering both wrought and cast grades of stainless steel
are available.5-6
Figure 1. Available stainless steel alloy systems.3
There are hundreds of commercially available stainless steel compositions, fabricated by
multiple processing steps which modify their properties. Fortunately these stainless
steels can be classified into several distinct categories. These include austenitic, ferritic,
martensitic, precipitation hardening, and duplex stainless steels. For convenience the
development of additional PM grades of stainless steel will adhere to these categories.
ALLOY PREPARATION AND TESTING
The powders used in this study were produced by water atomization with a typical
particle size (100 w/o) <150 µm (–100 mesh) and with 38 to 48 w/o <45 µm (-325
mesh). All the alloying elements were prealloyed into the melt prior to atomization,
unless otherwise noted. Admixed copper, molybdenum and nickel powders were used to
make some compositions and are so designated in the tables of chemical composition.
The stainless powders were mixed with 0.75 w/o Acrawax C lubricant. Samples for
transverse rupture (TR) and tensile testing were compacted uniaxially at 690 MPa (50
tsi). All the test pieces were sintered in a high temperature Abbott continuous-belt
furnace at 1260 °C (2300 °F) for 45 min in hydrogen with a dewpoint of –40 oC (-40 °F),
unless otherwise noted.
Prior to mechanical testing, green and sintered density, dimensional change (DC), and
apparent hardness, were determined on the tensile and TR samples. Five tensile
specimens and five TR specimens were tested for each composition. The densities of the
green and sintered steels were determined in accordance with MPIF Standard 42 and
tensile testing followed MPIF Standard 10. Impact energy specimens were tested in
accordance with MPIF Standard 40. Apparent hardness measurements were conducted
on tensile, TR and impact specimens, following MPIF Standard 43.
Rotating bending fatigue (RBF) specimens were machined from test blanks that were
pressed at 690 MPa (50 tsi) and sintered at 1260 oC (2300 oF). The dimensions of the test
blanks were 12.7 mm x 12.7 mm x 100 mm. RBF tests were performed using rotational
speeds in the range of 7,000-8000 rpm at R equal -1 using four fatigue machines
simultaneously. Thirty specimens were tested for each alloy composition, utilizing the
staircase method to determine the 50% survival limit and the 90% survival limit for 107
cycles (MPIF Standard 56).
Metallographic specimens of the test materials were examined by optical microscopy in
the polished and etched (glyceregia) conditions. Etched specimens were used for
microindentation hardness testing, per MPIF Standard 51.
Salt spray testing on TR bars was performed in accordance with ASTM Standard B 11703. Five TR bars per alloy (prepared as previously described) were tested. The percent
area of the bars covered by red rust was recorded as a function of time. The level of
corrosion was documented photographically.
RESULTS AND DISCUSSION
Ferritic Stainless Steels
For PM applications, the ferritic stainless steels are by far the most widely used grades,
reflecting their application in the automotive industry. Examples are ABS sensor rings
and muffler exhaust flanges. Chromium is the major alloy constituent of the ferritic
grades along with minor additions of other ferrite stabilizers such as silicon and niobium
(Table I). In general, the 400 series, ferritic stainless steels contain 11 to 27 w/o Cr, are
magnetic, have moderate ductlity and corrosion resistance and are relatively weak at high
temperatures.7-8 In order to form the passive oxide layer a minimum of about 11 w/o Cr is
required.
Table I: Composition of PM Ferritic Stainless Steels (w/o)
AISI
S40940
410L1
S41000
416L
S41603
430L1
S43000
434L1
S43400
436
S43600
439
S43035
441
S44100
446
S44600
446
S44626
409L
1
UNS
1
C
0.03
Max.
0.03
Max.
0.03
Max.
0.12
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.03
Max.
S
.030
Max.
.030
Max.
.150
.300
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
P
.040
Max.
.040
Max.
.040
Max.
.040
Max.
.040
Max.
.040
Max.
.040
Max.
.040
Max.
.040
Max.
.040
Max.
Si
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
Cr
10.5
11.7
11.5
13.0
11.5
13.0
16.0
18.0
16.0
18.0
16.0
18.0
16.0
18.0
16.0
18.0
23.0
27.0
25.0
27.5
Ni
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
1.0
Max.
