IMPROVED STAINLESS STEEL PROCESSING ROUTES
Chris Schade and Robert Causton
Hoeganaes Corporation, Cinnaminson, NJ
Tina Cimino-Corey
ANCOR Specialties, Ridgway, PA
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 these processing changes along with the potential new products that can be made with
this technology.
INTRODUCTION
Hoeganaes Corporation has made stainless steel powders via induction furnace technology since the
1960’s. The process, shown in Figure 1, has changed little over the years.1 Relatively low volumes of
stainless steel powders had dictated that small induction furnaces (typically 1 to 2 ton in capacity) are
commonly used. The raw materials are melted in the induction furnace and atomized using highpressure water and a V-Jet configuration. The chamber in which the powder is atomized is normally
purged with nitrogen to prevent excessive oxidation of the powder. After atomizing the material is
screened to the desired particle size distribution.
Figure 1: Process route for producing stainless powders using the induction furnace.2
AOD
High
Performance
Atomizing
High
Performance
Annealing
Figure 2: High Performance Stainless Processing Route
This method of manufacturing stainless steel powders was sufficient to meet the early demand for
stainless steel powder. However, the recent growth in the stainless powder metallurgy has led Hoeganaes
Corporation to develop a high volume, high performance, processing route. The new processing route is
shown in Figure 2. 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.
MELTING AND REFINING
EAF/AOD
One of the most obvious advantages of this EAF/AOD process is the size. The EAF can produce 45,000
pounds of powder; this is compared to 1000 pounds to 4000 pounds typically melted in an induction
furnace. The larger heat size associated with the EAF allows for truckload quantities to be shipped from
one batch. Needless to say, the properties such as apparent density, flow, green and sintered properties
are much more consistent when 45,000 pounds of powder are made from one batch as opposed to
multiple heats required by induction melting.
The other major advantage of the EAF/AOD combination is the principal of decarburization. The AOD
allows for the preferential oxidation of carbon over chromium. This allows for the use of a broader range
of materials such as high-carbon ferro-chromium and green scrap. The process of decarburization is
achieved by oxidation of carbon in the liquid steel to carbon monoxide that escapes from the melt. In
EAF steelmaking, if oxygen is used for decarburization in steels with high chromium levels
unacceptable, chromium oxidation occurs and valuable chromium units are lost to the slag. In the AOD
process, argon and nitrogen are bubbled through a set of tuyeres in the bottom of the vessel along with
oxygen. The partial pressure of carbon monoxide is adjusted by altering the ratios of oxygen, argon and
nitrogen, which are injected simultaneously through tuyeres into the bath. In this manner, the oxygen
combines with carbon rather than chromium, leading to decarburization of the melt. During the process,
as the aim carbon level is reached, the ratio of oxygen to inert gas is reduced in a stepwise manner from
3:1 to 1:3.
After a time period of oxygen injection through the tuyeres, the stainless steel and slag become saturated
in oxygen. At this stage the gas flow to the tuyeres is altered to 100 percent argon for refining. The
oxygen dissolved in the steel combines with any remaining carbon and forms carbon monoxide. At this
stage the carbon is reduced to extremely low levels. As the carbon monoxide bubbles travel to the
surface of the melt, nitrogen attaches to the bubbles and is removed from the liquid steel. Figure 3 shows
the levels of carbon and nitrogen achieved in AOD processed stainless versus induction melted stainless.
As can be seen, these powders reflect lower and more consistent levels of carbon and nitrogen. Various
authors have shown the benefit of lower carbon and nitrogen levels in stainless steels (austenitic and
ferritic). 3-4 Figure 4 illustrates the effect of carbon and nitrogen on the green density of 316L.
0.04
Induction Melting
0.035
Nitrogen (%)
0.03
EAF/AOD Processing
0.025
0.02
0.015
0.01
0.005
0
0
0.005
0.01
0.015
0.02
Carbon (%)
0.025
0.03
0.035
Figure 3: Carbon and Nitrogen levels for EAF/AOD processed material versus conventional induction
melted material.
6.36
Green Density g/cm 3
6.34
6.32
6.3
6.28
6.26
6.24
6.22
0.02
0.03
0.04
0.05
0.06
0.07
Carbon + Nitrogen (%)
Figure 4: Effect of Carbon and Nitrogen on the compressibility of 316L Stainless Steel.
