Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• 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.
• 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 main contain up to 10-15% ferrite.
• the ferrite levels are not specified, but listed alloys will develop a range of approximately 30 to 60 % ferrite with the balance austenite.
• 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.
• duplex materials are high mechanical strength, superior fatigue strength, good wear resistance and good corrosion resistance.
• cast and wrought products have found many attractive applications.
• duplex alloy compositions have been described with various optimisations.
• cast articles have been included as articles in patents of duplex compositions.
• special concern has peen paid to reduce the nickel and molybdenum levels in the alloys and still maintain appropriate properties.
• 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.
• 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 preferentially in the ferrite.
• the addition of nitrogen changes the phase balance in favourable way to avoid formation of such phases.
• the US patent 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 0 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 US patent 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 US patent 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 N/mm 2 ), a toughness of at least 20ft.-lbs (30 Nm) at 0 0 C and no nitrogen porosity.
• the US patent 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.
• the steel grade is treated by an accelerated in-mould heat treatment after casting without using a separate and slow heat 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 0 C to about 1090 0 C and keeping the temperature of the alloy in the mould within about 450 0 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 % 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 2101 ® 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.
• 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 2101 ® of the EP patent
• the object of the present invention is to eliminate some drawbacks of the prior art and to achieve 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.
• 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 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 + Vi 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.
• 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.
• 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 + Vz W) less than 1 percent, the remainder being iron and incidental impurities.
• Fig. 1 shows the test results when compared the machinability 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.
• duplex stainless steel casting of the present invention was tested in machinability and welding, especially in weld repair.
• 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:
• 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 Figure 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 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.
• 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 0 C followed by rapid cooling. However, thinner sections can be used in as-cast condition.
• 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.
• 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 needed as as- cast condition or after some further treatment, such as solution-annealed and quenched condition.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Articles (AREA)
• Heat Treatment Of Steel (AREA)
• Moulds For Moulding Plastics Or The Like (AREA)
• Treatment Of Steel In Its Molten State (AREA)

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Publications (1)

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• 2008
  • 2008-05-16 FI FI20080360A patent/FI125458B/fi active IP Right Grant
• 2009
  • 2009-05-14 CN CN2009801177006A patent/CN102027147A/zh active Pending
  • 2009-05-14 CN CN201510326076.8A patent/CN104988427A/zh active Pending
  • 2009-05-14 KR KR1020107025472A patent/KR20100133487A/ko active Search and Examination
  • 2009-05-14 JP JP2011508964A patent/JP5613152B2/ja active Active
  • 2009-05-14 EP EP09745895.4A patent/EP2279276B1/en active Active
  • 2009-05-14 PL PL09745895T patent/PL2279276T3/pl unknown
  • 2009-05-14 EA EA201001571A patent/EA027733B1/ru not_active IP Right Cessation
  • 2009-05-14 BR BRPI0912807-7A patent/BRPI0912807B1/pt active IP Right Grant
  • 2009-05-14 CA CA2722236A patent/CA2722236C/en active Active
  • 2009-05-14 SI SI200932070T patent/SI2279276T1/sl unknown
  • 2009-05-14 US US12/991,899 patent/US20110064601A1/en not_active Abandoned
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  • 2009-05-14 MX MX2010012226A patent/MX343938B/es active IP Right Grant
  • 2009-05-14 ES ES09745895T patent/ES2797953T3/es active Active
  • 2009-05-14 WO PCT/FI2009/050397 patent/WO2009138570A1/en active Application Filing
  • 2009-05-15 TW TW098116115A patent/TWI490345B/zh active

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