PRODUCTION OF STAINLESS STEEL POWDERS BY ADVANCED STEELMAKING
TECHNOLOGY
Christopher T. Schade and John Schaberl
Hoeganaes Corporation
1001 Taylors Lane
Cinnaminson, NJ 08077 USA
ABSTRACT
Advanced melting technology is now being employed in the manufacture of stainless steel
powders. The new process currently includes electric arc furnace (EAF) technology in
concert with Argon Oxygen Decarburization (AOD), High Performance Atomizing (HPA)
and hydrogen annealing. The new high performance processing route has allowed Hoeganaes
Corporation to provide not only a more consistent product, but has allowed enhanced
properties, such as improved green strength and green density. This paper will review the
potential to use this processing route to provide products with improved properties and
performance.
INTRODUCTION
The recent growth in stainless powder metallurgy has led Hoeganaes Corporation to develop a
high volume, high performance, processing route. The new processing route is shown in
Figure 1. A twenty-five ton electric arc furnace is used in conjunction with an argon-oxygen
decarburization unit to melt and refine stainless steel to the required composition and
temperature. The steel is then transferred by a bottom pour ladle to the High-Performance
Atomizer were it is converted into powder. Ferritic grades are then annealed in the hydrogen
atmosphere, annealing furnace. Powders are then screened and premixed according to
customer specifications. As reviewed in a previous paper [1], this new processing route has
led to the production of powders with improved chemistry control and enhanced mechanical
properties. This paper explores new grades of stainless steel and high alloy powders that can
be made economicaly through the new process.
AOD
High
Performance
Atomizing
High
Performance
Annealing
Figure 1. High Performance Stainless Processing Route
STAINLESS STEEL POWDERS CONTAINING BORON
The refining process in the AOD lends itself to the development of new grades. In wrought
stainless production the AOD is used in producing stainless alloys with low levels of elements
such as titanium, vanadium, tantalum and boron, many times consistently holding levels of
0.01%. to 0.10%. The use of argon stirring and real-time chemistry control give the AOD
advantages over induction melting.
Boron has long been of interest to producers of stainless P/M because of its ability to provide
high densities through liquid phase sintering [2-3]. The need to control boron levels
accurately is important because excessive boron leads to precipitates or excessive eutectic
phase at the grain boundary, both of which embrittle and deteriorate performance. Previous
work has suggested that boron levels of 0.25 weight percent can be used favorably in 316L to
enhance mechanical properties [4] An experimental heat of 316L was made (Table I) to
examine the effects of such an addition on physical properties.
The microstructures show a nearly 100% dense structure (Figure 2). The use of the AOD in
achieving tight boron levels is certainly advantageous for this grade. The proper level of
Table I. Effects of pre-alloyed boron on the mechanical properties of 316L.
Green
Sintered
Green Strength
Density
Density
(MPa) (g/cm3)
(psi) (MPa) (g/cm3)
690
6.74
1956
13.5
7.13
690
6.54
3203
22.1
7.79
Pressure
Material
316L
316L with B
(tsi)
50
50
D.C.
TRS
Hardness
(%)
-1.68
-5.90
(ksi) (MPa)
138
952
183 1262
(HRB)
56
72
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC) in 100% Hydrogen.
boron is needed. Insuffcient boron and sintering is not activated, whereas to high a level of
boron reults in excessive precipitates which decrease ductility and corrosion resistance.
Metallographic examination of the specimens showed no eutectic liquid around the pores and
very few precipitates.
(a)
(b)
Figure 2. Porosity distribution in (a) 316L with boron and (b) conventional 316L.
The corrosion behavior of austenitic stainless steels with boron is dependent on the amount of
the eutectic phase present and the final sintered density. If too much eutectic phase is present,
it depletes the surrounding matrix of chromium and leads to sensitization. The lower the
density of the material the more porosity exists and the corrosion resistance (especially
pitting) of the material is reduced. Corrosion testing via salt spray (ASTM B117-02) is shown
in Table II. The 316L with a boron addition is compared to conventional 316L and 316L with
copper and tin (a grade developed for enhanced corrosion resistance). The 316L with the
boron addition showed superior corrosion resistance.
Table II. Salt Spray testing of 316L P/M materials.
