Note: Descriptions are shown in the official language in which they were submitted.
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STAINLESS STEEL PRODUCT, USE OF THE PRODUCT AND METHOD
OF ITS MANUFACTURE
FIELD OF THE INVENTION
The invention relates to a casting produced of stainless steel, which has a
duplex
ferrite-austenite microstructure and which has high structural stability and
an improved
combination of properties, particularly machinability and weldability. The
invention further
relates to a use of the product and to a method of manufacturing the casting.
.. BACKGROUND OF THE INVENTION
Ferritic-austenitic or duplex stainless steel castings are in general defined
as alloys
with a mixture of almost equal proportions of ferrite and austenite in
contrast to austenitic
castings that may contain up to 10-15% ferrite. For duplex castings according
to ASTM
A890 standard the ferrite levels are not specified, but listed alloys will
develop a range of
approximately 30 to 60% ferrite with the balance austenite. With the two-phase
structure
interesting property profiles can be designed. The first duplex stainless
steels were
developed almost 80 years ago and most probably they emerged from austenitic
castings
where certain amounts of ferrite in the microstructure proved to be
advantageous. In fact
duplex compositions generally show better castability than austenitic ones.
Other favourable
properties of duplex materials are high mechanical strength, superior fatigue
strength, good
wear resistance and good corrosion resistance. Hence, both cast and wrought
products
have found many attractive applications. Several duplex alloy compositions
have been
described with various optimisations. In many cases also cast articles have
been included
as articles in patents of duplex compositions. In recent years, with greatly
increased raw
material costs, special concern has peen paid to reduce the nickel and
molybdenum levels
in the alloys and still maintain appropriate properties.
The favourable properties of the duplex stainless steels can be achieved for
phase
balances in the range of 30 to 70% ferrite and austenite. The interactions of
the major
alloying elements, particularly chromium, nitrogen, nickel and molybdenum are
quite
complex. To achieve a stable duplex structure that responds well to processing
and
fabrication, care must be taken to obtain the correct level of each of these
elements. Beside
the phase balance, the formation of detrimental intermetallic phases at the
elevated
temperatures is the second major concern with duplex stainless steels. Sigma
and chi
phases form in high chromium, high molybdenum stainless steels and precipitate
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preferentially in the ferrite. The addition of nitrogen changes the phase
balance in
favourable way to avoid formation of such phases.
The U.S. Pat. No. 4,500,351 relates to a cast duplex stainless steel, in which
the
microstructure in a casting comprises a ferritic matrix containing at least
about 30%
austenite after the solution treatment at 1200 C. and rapid cooling with
water quenching
in order to avoid the formation of sigma phase. The casting contains in weight
percent
about 0.02 percent carbon, 24 percent chromium, about 9.5 percent nickel,
about 6 percent
molybdenum, about 0.5 percent manganese, about 0.2 percent silicon and about
0.25
percent nitrogen. The castings of this U.S. Pat. No. 4,500,351 are useful in
pump parts
such as impellers and housings and in valve parts such as seats and gates.
A duplex stainless steel having good combination of properties in the as-cast
condition and resisting thermal transformation to martensite is described in
the U.S. Pat.
No. 4,828,630. The steel contains in weight percent up to 0.07 percent carbon,
17 to 21.5
percent chromium, 1 to 4 percent nickel, 4 to 8 percent manganese, 0.05 to
0.15 percent
nitrogen, less than 2 percent silicon, less than 2 percent molybdenum and less
than 1.5
percent copper. The steel of this patent contains 30 to 60% ferrite and it is
particularly
suited for thin-walled castings for automotive underbody components. The steel
has as-cast
properties including a 10% minimum elongation, a 0.2% yield strength greater
than 50 ksi
(350 Nimm2), a toughness of at least 20 ft.-lbs (30 Nm) at 0 C. and no
nitrogen porosity.