1.0
Max.
Cu
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
Mn
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
Mo
.50
Max.
.50
Max.
.50
Max.
.50
Max.
0.75
1.25
0.75
1.25
.50
Max.
.50
Max.
Nb
.40
.60
--------.40
.60
.40
.60
1.00
---
---
0.75
1.50
.05
.20
Covered by MPIF Standard 35
The early use of these grades was limited by the amount of carbon and nitrogen in the
alloys. With higher levels of carbon and nitrogen, the ductile to brittle transition can
occur at low temperatures. However, with the advent of argon-oxygen-decarburization
(AOD), lower values of nitrogen and carbon have been acheieved and the ductility of
these grades has been greatly enhanced.9 The effect of carbon and nitrogen can furthur be
reduced by the addition of niobium which combines with the interstitial elements to
prevent sensitization. Niobium is also a ferrite stabilizer which helps to prevent the
formation of martensite in the alloys.
In general, the oxidation resistance and mechancial properties (Table II) increase as the
chromium level increases. The addition of other alloying elements to the base
compositions can enhance certain properties. For example, in the case of 434L, when
molybdenum is added, the resistance of the alloy to corrosion by road salt is increased.
Niobium is added to several stainless steel grades to prevent the formation of chromium
carbides which leads to intergranular corrosion (409L,436 and 439). This is particularly
important when welding ferritic stainless steels, since the formation of chromium
carbides is rapid and difficult to avoid. Sulfur can be added to enhance the machinability
of ferritic stainless steels. In AISI 416L, sulfur is prealloyed prior to atomization, and the
element combines with manganese during solidification to form managnese sulfides that
assist in machining. This technique has been used in the PM grade of 303L for many
years.
Table II: Mechanical Properties of PM Ferritic Stainless Steels
Impact
AISI
409L
410L
416L
430L
434L
436
439
441
446
446
UNS
S40940
S41000
S41603
S43000
S43400
S43600
S43035
S44100
S44600
S44626
ft.lbs.f
107
164
110
121
140
42
37
9
67
46
(J)
143
220
147
162
188
56
50
12
90
62
Green Sintered
Density Density
(g/cm3) (g/cm3)
6.56
7.08
6.66
7.25
6.52
7.16
6.59
7.15
6.52
7.11
6.40
6.81
6.32
6.81
6.29
6.38
6.25
6.96
6.23
6.99
TRS
(ksi)
147
177
128
193
141
122
109
75
154
127
(MPa)
1011
1218
881
1328
970
839
750
516
1060
874
Apparent
Hardness
(HRB)
63
64
56
60
61
59
56
32
67
75
UTS
(ksi)
56
57
51
52
57
46
48
29
55
62
(MPa)
385
392
351
358
392
316
330
200
378
427
0.20% OFFSET Elongation
(ksi)
35
33
29
31
36
33
33
23
40
50
(MPa)
241
227
200
213
248
227
227
158
275
344
(%)
15.4
17.0
16.3
14.8
13.7
7.7
11.6
3.5
12.0
11.6
Superferritics have been developed for increased oxidation or scaling resistance. In
general, the higher the chromium content the higher the oxidation rsistance. Additions of
molybdenum and niobium can enhance oxidation resistance even further. Figure 2 shows
that the oxidation resistance of S44626 (containing molybdenum) and S44100
(containing 1 w/o niobium) approach that of high chromium-nickel grades such as 310L
and Hasteloy X (a superalloy).
20
Hastelloy X
Super Ferritic
310L
441
409Cb
% of Initial Weight
15
10
5
0
800
850
900
950
1000
1050
1100
1150
1200
o
Temperature ( C)
Figure 2. Weight gain (in air) as a function of temperature for selected oxidation resistant alloys.
Austenitic Stainless Steels
The AISI 300 austenitic series stainless steels contain nickel and chromium and have
excellent corrosion resistance in diverse environments. The properties of austenite are
generally described as nonmagnetic, with a relatively low yield strength, high ductility
and excellent impact toughness. Austenitic stainless steels behave in a manner similar to
that of low carbon steels but with enhanced high temperature strength and oxidation
resistance. Depending on chemical composition, these stainless steels can resist scaling
up to 1095 oC (2000 oF). Conversely, austenitic stainless steels can be used in low
temperature applications where their high toughness levels are compatible with cryogenic
applications.
