TECHNICAL FIELD
[0001] Embodiments of the present invention may provide a new stainless steel powder suitable
for manufacturing of duplex sintered stainless steels. Embodiments of the present
invention may also relate to a method for producing the stainless steel powder, the
duplex sintered stainless steel as well as methods for producing the duplex sintered
stainless steel.
BACKGROUND
[0002] Duplex stainless steels have been known to the industry for more than 60 years. They
are widely used in heat-treated cast, wrought and gas atomized powder forms, in many
applications that require a combination of high strength and high corrosion resistance.
However, they are unavailable today, in the water atomized powder form for use in
press and sinter applications.
[0003] Common uses for duplex stainless steels include chemical process plants pipeline,
petrochemical industry, power plants and automobiles. They are also used in food processing
industry, pharmaceutical process components, paper and pulp industry, in desalination
plants and in the mining industry. Duplex stainless steels are known for their high
resistance to inter granular corrosion (IGC) and stress corrosion cracking (SCC) in
chloride media. Chloride is severe challenge that leads to rapid corrosion media for
iron-based alloys.
[0004] High strength and high corrosion resisting properties in duplex stainless steel are
believed to be acquired due to a presence of ferrite and austenite phases in equal
amounts. Such structure is generally achieved by using a balance of austenite stabilizers,
e.g., nickel (Ni), manganese (Mn), carbon (C), nitrogen (N), copper (Cu) and cobalt
(Co), and ferrite stabilizers, e.g., chromium (Cr), silicon (Si), molybdenum (Mo),
tungsten (W), titanium (Ti) and niobium (Nb).
[0005] As mentioned previously, the high strength and high corrosion resistance of duplex
stainless steel is believed to come from a balance of ferrite and austenite in the
microstructure. The microstructure depends not only on the chemistry but also on the
heat treatment carried out on the material. All duplex steel compositions today make
use of N in the chemistry, as N is a strong austenite stabilizer. N, when present
in the alloy along with Cr, poses problem of forming nitrides which are deleterious
to the properties such as strength and corrosion resistance. Further, during welding
duplex stainless steels, an intermetallic phase known as "Sigma" is formed in a heat
affected zone (HAZ) due to slower cooling rates. This Sigma phase is a hard, supersaturated,
intermetallic phase containing Cr and Mo. The area around the Sigma phase is depleted
of Cr and Mo and becomes weak and less resistant to corrosion. Often duplex stainless
steels need annealing and quenching process to reduce or eliminate this Sigma phase.
[0006] In wrought or cast duplex stainless steels, the steel is solidified as ferritic steel
and the austenite phase is precipitated out from ferrite during cooling of the alloy.
The cooling rate is critical after casting or any heat treatment, as the cooling rate
determines the percentage of austenite and any intermetallic phases precipitated within
the structure.
[0007] Although wrought duplex stainless steels, in particular 'hot rolled' duplex stainless
steels, have been common in industrial use since 1930s, they were hardly used in the
Powder Metallurgy industry. There are a few applications where gas atomized duplex
stainless steel powders are used in hot isostatic pressed (HIP) condition. Powders
produced by gas atomizing have spherical morphology. Such powders are less suitable
for conventional press and sinter applications. Due to the spherical shape, they have
insufficient green strength, which is required to handle green press and sinter parts.
Irregular shaped powders, such as those produced with water atomization, have much
higher green strength as the irregular shape of the powders tends to bind together.
Currently there is no water atomized stainless steel powder available for producing
sintered duplex stainless steel components. The current chemical compositions used
in gas atomized powders, and also in wrought steels, use N as a major alloying element
to achieve austenite-ferrite balance and achieve required mechanical strength. Inclusion
of N in the powder increases the hardness of the powder reducing the compressibility
in conventional press and sinter applications. This may result in reduced green density
and subsequently reduced sinter density.
[0008] There have been several attempts to develop sintered duplex stainless steels made
from water atomized powders. Lawley et al
1 attempted to develop equivalent grades of AISI 329 and AISI 2205 with maximum tensile
strength of 578 MPa. Dobrzanski et al
2 mixed ferritic and austenitic powders to produce duplex structure with tensile strength
650 MPa. The same group also studied the corrosion properties of duplex stainless
steel with electrochemical method and concluded that the duplex stainless steels show
better corrosion resistance than their austenitic counterpart
3. Due to their high alloy content these steels are sensitive to the composition and
also the processing parameters. These alloys form intermetallic phases known as sigma,
chi and gamma prime which are rich in Mo, W, N, Ni and Cr and reduce both mechanical
properties and corrosion properties. Sigma phase forms in a temperature range 700°C
to 1000°C whereas Chi phase forms within range 300°C to 450°C. The Gamma (austenite)
phase may start forming at around 600°C.
