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EP0107489A1 - Stainless steel alloy - Google Patents

Stainless steel alloy Download PDF

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Publication number
EP0107489A1
EP0107489A1 EP83306395A EP83306395A EP0107489A1 EP 0107489 A1 EP0107489 A1 EP 0107489A1 EP 83306395 A EP83306395 A EP 83306395A EP 83306395 A EP83306395 A EP 83306395A EP 0107489 A1 EP0107489 A1 EP 0107489A1
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Prior art keywords
alloy
stainless steel
alloys
austenite
nitrogen
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EP83306395A
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German (de)
French (fr)
Inventor
Kevan John Gradwell
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Mather and Platt Ltd
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Mather and Platt Ltd
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N

Definitions

  • the present invention relates to stainless steel alloys.
  • Duplex stainless steel alloys of the ferritic austenitic type have been developed over recent years for their combination of mechanical properties, high resistance to localized corrosion and their resistance to stress corrosion cracking in a seawater environment. Such steels are described for example in U.K. Patent No. 1,158,614 and U.S. Patent No. 3,065,119 and are available under the designations CD4MCu and Ferralium.
  • a typical composition of such an alloy is as follows:
  • compositional levels of the elements promoting either austenite or ferrite in the microstructure are controlled to produce a structure which is balanced and which contains a minimum of 65% austenite.
  • This microstructural control provides an alloy which has a good combination of toughness and corrosion resistance in thick section castings when adopting air cooling from solution heat treatment temperatures.
  • austenite levels of 65% and above result in an overall reduction in tensile strength since the latter is sensitive to changes in the austenite/ferrite balance.
  • Nitrogen is used in amounts up to 0.5% in austenitic stainless steels of the A.I.S.I. 200 series to impart high strength levels.
  • solubility of nitrogen in the conventional duplex 25 Cr/5Ni steels reaches a maximum of approximately 0.2% (J.F. Elliot et al Thermochemistry for Steelmaking, Vol II, Adison and Wesley Publishers, 1963).
  • the inclusion of amounts of nitrogen in excess of 0.2% in the conventional duplex 25Cr/5Ni steels results in severe gassing problems in castings.
  • said alloy having a duplex microstructure of austenite and ferrite.
  • the alloy is of the following composition
  • the alloy has a microstructure comprising at least 60% and more preferably at least 65% austenite
  • N 2 0.15 - 0.3 % N 2 in the alloy provides an increase in tensile and proof stresses as well as in localised corrosion performance. Amounts of nitrogen less than 0.15% do not give the required properties for the alloy whereas amounts greater than 0.3% cannot be obtained due to its limited solubility. Amounts of manganese less than 3.5% do not give the required N 2 solubility whereas amounts greater than 5.0 % lead to excessive formation of sigma phase.
  • the alloy may be prepared from a melt containing the various components in the required amount by casting of the melt and subsequent heat treatment in the manner known for the production of duplex alloys of the austenite ferrite type.
  • the nitrogen for the alloy may be provided by high nitrogen ferrochrome.
  • the alloy with above 60% austenite content has a tensile strength of 650 MPa or less and a 0.2% Proof Strength of 490 MPa or less.
  • ferritic austenitic alloys which contained nominally 25% Cr, 6-7% Ni, 2% Mo, 0.05% N 29 amounts of manganese from 1 to 15%, and a balance of iron plus unavoidable impurities.
  • Fig. 2 illustrates this formation of sigma phase and is a Time Temperature Transformation curve showing the rate of sigma phase formation in a conventional duplex base alloy (nominally 25Cr/5Ni) (curve A), an alloy containing 3% Mn (curve B), and one containing 5% Mn (curve C).
  • Curve A a conventional duplex base alloy
  • Curve B an alloy containing 3% Mn
  • curve C a continuous cooling path typical of the cooling rate 'seen' at the centre of a 6" diameter billet.
  • Fig. 2 also shows the TTT curve showing rate of formation of sigma phase in Uranus 50, a typical duplex stainless steel alloy with a 50/50 austenite/ferrite microstructure.
  • duplex alloys (nominally 25Cr/5Ni) were prepared.
  • One series of alloys contained a nominal 1% Mn and the other contained 4.5% Mn.
  • the alloys of each series were prepared with nitrogen levels of between 0.2 and 0.35%.
  • Fig. 3 is a graph showing the relationship of increasing manganese content on the critical crevice and pitting temperatures in artificial seawater of two nominally 25Cr/5Ni duplex alloys, one containing 0.1% N 2 and the other 0.23% N 2 .
  • the solid curves represent pitting temperatures and the dashed lines represent crevice temperatures.
  • the alloy with the higher nitrogen content has the higher values of critical pitting tmmperature and critical crevice temperature; the alloy with 0.23% nitrogen maintains a critical pitting temperature of over 60°C up to a manganese addition of 5% and also shows a rise in critical crevice temperature for manganese additions of 3 to 4%.
  • Fig. 4 is a graph showing the relationship of increasing levels of nitrogen on the pitting and crevice temperatures of a series of duplex alloys (nominally 25Cr/5Ni) containing 4.5% Mn. The alloys were subjected to cyclic anodic polarisation measurement in artificial seawater to determine their localised corrosion performance.
  • the alloy may include copper in an amount up to 1% by weight. If greater than 0.5% by weight of copper is added, the active dissolution rate of a duplex alloy in boiling hydrochloric acid and the crevice corrosion loss in a chloride solution is also decreased.
  • Tungsten upto 1% by weight increases the immunity potentials to crevice corrosion of a duplex alloy in high temperature chloride solutions above which crevice corrosive starts.