0.08
After the decarburization stage in the AOD is complete, silicon is added to reduce any chromium that
was oxidized during the decarburization period. At this point the flow rate of argon is increased to
promote slag/metal interaction for refining. Mixtures of lime and spar are added to form a reducing slag
that removes impurities. This stage of the AOD process is called the reduction stage. During this stage
sulfur and other impurities, such as inclusions, are removed from the liquid steel and captured in the slag.
The slag is later removed by decanting from the AOD. Sulfur levels of less than 0.01% are typically
achieved, and levels less then 0.005% can be achieved with a revised slag practice.
New Grade Development
The ability of the EAF/AOD combination to remove significant levels of carbon has enabled the
economical production of new grades of stainless. There exists today, in conventional PM, many copper
and nickel containing scrap materials. The use of the green scrap is limited in normal induction furnace
melting of stainless because the induction furnace cannot remove carbon. The lower bulk density of the
parts is also an issue. Induction furnaces cannot handle large quantities of P/M scrap parts due to the
relatively small charge crucible. It is also difficult to guarantee the chemistry of large quantities of scrap,
which is a problem since adjusting the chemistry on a real-time basis is not generally done with induction
furnace melting. Generally, feedstock charge composition determines the final chemistry.
The combination of the EAF and AOD eliminate these three problems. Density of the parts is not an
issue for the EAF since the melt size is over 20 tons. Adjusting chemistry, such as carbon, is also not an
issue for the EAF/AOD combination since multiple chemistry tests are taken during the melting and
refining stages and large volume of trim additions can be made. With this process, green scrap (with both
graphite and lubricant) can be utilized because the AOD is capable of removing undesirable elements,
such as carbon, from the liquid metal. Carbon content at the start of AOD refining can be as high as
1.00%. Using the High Performance processing route, Hoeganaes Corporation has developed a 17-4PH
stainless that utilizes the scrap from various copper and nickel containing materials (including green
scrap). The typical chemistry of 17-4PH is shown in Table I.
Table I. Nominal chemistry of 17-4PH stainless steel for P/M.
Type
C
S
O
N
P
Si
Cr
Ni
(%) (%) (%) (%) (%) (%) (%) (%)
17-4pH 0.018 0.010 0.300 0.020 0.025 0.85 17.00 4.00
Cu
(%)
3.55
Mn
(%)
0.15
Mo
(%)
0.03
Cb
(%)
0.25
Adding copper that forms intermetallics, which precipitate during the secondary aging process,
strengthens precipitation-hardening steels. Advantages of 17-4PH include high strength, good corrosion
resistance and good toughness at temperatures up to 600 oF. This grade is commonly used in contact
with fluids at lower temperatures such as those in the chemical, food processing, hydraulic and paper
industries. Key automotive applications include: valve stems, fasteners, suspension knuckles, as well as
engine/power-train components such as sensors, cams and turbocharger rotors. 17-4PH is commonly
used in marine applications such as valve and pump parts and has corrosion resistance almost equal to
304L.5
Currently, most PM 17-4PH parts are manufactured via metal injection molding. This is predominantly
due to the high degree of difficulty in processing 17-4PH by the standard (-100 mesh) press and sinter
route. Due to its high alloy content, 17-4PH exhibits less compressibility than common P/M stainless
grades. Due to the fact that green strength is a partial function of green density, the resulting lower
strength leads to difficulties in handling green parts. However, with the increased carbon and nitrogen
removal from the AOD (and increased green strength from atomizing as discussed later,) a reasonable
powder for press and sinter applications can be produced. Figure 5 shows the compressibility of 17-4PH
in comparison with 304L and 434L. This enhancement in powder properties allows parts to be made via
conventional compaction methods, thus avoiding the high cost associated with metal injection molding.
Compaction Pressure (MPa)
Greeen Density (g/cm3)
345
420
495
570
6.8
6.6
6.4
6.2
6
5.8
5.6
5.4
5.2
5
645
434L
25
30
35
40
45
304L
720
17-4PH
50
55
Compaction Pressure (tsi)
Figure 5: Compressibility of 17-4PH compared to 434L and 304L.
In the wrought form, 17-4PH can be heat treated to develop a wide range of properties. The basic
premise of 17-4PH is that the copper added in the melt is in solution after sintering. Increases in
hardness and strength can be achieved by inducing the copper to precipitate out of the base metal as fine
precipitates.6 The amount of precipitation hardening can be varied by influencing the size, character and
spacing of the precipitates. Chemistry control is critical because the composition influences the amount
of precipitates. In order to examine the effects of heat treatment on P/M 17-4PH stainless, a series of
standard heat treatments were performed on test specimens. The microstructure of 17-4PH is austenitic
at elevated temperatures but forms martensite on cooling. The results of these heat treatments are shown
in Table II.
