DEVELOPMENT OF A HIGH-STRENGTH-DUAL-PHASE P/M STAINLESS STEEL
Chris Schade
Hoeganaes Corporation
Cinnaminson, NJ 08077
Alan Lawley
Emeritus Professor
Department of Materials Science and Engineering
Drexel University
Philadelphia, PA
19104
215-895-2326 (T)
215-895-6760 (F)
alan.lawley@drexel.edu
Eva Wagner
Drexel University
Philadelphia, PA 19104
ABSTRACT
Applications requiring high strength stainless steels are growing at a fast pace. Typical alloys
used for these applications are either highly alloyed materials such as 17-4PH or materials
that require a secondary heat treatment such as SS-410HT. A new dual-phase stainless steel
has been developed as a lower cost option. The microstructure of the dual-phase stainless
steel consists of a mixture of ferrite and martensite, the proportions of which are dictated by
the chemical composition of the alloy. This unique microstructure results in high strength and
hardness, while maintaining ductility. The mechanical properties of this new alloy are
compared with those of competing materials such as 17-4PH, SS-409LE and SS-410HT.
Potential applications for this new material are reviewed.
INTRODUCTION
Dual-phase stainless steels have been developed in the wrought steel industry to replace
carbon and galvanized steels. The purpose of dual-phase stainless steels is to provide the
corrosion resistance of the ferritic stainless steels, while maintaining the strength, toughness
and ductility of the carbon steels.1 Dual-phase stainless steels are commonly used in
applications where the mechanical properties of carbon steels are acceptable, but the corrosion
resistance is marginal. Typical appliations for this grade of steel in plate form, include coal
cars, coal handling equipment, electrical transmission towers, floor plate, tubing, and buckets
for front-end loaders. The unique properties of dual-phase stainless steels are a consequence
of the microstructure. Unlike duplex stainless steels, which exhibit a microstructure of
austenite and ferrite, dual-phase stainless steels have a microstructure consisting of martensite
and ferrite. It is the combination of these two phases that results in this unique combination
of properties, Figure 1.
(a)
(b)
Figure 1. Representative microstructure of a dual-phase stainless steel: (a) Durracor cast and and hot-rolled
plate. Note the banded microstructure due to rolling. (b) press + sinter P/M dual-phase stainless steel; sintered
density 7.20 g/cm3.
BACKGROUND
To predict the microstructure of the dual-phase stainless steels, it is necessary to understand
the attendant phase equilibria. The pseudo-phase diagram in Figure 2 shows the dependence
of phase stability on the ferrite factor (Km ), to be defined, and temperature. At the normal
sintering temperatures for stainless steels (~ 1260 oC (2300 oF)), the microstructure consists
of a mixture of ferrite and austenite. Upon cooling to room temperature, the austenite
transforms to martensite. The proportions of austenite and ferrite can be determined by the
lever rule. The addition of other elements to the iron-chromium system broadens the ferrite +
austenite region, and makes it difficult to determine the phases present accurately.2 Since the
transformation of austenite to martensite in ferritic stainless steels results in weld
embrittlement, several empirical methods and tools, such as the Schaeffler diagram, the
Delong diagram, and the ferrite factor, have been developed to predict the microstructure.
These predictions can also be used in the development of the microstructure in dual-phase
stainless steels. The most commonly used tool is the ferrite factor (Km).
Figure 2: Phase relations in the iron-chromium system as a function of the ferrite factor (Km) and temperature.
Several authors developing dual-phase-stainless steels have used the ferrite factor.2-4. This
factor accounts for the influence of the alloying elements in stabilizing austenite or ferrite; the
empirical equation for the ferrite factor Km is:
Km = Cr + 6Si + 8Ti + 4Mo + 2Al – 2Mn – 4Ni – 40(C+N) – 20P – 5Cu
(1)
where Cr, Si, etc. are the levels of each element present in the alloy in weight %.
Chromium, silicon, titanium, molybdenum, and aluminum stabilize the ferrite. Manganese,
nickel, carbon, nitrogen, phosphorus and copper promote the formation of high temperature
austenite, which transforms to martensite during cooling. In established wrought grades of
stainless steel, the optimum ferrite factor has been determined to lie between 8 and 13.