Material
Test Duration
Time for 1% Red Rust Rusted Area at Test Completion
(Hours)
(Hours)
(%)
Conventional 316L
240
168.00
79.00
316L with Cu and Sn
240
216.00
12.00
316Lwith Boron
240
0.20
NA1
Salt Spray Testing per ASTM B117-02. Five TRS bars per alloy.
1
At 240 hours, one bar had less than 1% red rust and 4 bars had no red rust
Since the density of the 316L with boron is 98% of theoretical, the mechanical properties of
this grade can approach that of wrought materials. Table III shows tensile results compared to
the ASTM standard for annealed bars.
Table III. Tensile properties of P/M 316L with boron and wrought 316L.
Pressure
UTS
0.20% OFFSET
Elongation
Material
(tsi)
(MPa)
(ksi)
(MPa)
(ksi)
(MPa)
(%)
Wrought 316L 1
P/M 316L with Boron
---50
---690
70
71
483
490
25
32
172
220
30
28
1
ASTM A479 (Specification for Annealed Bar)
Preliminary experiments with ferritic grades (409Cb and 410L) have shown similar response
and densities in excess of 98% of the theoretical have been obtained. Work has shown that as
the boron level is increased from .05% to .25% the sintering temperature over which
densification occurs is widened. For example, at .15% boron addition the window for
densification is 14 oC (25 oF), while at .25% boron the window is extended to 28 oC (50 oF).
However, excessive amounts of boron lead to the rich eutectic phase, which degrades
mechanical properties and corrosion resistance.
DUAL PHASE STAINLESS STEELS
Traditionally, when stainless parts producers have needed high strength and hardness,
graphite has been added to a ferritic grade to promote martensite, or a more highly alloyed
material, such as 17-4PH has been used. In applications that require abrasion resistance,
hardness, ductility and high strength, wrought producers of stainless steel have developed a
dual phase stainless steel consisting of a microstructure of martensite and ferrite. The level of
martensite is controlled according to a chemical balance:
Km = Cr + 6 x (Si) + 8 x (Ti) + 4 x (Mo) + 2 x (Al) – 2 x (Mn) – 4 x (Ni) – 40
(C + N) – 20 x (P) – 5 x (Cu)
In this equation, chromium, silicon, titanium, molybdenum, and aluminum are used to
stabilize the ferrite. Manganese, nickel, carbon, nitrogen, phosphorus and copper promote
formation of high temperature austenite, which transforms to martensite during cooling.
Using AOD technology, the chemistry control to obtain an optimum Km is now possible. By
adjusting elements in real time in the AOD, a consistent value of Km can be maintained. Also,
with the use of hydrogen annealing, it is possible to increase the alloy content of certain
grades without severely reducing the green density.
Table IV shows the chemical composition of a newly developed dual phase stainless steel
along with the two high strength options discussed earlier. The chemistry of the dual phase
stainless steel has been optimized leading to a microstructure consisting of ferrite and
martensite (Figure 3a). Table V shows the physical properties of the aforementioned grades.
Using conventional sintering (typically 1260 oC [2300 oF] in hydrogen) 17-4PH is a poor
option. The high alloy content is not only costly but leads to poor both green and sintered
density. The lower sintered density leads to mechanical properties that are lower than
expected for the level of alloy content. Vacuum sintering with long times at temperature may
improve the properties of this alloy, but this is a costly option. The precipitation-hardening
grade also requires secondary heat treatment to achieve its high strength.
Table IV. Chemistry of high strength-stainless grades.
Type
C
S
O
N
P
Si
Cr
Ni Cu
(%)
(%)
(%)
(%)
(%) (%) (%) (%) (%)
0.060 0.008 0.30 0.018 0.013 0.85 12.50 0.09 0.08
0.015 0.007 0.21 0.009 0.014 0.84 11.60 1.03 0.03
0.018 0.010 0.300 0.020 0.025 0.85 17.00 4.00 3.55
410L + C
Dual Phase
17-4PH
Mn
(%)
0.15
0.10
0.15
Mo Cb
(%) (%)
0.02 --0.34 --0.03 0.25
The graphite addition to the 410L increases the formation of chromium carbides and leads to
sensitization. The net effect is the depletion of chromium in solution and the lowering of
corrosion resistance. Graphite also reduces the compressibility of the powder and requires
strict atmosphere control in the sintering furnace for optimum carbon control. Even with the
higher carbon levels, hardness and tensile properties of the 410L are inferior to the dual phase
stainless steel.