Based on Table III, there exists a wide range of 300 series stainless steels suitable for a
variety of applications. This table also includes stainless steel grades commonly used by
the PM industry and detailed in MPIF Standard 35, namely 303L, 304L and 316L. With
the increased use of PM stainless parts an exploration of other grades listed in Table III
would appear to be timely.
Table III: Composition of PM Austenitic Stainless Steels (w/o)
1
AISI
UNS
302B
S30215
303L1
S30300
304L1
S30403
304Cu
S30430
309Cb
S30940
310S
S31008
316L1
S31603
316Cb
S31640
317L
S31703
321L
S32100
904L
N08904
C
0.03
Max.
0.03
Max.
0.03
Max.
0.12
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.03
Max.
0.02
Max.
S
.030
Max.
.150
.300
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
.030
Max.
Si
2.0
3.0
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.5
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
1.0
Max.
Cr
17.0
19.0
17.0
19.0
17.0
19.0
17.0
19.0
22.0
24.0
24.0
26.0
16.0
18.0
16.0
18.0
18.0
20.0
17.0
19.0
19.0
23.0
Ni
8.0
10.0
8.0
10.0
8.0
10.0
8.0
10.0
12.0
16.0
19.0
22.0
10.0
14.0
10.0
14.0
11.0
15.0
9.0
12.0
23.0
28.0
Cu
.50
Max.
.50
Max.
.50
Max.
3.0
4.0
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
1.0
2.0
Mo
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
.50
Max.
2.00
3.00
2.00
3.00
3.00
4.00
.50
Max.
4.0
5.0
Nb
--------.40
.60
----.40
.60
--.40
.60
---
Covered by MPIF Standard 35
There is a growing need to weld PM austenitic stainless steel parts to other structures. In
doing so the normal grades of stainless steel (304L and 316L) are susceptible to
sensitization, particularly in areas adjacent to the weld. Sensitization is the process by
which chromium combines with carbon to form chromium carbides. The chromium is
removed from areas close to the grain boundaries and leaves these areas depleted of
chromium, with attendant susceptibility to intergranular corrosion. The formation of
chromium carbides is enhanced by temperature and generally occurs in austenitic
stainless steels at temperatures between 480 oC and 815 oC (900 oF and 1500 oF). The
cooling rate resulting from the welding process is generally slow which increases the
likelihood of chromium carbide formation. Increased carbon levels due to insufficient
lubricant burn-off can also increase the chance of sensitization. In order to avoid
postweld heat-treatment, stabilized grades of austenitic stainless steels have been
developed.
In order to prevent the chromium from forming carbides, strong carbide-forming
elements have been added to the austenitic grades of stainless steel. In AISI grades
309Cb, 316Cb and 321L, titanium and niobium were added for this reason. In PM
products, the use of niobium is preferred because water atomization oxidizes the titanium.
Table IV lists the mechanical properties of these niobium-stabilized austenitic PM grades.
In general, due to the addition of niobium and the formation of carbides, the ductility and
impact toughness of these grades are slightly lower than those of the non-stabilized
grades. However, in general, this decline in mechanical properties is small, and can be
compensated for by an increase in the level of other elements (such as nickel and
chromium). Due to the formation of the carbides, the creep resistance of these grades of
stainless steel is improved.