However, it is difficult to compare ferrite factors for alloys of substantially different base
compositions.5-7 In general, a low Km (< 6) reflects a stainless steel with a predominantly
martensitic microstructure, whereas a high Km (> 15) is indicative of a stainless steel with a
predominantly ferritic structure. Ferrite factors between these values result in mixed
microstructures of martensite and ferrite. It is this mixed (or dual-phase) microstructure that
promotes the unique combination of high strength and toughness, while maintaining ductility
and fatigue strength.
ALLOY DEVELOPMENT
Representative chemical compositions of wrought dual-phase-stainless steels are shown in
Table I. In order to develop a P/M dual-phase stainless steel, changes have to be made to the
overall chemical composition. A review of the role and influence of pertinent elements, in
relation to P/M dual-phase stainless steels, follows:
Carbon, Sulfur, Oxygen and Nitrogen- The levels of these elements are generally kept as
low as possible in order to improve compressability and sinterability. Nitrogen stabilizes the
austenite.
Silicon- In order to produce a powder that is low in oxygen, the silicon level in the melt,
prior to atomization, needs to be relatively high (~0.85 w/o). Silicon adds strength to the
alloy, and also stabilizes the ferrite.
Chromium- This element is a ferrite stabilizer and imparts corrosion resistance. This is
accompanied by small increases in strength, hardenability, and wear resistance.
Nickel- The primary influence of nickel is to promote the formation of high temperature
austenite. In addition, nickel is known to improve toughness, impact resistance and corrosion
resistance. The element can be used at moderate levels in P/M steels without dramatically
decreasing compressibility.
Manganese- This element is generally kept at low levels in stainless steel, due to the
formation of a porous oxide on the powder surface. The porous oxide leads to a high level of
oxygen on the powder surfaces, which impedes sintering. Manganese also enhances work
hardening capacity, and decreases the compressibilty of the powder.
Copper- Copper increases corrosion resistance, while providing solid solution strengthening.
If it is kept at low levels, copper does not dramaticlly impair the compressibility of the
powder.
Molybdenum- Hardenability and high temperature strength are enhanced by molybdenum.
The element also aids high-temperature oxidation resistance. Research on wrought dualphase-stainless steels has shown that molybdnum increases impact toughness.1,5
Table I: Composition of Representative Wrought Dual-Phase Stainless Steels (UNS Designation 41003).
(Weight %)
Type
UNS41003
3CR12
Durracorr
C
0.026
0.025
S
0.015
0.008
N
0.014
0.030
P
0.023
0.040
Si
0.44
0.70
Cr
Ni
Cu Mn Mo
11.26 0.68 0.06 1.16 0.02
11.60 1.00 0.03 1.50 0.25
Ti
0.35
---
Based on this review, and the fact that the best combinations of properties in the wrought
grades of stainless steels were achieved with ferrite factors between 8 and 13, a series of P/M
dual-phase stainless steel alloys were fabricated. All the powders were produced by water
atomization with a typical particle size <150 µm (–100 mesh) with 38 to 48% <45 µm (-325
mesh). All the alloying elements were prealloyed into the melt prior to atomization. Table II
shows the chemical composition of the dual-phase (DP) alloys, along with those of two
standard stainless grades (410L and 430L).
Table II: Composition of P/M Dual-Phase Stainless Steels
(Weight %)
Alloy
DP1
DP2
DP3
DP4
DP5
410L
430L
C
0.004
0.004
0.005
0.003
0.013
0.011
0.011
S
0.003
0.014
0.015
0.013
0.006
0.010
0.010
O
0.04
0.10
0.20
0.24
0.16
0.14
0.26
N
0.0483
0.0319
0.0445
0.0133
0.0139
0.0246
0.0270
P
0.014
0.014
0.014
0.013
0.012
0.014
0.014
Si
Cr
0.60 11.7
0.85 11.8
0.84 11.6
0.80 12.0
0.81 12.0
0.85 12.5
0.85 16.5
Ni
1.52
1.03
1.03
0.52
0.58
0.03
0.03
Cu
0.27
0.29
0.03
0.48
0.01
0.02
0.02
Mn
0.11
0.13
0.10
0.10
0.11
0.11
0.11
Mo
0.22
0.22
0.34
0.52
0.32
0.03
0.03
Km
6.1
10.2
11.3
13.3
14.2
15.6
19.5
ALLOY PREPARATION AND TESTING
The prealloyed powders were mixed with 0.75w/o Acrawax C lubricant. Samples for
transverse rupture (TR) and tensile testing were uniaxially compacted at 690 MPa (50 tsi).