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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 Steel (AREA)

Abstract

A stainless steel alloy has the following composition
Figure imga0001
The alloy has a duplex microstructure of austentie and ferrite and has good corrosion resistance.

Description

  • The present invention relates to stainless steel alloys.
  • Duplex stainless steel alloys of the ferritic austenitic type have been developed over recent years for their combination of mechanical properties, high resistance to localized corrosion and their resistance to stress corrosion cracking in a seawater environment. Such steels are described for example in U.K. Patent No. 1,158,614 and U.S. Patent No. 3,065,119 and are available under the designations CD4MCu and Ferralium.
  • A typical composition of such an alloy is as follows:
    Figure imgb0001
  • The compositional levels of the elements promoting either austenite or ferrite in the microstructure are controlled to produce a structure which is balanced and which contains a minimum of 65% austenite. This microstructural control provides an alloy which has a good combination of toughness and corrosion resistance in thick section castings when adopting air cooling from solution heat treatment temperatures.
  • However austenite levels of 65% and above result in an overall reduction in tensile strength since the latter is sensitive to changes in the austenite/ferrite balance.
  • There is a need for stainless steel alloys which have an improved tensile strength as compared to the above alloys and which still have a good combination of toughness and corrosion resistance.
  • It is known that the addition of carbon and nitrogen as interstitial solid solution strengthening elements is the most effective way of strengthening in both ferrite and austenite (see for example K.J. Irvine, et al, J. Iron Steel Inst. 1961, Vol 199, p 153 and K.J. Irvine et al, ibid, 1963, Vol 201, p 944).
  • However increased carbon in a stahless steel is undesirable as it has an adverse effect on corrosion resistance.
  • Nitrogen is used in amounts up to 0.5% in austenitic stainless steels of the A.I.S.I. 200 series to impart high strength levels. However the solubility of nitrogen in the conventional duplex 25 Cr/5Ni steels reaches a maximum of approximately 0.2% (J.F. Elliot et al Thermochemistry for Steelmaking, Vol II, Adison and Wesley Publishers, 1963). The inclusion of amounts of nitrogen in excess of 0.2% in the conventional duplex 25Cr/5Ni steels results in severe gassing problems in castings.
  • It is known that manganese additions are effective in several ferrous alloys in promoting nitrogen solubility. However manganese has been shown adversly to effect localised corrosion resistance in austenitic stainless steels (R.J. Brigham and E.W. Tozer, Corrosion N.A.C.E., Vol 30, No. 5, May 1974, p 161) and also to promote formation of the embrittling sigma phase.
  • It is also clear from the second K.J. Irvine et al reference quoted above that an increase in the austenite content in the microstructure will increase nitrogen solubility. However this results in a net decrease in mechanical properties arising from a loss of dispersion strengthening reflected in the sensitivity of the proof stress value with ferrite volume fraction at higher austenite contents.
  • It is an object of the present invention to provide a stainless steel alloy which has an improved combination of mechanical properties, resistance to localised corrosion, and resistance to stress corrosion cracking in a sea water environment than the conventional duplex alloys.
  • According to the present invention there is provided a stainless steel alloy having the following composition
    Figure imgb0002
  • said alloy having a duplex microstructure of austenite and ferrite. Preferably the alloy is of the following composition
    Figure imgb0003
  • Preferably the alloy has a microstructure comprising at least 60% and more preferably at least 65% austenite
  • The inclusion of 0.15 - 0.3 % N2 in the alloy provides an increase in tensile and proof stresses as well as in localised corrosion performance. Amounts of nitrogen less than 0.15% do not give the required properties for the alloy whereas amounts greater than 0.3% cannot be obtained due to its limited solubility. Amounts of manganese less than 3.5% do not give the required N2 solubility whereas amounts greater than 5.0 % lead to excessive formation of sigma phase.
  • The alloy may be prepared from a melt containing the various components in the required amount by casting of the melt and subsequent heat treatment in the manner known for the production of duplex alloys of the austenite ferrite type. The nitrogen for the alloy may be provided by high nitrogen ferrochrome.
  • The invention will be further described by way of example only with reference to the accompanying drawings, in which;
    • Fig. 1 is a graph showing the effect on Ultimate Tensile Stress, 0.2% Proof Stress and Elongation of increasing the austenite content of a conventional duplex stainless steel alloy of the ferritic, austenitic type;
    • Fig. 2 is a TTT curve illustrating the rate of formation of sigma phase in various alloys of differing manganese content;
    • Fig. 3 is a graph showing the effect on corrosion properties of increasing the manganese content of a stainless steel alloy at various fixed nitrogen contents; and
    • Fig. 4 shows the effect on corrosion properties of increasing the nitrogen content in a stainless steel alloy of fixed manganese content.
    • Fig. 1 illustrates the variation in Ultimate Tensile Stress (UTS), 0.2% Proof Stress (PS) and Elongation with increasing austenite content in a duplex alloy of nominally 25% Cr obtained by heat treatment at 1120°C with quenching. The variation in austenite content is obtained by varying the nickel content of the alloy.