Table IV: Mechanical Properties of PM Austenitic Stainless Steels
Impact
AISI
302B
303L
304L
304Cu
309Cb
310S
316L
316Cb
317L
321L
904L
UNS
S30215
S30300
S30403
S30430
S30940
S31008
S31603
S31640
S31703
S32100
N08904
ft.lbs.f
70
63
60
64
25
38
77
73
66
53
44
(J)
94
84
80
86
34
51
103
98
88
71
59
Green Sintered
Density Density
(g/cm3)
6.44
6.60
6.57
6.73
6.45
6.42
6.74
6.56
6.72
6.66
6.52
(g/cm3)
6.72
6.97
6.90
6.86
6.76
6.80
7.13
6.88
7.01
6.90
6.80
TRS
(ksi)
210
205
180
129
119
139
138
137
162
128
130
(MPa)
1445
1410
1238
888
819
956
949
943
1115
881
894
Apparent
Hardness
(HRB)
56
48
55
28
62
63
56
51
61
57
56
UTS
(ksi)
59
55
61
55
63
61
61
51
70
54
55
(MPa)
406
378
420
378
433
420
420
351
482
372
378
0.20% OFFSET Elongation
(ksi)
32
28
35
30
41
41
32
29
42
32
33
(MPa)
220
193
241
206
282
282
220
200
289
220
227
(%)
24.8
25.4
22.4
20.0
14.9
14.5
27.0
15.0
22.0
16.2
14.8
Other alloying elements, such as molybdenum, can be added to the austenitic stainless
steels to improve corrosion resistance. Molybdenum, when added at levels between 2
w/o and 4 w/o improves resistance to oxidation, pitting and crevice corrosion. The
addition of molybdenum also tends to improve both room and high temperature
properties such as tensile strength and creep. The mechanical properties of AISI 317L are
cited in Table IV and the corrosion resistance is illustrated in Figure 3. Currently PM
fabricators sinter austenitic grades in a hydrogen/nitrogen atmosphere to increase strength
(MPIF Standard 35: grades SS-304N and SS-316N). In so doing, significant chromium
nitride formation occurs, which is detrimental to the overall corrosion resistance of the
alloy. The addition of molybdenum is beneficial if both corrosion resistance and strength
are required.
(a)
(b)
(c)
Figure 3. Representative appearance of salt spray specimens: (a.) 304L, (b.) 316L, (c.) 317L.
More highly alloyed 300 series stainless steels are available that are designed to resist
oxidation at high temperatures while maintaining a high degree of tensile strength and
creep resistance. These alloys rely on the formation of the chromium oxide film for
protection from corrosion, but the additional nickel and silicon in these alloys helps to
form a more ductile scale, which increases its adherence to the base metal. The adherent
scale is particularly important when service conditions involve cyclic temperatures. The
properties of several of these PM grades (302B, 304L and 310L) are listed in Table IV
and the relative oxidation resistance of these grades is shown in Figure 4.
1.4
304L
302B
310L
1.2
% of Initial Weight
1
0.8
0.6
0.4
0.2
0
600
700
800
900
1000
1100
o
Temperature ( C)
Figure 4. Oxidation of 300 series stainless steels.
1200
As with the other categories of stainless steels, there exists “super austenitics” where
increased levels of nickel, copper, and molybdenum provide superior or specialized
corrosion resistance. However, for the PM grades of these alloys the increased alloy
content has a negative impact on powder compressibility and therefore on overall density.
In consequence, care has to be taken to ensure that the increased corrosion resistance and
enhanced mechanical properties gained by the increase in alloy content are not offset by a
reduction in achievable density. Type 904 stainless steel is considered a “super
austenitic” stainless steel.
Martensitic Stainless Steels
Martensitic steels in the 400 series are similar to the ferritic stainless steels in that they
contain chromium in the range of 11 w/o to 18 w/o but also contain other elements such
as nickel Table V. The martensitic stainless steels are magnetic and are generally used in
applications where hardness and/or wear resistance is required. When heat-treated they
can achieve high strength and when tempered, they can exhibit some ductility.
Essentially, these steels achieve mechanical properties comparable with those of a heattreatable low alloy steel but with enhanced corrosion resistance, although their corrosion
resistance is the lowest of any of the stainless steel categories. MPIF standard 35
recognizes SS-410-90HT as a martensitic alloy. For this grade, the sintering atmosphere
contains a high level of nitrogen, and the alloys form high temperature austenite, which
transforms to martensite on cooling.
Table V: Composition of PM Martensitic Stainless Steels (w/o)
C
S
Si
Cr
Ni
Cu
Mo
Nb
0.03
.030
1.0
12.0
1.25
.50
.50
S41400
--414M
Max.
Max.
Max.
15.0
2.50
Max.
Max.
0.03
.030
1.0
12.0
4.0
1.5
1.5
S41426
--414M
Max.
Max.
Max.
15.0
7.0
2.0
2.0
0.03
.030
1.0
11.5
3.5
.50
.5
S41500
--415M
Max.