Table V. Mechanical propety data for high strength stainless steels.
Sintered
Density
(g/cm3)
7.05
7.15
6.67
Pressure
Material
410L + C
Dual Phase
17-4PH
(tsi)
50
50
50
(MPa)
690
690
690
D.C.
Hardness
TRS
(%)
-2.17
-2.99
-1.80
(ksi)
181
238
206
(MPa)
1248
1641
1421
o
UTS
(HRA)
49
56
53
(ksi)
92
119
110
0.20% OFFSET
(MPa)
633
819
757
(ksi)
52
89
91
(MPa)
358
612
627
Elongation
(%)
4.9
2.5
1.4
o
.75% Acrawax C used as lubricant and sintered at 2300 F (1260 C) in 100% Hydrogen.
o
o
Note: 17-4PH was heat treated at 900 F (482 C) for 30 minutes.
Another advantage of the Dual Phase stainless steel is that the amount of martensite, and
hence hardness, can now be controlled by the elements in solution. Incorporating various
sintering atmospheres the amount of nitrogen in the alloy can be used to alter the Km factor
and change the amount of martensite present in the microstructure. Nitrogen can be used to
increase strength, and since it is added during sintering, it does not impact the green density of
the material. When this alloy is sintered in 90% hydrogen and 10% nitrogen, the level of
marteniste in the microstructure is increased and the properties respond accordingly (Table
VI). The use of nitrogen as an alloying element during sintering can be anticipated, and the
overall alloy content in the base material can be reduced.
Table VI. Effect of sintering atmosphere on the mechanical properties of dual phase stainless.
Pressure
Material
100% Hydrogen
90% Hydrogen/ 10% Nitrogen
Surface Nitrided
(tsi)
(MPa)
50
50
50
690
690
690
Sintered
Density
3
(g/cm )
7.15
6.99
7.25
TRS
D.C.
UTS
Hardness
0.20% OFFSET
Elongation
(%)
(ksi)
(MPa)
(HRA)
(ksi)
(MPa)
(ksi)
(MPa)
(%)
-2.99
-2.33
-3.76
238
248
214
1638
1708
1473
53
60
54
119
126
97
819
867
668
89
95
71
612
654
489
2.5
2
2.8
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC) in 100% Hydrogen. Surface nitrided material was treated in 90% Nitrogen and 10% Hydrogen after sintering.
Another advantage of the dual phase stainless steel is the ability to “case harden” with
nitrogen. Similar to carburizing, a sintered P/M component can be treated in a nitrogen
atmosphere to form a hard martensitic case (Figure 3c). Heating the material in the
austenite/ferrite region allows nitrogen to diffuse into the austenite, and upon quenching,
forms martensite. The amount of nitrogen diffusion controls the depth of the martensite
formation. The solubility of nitrogen in austenite is greater than carbon and therefore the
formation of chromium nitrides, which subsequently depletes the matrix of chromium and
reduces the corrosion resistance, does not occur. This technique provides a material with high
surface hardness and high matrix strength with reasonable ductility [5]
a.)
b.)
Figure 3.
c.)
Etched optical photomicrographs of dual phase stainless: a.) Sintered in 100% hydrogen, b.)
Sintered in 90% hydrogen and 10% nitrogen, and c.) surface-nitrided using a second treatment in
90% nitrogen and 10% hydrogen (note: nitrided surface is at the bottom of the photomicrograph).
DUPLEX STAINLESS STEELS
While dual phase stainless steels provide high strength and corrosion resistance slightly better
than typical ferritic stainless steels, its use in highly corrosive environments is somewhat
limited. Duplex stainless steels display a corrosion resistance equal to or better than that of
the austenitic grades while their mechanical properties range between ferritic and austenitic. A
typical chemistry for 2205 (duplex stainless steel) is shown in Table VII.
