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    • 专利摘要:
    17 ABSTRACT A powder metallurgy duplex stainless steel,percent by weight: C max 0.030 wt.%, Ni 4.5-6.5 wt.%, N 0.21-0.29 wt.%, Mo 3.0-3.5 wt.%, Cr 21-24 wt.%, optionally one or more of the following elements:Cu 0-1.0 wt.%, W 0-1.0 wt.%, Mn 0-2.0 wt.%, Si 0-1.0 wt.%, comprising, in wherein, in percent by weight, the amount of N 2 0.1*Cr,the balance being Fe and unavoidable impurities. The stainlesssteel has excellent corrosion resistance and is a cost efficient alternative to super duplex stainless steels. (Fig. 1)
    公开号:SE1450577A1
    申请号:SE1450577
    申请日:2014-05-16
    公开日:2015-11-17
    发明作者:Karin Jakobsson；Christophe Pellegrini
    申请人:
    IPC主号:
    专利说明:

    A powder metallurgy duplex stainless steelTECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a powder metallurgy duplexstainless steel, a method of manufacturing such a stainless steeland an article comprising the stainless steel. A powdermetallurgy duplex stainless steel is here defined as a duplexstainless steel produced by means of powder metallurgy.
    Duplex stainless steels, and in particular duplex stainless steelsproduced by powder metallurgy, are known for having highstrength and good corrosion resistance. They provide acorrosion resistance similar to austenitic steel grades at a lowercost. By the use of duplex steel instead of austenitic steel, it isoften possible to significantly reduce material costs and weightin articles for use in corrosive environments, since the high yieldstrength of duplex stainless steels often enables reduction ofthe section thickness. Duplex stainless steels produced bypowder metallurgy additionally have isotropic mechanicalproperties, i.e. identical mechanical properties in all directions.lt is therefore often possible to reduce the amount of material inconstructions using powder metallurgy duplex stainless steelinstead of conventionally produced duplex stainless steels,thereby further reducing material costs and weight.
    The duplex stainless steel UNS S32205 is a standard gradecomprising, in percent by weight (wt.%), C max 0.030 wt.%,Ni 4.5-6.5 wt.%, N 0.14-0.20 wt.%,l/lo 3.0-3.5 wt.%,Cr 22-23 wt.%, Cu max 1.0 wt.%, W max 1.0 wt.%, Mn max 2.0 wt.%, Si max 1.0 wt.%, the balance being Fe and unavoidable impurities. This duplexstainless steel is often used in seawater applications and inother corrosive environments. lt may be produced byconventional melting and forging processes or by powdermetallurgy processes.
    The corrosion properties of stainless steel alloys are largelygoverned by the alloying contents of the alloy. The PittingResistance Equivalent (PRE) number is often used tocharacterize the alloy and is defined as, in percent by weight: PRE=Cr+3.3 M0+ 16 N.
    A higher PRE number normally means better corrosionresistance. A commonly used method of increasing the corrosionresistance of stainless steels is therefore to increase the amountof alloying elements. A so called super duplex stainless steel,such as UN8 832760 or UN8 832505, comprises a higheramount of alloying elements than UN8 832205 and has a PREnumber of at least 40, as compared to a PRE number of typically35 for UN8 832205. However, due to the larger amount ofalloying elements, super duplex stainless steels are associatedwith relatively high material costs.
    SUMMARY OF THE INVENTION lt is an object of the present invention to provide a duplexstainless steel with in at least some aspect improved propertieswith respect to known stainless steels and which can offerexcellent corrosion resistance at a reasonable cost.
    This object is achieved by a powder metallurgy duplex stainlesssteel comprising, in percent by weight: C max 0.030 wt.%, Ni 4.5-6.5 wt.%, N 0.21-0.29 wt.%, Mo 3.0-3.5 wt.%, Cr 21-24 wt.%, optionally one or more of the following elements:Cu 0-1.0 wt.%, W 0-1.0 wt.%, Mn 0-2.0 wt.%, Si 0-1.0 wt.%, wherein, in percent by weight, the amount of N 2 0.1*Cr,the balance being Fe and unavoidable impurities. lt has been surprisingly found that a powder metallurgy duplexstainless steel with this composition has corrosion propertieswhich are superior to what can be expected from the PREnumber of the steel. The duplex stainless steel according to theinvention has a corrosion resistance with regard to pittingcorrosion as well as crevice corrosion which is significantlyhigher than for the standard duplex stainless steel alloy UNSS32205. The inventive steel also has significantly better stresscorrosion cracking resistance. The steel according to theinvention is therefore a cost efficient alternative to super duplexstainless steel alloys for demanding applications in corrosiveenvironments. Since the stainless steel according to theinvention is a powder metallurgy stainless steel, it is isotropicand thereby has identical properties in all directions. This is incontrast with conventional forged stainless steels, which areanisotropic.