Table II. Heat treatment Data for 17-4PH, sintered at 2300 oF in H2.
ID
SD
DC
TRS
HRA
YS
UTS
(g/cm3) (%)
(MPa/103
(MPa/103 (MPa/103
psi)
psi)
psi)
As Sintered
6.65
-1.75 1247/181
47
413/60
627/91
900 oF (30 mins.)
6.67
-1.80 1420/206
56
627/91
758/110
900 oF (60 mins.)
6.67
-1.77 1578/229
57
517/75
593/86
1050 oF (4hrs.)
6.67
-1.82 1495/217
50
538/78
682/99
1150 oF (4 hrs.)
6.68
-1.86 1440/209
50
572/83
634/92
E
(%)
2.4
1.4
.9
1.1
1.0
The data suggest that for maximum strength and hardness a treatment of 30 minutes at 900 oF is useful.
Extended times at temperature (four hours) can lead to higher strengths than the as sintered condition but
at the expense of elongation. The microstructures of the various heat treatments are shown in Figure 6.
The finest distribution of precipitates is shown with the 900 oF treatment for 30 minutes (Figure 6B). As
the time and temperature is increased, the precipitates can be seen to coarsen (Figure 6C and 6D).
(a)
(b)
(c)
(d)
Figure 6: Microstructure of 17-4PH showing precipitate formation and aging; (a) as sintered; (b) treated
at 900 oF for 30 minutes; (c) treated at 900 oF for 60 minutes and (d) treated at 1050 oF for 4 hours.
The preceding discussion shows the utility of the electric arc furnace in producing an alloy requiring tight
chemistry control such as 17-4PH. The refining process in the AOD also 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. The need to control boron levels accurately is important because
excessive boron leads to precipitates, 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.7 An experimental heat of 316L was made (Table III) to examine the effects on physical
properties.
Table III. Effects of pre-alloyed boron on 316L.
ID
316L
316L +
.25%B
Press.
(MPa /
tsi)
690 / 50
690/ 50
GD
(g/cm3)
GS
(psi)
SD
(g/cm3)
DC
(%)
6.77
6.54
1956
937
7.13
7.79
-1.68
-5.90
Apparent
Hardness
(HRB)
56
72
TRS
(MPa / 103 psi)
951 / 138
1261 / 183
The 316L, with the 0.25 weight percent pre-alloyed boron, showed increases in sintered density, hardness
and TRS. The microstructures show a nearly 100% dense structure (Figure 7). The green density is
slightly lower then conventional 316L while the green strength is substantially reduced, despite having
the same apparent density. The use of the AOD in achieving tight boron levels is certainly advantageous
for this grade and future development work is warranted.
(a)
(b)
Figure 7: Porosity distribution in (a) 316L with boron and (b) conventional 316L.
HIGH PERFORMANCE ATOMIZING (HPA)
Due to the large volume of material to be atomized, over 20 tons, Hoeganaes Corporation had to develop
atomizing technology that could handle the high metal flow rates and minimize the oxygen and nitrogen
pick-up in the powder. This led to the development of Hoeganaes Corporation’s HPA system. Not only
is this atomizing system capable of handling high metal throughputs, but HPA also provides some
significant improvements in the properties of the stainless steel powder.
Green Strength
Due to the better energy transfer from the water jet to the molten metal stream, the HPA system gives
better green strength when compared to V-jet atomized stainless powders. Figure 8 shows an increase in
green strength of about 40% for 316L stainless steel. Since the increase in green strength is related to the
atomizing jet system, the increase in green strength holds for all grades of stainless steel. The HPA
process does not adversely impact properties such as green density and sintered density.
Green Strength (psi)
Compaction Pressure (MPa)
345
2000
1800
1600
1400
1200
1000
800
600
400
200
0
25
420
495
570
645
720
Conv.
30
35
40
45
HPA
50
55
Compaction Pressure (tsi)
Figure 8: Green Strength of 316L using conventional atomizing (V-Jet) and High Performance
Atomizing (HPA).