Max.
Max.
14.0
5.5
Max.
1.0
.15
.030
1.0
12.0
.50
.50
.50
S42000
--420M
.30
Max.
Max.
14.0
Max.
Max.
Max.
0.75
.030
1.0
16.0
.50
.50
.75
S44003
--440BM
0.95
Max.
Max.
18.0
Max.
Max.
Max.
0.95
.030
1.0
16.0
.50
.50
.75
S44004
--440CM
1.20
Max.
Max.
18.0
Max.
Max.
Max.
0.15
.030
1.5
11.5
1.0
3.0
.50
410LCu
J91151
--Max.
Max.
Max.
14.0
Max.
5.0
Max.
M
Designates material made from a mix of a base powder and additives such as nickel, graphite, copper and
molybdenum.
AISI
UNS
Other AISI martensitic grades of stainless steel, such as 420,440A, 440B and 440C, can
be processed by adding graphite to ferritic grades of stainless steels such as 410L and
430L. Table VI gives the properties of 420L, 440B and 440C made by this approach.
The level of carbon added to the alloy dictates the mechanical properties of the
martensitic stainless steel. The higher the carbon content, the larger the extent of
chromium carbide formation, and the higher the strength and apparent hardness of the
alloy.
Table VI: Mechanical Properties of PM Martensitic Stainless Steels
Impact
AISI
UNS
ft.lbs.f
(J)
410LCu
414
414
415M
J91151
S41400
S41426
S41500
13
50
21
26
17
67
28
35
415PA
420
440B
440B HT
440C
440C HT
S41500
S42000
S44003
S44003
S44004
S44004
11
24
20
3
13
3
15
32
27
4
17
4
Green Sintered
Density Density
(g/cm3) (g/cm3)
6.43
6.84
6.55
6.95
6.62
7.00
6.69
7.03
6.17
6.68
6.58
6.58
5.56
5.56
6.52
6.86
6.82
6.76
6.91
6.90
(ksi)
(MPa)
Apparent
Hardness
(HRB)
(ksi)
(MPa)
(%)
249
223
209
253
1713
1534
1438
1741
95
90
89
96
116
98
100
108
798
674
688
743
89
75
76
80
612
516
523
550
2.7
3.7
2.5
2.9
178
172
134
128
169
155
1225
1183
922
881
1163
1066
83
67
93
29/c
94
37/c
98
57
67
74
79
65
674
392
461
509
544
447
73
37
42
70
61
62
502
255
289
482
420
427
2.3
2.2
2.0
0.6
2.2
0.6
TRS
0.20% OFFSET Elongation
UTS
(ksi)
(MPa)
AISI 415M = Powder Mix, AISIPA = Prealloy.
A major drawback to carbon-containing martensitic stainless steels is their relatively poor
corrosion resistance and ductility. Low carbon-containing martensitic stainless steels can
be produced by adding nickel, molybdenum and copper to form martensitic stainless
steels with improved toughness and corrosion resistance. Table V cites the composition
of several martensitic stainless steels made by adding nickel, copper and molybdenum.
Nickel and copper are austenite formers, while molybdenum improves properties via
solid solution strengthening; this element is responsible for improving high temperature
properties. While the apparent hardness of these alloys is slightly inferior to that of heattreated carbon-containing martensitic stainless steels, other mechanical properties such as
tensile strength, toughness and ductility are superior. These grades of stainless steel also
exhibit superior corrosion resistance which reflects the absence of carbide formation and
hence sensitization, as shown in Figure 5.
Figure 5.
(a)
(b)
(d)
(e)
(c)
Representative appearance of salt spray specimens: (a.) 440B, (b.) 440C, (c.) 410Lcu,
(d.) super-martensitic admixed, (e.) super-martensitic prealloyed
As with other categories of stainless steels, super-martensitic stainless steels can be
formed by adding high levels of nickel, copper and molybdenum. For PM alloys the
prealloyed materials are usually low in compressibility but can exhibit superior corrosion
resistance due to their high alloy content.