[0009] Typical composition of wrought duplex stainless steel is Fe with 32-23wt% Cr, 4.5-6.5wt%
Ni, 2.5-3.5wt% Mo, and 0.08-0.2wt% N, such as for SAF 2205. There are numerous patents
for duplex stainless steel composition close to this composition. Almost all of the
duplex stainless steels rely on the N content for increased corrosion resistance and
increased strength. So far the commercial uses of sintered powder metallurgy (PM)
duplex stainless steels are limited to the use of gas atomized fine powders that can
be used for mainly HIP process. The main obstacle in using low cost water atomized
powders for conventional
1 A. Lawley, E. Wagner, C.T. Schade, Advances in Powder Metallurgy and Particulate Materials
2005 Part 7 pp 78-89
2 L.A. Dobrzanski, Z. Brytan, M. Actis Grande, M. Rosso, Archives of Materials Science
and Engineering, Vol 28 Iss 4, April 2007 PP 217-223
3 L.A. Dobrzanski, Z. Brytan, M. Actis Grande, M. Rosso, Journal of Achievements in
Materials and Manufacturing Engineering, Vol 17 Iss 1-2 pp 317-320 PM use is increased N and possibility of intermetallic and carbide precipitation
due to cooling rate during the sintering. Also conventional sintering needs some wetting
agents or low temperature melting constituents to increase free energy and accelerate
the kinetics of austenite phase precipitation within ferritic matrix.
[0010] In the patent literature there are some documents disclosing sintered duplex stainless
steel structures.
[0011] SE538577C2 (Erasteel) discloses a sintered duplex stainless steel made from gas atomized powder
and having a chemical composition with a max 0.030wt% C, 4.5-6.5wt% Ni, 0.21-0.29wt%
N, 3.0-3.5wt% Mo, 21-24wt% Cr, and optionally one or more of 0-1.0wt% Cu, 0-1.0wt%
W, 0-2.0wt% Mn, 0-1.0wt% Si wherein N is equal or greater than 0.01*Cr and the remaining
elements are Fe and unavoidable impurities.
[0012] EP0167822A1 (Sumitomo) discloses a sintered stainless steel comprising a matrix phase and a dispersed phase
and a process for manufacturing. The dispersed phase is an austenite metallurgical
structure and is dispersed throughout the matrix phase, which is comprised of an austenitic
metallurgical structure having a steel composition different from that of the dispersed
phase or a ferritic-austenitic duplex stainless steel.
[0013] JP5263199A (Sumitomo) discloses production of a sintered stainless steel comprising a matrix phase and
a dispersing phase. The method includes mixing a ferritic stainless steel powder with
a powder selected from an austenitic stainless steel powder, an austenitic-ferritic
duplex stainless steel powder, an austenitic-martensitic duplex stainless steel powder
and austenitic-ferritic-martensitic stainless triple phase stainless steel powder.
The powder mixture being compacted and sintered.
[0014] EP0534864B1 (Sumitomo) discloses a sintered stainless steel having a content of N of 0.10-0.35wt% and made
from gas atomized steel powder having the same chemical composition as the sintered
stainless steel.
SUMMARY
[0015] Almost all duplex grades available have N content between 0.18- 0.40wt% in order
to balance austenite-ferrite balance in the structure and increase the strength. Although
N content helps the above properties, it can pose hurdles in post processing, such
as heat treatment and welding operations, by forming chromium nitrides, which limits
the use of duplex stainless steels in many applications. In powder form N increases
the powder hardness making it less suitable for press and sinter applications.
[0016] Embodiments of the invention overcome the problem with nitrides by avoiding the use
of N in the chemistry, for example, having less than 0.10 wt% N or less than 0.07
wt% N, or less than 0.06 wt% N, or less than 0.05 wt% N, or less 0.04 wt% N, or less
than 0.03 wt% N, and achieving phase balance and strength by alternative elements.
Embodiments of the invention may enable production of water atomized powder with moderate
compressibility for use in conventional press and sinter applications. Embodiments
of this composition may also reduce precipitation of a deleterious 'Sigma' phase;
irrespective of rate of cooling during sintering or annealing, mainly due to lower
Mo content. Thus minimizing post sintering heat treatments necessary to eliminate
"Sigma" phase and minimizing Sigma phase precipitation during welding.
[0017] Embodiments of the composition may offer similar advantages when formed by gas atomization.
[0018] Other than conventional powder metallurgy, embodiments of the composition yield similar
properties when processed with Casting, Direct Metal Deposition and Additive Manufacturing
techniques.
DETAILED DESCRIPTION
[0019] One object of certain embodiments of the invention is to provide an alloy powder
for conventional PM that will produce a duplex structure during a sintering cycle.
[0020] Another object of certain embodiments of the present invention is to provide a duplex
sintered stainless steel.
[0021] Another object of certain embodiments of the present invention is to obtain at least
35% higher tensile strength than ferritic steels such as 430L and double the corrosion
resistance than austenitic steels such as 316L.
[0022] Still another object of certain embodiments of the present invention is to provide
a method for producing a duplex sintered stainless steel without the need of post
sintering heat treatment.