    According to one embodiment of the invention, the stainlesssteel comprises 0.23-0.27 wt.% N. ln this embodiment, thestainless steel has particularly good corrosion properties.
    According to one embodiment of the invention, the stainlesssteel comprises 22-23 wt.% Cr. Keeping the Cr content within this interval ensures a good resistance to localized corrosion,while limiting the risks of precipitation of chromium carbides.
    According to one embodiment of the invention, the proportion ofaustenite in the stainless steel is, in percent by volume, 45-65 %, preferably 50-60 %, the rest being ferrite. For optimalcorrosion and mechanical properties, the proportion of austeniteis within the range 50-60 % by volume.
    According to one embodiment of the invention, the stainlesssteel has an austenite spacing of less than 30 pm, preferablyless than 20 pm. The stainless steel with such a small austenitespacing has a better resistance to cracks and also bettercorrosion properties than steels with a similar composition butwith a larger austenite spacing, since the grain boundariesbetween austenite and ferrite phases generally act to arreststress induced cracks propagating in the ferrite phase.
    According to one embodiment of the invention, the stainlesssteel comprises less than 1 wt.% unavoidable impurities,preferably less than 0.5 wt.%. Preferably, unavoidable impuritiesare kept to a minimum. ln this way, impurities will have aminimum impact on the final properties of the stainless steel.
    According to one embodiment of the invention, the stainlesssteel comprises 0.3-1.0 wt.% Si, preferably 0.5-1.0 wt.% Si. lnthis embodiment, Si is used as a deoxidisation agent in themanufacturing process of the stainless steel and therebycontributes to a low oxygen level, which in turn provides goodmechanical properties such as strength and fatigue resistance.
    According to one embodiment of the invention, the stainlesssteel has a critical pitting corrosion temperature of at least60°C, as determined in a solution of 2 l/I NaCl at 700 mV vs.SCE (saturated calomel electrode) according to ASTM G150,which is a standard test method for electrochemical criticalpitting temperature testing of stainless steels. The stainless steel thereby has significantly better corrosion properties thanthe standard duplex stainless steel alloy UNS S32205.
    The invention also relates to an article comprising the stainlesssteel according to the invention for use in corrosiveenvironments such as in oil and gas industry, process industry,chemical industry, and nuclear power applications. Suchapplications require highly corrosion resistant alloys and thestainless steel according to the present invention is thereforesuitable for use in said articles.
    According to another aspect of the invention, the inventionrelates to a method of manufacturing a powder metallurgyduplex stainless steel according to the invention, comprising thesteps of: - preparing a stainless steel alloy powder using anatomisation technique giving spherical particles, such asgas atomisation, ultrasonic atomisation or centrifugalatomisation, - hot isostatic pressing of the prepared stainless steel alloypowder to obtain a dense stainless steel material, - solution annealing and water quenching of the densestainless steel material so as to form a duplex stainlesssteel.
    The atomisation can be carried out using nitrogen or argon gas,or possibly helium. The heat treatment should preferably, for astainless steel capsule with a diameter of 200 mm, be carriedout at 1050-1130°C, for example for eight hours at atemperature of 1070°C. The exact temperature used and theduration of the solution annealing depends on the desired phasebalance in the duplex stainless steel and the thickness of thecomponent.
    According to a preferred embodiment of this aspect of theinvention, gas atomisation is used for preparing the stainless steel alloy powder. This efficient preparation process. gives a cost powder According to a further embodiment of this aspect of theinvention, the gas atomisation is carried out using nitrogen gas.Gas atomisation using nitrogen gas is a cost effective way ofobtaining spherical particles with low levels of impurities.
    Further advantages and advantageous features of the inventionwill appear from the following description of the invention andembodiments thereof.
    BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will in the following be describedwith reference to the appended drawings, in which: Fig. 1 is a diagram showing the resistance to stresscorrosion cracking for the stainless steel according toan embodiment of the invention and a stainless steelaccording to UNS 832205, Fig. 2 is a micrograph showing a stainless steel according tothe invention, and Fig. 3 is a micrograph showing a prior art stainless steelaccording to UNS S32205.
    DETAILED DESCRIPTION OF EMBODIMENTS OF THEINVENTION The powder metallurgy duplex stainless steel according to theinvention comprises, in percent by weight: C max 0.030 wt.%, Ni 4.5-6.5 wt.%, N 0.21-0.27 wt.%, Mo 3.0-3.5 wt.%, Cr 21-24 wt.%, optionally one or more of the following elements:Cu 0-1.0 wt.%, W 0-1.0 wt.%, Mn 0-2.0 wt.%, Si 0-1.0 wt.%, wherein, in percent by weight, the amount of N 2 0.1*Cr, andwherein the balance is Fe and unavoidable impurities.
    Carbon (C) is detrimental to the corrosion properties of the alloyas it increases the risk of forming chromium carbides in grainboundaries. Carbon is therefore limited to 0.030 wt.%.
    Nickel (Ni) is an indispensable addition to the alloy in that itstabilizes the austenite phase to ensure the duplex structure ofthe steel. This also has the indirect effect of increasing thesolubility of nitrogen. Nickel also contributes to the corrosionresistance but is an expensive alloying element. ln order toreach a desired austenite/ferrite balance, Ni is present in anamount of 4.5-6.5 wt.%.
    Nitrogen (N) promotes the formation of austenite. Nitrogen alsopermits obtaining high strength and a good resistance tocorrosion. Nitrogen should therefore be present in an amount ofat least 0.21 wt.%, preferably at least 0.23 wt.%. However,nitrogen may form nitrides, which negatively influence thecorrosion resistance and toughness of the alloy. The amount ofnitrogen in the steel according to the present invention istherefore limited to 0.29 wt.%, preferably 0.27%.
    Molybdenum (Mo) improves the corrosion resistance of the steelalloy. The Mo content is at least 3.0 wt.% to obtain the desired effect. Above 3.5 wt.%, Mo-rich intermetallic phases, which aredetrimental to corrosion resistance as well as to mechanicalproperties, become stable up to temperatures closer to thesolution annealing temperature. Therefore, the content of Mo islimited to 3.5 wt.%.
    Chromium (Cr) stabilizes the ferrite phase in the duplex steeland contributes strongly to the corrosion resistance of the alloyby forming stable oxides. At least 21 wt.% Cr, preferably at least22 wt.% Cr, is needed so as to ensure a good resistance tolocalized corrosion, but not more than 24 wt.%, preferably notmore than 23 wt.%, so as to limit the risks of precipitation ofchromium carbides.
    Copper (Cu) contributes to the corrosion resistance of the steel.lt may be present in a small amount in steel scrap. However, Cualso promotes precipitation of low temperature intermetallicphases and is therefore limited to 1.0 wt.%.
    Tungsten (W) is a ferrite stabilizer and present to a smallamount in steel scrap. ln order to keep the austenite/ferritebalance in the steel, W is limited to 1.0 wt.% in the steelaccording to the present invention.
    Manganese (Mn) stabilizes austenite and is used as adeoxidisation agent. Mn also increases the solubility of nitrogenin the steel. Mn should be limited to less than 2.0 wt.% so as tolimit the risks of precipitation of non-wanted phases, for exampleMnS.
    Silicon (Si) is used as a deoxidisation agent and also stabilizesferrite. Si is therefore optionally present in an amount of at least0.3 wt.%, preferably at least 0.5 wt.%. However, Si promotesprecipitation of intermetallic inclusions and should therefore belimited to 1.0 wt.%. lmpurities, such as contamination elements, can be present inthe alloy in an amount of maximum 1 wt.%, preferably maximum0.75 wt.% and more preferably maximum 0.5 wt.%. Examples ofimpurities that may be present are titanium (Ti), sulphur (S),phosphorus (P), tin (Sn), lead (Pb), etc. Oxygen (O) should bekept to a minimum. The impurities may be naturally-occurring inthe raw material used to produce the steel alloy, or may resultfrom the production process.