All 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).
Prior to mechanical testing, green and sintered density, dimensional change (DC), and
apparent hardness, were determined on the tensile and TRS samples. Five tensile and five
transverse rupture specimens (TRS) were tested at each composition. The densities of the
green and sintered steels were determined in accordance with MPIF Standard 42, while tensile
testing followed MPIF Standard 10. Impact energy specimens were tested according to MPIF
40. Apparent hardness measurements were conducted on tensile,TRS and impact specimens
according to MPIF 43.
Rotating bending contact fatigue 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 7,000-8000 rpm at R = -1 using four machines simultaneously. Thirty specimens were
tested for each alloy and the staircase method was utilized to determine the 50% survival limit
and the 90% survival limit for 107 cycles (MPIF 56).
Metallographic specimens of all the test materials were examined by optical microscopy in
the polished and etched conditions. Etched specimens were used for microhardness testing as
per MPIF 51.
Two corrosion tests were performed. Salt spray testing on TRS bars was performed
according to (ASTM B 117-03). Five TRS bars (prepared as previously described) per alloy
were tested. The percent area of the bars covered by red rust was recorded over time. Since
ferritic steels are frequently used in conditions where they experience high-temperature
oxidation (such as exhaust flanges) a test have been devised to measure the oxidation during
cycling from room temperature to 1200 °C (2192 oF). The technique developed was very
similar to previous tests in which sintered impact bars (sintered) were cycled from room
temperature to 676 °C (1249 oF) in air.8-10 Each heating cycle lasted two to four hours, and
this was repeated as much as 400 times with changes in mass recorded. In this study, sintered
cross-sections of tensile bars were used in a thermal gravimetric (TG) unit to measure the
mass gain over time. Each of the four alloys was heated from room temperature to 1200 °C
(2192 oF) in air. This cycle was repeated several times, and the weight gain per cycle
monitored.
RESULTS AND DISCUSSION
Table III cites the green density, sintered density, transverse rupture strength, apparent
hardness, yield strength, ultimate tensile strength, elongation, and ferrite factors, for the P/M
experimental alloys, and two standard P/M ferritic grades.
Table III: Mechanical Properties of P/M Dual-Phase Stainless Steel.
Green Sintered
Density Density
Alloy
DP1
DP2
DP3
DP4
DP5
410L
430L
Km
6.2
10.2
11.3
13.3
14.2
15.6
19.5
(g/cm3)
6.69
6.68
6.62
6.58
6.59
6.66
6.61
(g/cm3)
7.03
7.34
7.38
7.35
7.31
7.3
7.16
TRS
(MPa)
1578
1914
1730
1201
1274
1430
905
(103 psi)
229
278
251
175
185
208
132
Apparent
Hardness
(HRB)
84
88
83
66
68
54
54
UTS
(MPa)
687
736
674
444
478
377
358
(103 psi)
100
107
98
64
69
55
52
0.20% OFFSET
(MPa)
538
564
509
284
297
213
202
(103 psi)
78
82
74
41
43
31
29
Elongation
(%)
2.3
3.1
4.0
8.5
10.0
16.2
15.3
Km in the experimental dual-phase P/M alloys was varied by changing the levels of nickel,
copper, and molybdenum. Quantitative metallography was performed to measure the actual
ferrite content of the P/M alloys. Figure 3 shows the relationship between the amount of
ferrite present and Km.
There is an increase in ductility and a decrease in strength, as the level of ferrite in the
microstructure increases, Table III. Apparent hardness appears to be directly related to the
amount of martensite in the microstructure and is maximized at Km values between 6 and 11.