[0023] The above objectives may be accomplished by the following aspects and embodiments.
[0024] In a first aspect of the present invention there is provided a stainless steel powder
comprising, or consisting of, in weight percent:
up to 0.1 % of C,
0.5-3% of Si,
up to 0.5% of Mn,
20-27% of Cr,
3-8% of Ni,
1-6% of Mo,
up to 3% of W,
up to 0.1 % N,
up to 4% of Cu,
up to 0.04% of P,
up to 0.04% of S,
unavoidable impurities up to 0.8%,
optionally one or more of up to 0.004% B, up to 1 % Nb, up to 0.5% Hf, up to 1 % Ti,
up to 1% Co,
rest Fe.
[0025] The unavoidable impurities do not include the listed elements of C, Si, Mn, Cr, Ni,
Mo, W, N, Cu, P, S, B, Nb, Hf, Ti, or Co. Unavoidabale impurties may include impurities
that cannot be controlled, or controlled with difficulty, during manufacture of steels.
These can come from the raw materials used and also from the process. These include,
Al, O, Mg, Ca, Ta, V, Te, or Sn. The unavoidable impurities may be up to 0.8%, up
to 0.6%, up to 0.3%. An unavoidable impurity may be O. O may be present up to 0.6%,
up to 0.4%, or up to 0.3%.
[0026] In a preferred embodiment of the first aspect there is provided a stainless steel
powder consisting of, in weight percent:
up to 0.06% of C,
1-3% of Si,
up to 0.3% of Mn,
23-27% of Cr,
4-7% of Ni,
1-3% of Mo,
0.8-1.5% of W,
up to 0.07% N,
1-3% of Cu,
up to 0.04% of P,
up to 0.03% of S,
unavoidable impurities up to 0.8%,
optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti,
up to 1% Co,
rest Fe.
[0027] In another preferred embodiment of the first aspect there is provided a stainless
steel powder comprising in weight percent:
up to 0.03% of C,
1.5-2.5% of Si,
up to 0.3% of Mn,
24-26% of Cr,
5-7% of Ni,
1-1.5% of Mo,
1-1.5% of W,
up to 0.06% N,
1-3% of Cu,
up to 0.02% of P,
up to 0.015% of S,
unavoidable impurities up to 0.8%,
optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti,
up to 1% Co,
rest Fe.
[0028] In embodiments of the first aspect the powder is ferritic. For example, 99.5% ferritic.
Slight amounts of austenite, e.g., up to 0.5% may be tolerated.
[0029] In embodiments according to the first aspect the powder is produced by water atomization.
[0030] In embodiments of the first aspect the powder is produced by gas atomization.
[0031] In embodiments of the first aspect the particle size of the powder is between 53
microns and 18 microns such that at least 80wt% of the particles are less than 53
microns and at most 20wt% of the particles are less than 18 microns.
[0032] In embodiments of the first aspect the particle size of the powder is between 26
microns and 5 microns such that at least 80wt% of the particles are less than 26 microns
and at most 20wt% of the particles are less than 5 microns.
[0033] In embodiments of the first aspect the particle size of the powder is between 150
microns and 26 microns such that at least 80wt% of the particles are less than 150
microns and at most 20wt% of the particles are less than 26 microns.
[0034] In a second aspect of the present invention there is provided a method of producing
a stainless steel powder according to the first aspect comprising the steps of:
- providing a molten metal of having a chemical composition corresponding to the chemical
composition of the stainless steel powder according to the first aspect;
- subjecting a stream of the molten metal to water atomization; and
- recovery of the obtained stainless steel powder.
[0035] In a third aspect of the present invention there is provided a sintered duplex stainless
steel having a chemical composition according to the first aspect and embodiments
thereof.
[0036] In embodiments of the third aspect the Ni equivalent (Ni
eq) is such that 5 < Ni
eq < 11 and the Cr equivalent (Cr
eq) is such that 27 < Cr
eq <38.
[0037] In embodiments of the third aspect the pitting resistance equivalent number (PREN)
is 28 < PREN < 33.
[0038] In embodiments of the third aspect, the microstructure of the sintered duplex stainless
steel is characterized by austenite phase precipitated within ferrite phase.
[0039] In embodiments of the third aspect, the microstructure of the sintered duplex stainless
steel contains 30-70% austenite and 30-70% ferrite. In embodiments of the third aspect,
the microstructure of the sintered duplex stainless steel contains at least 99.5%
austenite and ferrite, for example, at least 99.8% austenite and ferrite. The percentage
of austenite and ferrite may be determined by ASTM E 562-11 and ASTM E 1245 -03.
[0040] In embodiments of the third aspect the microstructure of the sintered duplex stainless
steel is characterized by being free from sigma phases and nitrides, for example,
having less than 1% of sigma phases and nitrides.