    A powder metallurgy duplex stainless steel alloy A according tothe invention was produced in the form of 200 mm diametercapsules. The alloy A was produced by preparing a stainlesssteel alloy powder using nitrogen gas atomisation, followed byhot isostatic pressing (HIP) of the prepared powder to formcapsules with a diameter of 200 mm. The dense stainless steelcapsules were thereafter subjected to heat treatment so as toform the duplex stainless steel alloy A. The heat treatment wasa solution annealing at 1070°C for 8 hours, followed by waterquenching. The alloy A was tested together with aconventionally produced duplex stainless steel alloy B accordingto UNS S32205 in the form of a forged bar with a diameter of250 mm and a conventionally produced superduplex stainlesssteel alloy C according to UNS S32505 in the form of a forgedbar with a diameter of 260 mm, wherein alloys B and C arecommercially available from well-known high quality producers.The compositions of the alloys A, B and C are listed in Table l.
    Alloy C Ni N M0 Cr Cu Mn Si PREA 0.019 6.1 0.23 3.2 23.0 0.2 0.7 0.88 37B 0.025 6.0 0.17 3.2 22.3 0.4 1.5 0.48 36C 0.033 6.2 0.26 3.29 25.9 1.47 0.89 0.40 41Tablel The critical pitting corrosion temperatures (CPT) of alloys A, Band C were determined using the ASTM G150 standard test in 2M NaCl with an anodic potential of 700 mV vs. SCE, starting from room temperature. The results of the test are shown in Table ll, listing average values of the measured CPT.
    Alloy Test position CPT [°C] in 2M NaCl A Centre 65.5 B Centre 53.9 B Surface 49.7 C Centre 71.8 C Surface 65.7 Table ll As can be seen from Table ll, alloy A has a critical pitting corrosion temperature which is significantly higher than thecritical pitting corrosion temperature of alloy B, and which isalmost in parity with alloy C. These results cannot beunderstood from the PRE numbers listed in Table I alone.
    The resistance to stress corrosion cracking for samples of alloysA and B was also tested and the resulting stress versus straindiagrams are shown in Fig. 1. The samples were first tested at80°C, where there were no signs of stress corrosion on any ofthe samples. The samples were thereafter tested at 110°C,where the sample from alloy B broke at an early stage and alsoshowed signs of corrosion cracks at the surface. The samplefrom alloy A according to the invention was still unaffected at110°C.
    The resistance to crevice corrosion for three different samplesfrom each of alloys A and B was also tested at 20°C in naturalseawater. Three different samples from each of alloys A, B andC were also tested at 20°C in chlorinated seawater. The resultsfrom the tests are shown in Table lll and Table IV, respectively. 11 Test conditions Alloy Sample 1 Sample 2 Sample 320°C, natural A OK OK CORseawater B COR COR++ COR++Table |Test conditions Alloy Sample 1 Sample 2 Sample 330°C, seawater A OK COR OKwith 0.5 ppm Cl B COR COR COR C OK* COR OK* Table IV As can be seen from tables lll and IV, all alloy B samples showcorrosion (COR) or severe corrosion (COR++) at the testconditions for both natural and chlorinated seawater, while twoout of three alloy A samples show no signs of corrosion (OK),neither for natural seawater, nor for chlorinated seawater. Thealloy A samples thereby show significantly better crevicecorrosion resistance than the alloy B samples at both testconditions. From alloy C, tested in chlorinated seawater, two outof three samples show non-propagating signs of corrosion (OK*)and the third sample shows corrosion. Thus, the crevicecorrosion resistance of alloy A according to the invention is atleast in parity with that of the more expensive superduplex alloyC.
    The maximum corrosion depth in natural seawater wasdetermined to be, in average, 19 um for the alloy A samples and719 um for the alloy B samples, thus significantly better for alloyA.
    Yield strength, tensile strength, elongation and area reduction ofthe three alloys A, B and C were determined in a tensile test atroom temperature according to the standard lSO6892. Theresults of the test are shown in Table V. As can be seen, alloy Aaccording to the invention has mechanical properties which arein parity with the mechanical properties of at least alloy B. Thus, 12 the alloy according to the invention combines excellentcorrosion resistance with good mechanical properties.