Nevertheless, based on Figure 3, there is very little ferrite in the microstructure with Km
values <9.
Notwithstanding, the presence of prealloyed nickel, copper, and molybdenum, the dual-phase
P/M steels exhibit a level of compressibility close to that of 410L. It was expected that the
increase in the level of alloying elements in the prealloy would harden the powders, with an
adverse effect on green density; this does seem to be the case, Table III. There also appears to
be a synergistic effect of prealloyed nickel, molybdenum and copper on sintered density; with
the exception of DP1, the sintered density of the dual-phase stainless steels exceeded
7.3 g/cm3.
16.00
Volume Ferrite (%)
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
6.00
7.00
8.00
9.00
10.00
11.00
12.00
13.00
Km
Figure 3. Relationship between measured ferrite content and Km in experimental P/M dual-phase
stainless steels
From the results of the static mechanical tests, it was determined that alloy DP2 was the most
promising, in terms of its combination of physical and mechanical properties. The purpose of
developing the dual-phase P/M stainless steel was to provide a high-strength material that had
good corrosion resistance. In order to determine the relative success of the alloy, it was
necessary to compare its properties with those of existing stainless steels, which are typically
used in high-strength applications. These stainless grades include 17-4PH, 409LNi, and 410L
with a graphite addition. The compositions of these three alternative materials, and the DP2,
are listed in Table IV.
Table IV: Chemical Composition of High-Strength Stainless Grades.
(Compositions in w/o)
Alloy
410+ Graphite
17-4PH
409LNi
DP2
C
0.100
0.018
0.013
0.015
S
0.008
0.010
0.004
0.007
O
0.30
0.30
0.19
0.21
N
0.0180
0.0200
0.0160
0.0090
P
0.013
0.025
0.01
0.014
Si
Cr
0.85 12.5
0.85 17.0
1.00 11.3
0.84 11.6
Ni
0.09
4.00
1.30
1.03
Cu
0.08
3.55
0.04
0.29
Mn
0.15
0.15
0.12
0.10
Mo
0.02
0.03
0.05
0.22
Cb
--0.25
0.56
---
When graphite is added to 410L stainless steel, the microstructure becomes martensitic, with
increasing strength and hardness. This grade of stainless steel is used in applications where
high strength is required. The disadvantage of using graphite is that it negatively impacts the
corrosion resistance.
17-4PH is a precipitation-hardening, martensitic stainless steel that combines high strength
and hardness with excellent corrosion resistance. The alloy is age-hardened to achieve
maximum strength. This compressibility of this material is compromised somewhat,
compared with the other alloys studied due to its high alloy content.
409LNi is a nickel-modified 409Cb stainless steel, which exhibits excellent mechanical
properties, and is commonly used for automotive exhaust flange applications.11-13 In this
study, nickel powder (Inco-123) was admixed with a 409Cb powder. The addition of nickel
has been shown to improve the high temperature strength of the stainless steel.
Table V: Physical and Mechanical Properties of High Strength Stainless Steels.
Sintered
Density
(g/cm3)
7.05
6.67
7.01
7.15
Alloy
410L + graphite
17-4PH
409LNi
DP2
D.C.
(%)
-2.17
-1.80
-2.10
-2.99
o
Apparent
Hardness
(HRB)
(103 psi)
181
79
206
86
182
79
238
91
TRS
(MPa)
1248
1421
1255
1641
UTS
(MPa)
633
757
612
819
0.20% OFFSET
3
(10 psi)
92
110
89
119
(MPa)
358
627
482
612
Elongation
3
(10 psi)
52
91
70
89
(%)
4.9
1.4
1.8
2.5
o
0.75% Acrawax C lubricant and sintered; sintered at 1260 C (2300 F) in 100% hydrogen.
o
o
Note: 17-4PH was heat-treated at 482 C (900 F) for 30 min.
The static mechanical properties of these three alloys are compared with those of DP2 in
Table V. From Table V, it is seen that DP2 is superior, in both strength and apparent
hardness, to the other alloys. One reason for this is the higher sintered density of DP2.