[0041] In a fourth aspect of the present invention there is provided a method for producing
a sintered stainless steel comprising the steps of:
- providing a stainless steel powder according to the first aspect,
- optionally mixing the stainless steel powder with a lubricant and optionally other
additives,
- subjecting the stainless steel powder or the mixture to a consolidation process forming
a green component,
- subjecting the compacted green component to a sintering step in an inert or reducing
atmosphere or in vacuum at a temperature between 1150°C to 1450°C, preferably at a
temperature between 1275°C to 1400°C for a period of time of 5 minutes to 120 minutes
,
- subjecting the sintered component to a cooling step down to ambient temperature.
[0042] Examples of an inert atmosphere include nitrogen, argon, and vacuum with argon backfill.
[0043] An example of a reducing atmosphere is a hydrogen atmosphere, an atmosphere of a
mixture of hydrogen and nitrogen, or an atmosphere of dissociated ammonia. In limited
examples, carbon dioxide or carbon monoxide atmospheres may be used.
[0044] In embodiments of the fourth aspect said consolidation process includes the steps
of:
- uniaxial compaction at a compaction pressure of up to 900 MPa in a die to form a green
component,
- ejecting the obtained compacted green component from the die.
[0045] In embodiments of the fourth aspect said consolidation process includes one of: Metal
Injection Molding (MIM), Hot Isostatic Pressing (HIP) or Additive Manufacturing techniques
such as Binder Jetting, Laser Powder Bed Fusion (L-PBF), Direct Metal Laser Sintering
(DMLS) or Direct Metal Deposition (DMD).
[0046] In embodiments of the fourth aspect forced cooling or quenching is excluded from
the cooling step.
Effect of alloying elements
[0047] The effect of common alloying elements in stainless steels is well known. Cr is a
major element in stainless steels which forms a Cr
2O
3 layer on the surface which then prevents further oxygen passing the layer, therefore
providing an increased corrosion resistance. Ni is another major element which affects
the properties of stainless steel. Ni increases the strength and toughness of the
steel and also when present with Cr, enhances the corrosion resistance. Mo and W both
impart the strength and toughness when present along with Ni. Mo also enhances the
corrosion resistance along with Cr and Ni. Si acts as deoxidizer preventing O combining
in the steel during melting and also Si is strong ferrite former. Cu is austenite
stabilizer. Cu also increases the corrosion resistance of stainless steel. Especially
in conventional PM, Cu helps sintering promoting liquid phase sintering.
[0048] Embodiments of the invention provide a powder suitable for producing sintered duplex
stainless steel, as well as the sintered stainless steel. The powder and the sintered
stainless steel having a low or neglectable content of N. This eliminates the problem
of formation of deleterious nitrides during fabrication of the sintered stainless
steel. The sintered stainless steel is preferably produced from a compacted and sintered
water-atomized powder since the low N content makes it possible to produce water-atomized
powder with reasonable compressibility.
[0049] Mo is normally present in stainless steel as it strongly promotes the resistance
to both uniform and localized corrosion. Mo strongly stabilizes ferritic microstructure.
At the same time Mo is prone to precipitate Mo rich "Sigma" and "Chi" phases at Ferrite-
Austenite grain boundary. These are deleterious phases and affect strength and corrosion
resistance adversely. However, due to lower Mo content in embodiments of the powder
of the present invention, the possibility of forming sigma phase at any cooling rate
is reduced, eliminating or reducing the need for the post processing heat treatment
of annealing. This also means that the sigma phase will not likely form during welding
operation, which is a common fabrication process for duplex stainless steels.
[0050] Cr gives stainless steels their basic corrosion resistance and increases the resistance
against high temperature corrosion.
[0051] Ni promotes an austenitic microstructure and generally increases ductility and toughness.
Ni has also a positive effect as it reduces the corrosion rate of stainless steels.
[0052] Cu promotes an austenitic microstructure. The presence of Cu in the powder of the
present invention facilitates the sintering process by enabling liquid phase sintering.
[0053] W is expected to improve the resistance against pitting corrosion.
[0054] Si increases strength and promotes a ferritic microstructure. It also increases oxidation
resistance at high temperatures and in strongly oxidizing solutions at lower temperatures.
[0055] When present in the powder according to certain embodiments of the present invention
B, Nb, Hf, Ti, Co may enhance the properties. B when added in small % may help in
liquid phase sintering. However, excess B, if present, may form borides, which are
deleterious to both mechanical, and corrosion properties. Nb and Hf when present may
stabilize the microstructure by preferentially combining with carbon forming fine
carbides freeing Cr for the corrosion resistance. Ti in stainless steels may increase
the tensile strength and toughness. Co increases the high temperature mechanical properties.
[0056] Elements such as C, Mn, S and P should be kept at a level as low as possible in the
powder of embodiments of the present invention as they may have a negative effect
to various extent on compressibility of the powder and/or mechanical and corrosion
preventive properties on the sintered component.
[0057] Other elements, here designated as unavoidable impurities, may be tolerated up to
a content of 0.8% by weight of the powder according to the present invention.