New Grade Development
Atomization of large batches of stainless (20 tons) required changes in the method that molten metal was
delivered to the atomizer. A delivery system that provides high metal flow rates (over 400 lbs/min) while
protecting the stainless steel from oxidation was developed. The effectiveness of this system has shown
the potential for atomizing grades that have been troublesome in the past. For example, the original
development of 409L for the P/M industry was done exclusively with columbium as the stabilizing agent.
It has been shown (in wrought stainless8-10) that a balance of titanium and columbium improve the
resistance to sensitization and intergranular corrosion and also improve formability, toughness and
machining. However, with the low metal flow rates (85 lbs/min) used in V-jet atomizing, titanium can
lead to nozzle clogging, consequently columbium has been used exclusively in 409L for the P/M
industry. In order to use titanium as a stabilizing agent, careful control of oxygen in the melt and during
atomizing is necessary. Titanium oxides are solid at steelmaking temperatures and can build up on the
nozzle and cause clogging. With the high metal flow rate delivery system, in particular the shrouding
developed, Hoeganaes has experimented with dual stabilization of 409L. Table IV shows the chemical
composition of three experimental heats: (1) 409Cb with the conventional level of columbium, (2) a
409Cb with a reduced level of columbium and (3) and heat of 409L stabilized with both columbium and
titanium.
Table IV. Chemistry of 409L stabilized with columbium and titanium.
Type
409L
Normal Cb
Reduced Cb
Cb + Ti
C
(%)
0.008
0.009
0.008
S
(%)
0.009
0.006
0.006
O
(%)
0.300
0.240
0.230
N
(%)
0.015
0.013
0.014
P
(%)
0.012
0.013
0.013
Si
Cr
Ni
(%) (%) (%)
0.95 12.40 0.09
0.90 12.70 0.05
0.80 12.60 0.04
Cu
(%)
0.08
0.07
0.02
Mn
(%)
0.15
0.18
0.11
Mo
(%)
0.02
0.10
0.03
Cb
(%)
0.50
0.26
0.17
Ti
(%)
-----
0.10
Stabilizing elements such as columbium and titanium are added to stainless steels to reduce sensitization,
typically during the welding operation. The material becomes sensitized when the chromium combines as
carbides and/or nitrides. If the free chromium content available to form an oxide coating is reduced
significantly, corrosion resistance can be reduced.11 Titanium and columbium combine with the carbon
and nitrogen during the welding operation, leaving the chromium to form a surface oxide which provides
corrosion resistance. The MPIF specification for the minimum amount of columbium added is eight
times the percent carbon with a maximum value of .8%. The designation used by most other stainless
steel producers is a minimum of:
(Ti + Cb) = .08 + 8 x (C + N).
Typical values of columbium currently used in PM are between .50% and .60%. With the low levels of
carbon and nitrogen produced in the AOD, and because of the real time chemistry control that is now
possible, more exacting amounts of columbium can be added. The reduced columbium heat has a better
ratio to stabilize the actual carbon and nitrogen in the 409L. Mechanical properties of the materials are
shown in Tables V and VI. This material exhibits superior sintered density and elongation than the over
stabilized heat (Normal Cb). Over stabilization can lead to inclusion formation and weld embrittlement.
The dual stabilized heat, with additions of titanium and columbium, shows an improvement in
compressibility while maintaining elongation. Strength and hardness are slightly lower with the dual
stabilized material. However, it has been shown that high temperature creep and oxidation resistance of
dual stabilized steel is higher then columbium stabilized materials. More work is warranted with dual
stabilization practices for P/M grades. Physical properties of the three materials are shown in Tables V
and VI.
ID
Normal
Cb
Reduced
Cb
Cb + Ti
Table V. Physical properties for 409L
sintered at 1260°C (2300°F) in Hydrogen.
Press.
GD
SD
DC
(MPa /
(g/ cm3) (g/ cm3)
(%)
tsi)
690 / 50
6.56
7.12
-2.77
Apparent
Hardness
(HRB)
62
690/ 50
6.53
7.23
-3.51
61
690 / 50
6.65
7.00
-2.38
55
Table VI. Physical properties for 409L
sintered at 1260°C (2300°F) in Hydrogen.
ID
Normal
Cb
Reduced
Cb
Cb + Ti
Press.
(MPa /
tsi)
690 / 50
Yield
(MPa / 103 psi)
UTS
(MPa / 103 psi)
Elong.