Precipitation-Hardening Stainless Steels
Precipitation hardening stainless steels are not defined by their microstructure, but rather
by strengthening mechanism. These grades may have austenitic, semi-austenitic or
martensitic microstructures and can be hardened by aging at moderately elevated
temperatures, 480 oC to 620 oC (900 oF to 1150 oF). The strengthening effect is due to
the formation of intermetallic precipitates from elements such as copper or aluminum.
These alloys generally have high strength and high apparent hardness while exhibiting
superior corrosion resistance compared with martensitic stainless steels. Heat treatments
can be used to vary the properties of the alloys and involve short times (1 h) at
temperatures ranging from 480 oC to 620 oC (900 oF to 1150 oF). The aging treatment can
take place in either air or in nitrogen, depending on the surface appearance required.
However, these alloys should not be subjected to welding or in service temperatures
above the heat-treatment temperature because strength can be lost due to overaging.
The AISI designation for these alloys is the 600 series of stainless steels, but most are
more commonly known by their alloy name, for example, 15-5PH, 17-4PH and17-7PH.
The aluminum-containing precipitation hardening alloys are difficult to process by the
PM route due to their high affinity for nitride formation and the difficulty in reducing
aluminum oxide during sintering.
Table VII and VIII give the chemical compositions and mechanical properties of several
precipitation hardening alloys produced by conventional PM techniques. 17-4PH is a
martensitic grade in which ductility and toughness are generally higher than in the
carbon-containing martenstitc grades. The mechanical properties of 17-4PH can be
increased by 15% by aging at 538 oC (1000 oF) for 1 h. Applications for this alloy exist
in the food, chemical and aerospace industries.
Table VII: Composition of PM Precipitation Hardening Stainless Steels (w/o)
AISI
UNS
17-4PH
S17400
410LCu
J91151
633M
S35000
C
0.03
Max.
.15
.30
0.07
0.11
S
.030
Max.
.030
Max.
.030
Max.
Si
1.0
Max.
1.0
Max.
1.0
Max.
Cr
15.0
17.0
12.0
14.0
16.0
17.0
Ni
3.0
5.0
.50
Max.
4.0
5.0
Cu
3.0
5.0
.50
Max.
.50
Max.
Mo
.50
Max.
.50
Max.
2.50
3.25
Nb
.15
.45
-----
633 is a semi-austenitic precipitation hardening stainless steel offering improved
corrosion resistance compared with martensitic precipitation hardening alloys. These
alloys are used for parts requiring high strength at moderately elevated temperatures.
Depending on the aging treatment, the ductility and toughness of this alloy can approach
those of the austenitic stainless steels. The microstructure of the alloy is a mixture of
austenite, martensite and small quantities of ferrite.
Table VIII: Mechanical Properties of PM Precipitation Hardening Stainless Steels
Impact
AISI
17-4PH
17-4PH Aged
410LCu
410LCu Aged
633
633 AGED
UNS
S17400
S17400
J91151
J91151
S35000
S35000
ft.lbs.f
10
6
21
22
65
67
(J)
13
8
28
29
87
90
Green Sintered
Density Density
(g/cm3) (g/cm3)
6.39
6.66
6.39
6.66
6.50
6.98
6.50
6.98
6.57
7.23
6.57
7.24
TRS
(ksi)
220
273
219
245
218
265
(MPa)
1514
1878
1507
1686
1500
1823
Apparent
Hardness
(HRB)
81
93
88
94
94
96
0.20% OFFSET Elongation
UTS
(ksi)
92
119
99
113
111
113
(MPa)
633
819
681
777
764
777
(ksi)
60
100
76
94
63
85
(MPa)
413
688
523
647
433
585
(%)
2.8
1.7
3
3.5
4.5
8.7
Usage of the precipitation-hardening alloys is generally limited by the high cost of the
alloying elements. Recently, a lower cost PM precipitation hardening alloy has been
introduced based on UNS J91151 (a cast grade). 10 This alloy has only 13 w/o chromium
and utilizes the precipitation of copper to provide a low cost-high strength alloy with
moderate corrosion resistance. Table VIII shows that the mechanical properties approach
those of 17-4PH, while still maintaining a level of corrosion resistance that is better than
that of the high carbon martensitic grades.
Duplex Stainless Steels
Table IX: Composition of PM Duplex/Dual Phase Stainless Steels (w/o)
AISI
UNS
Duplex- 329M
S32900
Duplex- 2205
S32205
Duracor/3Cr12
S41003
C
0.08
Max.