[0058] The composition of the powder according to embodiments of the present invention is
designed such that the produced powder will have fully (e.g., at least 99.5%) ferritic
structure in the powder form and austenitic phase is precipitated out during sintering
cycle. This will allow controlling the ratio of ferrite and austenite by adjusting
the sintering parameters.
[0059] Ni and Cr equivalents are calculated based on following empirical formulae:
Where Cr, Ni, etc. are the level of each element in the alloy in weight %.
[0060] Further Pitting Resistance Equivalent Number is calculated as:
Where Cr, Mo and N are the level of each element in the alloy in weight %.
[0061] The composition is targeted such that 5 < Ni
eq < 11 and 27 < Cr
eq <38. This places the alloy in at the border of Ferritic - Duplex region on Schaeffler
Diagram. At this point the alloy is almost entirely ferritic (e.g., at least 99.5%).
Elements like Mo, W and Si are supersaturated in the ferritic matrix.
[0062] The powder of embodiments of the present invention may be produced by conventional
powder manufacturing processes. Such processes may encompass melting of the raw materials
followed by water or gas atomization, forming a so called prealloyed powder wherein
all elements are homogeneously distributed within the iron matrix. A major advantage
with a prealloyed powder in contrast to a premixed powder, wherein two or more powders
are mixed together, is that segregation is avoided. Such segregation may cause variation
in mechanical properties, corrosion resistance etc.
[0063] When used for the production of sintered components, the powder of embodiments of
the present invention may be compacted in a conventional uniaxial compaction equipment
at a compaction pressure up to 900 MPa.
[0064] Suitable particle size distribution of the stainless steel powder to be used at conventional
uniaxial compaction is such that the particle size of the powder is between 53 microns
and 18 microns such that at least 80wt% of the particles are less than 53 microns
and at most 20wt% of the particles are less than 18 microns. Before compaction, the
powder of embodiments of the present invention may be mixed with conventional lubricants,
such as, but not limited to, Acrawax, Lithium Stearate, Intralube at a content up
to 1wt%. Other additives mixed in, up to 0.5wt%, may be machinability enhancing agents
such as CaF
2, muscovite, bentonite or MnS.
[0065] Other methods of consolidation techniques may be utilized such as Metal Injection
Molding (MIM), Hot Isostatic Pressing (HIP), extrusion or Additive Manufacturing techniques
such as Binder Jetting, Laser Powder Bed Fusion (L-PBF), Direct Metal Laser Sintering
(DMLS) or Direct Metal Deposition (DMD)
[0066] In a MIM process, suitable particle size distribution of the stainless steel powder
to be used is such that the particle size of the powder is between 26 microns and
5 microns such that at least 80wt% of the particles are less than 26 microns and at
most 20wt% of the particles are less than 5 microns.
[0067] In a HIP or extrusion process suitable particle size distribution of the stainless
steel powder to be used is such that the particle size of the powder is between 150
microns and 26 microns such that at least 80wt% of the particles are less than 150
microns and at most 20wt% of the particles are less than 26 microns.
[0068] The particle size distribution may be measured by a conventional sieving operation
according to ISO 4497:1983 or by laser diffraction (Sympatec) according to ISO 13320:1999.
[0069] After compaction or consolidation, the compacted or consolidated body is subjected
to a sintering process at sufficiently high temperatures in the range of 1150°C to
1450°C, preferably at sufficiently high temperatures in the range of 1275°C to 1400°C
for a period of time of 5 minutes to 120 minutes. Depending of shape and size of parts
to be sintered, other period of sintering time such as 10 minutes to 90 minutes or
15 minutes to 60 minutes may be applied. The sintering atmosphere may be vacuum, inert
or reducing such as a hydrogen atmosphere, an atmosphere of a mixture of hydrogen
and nitrogen or dissociated ammonia. During the sintering process, the supersaturated
elements in ferrite matrix precipitate out as an austenitic phase. Austenite will
start precipitating out at the grain boundaries and with further sintering will grow
and precipitate within the grain itself.
[0070] In contrast to other known duplex stainless steel materials, the composition of embodiments
of the present invention should not form sigma phases or other hard and deleterious
phases, e.g., Chi phase and nitrides, during cooling from an elevated temperature,
irrespective of the cooling rate. For example, the amount of sigma phase or other
hard and deleterious phases is less than 0.5%. Forced cooling or quenching is thus
not necessary to apply. In this context forced cooling means that the sintered parts
are subjected to a cooling gas at a pressure above atmospheric pressure. Quenching
means that the sintered parts are submerged into a liquid cooling media.
[0071] A microstructure as shown in Figure 1 will typically be formed containing ferrite
and austenite. Presence of both phases is responsible for elevated mechanical and
corrosion properties. No, or significantly limited amounts of, deleterious phases
such as sigma and chi are formed during cooling which are normal for current known
duplex stainless steels. As another consequence, this property will reduce or eliminate
the formation of such phases during welding where the heat affected zone (HAZ) experience
varying cooling rates. In another consequence, this composition will limit the precipitation
of such phases during processes such as casting, extrusion, MIM, HIP and additive
manufacturing.