17-4PH, because of its high alloy content, is inherently less compressible and this translates
into a low sintered density. An additional heat treatment is required for 17-4PH in order to
maximize strength and hardness. 409LNi generally requires longer times at temperature than
DP2 to achieve acceptable diffusion of the admixed nickel. The advantage of the prealloyed
nickel, copper and molybdenum in DP2 is evident when the alloys are sintered under similar
conditions. Due to the high compressibility of 410L with graphite, the alloy has a high
sintered density, and acceptable properties. However, in order to achieve consistent carbon
levels, tight control in the furnace is necessary. Graphite additions also reduce the corrosion
resistance of the stainless steel.
Table VI: Dynamic Properties of High Strength Stainless Steels.
Sintered Apparent
Density Hardness
(HRB)
Material
(g/cm3)
410L + graphite
7.05
79
17-4PH
6.67
86
409LNi
7.01
79
DP2
7.15
91
Impact Energy
(J)
39
26
32
65
(ft.lbf)
29
19
24
48
Fatigue Strength
(90% Survival)
(MPa)
(103 psi)
330
47.8
244
35.4
249
36.0
315
45.6
The dynamic properties of the four alloys are cited in Table VI. It has been found that in
wrought dual-phase stainless steels, the addition of molybdenum has a dramatic effect on
dynamic properties. Davison et al. have shown that adding molybdenum, and eliminating
titanium in the 3CR12 chemistry, increases impact toughness.5 Titanium can lead to the
formation of titanium carbides and nitrides, which reduce impact energy. This may be one
explanation why DP2 significantly outperforms 409LNi. This grade contains niobium, which
is more conducive to the atomizing process than is titanium. The ferrite content of DP2 also
enhances the impact properties, compared with 17-4PH and 410L + graphite, both of which
have martensitic microstructures.
Since sintered density has been proven to be an important factor in fatigue response, one may
question the fatigue strength of DP2 in respect to the other alloys. Although DP2 and the
other alloys were sintered under identical conditions, DP2 achieved a higher sintered density.
Shah et al. compared the fatigue strengths of various stainless steels, as a function of tensile
strength, Figure 4.13 Data from this study are included in Figure 4 for 409LNi, at a density of
7.20 g/cm3 (similar density to that of the DP2 alloy).
Tensile Strength (MPa)
0
100
200
300
400
500
600
700
800
900
345
50
410L+Graphite
45
DP2
295
Fatique Endurance Limit (KSI)
409LNi
35
410HT
409LNi-HC
245
17-4PH
409LE
30
195
25
430N2
430L
434L
20
145
434N2
410L
15
95
Fatique Endurance Limit MPa)
40
10
45
5
0
-5
0
20
40
60
80
100
120
Tensile Strength (KSI)
Figure 4. Fatigue endurance limit as a function of tensile strength for various P/M stainless alloys.
The excellent fatigue response of the DP2 alloy appears to be related its high tensile strength.
In general, fatigue propagation rates in P/M steels are high and the fatigue limit is dictated by
crack initiation rather than crack propagation. Thus, resistance to crack initiation increases as
the tensile strength increases. Both the 410L + graphite and the DP2 alloy have high tensile
strengths and therefore higher fatigue endurance limits. Notwithstanding, having a high
tensile strength, the 17-4PH has a lower fatigue endurance limit due to its low sintered density
(high level of porosity).
Micro-indentation hardness measurements of the martensite (along with the corresponding
microstructures) are shown in Figure 5. The micro-indentation hardness of the martensite in
the 410L + graphite was 372 HV 50 gf. The 410L with graphite had the highest fatigue
endurance limit however, the martensite was formed with the use of carbon and the alloy is
fairly brittle and had lower impact energy than the DP2. The martensite grains in the DP2 had
a micro-indentation hardness of 318 HV 50 gf, while the 409LNi had a martensite microindentation hardness of 250 HV 50 gf. It appears the addition of copper (along with the
nickel) leads to a harder martensite. The micro-indentation hardness of the martensite in the
17-4PH was lower (245 HV 50 gf). The combination of the strong martensite and the ductile
ferrite allows the DP2 alloy to have a high strength, but superior ductility and impact
toughness than the other alloys.