  • It will be seen from Fig. 1 that the alloy with above 60% austenite content has a tensile strength of 650 MPa or less and a 0.2% Proof Strength of 490 MPa or less.
  • In order to improve the properties of the known alloy it was decided to investigate whether a level of manganese addition could be found which would aid nitrogen solubility without loss of toughness or corrosion performance thereby resulting in improved mechanical properties.
  • Initially a series of ferritic austenitic alloys were cast which contained nominally 25% Cr, 6-7% Ni, 2% Mo, 0.05% N29 amounts of manganese from 1 to 15%, and a balance of iron plus unavoidable impurities.
  • A study was then made on the effects on microstructure, and tensile and hardness properties.
  • Additions of manganese up to 8% were found not to effect the austenite/ferrite phase balance. Proof stress and ultimate tensile strengths were found not to be affected up to a level of 12% Mn.
  • A progressive increase in hardness was recorded at manganese levels of 5% and above in the solution treated and air cooled condition and was attributable to hard, brittle sigma phase.
  • Fig. 2 illustrates this formation of sigma phase and is a Time Temperature Transformation curve showing the rate of sigma phase formation in a conventional duplex base alloy (nominally 25Cr/5Ni) (curve A), an alloy containing 3% Mn (curve B), and one containing 5% Mn (curve C). Superimposed on Fig. 2 is a continuous cooling path typical of the cooling rate 'seen' at the centre of a 6" diameter billet.
  • It will be seen from Fig. 2 that rate of formation of sigma phase increases with increasing manganese cortent, and above 5% Mn the ferritic microstructure at the centre of a 6" billet will transform partially or wholly to sigma phase upon cooling from the solution heat treatment temperature of 1100°C.
  • By way of example, Fig. 2 also shows the TTT curve showing rate of formation of sigma phase in Uranus 50, a typical duplex stainless steel alloy with a 50/50 austenite/ferrite microstructure.
  • From the above discussion it will be appreciated that amounts of manganese of less than 5% should. be used in the alloys of the invention to ensure that undesirable amounts of sigma phase are not formed.
  • In order to study the effect of manganese addition on nitrogen solubility, two series of duplex alloys (nominally 25Cr/5Ni) were prepared. One series of alloys contained a nominal 1% Mn and the other contained 4.5% Mn. The alloys of each series were prepared with nitrogen levels of between 0.2 and 0.35%.
  • On visual examination it was evident that the alloys containing 1% Mn had gassed severely once a level of 0.15 N2 was exceeded, whereas the higher manganese alloys showed good soundness up to 0.3% N 2.
  • Tests were conducted to determine the effect of manganese and nitrogen additions on the critical crevice and pitting temperatures of the alloys.
  • Fig. 3 is a graph showing the relationship of increasing manganese content on the critical crevice and pitting temperatures in artificial seawater of two nominally 25Cr/5Ni duplex alloys, one containing 0.1% N2 and the other 0.23% N2. In Fig. 3, the solid curves represent pitting temperatures and the dashed lines represent crevice temperatures.
  • It will be seen from Fig. 3 that the alloy with the higher nitrogen content has the higher values of critical pitting tmmperature and critical crevice temperature; the alloy with 0.23% nitrogen maintains a critical pitting temperature of over 60°C up to a manganese addition of 5% and also shows a rise in critical crevice temperature for manganese additions of 3 to 4%.
  • Fig. 4 is a graph showing the relationship of increasing levels of nitrogen on the pitting and crevice temperatures of a series of duplex alloys (nominally 25Cr/5Ni) containing 4.5% Mn. The alloys were subjected to cyclic anodic polarisation measurement in artificial seawater to determine their localised corrosion performance.
  • It will be seen from Fig. 4 that increasing the nitrogen content to 0.3% improves the critical pitting and crevicing temperatures by more than 20°C as compared to an alloy containing only 0.1% N2.
  • The inclusion of amounts of manganese of from 3.5 to 5% and amounts of nitrogen of from 0.15 to 0.3% in the alloys of the invention results in numerous advantageous tensile properties for the alloys of the invention as will be appreciated from the following.
  • The tensile properties of an alloy of the invention and containing 0.28% N2 and 4.5% Mn were compared with those of a conventional duplex alloy of the type set out in Table I above. The comparison for the two alloys for the solution heat treated and water quenched conditions is given in Table II below.
    Figure imgb0004
  • It will be seen from Table II that the properties of the alloy are much improved as compared to the conventional alloy. In particular the increase in proof stress, in excess of 100 MPa is totally due to the increased nitrogen addition held in solid solution in the austenite.
  • It should be appreciated that a number of modifications may be made to the composition of the alloy of the invention.
  • The addition of copper will have a beneficial effect on corrosion resistance. Thus, for example, the alloy may include copper in an amount up to 1% by weight. If greater than 0.5% by weight of copper is added, the active dissolution rate of a duplex alloy in boiling hydrochloric acid and the crevice corrosion loss in a chloride solution is also decreased.
  • Tungsten upto 1% by weight increases the immunity potentials to crevice corrosion of a duplex alloy in high temperature chloride solutions above which crevice corrosive starts.
  • Tin and antimony in amounts upto 0.10% and 0.2% by weight respectively improve the stress corrosion cracking and pitting corrosion in environments containing significant concentrations of H2 S.
  • Such modified alloys are also within the scope of the invention.