The U.S. Pat. No. 6,033,497 relates to a pitting resistance duplex steel alloy
with
improved machinability containing in addition to iron in weight percent less
than 0.1 percent
carbon, 25-27 percent chromium, 5-7.5 percent nickel, less than 0.5 percent
molybdenum,
less than 0.15 nitrogen, less than 1.5 percent silicon, less than 2.0 percent
manganese,
1.5-3.5 percent copper. In the prior art of this US patent it is said, that
machinability of
austenitic stainless steels can be enhanced by additions of alloying elements
such as
sulphur and selenium that may reduce the corrosion performance. Further, it is
said that the
addition of copper without molybdenum allows the duplex stainless steel alloy
to be very
slowly control cooled in a tightly closed heat treatment furnace so that
harmful tensile
residual stresses are minimized while excellent ductility and corrosion
resistance were
retained.
In accordance with the U.S. Pat. No. 6,033,497 the steel grade is treated by
an
accelerated in-mould heat treatment after casting without using a separate and
slow heat
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treatment step. The steel grade of the patent is particularly for a hollow
cylindrical
centrifugal casting and it is used for instance for paper machine suction roll
shell
applications. The in-mould heat treatment comprises controlling the rate of
cast cooling in
the temperature range of about 260 C. to about 1090 C. and keeping the
temperature of
the alloy in the mould within about 450 C. of the temperature outside of the
mould. The
steel grade has improved machinability when treated in the mould after casting
by an
accelerated heat treatment as compared to the same alloy composition that is
slowly control
cooled in a tightly closed heat treatment furnace. The alloy without the in-
mould treatment
has a nominal internal diameter tensile residual stress of 24 MPa, while the
respective value
for the alloy treated in the mould after casting is 52 MPa.
The EP patent 1,327,008 describes a ferritic-austenitic stainless steel having
a
microstructure, which essentially contains of 35-65 vol % ferrite and 35-65
vol % austenite.
The composition of this steel grade contains as main alloying components in
weight percent
0.02-0.07 percent carbon, 19-23 percent chromium, 1.1-1.7 percent nickel, 0.15-
0.30
percent nitrogen, 3-8 percent manganese, optionally molybdenum and/or copper
less than
1 percent. The steel of this EP patent is produced by Outokumpu under the
trademark LDX
21010 and wrought products have been received with great commercial interest.
Duplex stainless steel castings in general show good castability. However,
there is
a risk of formation of nitrogen gas pores during solidification because of
limited nitrogen
.. solubility in the ferrite phase that normally solidifies from a steel melt
with a composition of
a duplex stainless steel alloy. In general it can be stated that most
stainless steel castings
are subjected to various machining operations to be fitted into the system
wherein the
castings will be used. In this regard duplex stainless steels are considered
more difficult to
machine than for instance austenitic stainless steels. The higher strength
levels of the
former steel type explain this behaviour. Additions of carbon and nitrogen
both increase the
strength and the degree of strain hardening of the steel and should therefore
be kept low
to achieve good machinability. However, modern duplex stainless steels are
alloyed with
high nitrogen contents for good weldability and best weldability properties at
the sacrifice
of machinability.
One application where cast or wrought duplex stainless steels are used is a
steel
shell for a suction roll of paper machines. One important material property
for this
application is also machinability, because cast or wrought steel shells are
subjected to
substantial machining to produce the final suction roll. As said in connection
with the U.S.
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Pat. No. 6,033,497 one way to improve the machinability is to add sulphur or
selenium,
which elements, however, reduce the corrosion performance.
The WO publication 2006/041344 describes a steel shell for a suction roll of
paper
machines, in which the wrought steel grade LDX 21010 of the EP patent
1,327,008 is used
without any addition of sulphur. Further, any treatment improving
machinability is not carried
out as well as the optional additions of copper and molybdenum are remarkable
smaller
when compared the U.S. Pat. No. 6,033,497.