[0072] Embodiments of the invented alloy has shown mechanical and corrosion properties that
are comparable to or exceeding the wrought and PM products manufactured with known
duplex stainless steel alloys available.
[0073] In summary, certain advantages of embodiments of this invention may include fewer
tendencies to precipitate deleterious sigma and chi phases that affect the mechanical
and corrosion properties. This is particularly of interest in welding. Most of the
duplex stainless steel components are welded after they are formed. Welding imparts
different cooling rates in different parts of HAZ. These cooling rates tend to precipitate
sigma and chi phases along with nitrides due to nitrogen present in the current known
alloys. Absence of these phases may eliminate the post heat treatments, which normally
involve annealing at temperatures above 1200°C followed by rapid cooling. This in
most cases becomes difficult when parts are welded to a bigger structure limiting
use of duplex stainless steel.
FIGURE LEGENDS
[0074]
Figure 1 shows the microstructure of invented sintered stainless steel, austenite
and ferrite phases are present in equal proportions in as sintered condition, black
spots are porosity.
Figure 2 discloses a comparison of ultimate tensile strength (UTS) and corrosion properties
of the invented sintered stainless steel compared to alloy to 300 and 400 alloys
Figure 3 shows a comparison of mechanical properties of the invented sintered stainless
steel at different sintering conditions
EXAMPLES
EXAMPLE 1
[0075] A stainless steel powder, having a particle size below 325 mesh, i.e. 95wt% of the
particles passed 45µm sieve, was mixed with 0.75wt% of Acrawax as a lubricant. The
chemical analysis of the stainless steel powder was 0.01 wt% C, 1.52wt% Si, 0.2wt%
Mn, 0.013wt% P, 0.008wt% S, 24.9wt% Cr, 2.0wt% Cu, 1.3wt% Mo, 1.0wt% W, 0.05wt%N,
balance Fe.
[0076] The obtained powder mixture was pressed in a uniaxial press and compacted into transverse
rapture strength (TRS) bars, according to ASTM B528-16 at a compaction pressure of
750 MPa. The pressed TRS bars were then sintered in 100% hydrogen atmosphere at 1343°C
with ramp rate of 7°C/minute for 45 minutes. This was followed by furnace cooling
at rate 5°C/minute. The samples were then mounted and polished for microstructure
examination. The polished samples were then electro-etched with 33%NaOH at 3V for
15 sec. Electro-etch with NaOH reveals the ferrite phase as tan, austenite as white
(unaffected) and sigma phases in dark orange at grain boundaries within ferrite matrix.
The microstructure observed is as shown in Figure 1. The microstructure shows approximately
50/50 mixture of ferrite (tan) and austenite (white). There is no sign of any sigma
phase (dark orange) in the microstructure. The black spots are porosity in the sample.
EXAMPLE 2
[0077] Various stainless steel powders according to embodiments of the invention, and as
comparative samples, were produced by water atomizing. The chemical composition of
the stainless steel powders are shown in table 1. Stainless steel melts having various
chemical compositions were melted in an induction furnace, the molten metal was subjected
to water stream to obtain steel powder. The obtained powders was then dried and screened
to -325 mesh. The screened powder was -45 microns i.e. 95wt% of the powder particles
were less than 45 microns. The powders were then mixed with 0.75wt% of the lubricant
Acrawax.
[0078] In order to test the mechanical properties i.e. ultimate tensile strength (UTS),
yield strength (YS) and elongation, TS samples (dog bone) per ASTM B925-15 were pressed
with a compaction pressure of 750 MPa. The bars were then sintered as mentioned in
Example 1. The sintered bars were then tested for mechanical properties per ASTM E8/E8M-16a.
Metallographic examination was also conducted in order to establish the ratio between
austenite and ferrite in sintered samples. The test results are shown in table 2 in
comparison with published data from samples of known duplex stainless steels in wrought,
(DSS 329 Wrought), and gas atomized and hipped conditions (DSS 329 PM GA).
[0079] Table 2 shows that the stainless steel powders according to the present invention
can be used for producing sintered duplex stainless steel having desired mechanical
properties.
Table 1, chemical compositions of various stainless steel powders, there production
method and type of process for producing sintered samples.