(a)
(b)
(c)
(d)
Figure 5: Microstructure of alloys cited in Table V; (a) 410L + Graphite (microhardness of martensite 372 HV
50 gf); (b) 17-4PH (microhardness of martensite 245 HV 50 gf); (c) 409LNi (microhardness of martensite 250
HV 50 gf); (d) DP2 (microhardness of martensite 318 HV 50 gf).
In the wrought stainless steel industry, dual-phase-stainless steels were developed as a
replacement for weathering steels. The corrosion resistance was substantially improved when
the 12 w/o chromium steel was used. The dual-phase P/M stainless steels will compete with
ferritic grades of stainless steel. There are many applications that have less-severe corrosion
conditions, and, in such cases, ferritic stainless steels are preferred over the more expensive
austenitic stainless steels. The four stainless steels were subjected to salt spray testing (ASTM
B 117-03). Five TRS bars were tested from each alloy composition. The results are given in
Table VII.
Table VII: Results of Salt Spray Testing (ASTM B 117-03).
Material
410L + graphite
17-4PH
409LNi
Dual Phase 2
Red Rust After 24 h
(%)
84.00
1.00
20.00
5.00
Although the sintered density of the 17-4PH alloy was much lower than the other three
alloys the high alloy content (17 w/o chromium and 4 w/o nickel) resulted in superior
corrosion resistance, with an average of only 1% of the TRS bars showing red rust after 24 h
exposure. The dual-phase alloy, and the 409LNi alloy, performed at a similar level, as
expected, since each alloy has a similar chromium and nickel content. The addition of small
amounts of copper and molybdenum may lead to slightly better corrosion resistance for the
DP2. The 410L with graphite had the highest level of red rust after 24 h. This is not
surprising, since the addition of carbon leads to sensitization and makes the chromium less
effective in passivation.
Many of the P/M grades such as 410L and 409LNi are used in flange applications. A
described in the experimental procedure, cross-sections of the sintered tensile specimens were
heated in air from room temperature to 676 °C (1249 oF). This cycle was repeated several
times. Figure 6 shows the mass gain per cycle. The alloys behaved similarly, with a high
initial rate of oxidation. This was followed by saturation to a constant rate of oxidation.
5
4.5
4
Mass Gain (%)
3.5
3
2.5
2
1.5
1
0.5
0
0
1
2
3
4
5
6
Number of Oxidation Cycles
17-4
410L + Graphite
DP2
409LNi
Figure 6. Mass gain per cycle. TGA
As in the salt spray testing, despite its lower density, 17-4PH exhibited the lowest initial mass
gain, and saturated at the lowest rate of oxidation. DP2 exhibited the highest initial rate of
oxidation, but it saturated at a lower number of cycles than the other alloys. It also exhibited a
slightly lower oxidation rate than 409LNi and 410L with carbon. This may be due to the
additions of molybdenum and copper; these elements are known to improve high temperature
oxidation.
The corrosion testing highlights a final aspect of the DP2 alloy. Despite having a much leaner
chemistry than the 17-4PH and a lower nickel content han the 409LNi the corrosion resistance of the
DP2 alloy performs comparable to these materials. The excellent mechancial properties are
developed without need for secondary treatments (such as heat treating). Both these aspects make the
alloy a cost effective solution when a high strength-moderate corrosion resistant material is needed.
CONCLUSIONS
•
The chemical composition and physical properties of a new dual-phase P/M stainless
steel have been optimized by evaluating a series of prealloys produced by water
atomization.
•
The new alloy DP2 has a dual (mixed) microstructure of martensite and ferrite.
Quantitative metallography has shown the alloy contains approximately 5 to 8%, by
volume, ferrite in a martensitic matrix.
•
The static properties of the DP2 show improvements over competing alloys, in
particular higher tensile strength and hardness.
•
The fatigue and impact properties of DP2 are superior to those of existing alloys such
as 17-PH, 409LNi, and 410L with graphite.
•
Corrosion properties of DP2 show it to be a viable candidate for replacing ferritic
stainless steels.
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