0.03
Max.
.15
.30
S
.030
Max.
.030
Max.
.030
Max.
Si
1.0
Max.
1.0
Max.
1.0
Max.
Cr
23.0
28.0
21.0
23.0
10.5
12.5
Ni
2.5
5.0
4.5
6.5
1.50
Max.
Cu
.50
Max.
3.0
5.0
.50
Max.
Mo
1.00
2.00
2.50
3.50
.50
Max.
Nb
-------
Technically, duplex steels are stainless steels that contain two phases.3 Duplex stainless
steels are more-accurately defined as alloys containing a mixed microstructure of ferrite
and austenite. New alloys being developed that contain mixtures of ferrite and martensite
are generally termed dual-phase.11 Compositions of PM Duplex/Dual Phase stainless
steels are listed in Table IX. A major advantage of these stainless steel grades is that each
phase imparts improved properties to the alloy.
(a)
(b)
Figure 6. Representative microstructures of (a) duplex stainless steel: (b) dual-phase stainless steel.
Duplex stainless steels are ferritic stainless steels (Figure 6(a)) containing chromium and
molybdenum to which austenite formers (primarily nickel) have been added to ensure
that austenite is present at room temperature. Duplex stainless steels have several
advantages over the austenitic grades including high strength, acceptable toughness, and
superior corrosion resistance, particularly to chloride stress corrosion cracking. The
mechanical properties of a duplex stainless steel (2205) are shown in Table X.
Table X: Mechanical Properties of PM Duplex/Dual Phase Stainless Steels
Green Sintered
Density Density
Impact
AISI
UNS
ft.lbs.f
(J)
Duplex- 329M
Duplex- 2205
Duracor/3Cr12
S32900
S32205
S41003
46
82
49
62
110
66
3
(g/cm )
6.27
6.48
6.62
3
(g/cm )
7.07
7.20
7.34
(ksi)
(MPa)
Apparent
Hardness
(HRB)
179
202
278
1232
1390
1913
50
81
88
TRS
0.20% OFFSET Elongation
UTS
(ksi)
(MPa)
(ksi)
(MPa)
(%)
76
84
107
523
578
736
67
62
82
461
427
564
7.7
10.8
3.1
Dual phase stainless steels vary in composition but are generally non-austenitic (Figure
6(b)) and magnetic, containing 11 w/o Cr. The chemistry of the alloy is balanced by
ferrite formers and austenite formers. The austenite transforms to martensite upon cooling
resulting in a mixture of ferrite and martensite. Because of the low cost of the alloy it is
used as a replacement for plain carbon steels where increased corrosion resistance is
needed. The martensite in the alloy allows the material to be used in applications
requiring strength and wear resistance. The properties of a PM version of this stainless
steel (S41003) are cited in Table X.
FATIGUE BEHAVIOR OF PM STAİNLESS STEELS
Fatigue tests were performed on some of the high strength PM alloys developed. The results of
these tests,in terms of the 90% survival limit, are compared with those of other stainless steel
fatigue data by Shah et al. 12 in Figure 7. The latter study study compared the fatigue strength of
various stainless steels as a function of tensile strength.
Tensile Strength (MPa)
100
200
300
400
500
600
700
50
420
Dual Phase
Fatigue Endurance Limit (KSI)
45
800
410LCu
Aged
900
345
295
410LCu
Martensitic
40
409LNi
35
409LNi-HC
410HT
DUPLEX
245
17-4PH
409LE
30
195
430L
25
430N2
434L
20
145
434N
410L
15
95
10
Fatique Endurance Limit MPa)
0
45
5
0
40
50
60
70
80
90
100
110
120
130
-5
140
Tensile Strength (KSI)
Figure 7. Fatigue endurance limit (90% survival) as a function of tensile strength.
The excellent fatigue response of these alloys is attributed to their high tensile strength.
In general, fatigue crack propagation rates in PM steels are high and the fatigue limit is
dictated by crack initiation rather than by crack propagation. Resistance to crack
initiation increases as the tensile strength increases. All the PM alloys included in Figure
7 have high tensile strengths, and therefore high fatigue endurance limits. It appears that
the addition of copper, nickel, and molybdenum, results in harder martensite, which has a
positive effect on fatigue strength.