Chemical analysis [% by weight] |
|
Sample |
Type |
C |
Si |
Mn |
S |
P |
Cr |
Ni |
Mo |
W |
Cu |
O |
N |
Other |
Comparative |
DSS 329 Wrought |
Wrought steel |
0.08 |
|
1.00 |
|
|
23-28 |
2.5-5 |
1-2 |
|
|
|
0.08 |
|
Comparative |
DSS 329 PM WA |
Water atomized powder, HIP |
0.20 |
0.75 |
1.00 |
|
|
23-28 |
2.5-5 |
1-2 |
|
|
0.05 |
0.08 |
|
Comparative |
DSS 2205 PM GA |
Gas atomized powder HIP |
0.03 |
1.00 |
2.00 |
0.020 |
0.030 |
22.0-23.0 |
4.5-6.5 |
3.0-3.5 |
|
0.75 |
|
0.14-0.20 |
|
Premix |
XSS DP1 PM WA Premix |
Water atomized powders4, compacted and sintered |
0.03 |
2.00 |
0.10 |
0.006 |
0.008 |
25 |
5.5 |
1.3 |
1 |
2 |
0.2 |
0.06 |
|
Invention |
XSS DP1 PM WA Prealloy |
Water atomized powder, prealloyed compacted and sintered |
0.01 |
1.52 |
0.20 |
0.013 |
0.008 |
24.9 |
5.5 |
1.3 |
1 |
2 |
0.1 5 |
0.05 |
|
Invention |
XSS DP1-1 |
Water atomized powder, prealloyed compacted and sintered |
0.03 |
1.97 |
0.10 |
0.007 |
0.012 |
23.4 |
5.1 |
1.2 |
0.9 |
1.9 |
0.1 3 |
0.05 |
|
Invention |
XSS DP1-2 |
Water atomized powder, prealloyed compacted and sintered |
0.03 |
2.12 |
0.20 |
0.007 |
0.012 |
26.1 |
5.2 |
1.3 |
0.9 |
3 |
0.1 5 |
0.02 |
|
Invention |
XSS DP1-3 |
Water atomized powder, prealloyed compacted and sintered |
0.03 |
1.94 |
0.20 |
0.008 |
0.013 |
25.1 |
5.6 |
1.2 |
0.8 |
2 |
0.15 |
0.02 |
0.58 Nb |
Comparative |
XSS DP1-4 |
Water atomized powder, prealloyed compacted and sintered |
0.03 |
2.14 |
0.20 |
0.009 |
0.015 |
22.3 |
5.2 |
1.3 |
0.9 |
1.9 |
0.1 6 |
0.06 |
0.6 Sn |
4 Premix of 316L, 434L and elemental powders of Si, W and Cu. |
Table 2, mechanical properties and metallographic structure for sintered samples produced
from stainless steel powders according to table 1.
Mechanical Properties |
|
Sample |
Type |
Sintering time [minutes] |
TS [Mpa] |
YS [Mpa] |
TRS [Mpa] |
Elongation [%] |
% austenite in ferrite matrix |
Comparative |
DSS 329 Wrought |
Wrought steel |
Annealed |
725 |
550 |
|
25 |
-50 |
Comparative |
DSS 329 PM WA |
Water atomized powder, HIP |
45 |
523 |
460 |
180 |
7 |
0 |
Comparative |
DSS 2205 PM GA |
Gas atomized powder, HIP |
45 |
578 |
427 |
200 |
11 |
-50 |
Comparative |
XSS DP1 PM WA Premix |
Water atomized powders5, compacted and sintered |
45 |
720 |
700 |
220 |
2.5 |
-35 |
Invention |
XSS DP1 PM WA Prealloy |
Water atomized powder, prealloyed compacted and sintered |
45 |
776 |
617 |
278 |
8.6 |
-60 |
Invention |
XSS DP1-1 |
Water atomized powder, prealloyed compacted and sintered |
45 |
727 |
504 |
275 |
11.0 |
∼ 50 |
Invention |
XSS DP1-2 |
Water atomized powder, prealloyed compacted and sintered |
45 |
809 |
745 |
265 |
2.5 |
-50 |
Invention |
XSS DP1-3 |
Water atomized powder, prealloyed compacted and sintered |
45 |
843 |
691 |
257 |
6.5 |
-45 |
Comparative |
XSS DP1-4 |
Water atomized powder, prealloyed compacted and sintered |
45 |
749 |
743 |
218 |
0.5 |
-10 |
5 Premix of 316L, 434L and elemental powders of Si, W and Cu. |
[0080] An embodiment of the invented powder with composition as in Example 1 was also sintered
at various temperatures and atmospheres to show the effect on mechanical properties.
Such data is plotted in Figure 3.
EXAMPLE 3
[0081] In order to perform corrosion test, TRS bars as in Example 1 were produced along
with bars for 316L and 434L as representatives from austenitic and ferritic grades.
The samples were then tested for corrosion in 5% NaCl solution at room temperature
per ASTM B895-16. The corrosion was compared by the hours takes for onset of corrosion
on the samples. The comparative data is plotted in Figure 2 along with the UTS and
YS for these samples. The diameter of the bubbles in the Figure 3 represents the number
of hours taken for the start of the corrosion on the samples. The corrosion test for
the invented powder was discontinued after 3700 hours as there was no sign of corrosion
and it already exceeded 3 times that of 316L samples.