Table VII. Chemistry of Duplex Stainless Steel
Type
C
S
(%)
(%)
Duplex 2205 0.009 0.004
O
(%)
0.23
N
P
Si
Cr
Ni Cu Mn Mo
(%)
(%) (%) (%) (%) (%) (%) (%)
0.076 0.028 1.00 22.20 5.50 0.04 0.16 3.43
The microstructure of a duplex stainless steel is a mixture of austenite and ferrite with
optimum properties normally achieved within a range of 30% to 70% ferrite. Wrought
producers of stainless steel have found that the use of the AOD has been important in limiting
the amount of inter-metallic phases [6]. It has been shown that for optimum corrosion
resistance and to avoid sigma and chi phases the chemistry has to be controlled precisely, with
the chromium and molybdenum kept in the high end of their ranges.
An advantage of P/M duplex stainless steels is the availability of nitrogen during sintering as
an alloy element. Nitrogen is a strong austenite former and can replace nickel, thus lowering
the cost. Figure 4 shows the microstructure of the experimental heat sintered in atmospheres
ranging from pure hydrogen to nearly pure nitrogen. The nitrogen content varied from .076%
to 1.2%. Table VIII shows the corresponding mechanical properties,
Generally when nitrogen is introduced into stainless steels there is a concern for the formation of
chromium nitrides, which may adversely affect the corrosion resistance of the alloy. However, in
duplex stainless steels, the high nitrogen content promotes austenite which has a high solubility for
nitrogen and significantly reduces the formation of chromium nitrides. In this sense, the use of
nitrogen in duplex stainless steels increases pitting and crevice corrosion resistance.
a.)
b.)
Figure 4.
c.)
Etched optical photomicrographs of duplex stainless steel: a.) Sintered in 100% hydrogen, b.)
Sintered in 50% hydrogen and 50% nitrogen, and c.) Sintered in 90% nitrogen and 10% hydrogen.
Since nitrogen is a very effective solid-solution strengthening element, duplex stainless steels have
excellent strength. However, the presense of austentite in the microstructure allows the alloy to have
good toughness and ductility.
Table VIII. Mechanical property data for varoius sintering atmospheres of dulpex stainless.
Sintered
Density
Pressure
3
TRS
D.C.
UTS
Hardness
0.20% OFFSET
Elongation
Material
(tsi)
(MPa)
(g/cm )
(%)
(ksi)
(MPa)
(HRA)
(ksi)
(MPa)
(ksi)
(MPa)
(%)
100% Hydrogen
50% Hydrogen/ 50% Nitrogen
10% Hydrogen/ 90% Nitrogen
50
50
50
690
690
690
7.2
7.2
6.79
-5.33
-4.6
-2.70
202
197
214
1390
1356
1473
50
52
55
84
81
96
578
558
661
62
61
67
427
420
461
10.8
7.6
5.3
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC).
OXİDATİON RESİSTANT ALLOYS
High temperature alloys which provide strength and oxidation resistance at elevated temperatures
(300 oC to1200 oC) will be critical in new applications such as fuel cell technology and diesel
engines. These materials are generally used in the presence of combustion gases that may be
generated from exhaust and pollution control equipment. The opportunities for these materials are on
the rise. As Figure 5 shows the gasoline engine will decline in deference to the use of new
technology such as hybrids and diesel engines. The fuel cells used in the hybrid vehicles offer new
oportunities for P/M materials [7].
2020 Powerplant Mix
2000 Powerplant Mix
Gasoline
49%
Diesel
15%
Gasoline
Gasoline
98%
98%
Hybrid/
Electric/
Fuel Cell
31%
Diesel
2%
Figure 5. Projected growth of diesel and fuel cell components.
Alcohol/
Nat.Gas/
Propane
5%
These components will be exposed to high tempertures and various service atmospheres such as
superheated steam, hydrogen, and air. Particulate filters for capturing soot and ash from the exhaust
of diesel engines are being used to meet the Eurpoean environmental regulations. The ease of which
shapes can be made from P/M products make them extremely attractive for this application.
Table IX. Chemistry of oxidation resistant alloys.