CONCLUSIONS
•
Many AISI grades of stainless steel can be made via conventional water
atomization and press and sinter PM. These grades are not currently covered by
MPIF Standard 35, but provide a range of properties and corrosion resistance that
can lead to increased opportunities for PM parts producers.
•
These PM grades can be made as prealloys, or admixed nickel, copper and
molybdenum powders can be added to the base stainless steel.
•
In ferritic grades, higher levels of chromium, niobium and sulfur can lead to
improved mechanical properties, corrosion resistance and machinability.
•
The addition of carbon to low chromium PM alloys results in martensitic stainless
steels with increased strength and apparent hardness. Additions of nickel, copper
and molybdenum produce low carbon martensitic stainless steels with increased
toughness and corrosion resistance.
•
Niobium, when added to PM austenitic stainless steels, improves weldability.
Increases in the molybdenum content of austenitic stainless steels can increase
strength and enhance corrosion resistance. Increases in chromium, nickel and
silicon levels enhance oxidation resistance.
•
Several precipitation hardening alloys with a range of mechanical properties,
microstructure and attendant corrosion resistance can be produced by
conventional PM processes.
•
Mixed microstructure stainless steels exhibiting excellent mechanical properties
and corrosion resistance can be produced by conventional PM processes.
•
Fatigue response of the high strength PM alloys is a function of their tensile
strength.
REFERENCES
Materials Standards for PM Structural Parts, 2007, Metal Powder Industries
Federation, Princeton, NJ.
2. J.R. Davis, Alloying: Understanding the Basics, 2001, ASM International, Materials
Park OH.
3. J. Beddoes and J. G. Parr, Introduction to Stainless Steels, 1999, ASM International,
Materials Park, OH.
4. J.D. Redmond, Metals and Alloys in the Unified Numbering System, 10th Edition,
2004, Joint Publication of the Society of Automotive Engineers, Inc. and the
American Society for Testing and Materials.
5. J.E. Bringas, Stainless Steel Data Book, 1992, CASTI Publishing Inc., Edmonton,
Alberta, Canada.
6. H. Cobb, Steel Products Manual- Stainless Steels, 1999, Iron and Steel Society,
Warrendale, PA.
7. T.R. Albee, P. DePoutiloff, G. Ramsey and G.E. Regan, “Enhanced Powder Metal
Materials for Exhaust System Applications,” SAE Paper No. 970281, SAE
International, Warrendale, PA, USA.1997.
8. T. Hubbard, K. Couchman and C. Lall, “Performance of Stainless Steel PM Materials
in elevated Temperature Applications,” SAE Paper No. 970422, SAE International,
Warrendale, PA, USA.1997.
9. R.J. Causton, T. Cimino-Corey, and C.T. Schade “Improved Stainless Steel Process
Routes,” Advances in Powder Metallurgy and Particulate Materials – 2003, compiled
by R. Lawcock and M. Wright, Metal Powder Industries Federation, Princeton, NJ,
2003, part 2 pp.1-13.
10. A. Lawley, R.Doherty, P. Stears and C.T. Schade, “Precipitation Hardening Stainless
Steels,” Advances in Powder Metallurgy and Particulate Materials – 2006, compiled
1.
by W. R. Gasbarre and J.W. von Arx, Metal Powder Industries Federation, Princeton,
NJ, 2006, part 7 pp.141-153.
11. A. Lawley, E. Wagner, and C.T. Schade, “Development of a High-Strength-DualPhase P/M Stainless Steel,” Advances in Powder Metallurgy and Particulate
Materials – 2005, compiled by C. Ruas and T. Tomlin, Metal Powder Industries
Federation, Princeton, NJ, 2005, part 7 pp.78-89.
12. S.O. Shah, J.R. McMillen, P.K. Samal and L.F. Pease, “Mechanical Properties of
High Temperature Sintered P/M 409LE and 409LNi Stainless Steels Utilized in the
Manufacturing of Exhaust Flanges and Oxygen Sensor Bosses,” SAE Paper No.
2003-01-0451, SAE International, Warrendale, PA 15096, USA, 2003.