    Alloy Sampling Yield Tensile Elongation Areadirection strength strength [%] reductionRpO.2 [MPa] Rm [MPa] [%]A -- 564 811 38 78B Transversal 487 733 40 63B Longitudinal 520 713 41 76C Transversal 603 835 32 62C Longitudinal 622 816 36 70 Table V The austenite spacing, which is a measure of the averagedistance between austenite grains, was determined inaccordance with “Recommended practice DNV-RP-F112”(October 2008), i.e. by superimposing lines over a micrographand measuring the length of the lines falling within each ferriteunit, and therefrom determining the austenite spacing. Amicrograph showing the microstructure of the alloy A accordingto the inventions is shown in Fig. 2. The austenite phase isshown as light areas and the ferrite phase is shown as darkareas. ln comparison, Fig. 3 is a micrograph showing themicrostructure of the alloy B. As can be seen, the austenitegrain size, and the austenite spacing, is significantly smaller foralloy A according to the present invention than for the prior artalloy B. The austenite spacing of alloy A samples falls within therange 5-20 pm and was in the shown example determined to be14 pm, while the austenite spacing of alloy B samples can be upto 80 pm, and in the shown example was determined to be 28pm. 13 The invention is of course not in any way restricted to theembodiments disclosed, but may be varied and modified withinthe scope of the following claims.
    权利要求:
    Claims (13)
    [1] 1. A powder metallurgy duplex Stainless steel comprising, inpercent by weight: C max 0.030 wt.%, Ni 4.5-6.5 wt.%, N 0.21-0.29 wt.%, Mo 3.0-3.5 wt.%, Cr 21-24 wt.%, optionally one or more of the following elements:Cu 0-1.0 wt.%, W 0-1.0 wt.%, Mn 0-2.0 wt.%, Si 0-1.0 wt.%, wherein, in percent by weight, the amount of N 2 0.1*Cr,the balance being Fe and unavoidable impurities.
    [2] 2. The stainless steel according to claim 1, wherein thestainless steel comprises 0.23-0.27 wt.% N.
    [3] 3. The stainless steel according to claim 1 or 2, wherein thestainless steel comprises 22-23 wt.% Cr.
    [4] 4. The stainless steel according to any of the precedingclaims, wherein the proportion of austenite in the stainless steelis, in percent by volume, 45-65 %, preferably 50-60 %, the restbeing ferrite.
    [5] 5. The stainless steel according to any of the precedingclaims, wherein the stainless steel has an austenite spacing ofless than 30 pm, preferably less than 20 pm.
    [6] 6. The stainless steel according to any of the precedingclaims, wherein the stainless steel is isotropic.
    [7] 7. The stainless steel according to any of the precedingclaims, wherein the stainless steel comprises less than 1 wt.%unavoidable impurities, preferably less than 0.5 wt.%.
    [8] 8. The stainless steel according to any of the precedingclaims, wherein the stainless steel comprises O.3-1.0 wt.% Si,preferably O.5-1 .O wt.% Si.
    [9] 9. The stainless steel according to any of the precedingclaims, wherein the stainless steel has a critical pitting corrosiontemperature of at least 60°C, as determined in a solution of 2 l/INaCl at 700 mV vs. SCE.
    [10] 10. An article comprising the stainless steel according to anyof the preceding claims for use in corrosive environments suchas in oil and gas industry, process industry, chemical industry,and nuclear power applications.
    [11] 11. A method of manufacturing a powder metallurgy duplexstainless steel according to any of claims 1-9, comprising thesteps of: - preparing a stainless steel alloy powder using anatomisation technique giving spherical particles, such asgas atomisation, ultrasonic atomisation or centrifugalatomisation, - hot isostatic pressing of the prepared stainless steel alloypowder to obtain a dense stainless steel material, - solution annealing and water quenching of the densestainless steel material so as to form a duplex stainless steel.
    [12] 12. The method according to claim 11, wherein gasatomisation is used for preparing the stainless steel alloypowder. 16
    [13] 13. The method according to claim 12, wherein the gasatomisation is carried out using nitrogen gas.
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    同族专利:
    公开号 | 公开日
    SE538577C2|2016-09-27|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    PL3333275T3|2016-12-07|2021-05-17|Höganäs Ab |Stainless steel powder for producing sintered duplex stainless steel|
    法律状态:
    2021-12-28| NUG| Patent has lapsed|
    优先权:
    申请号 | 申请日 | 专利标题
    SE1450577A|SE538577C2|2014-05-16|2014-05-16|Powder metallurgical duplex stainless steel|SE1450577A| SE538577C2|2014-05-16|2014-05-16|Powder metallurgical duplex stainless steel|
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