T ype
409Cb
430C B
310L
S u p er F erritic
H astello y X
Si
(% )
0 .8 5
0 .8 5
1 .5 0
0 .9 0
1 .2 0
Cr
(% )
1 0 .8 0
1 8 .0 0
2 6 .5 0
2 4 .2 0
2 2 .0 0
Ni
(% )
0 .0 8
0 .0 8
2 1 .1 0
0 .0 5
4 9 .0 0
Mo
(% )
0 .0 8
0 .0 3
0 .0 2
0 .0 3
9 .0 0
Cb
(% )
0 .5 0
1 .0 0
-------
W
(% )
--------0 .6 0
Co
(% )
--------1 .3 0
The alloy selection must be based on both cost and performance. Certainly high levels of nickel and
chromium will aid in performace (Table IX) but the cost must be justfied. Figure 6 shows the TGA
results of several stainless compositions. The weight gain of various powder compositions was
measured in air while increasing the temperature from ambient to 1200 oC. The alloys tested ranged
from a 409Cb which is commonly used for exhaust flanges to Hastelloy X, which has been used for
industrial applications (furnaces) because it has unusual resistance to oxidizing, reducing and neutral
atmospheres. In general, as the alloy content increased (in particular nickel), the oxidation resistance
increased. The super ferritic alloy, which contained 24% chromium, is especially resistant to
oxidation. In the wrought industry, this grade is used in the manufacture of parts which must resist
scaling at high temperatures. This AOD processing of this alloy, which produces low carbon and
nitrogen levels, improves its relative corrosion resistance and mechanical properties. The high
temperature-mechanical properties of this alloy are far less than Hastelloy X.
20
18
% of Initial Weight
16
14
12
10
8
6
4
2
0
800
850
900
950
1000
1050
1100
1150
1200
Temperature (°C)
Hastelloy X
Super Ferritic
310L
18%Cr- 1%Cb
409Cb
Figure 6. Oxidation rate of alloys measured in air.
CONCLUSIONS
•
The high-performance processing route for stainless steel, which includes EAF/AOD
refining, High Performance Atomizing and Hydrogen Annealing have led Hoeganaes
to explore new grades of materials that can be economically manufactured. Advances
in chemistry control, improved green strength and green density have allowed for the
development of new stainless grades with improved performance.
•
Pre-alloyed boron containing stainless steel can achieve near full density leading to
corrosion resistance and mechanical properties that are superior to conventional P/M
materials and approach wrought performance.
•
Low cost, dual phase stainless steels can be manufactured that have excellent green
and sintered density while having strength that exceeds 17-4PH in the heat-treated
condition. The dual phase microstructure also allows for surface nitriding, leading to
improved wear resistance and corrosion resistance.
•
Duplex stainless steel can be manufactured by the new processing route, which can be
used in high strength applications where corrosion resistance of ferritics and dual
phase stainless is not adequate.
•
Super-Ferritic stainless steel shows promise in high temperature-oxidation resistant
applications such as fuel cells and diesel exhaust filters. For those grades that need
good mechanical properties at high temperatures, Hastelloy X is an excellent
candidate.
•
The relationship between processing and properties has allowed for the development
of potential new grades for the P/M industry. Properties such as density, tensile
strength, hardness, wear resistance, corrosion resistance, magnetic and electrical
response can all be tailored for the application.
REFERENCES
1. Causton, R.J., T. Cimino-Corey, and C.T. Schade “Improved Stainless Steel Process
Routes,” Advances in Powder Metallurgy and Particulate Materials – 2003, MPIF,
Princeton, NJ, USA, 1998.
2. Reen, O.R., “Sintered Liquid Phase Stainless Steel,” U.S. Patent No. 3,980,444.
3. Reen, O.R., “Sintered Liquid Phase Stainless Steel,” U.S. Patent No. 4,032,336.
4. Rajiv Tandon, Yixiong Liu and Randall German, High Density of Ferrous Alloys via
Supersolidus Liquid Phase Sintering, Advances in Powder Metallurgy and Particulate
Materials, Vol. 2 pp.5-27 to 5-36, 1995, MPIF- Princeton, NJ.
5. Hans Berns, “Case Hardening of Stainless Steel Using Nitrogen,” Industrial Heating
Website, 3/25/04.
6. The International Molybdenum Association, “Practical Guidelines for the Fabrication of
Duplex Stainless Steels. Edited by Technical Marketing Resources, Inc. Pittsburgh, PA,
2001.
7. G. Hoogers, Fuel Cell Technology Handbook, SAE International, Warrendale, PA, 2003,
pp. 4-1 to 4-27.