(%)
214 / 31
359 / 52
15.9
690 / 50
193 / 28
352 / 51
17.4
690 / 50
193 / 28
310 / 45
15.5
HIGH PERFORMANCE ANNEALING
Green Properties
The High Performance Processing route now incorporates hydrogen annealing. Work by previous
authors have shown benefits to annealing stainless in a hydrogen atomosphere.2, 12 Benefits in green
density have been linked to slight reductions in carbon and oxygen levels as well as grain growth in
ferritic stainless. Green strength and green density can be enhanced by high temperature agglomeration
of the powder, but usually at the expense of sintered density. Hoeganaes has experienced an increase in
green density of approximately .05% to .10% on various 400 series stainless materials. Table VII shows
some typical results when annealing 410Cb in hydrogen. Note the increase in green density, green
strength and oxygen, with slight decreases in carbon and nitrogen.
Table VII. Influence of High Performance Annealing Process on 410Cb.
ID
410Cb –Unannealed
410Cb -H2 Anneal
Press.
(MPa /
tsi)
550 / 40
550/ 40
GD
(g/cm3)
GS
(psi)
C
(%)
O2
(%)
N2
(%)
6.33
6.38
1247
1452
.021
.019
.22
.25
.019
.012
New Grade Development
This increase in compressibility by annealing has allowed some advances in alloy development.
Typically with 400 series stainless, as the alloy content increases the green density deteriorates. The use
of hydrogen annealing in the high-performance processing route has allowed the development of some
new grades of stainless, one of these is dual phase stainless.
In applications that require abrasion resistance, hardness and ductility, wrought producers of stainless
steel have developed a dual phase stainless steel 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, chromium, 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.
Traditionally, when stainless parts producers have needed high hardness, graphite has been added to
410L to promote martensite. This 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. Table VIII shows the chemistry of 410L, with graphite addition, and a heat produced using
the dual phase approach. The amount of martensite, and hence hardness, can now be controlled by the
elements in solution. Tables IX and X show that properties of ferritic stainless steels can be altered with
slight variations in chemistry. The dual phase chemistry provides physical properties that are equal to or
superior to the 410L with added carbon. The ability to meet the hardness without carbon additions
should lead to better corrosion resistance. Figure 9 shows the microstructure of the both steels. By using
the AOD to vary the Km factor, one can adjust the properties through microstructural changes. The 410L
with carbon addition is fully martensitic while the dual phase steel shows a structure of ferrite and
martensite.
.
Table VIII. Chemistry of 410L with graphite additions versus Dual Phase Stainless.
Type
C
S
O
N
P
Si
Cr
Ni
(%) (%) (%) (%) (%) (%) (%) (%)
410L + .3% C 0.060 0.008 0.300 0.024 0.013 0.85 12.50 0.09
Dual Phase 0.015 0.007 0.210 0.009 0.014 0.84 11.60 1.03
Cu
(%)
0.08
0.03
Mn
(%)
0.15
0.10
Mo
(%)
0.02
0.34
Km
10.30
12.29
Table IX. Transverse rupture properties for Dual Phase Stainless Steel and 410L with graphite addition.
Sintered at 1260°C (2300°F) in Hydrogen.
SD
DC
Apparent
TRS
ID
Press.
GD
Hardness (MPa / 103
(MPa /
(g/ cm3) (g/ cm3) (%)
(HRB)
psi)
tsi)
690 / 50
6.63
7.05
-2.17
79
1580 / 229
410L +C
690/ 50
6.57
7.40
-3.75
88
1565 / 227
Dual
Phase
Table X. Physical properties for 410L and Dual Phase Stainless
sintered at 1260°C (2300°F) in Hydrogen.
ID
410L + C
Dual
Phase
(a)
Press.
(MPa /
tsi)
690 / 50
690 / 50
Yield
(MPa / 103 psi)
UTS
(MPa / 103 psi)
Elong.
(%)
359 / 52
469 / 68
634 / 92
676 / 98
4.9
4.4
(b)
Figure 9: Microstructure of (a) 410L with graphite (martensitic); (b) Dual Phase Stainless (Martensitic
and Ferritic)
CONCLUSIONS
The new high performance processing route, which includes EAF/AOD melting and refining, High
Performance Atomizing and hydrogen annealing has improved the properties of conventional stainless
steel powders. The larger batch size produced by this process has led to a more consistent product. This
innovative process route has led to the development of powders with unique microstructures and
properties. Specifically, this new production system for stainless steel powders has led to the economic
manufacture of high performance materials.
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Electric Furnace Conference, San Antonio, Texas, 2002.
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