Claims (8)

1. A stainless steel alloy having the following composition
Figure imgb0005
said alloy having a duplex microstructure of austenite and ferrite.
2. An alloy as claimed in claim 1 and having the following composition
Figure imgb0006
3. An alloy as claimed in claim 1 or 2 and having a microstructure comprising at least 60% austenite.
4. An alloy as claimed in claim 3 and having a microstructure comprising at least 65% austenite.
5. A stainless steel alloy as claimed in any one of claims 1 to 4 including copper as an alloying addition in an amount of greater than 0.5% by weight.
6. A stainless steel alloy as claimed in any one of claims 1 to 4 including tungsten as an alloying addition in an amount of up to 1% by weight.
7. A stainless steel alloy as claimed in any one of claims 1 to 4 including tin as an alloying addition in an amount of up to 0.10% by weight.
8. A stainless steel alloy as claimed in any one of claims 1 to 4 including antimony as an alloying additions in an amount of up to 0.2% by weight.
EP83306395A 1982-10-23 1983-10-21 Stainless steel alloy Withdrawn EP0107489A1 (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1985005129A1 (en) * 1984-04-27 1985-11-21 Bonar Langley Alloys Limited High chromium duplex stainless steel
EP0455625A1 (en) * 1990-05-03 1991-11-06 BÖHLER Edelstahl GmbH High strength corrosion-resistant duplex alloy
EP2684973A1 (en) * 2011-03-09 2014-01-15 Nippon Steel & Sumikin Stainless Steel Corporation Two-phase stainless steel exhibiting excellent corrosion resistance in weld
US9862168B2 (en) 2011-01-27 2018-01-09 Nippon Steel & Sumikin Stainless Steel Corporation Alloying element-saving hot rolled duplex stainless steel material, clad steel plate having duplex stainless steel as cladding material therefor, and production method for same