1. A stainless steel powder comprising:
up to 0.1% of C,
0.5-3% of Si,
up to 0.5% of Mn,
20-27% of Cr,
3-8% of Ni,
1-6% of Mo,
up to 3% of W,
up to 0.1% N,
up to 4% of Cu,
up to 0.04% of P,
up to 0.04% of S,
unavoidable impurities up to 0.8%,
optionally one or more of up to 0.004% B, up to 1 % Nb, up to 0.5% Hf, up to 1 %
Ti, up to 1 % Co,
rest Fe.
2. A stainless steel powder comprising:
up to 0.06% of C,
1-3% of Si,
up to 0.3% of Mn,
23-27% of Cr,
4-7% of Ni,
1-3% of Mo,
0.8-1.5% of W,
up to 0.07% N,
1-3% of Cu,
up to 0.03% of P,
up to 0.03% of S,
unavoidable impurities up to 0.8%,
optionally one or more of up to 0.004% B, up to 1 % Nb, up to 0.5% Hf, up to 1 %
Ti, up to 1 % Co,
rest Fe.
3. A stainless steel powder comprising:
up to 0.03% of C,
1.5-2.5% of Si,
up to 0.3% of Mn,
24-26% of Cr,
5-7% of Ni,
1-1.5% of Mo,
1-1.5% of W,
up to 0.06% N,
1-3% of Cu,
up to 0.02% of P,
up to 0.015% of S,
unavoidable impurities up to 0.8%,
optionally one or more of up to 0.004% B, up to 1 % Nb, up to 0.5% Hf, up to 1 %
Ti, up to 1% Co,
rest Fe.
4. A stainless steel powder according to any of claims 1 to 3 wherein the stainless steel
powder is ferritic.
5. A stainless steel powder according to any of claims 1 to 4 wherein the stainless steel
powder is produced by water atomization.
6. A stainless steel powder according to any of claims 1 to 4 wherein the stainless steel
powder is produced by gas atomization.
7. A stainless steel powder according to any of claims 1 to 4 wherein the particle size
of the powder is between 53 microns and 18 microns such that at least 80% of the particles
are less than 53 microns and at most 20% of the particles are less than 18 microns.
8. A stainless steel powder according to any of claims 1 to 4 wherein the particle size
of the powder is between 26 microns and 5 microns such that at least 80% of the particles
are less than 26 microns and at most 20% of the particles are less than 5 microns.
9. A stainless steel powder according to any of claims 1 to 4 wherein the particle size
of the powder is between 150 microns and 26 microns such that at least 80% of the
particles are less than 150 microns and at most 20% of the particles are less than
26 microns.
10. A stainless steel powder according to any of claims 1 to 4 wherein the powder is a
prealloyed powder.
11. A method for producing a stainless steel powder by water atomization comprising the
steps of:
- providing a molten metal of having a chemical composition corresponding to the chemical
composition of the stainless steel powder according to claim 1,
- subjecting a stream of the molten metal to water atomization,
- recovery of the obtained stainless steel powder.
12. A sintered duplex stainless steel having a chemical composition according to any of
claims 1-3.
13. A sintered duplex stainless steel according to claim 12 wherein the Ni equivalent
(Nieq) is such that 5 < Nieq < 11 and the Cr equivalent (Creq) is such that 27 < Creq <38.
14. A sintered duplex stainless steel according to any of claims 12 to 13 wherein the
pitting resistance equivalent number (PREN) is 28 < PREN < 33.
15. A sintered duplex stainless steel according to any of claims 12 to 14 wherein the
microstructure of the sintered duplex stainless steel is characterized by austenite phase precipitated in ferrite phase.
16. A sintered duplex stainless steel according to claim 15 wherein the microstructure
of the sintered duplex stainless steel contains 30-70% austenite.
17. A sintered duplex stainless steel according to any of claims 16 to 16 wherein the
microstructure is characterized by being free from sigma phases and nitrides.
18. A method for producing a duplex sintered stainless steel comprising the steps of:
- providing a stainless steel powder according to according to any of claims 1 to
9,
- optionally mixing the stainless steel powder with a lubricant and optionally other
additives,
- subjecting the stainless steel powder or the mixture to a consolidation process
forming a green component,
- subjecting the compacted green component to a sintering step in an inert or reducing
atmosphere or in vacuum at a temperature between 1150°C to 1450°C, preferably at a
temperature between 1275°C to 1400°C for a period of time of 5 minutes to 120 minutes
,
- subjecting the sintered component to a cooling step down to ambient temperature.
19. A method for producing a duplex sintered stainless according to claim 18 wherein the
consolidation process includes:
- uniaxial compaction at a compaction pressure of up to 900 MPa in a die to form a
green component,
- ejecting the obtained compacted green component from the die.
20. A method for producing a duplex sintered stainless according to claim 18 wherein the
consolidation process includes one of:
Metal Injection Molding (MIM), Hot Isostatic Pressing (HIP), Additive Manufacturing
techniques such as Binder Jetting, Laser Powder Bed Fusion (L-PBF), Direct Metal Laser
Sintering (DMLS) or Direct Metal Deposition (DMD).