Schramm et al published in the presentation of "Lean Duplex Stainless Steels
for
Pump Applications" in Stainless Steel World 2007 Conference, Maastricht, 6-8
Nov. 2007,
results of studies on lean duplex materials for pump-specific applications.
One alloy "cast
2101" was made of cast bars having a composition in weight percent of 0.028
percent
carbon, 0.97 percent silicon, 5.04 percent manganese, 0.011 percent
phosphorus, 0.004
percent sulphur, 20.73 percent chromium, 0.31 percent molybdenum, 1.73 percent
nickel,
0.20 percent nitrogen and 0.30 percent copper. As results for this alloy "cast
2101" after
solution-annealing at the temperature of 1050 C. and water-quenching Schramm
et al
mention for instance the values of 473 MPa for 0.2% proof strength and 37.3%
A5
elongation. As to the corrosion properties Schramm et al say that the alloy
"cast 2101" has
the pitting potential lower than in the alloy 2304 having a composition in
weight percent of
0.024 percent carbon, 0.64 percent silicon, 1.32 percent manganese, 0.015
percent
phosphorus, 0.001 percent sulphur, 22.50 percent chromium, 0.28 percent
molybdenum,
4.92 percent nickel, 0.09 percent nitrogen and 0.26 percent copper. However,
Schramm
et al do not mention any information of the applicability of this alloy "cast
2101" for desired
applications.
.. SUMMARY OF THE INVENTION
The present invention seeks to address some drawbacks of the prior art by
achieving a casting of a duplex stainless steel, which in the method of
manufacturing a
casting is sufficiently stable against formation of detrimental precipitates,
such as
intermetallic phase and which has as properties a combination of high strength
and good
corrosion resistance, good castability and high machinability.
The present invention relates to a stainless steel product, preferably to a
duplex
stainless steel casting with high machinability comprising, in weight percent,
up to 0.07
percent carbon, up to 2 percent silicon, greater than 3 up to 8 percent
manganese, greater
than 19 up to 23 percent chromium, greater than 0.5 up to 1.7 percent nickel
and greater
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than 0.15 and up to 0.30 percent nitrogen. The alloys to be used in production
of the duplex
stainless steel casting with foresaid range may contain small amounts of other
elements
or impurities and optionally elements such as up to 1 percent copper, up to
totally 1 percent
of molybdenum and/or tungsten according to the formula (Mo +1/2 W) less than 1
percent,
the remainder being iron and incidental impurities. The microstructure of the
duplex
stainless steel casting of the invention contains 30-70 vol percent ferrite
and 30-70 vol
percent austenite. The invention also relates to a cast method for producing
the casting as
well as to the use of the casting.
In the manufacturing of large stainless steel castings it is important to have
a
microstructure that is sufficiently stable against formation of detrimental
precipitates such
as intermetallic phase as such phases have adverse effects on properties. For
this a lean,
balanced duplex composition of the casting of the invention is desirable.
Preferably the
microstructure of the duplex stainless steel of the invention contains 50 vol
percent ferrite
and 50 vol percent austenite.
Another important property for steel castings is the ease to perform repair
welding.
In addition to their good castability the casting of the invention is in
general quite resistant
to hot cracking during welding. If repair welding is needed it is in most
cases necessary to
perform a post weld heat treatment as weld metal and heat affected zone easily
will be
exposed to rapid cooling due to a small weld pool surrounded by a large cast
section. This
may result in a microstructure with high ferrite content that is sensitive to
cracking and
reduction in properties why heat treatment must follow. For this reason it is
desirable with
the duplex stainless steel composition of the invention having high austenite
reformation
during rapid thermal cycles such as in welding. To obtain such a feature high
nitrogen
content in the duplex stainless steel casting of the invention is advisable.