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2229065A (en) * 1938-12-14 1941-01-21 Electro Metallurg Co Austenitic alloy steel and article made therefrom
DE1214005B (en) * 1965-02-03 1966-04-07 Suedwestfalen Ag Stahlwerke Components made from austenitic steels
FR1444807A (en) * 1965-05-24 1966-07-08 Loire Atel Forges Improvements made to austenitic stainless steels with high creep resistance
FR2007566A1 (en) * 1968-04-30 1970-01-09 Armco Steel Corp
FR2045584A1 (en) * 1969-06-03 1971-03-05 Ugine Kuhlmann
DE2457089A1 (en) * 1973-12-10 1975-06-12 Schoeller Bleckmann Stahlwerke Austenitic - ferritic chromium-nickel-nitrogen steels - for chemical industry esp food, paper and fermentation industries
EP0060577A1 (en) * 1981-03-16 1982-09-22 BBC Brown Boveri AG Turbine blade material with high fatigue-corrosion resistance, method of production and use

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3306736A (en) * 1963-08-30 1967-02-28 Crucible Steel Co America Austenitic stainless steel
GB1158614A (en) * 1967-03-16 1969-07-16 Langley Alloys Ltd Improvement in Stainless Steels
ATA285473A (en) * 1972-04-24 1976-09-15 Bofors Ab AUSTENITIC-FERRITIC, STAINLESS STEEL WITH HIGH STRENGTH AND GOOD CORROSION RESISTANCE
GB1456634A (en) * 1972-09-13 1976-11-24 Langley Alloys Ltd High strength stainless steel having a high resistance to corro sive and abrasive wear in corrosive environments particularly chloride environments
JPS5424364B2 (en) * 1973-05-04 1979-08-21
DE2616599C3 (en) * 1976-04-13 1987-01-22 Mannesmann AG, 4000 Düsseldorf Use of a high-alloy steel to manufacture high-strength objects resistant to acid gas corrosion

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2229065A (en) * 1938-12-14 1941-01-21 Electro Metallurg Co Austenitic alloy steel and article made therefrom
DE1214005B (en) * 1965-02-03 1966-04-07 Suedwestfalen Ag Stahlwerke Components made from austenitic steels
FR1444807A (en) * 1965-05-24 1966-07-08 Loire Atel Forges Improvements made to austenitic stainless steels with high creep resistance
FR2007566A1 (en) * 1968-04-30 1970-01-09 Armco Steel Corp
FR2045584A1 (en) * 1969-06-03 1971-03-05 Ugine Kuhlmann
DE2457089A1 (en) * 1973-12-10 1975-06-12 Schoeller Bleckmann Stahlwerke Austenitic - ferritic chromium-nickel-nitrogen steels - for chemical industry esp food, paper and fermentation industries
EP0060577A1 (en) * 1981-03-16 1982-09-22 BBC Brown Boveri AG Turbine blade material with high fatigue-corrosion resistance, method of production and use

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1985005129A1 (en) * 1984-04-27 1985-11-21 Bonar Langley Alloys Limited High chromium duplex stainless steel
US4657606A (en) * 1984-04-27 1987-04-14 Bonar Langley Alloys Limited High chromium duplex stainless steel
EP0455625A1 (en) * 1990-05-03 1991-11-06 BÖHLER Edelstahl GmbH High strength corrosion-resistant duplex alloy
US9862168B2 (en) 2011-01-27 2018-01-09 Nippon Steel & Sumikin Stainless Steel Corporation Alloying element-saving hot rolled duplex stainless steel material, clad steel plate having duplex stainless steel as cladding material therefor, and production method for same
EP2684973A1 (en) * 2011-03-09 2014-01-15 Nippon Steel & Sumikin Stainless Steel Corporation Two-phase stainless steel exhibiting excellent corrosion resistance in weld
EP2684973A4 (en) * 2011-03-09 2015-04-15 Nippon Steel & Sumikin Sst Two-phase stainless steel exhibiting excellent corrosion resistance in weld
US9365914B2 (en) 2011-03-09 2016-06-14 Nippon Steel & Sumikin Stainless Steel Corporation Duplex stainless steel superior in corrosion resistance of weld

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GB2128632A (en) 1984-05-02

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