The duplex stainless steel casting of the invention can advantageously contain
in
weight percent, preferably up to 0.05 percent carbon and more preferably up to
0.03
percent carbon, preferably up to 1 percent silicon, preferably greater than 4
up to 6 percent
manganese, preferably greater than 21 up to 22 percent chromium, preferably
greater than
1.1 up to 1.7 percent nickel and more preferably greater than 1.35 up to 1.7
percent nickel
and preferably greater than 0.20 and up to 0.26 percent nitrogen, and
optionally elements
up to 1 percent copper, up to totally 1 percent of molybdenum and/or tungsten
according
to the formula (Mo + 1/2 W) less than 1 percent, the remainder being iron and
incidental
impurities.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in more details in the following referring to the
drawings
in which
FIG. 1 shows the test results when comparing the machinability of the casting
of the
invention with the prior art austenite stainless steel,
FIG. 2 shows the microstructure of a simulated weld repair in a casting of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
The duplex stainless steel casting of the present invention was tested in
machinability and welding, especially in weld repair.
For testing the machinability a casting was produced having the following
chemical
composition in weight percent in table 1:
C Si Mn P S Cr Ni Mo Cu
0.026 0.76 4.93 0.021 0.001 21.37 1.44 0.23 0.34 0.226
Table 1
A cast billet with a square section 140 mm was subjected to different tests in
the
as-cast condition without any previous heat treatment. The mechanical
properties of the
casting were as follows in table 2:
Yield strength Ultimate Tensile Strength Fracture Elongation
RID0.2 MPa RmMPa A5%
451 634 40
Table 2
The strength level is far above that for austenitic castings, which typically
exhibit
yield strengths of about 200 MPa and an ultimate strength of about 500 MPa.
Testing of
machinability was made with turning of cylindrical test pieces and results are
shown in FIG.
1. The figure illustrates allowable cutting speed for a tool life of 15
minutes in turning. The
tool insert was of cemented carbide type. The machinability of casting
according to the
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invention is superior to that of an austenitic steel of type 304L. This is in
contradiction to the
expected outcome where the austenitic steel is considered having better
machinability.
Further tests were performed with a casting according to present invention
which
was produced with the following chemical composition, in weight percent in
table 3:
Si Mn P S Cr Ni Mo Cu
0.024 0.69 4.89 0.020 0.001 21.45 1.60 0.20 0.25 0.230
Table 3
From a 140 mm thick cast section 30 mm thick square samples were removed and
the samples were subjected to simulated repair welding using shielded metal
arc welding.
The base metal was in the as-cast condition. Grooves were made in the sample
and
afterwards filled by welding using a filler material suited for this alloy.
The arc energy was
0.7 to 0.8 kJ/mm. The resulting welds were free of cracks and showed a normal
microstructure, also in the heat-affected zone. This is illustrated in FIG. 2.
Castings according to the present invention can be cast by different casting
processes such as centrifugal casting, chill casting, die casting, investment
casting,
pressure casting, permanent mould casting, sand casting and vacuum casting.
The
castability is good showing no tendency to cracking or pore formation in spite
of the high
nitrogen content. This is because of the high level of manganese, 3-8%, in the
steel and
preferably a range of 4-6% manganese can be used. Cast items are preferably
solution
annealed at a temperature of 1020 to 1100° C. followed by rapid
cooling. However,
thinner sections can be used in as-cast condition. Although microstructure is
not a property
and can be difficult to measure correctly the present invention will contain
roughly equal
amounts of austenite and ferrite, the allowable phase range being 30 to 70%.
Furthermore,
the microstructure is very resistant to precipitation of intermetallic phases,
which in turn
gives a low sensitivity to embrittlement. Castings of present invention
exhibit superior
machinability in as-cast as well as in solution annealed conditions.
Thus duplex castings of the present invention offer desirable and inexpensive
cost
alternatives to austenitic cast materials due to their high machinability,
high strength and
good weldability. Castings of the present invention can be especially
favourable for use in
various solutions and parts for pumps, valves, impellers or for use in other
solutions wherein
a combination of high machinability, high strength and good weldability in a
casting is
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needed as as-cast condition or after some further treatment, such as solution-
annealed and
quenched condition.
