WO2020176616A1 - Poudre d'acier à haute teneur en azote et procédés de fabrication de celle-ci - Google Patents

Poudre d'acier à haute teneur en azote et procédés de fabrication de celle-ci Download PDF

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Publication number
WO2020176616A1
WO2020176616A1 PCT/US2020/019894 US2020019894W WO2020176616A1 WO 2020176616 A1 WO2020176616 A1 WO 2020176616A1 US 2020019894 W US2020019894 W US 2020019894W WO 2020176616 A1 WO2020176616 A1 WO 2020176616A1
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Prior art keywords
nitrogen
powder
precursor
alloy
optionally
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Pravansu S. Mohanty
Vikram VARADARAAJAN
Sharan NAGENDIRAN
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Somnio Global Holdings LLC
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Somnio Global Holdings LLC
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Priority to CA3131528A priority Critical patent/CA3131528A1/fr
Priority to US17/434,273 priority patent/US20220134424A1/en
Priority to EP20763825.5A priority patent/EP3930942A4/fr
Priority to CN202080031000.1A priority patent/CN113840673A/zh
Priority to AU2020228291A priority patent/AU2020228291A1/en
Publication of WO2020176616A1 publication Critical patent/WO2020176616A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/14—Treatment of metallic powder
    • B22F1/142—Thermal or thermo-mechanical treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/14—Treatment of metallic powder
    • B22F1/145—Chemical treatment, e.g. passivation or decarburisation
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0207—Using a mixture of pre-alloyed powders or a master alloy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
    • C22C33/0285—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5% with Cr, Co, or Ni having a minimum content higher than 5%
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/60—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using solids, e.g. powders, pastes
    • C23C8/62—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using solids, e.g. powders, pastes only one element being applied
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/05—Metallic powder characterised by the size or surface area of the particles
    • B22F1/052—Metallic powder characterised by the size or surface area of the particles characterised by a mixture of particles of different sizes or by the particle size distribution
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2301/00—Metallic composition of the powder or its coating
    • B22F2301/35—Iron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy

Definitions

  • This disclosure relates to steel powders having dissolved nitrogen in excess of the solubility limit at solidification temperature and methods for making such powders with tailored phase constituents.
  • Nitrogen (N) is effective in improving the mechanical, wear and corrosion properties of steels if it remains in solid solution, specifically in the form of coherent Cr-N short range order (SRO).
  • HNS high nitrogen steels
  • wt% dissolved N
  • Austenitic stainless steels with dissolved nitrogen contents up to 0.60 wt% have successfully been utilized in applications involving pitting corrosion, crevice corrosion and stress corrosion cracking in hot chloride solutions, such as NaCl and MgCI 2 .
  • precipitation of nitride particles e.g., Cr 2 N, TiN, VN
  • pressurized metallurgy namely melting and solidifying under high N partial pressure
  • these procedures are very expensive and require sophisticated equipment.
  • HNS bulk high nitrogen steel
  • powders of HNS can find many beneficial applications such as coatings and sintered metal products due to their excellent mechanical and corrosion properties.
  • Steel powders are commonly fabricated by atomization of liquid steel.
  • the melting procedure needs to use a high nitrogen partial pressure environment along with appropriate alloy composition to avoid an L ®d reaction and then atomizing the liquid with high pressure nitrogen gas jets. While this procedure is feasible, it is complicated and expensive.
  • mechanical alloying by attrition milling can introduce high levels of nitrogen into powders; however, typical time requirements are in excess of 100 hours and only a limited amount of material can be processed at one time.
  • the process introduces undesirable impurities and hard powders. 'This is especially true for fabricating coatings using HNS powder, where there are not many good fabrication options available. These powders cannot be rerneited using high temperature coating processes such as plasma spray’ as the dissolved nitrogen will be lost unless high nitrogen partial pressure is maintained during deposition process.
  • Cold spray processes that employ solid state fusion to fabricate coatings can be effective in this case, however, it requires that the powders possess sufficient deformability (plasticity) that necessitates well controlled nitrogen dissolution and phase constituents, preferably g phase free of Cr 2 N precipitates.
  • HNS high nitrogen steel
  • the precursor optionally comprising, ferrite (a) phase, or austenite (g) phase or a mixture of a + g phase; and the HNS powder comprising a mechanically tough alloy having dissolved nitrogen and optionally having a substantially homogeneous composition, in weight percent, of from 0.1 to 6.0 wt% nitrogen.
  • the HNS powder may optionally include a single phase nitrogen alloy, optionally a g phase alloy.
  • an HNS powder is provided comprising a dissolved nitrogen content significantly in excess of what could have been achieved through atomizing liquid steel at atmospheric pressures.
  • an object of the present disclosure is to provide methodologies to remove preexisting oxides from the precursor steel powder to accelerate diffusion of nitrogen into the powder and prevent precipitation of incoherent nitride precipitates to promote plastic deformability.
  • Methodologies as provided herein include exposing the precursor steel powder to a reducing gas environment at elevated temperatures and hydrogen from the reducing gas environment combines with the preexisting oxygen of the precursor, resulting in a volatile byproduct which is removed from the atmosphere, and further quenching the pow'ders quickly to ambient temperatures after nitrogen dissolution to prevent precipitation of nitrides or formation of oxides.
  • the oxygen removal methods optionally further include using a different gas composition from the ones used to introduce dissolved nitrogen or the same gas composition during the entire treatment cycle.
  • the methods as provided herein include continuously agitating the powder to prevent necking or joining between powders and thus maintaining supply route of nitrogen around the surface of the powder.
  • the methods optionally include providing a rotary hot tube comprising baffles that prevent formation stratified layers and continuously breakdown any lumps formed.
  • the methods include a fluidized bed reactor that uses the nitrogen containing gas and agitates, optionally continuously, the powder mass until the powder is quenched.
  • FIG. 1A is a schematic description of the solidification process of steel involving liquid to d-ferrite transformation, followed by austenite and the associated rejection of nitrogen gas forming pores;
  • FIG. IB is a schematic description of solidification process of steel involving liquid to austenite transformation and the associated retention of dissolved nitrogen gas in the solid precursor material according to the teachings of the current disclosure (exemplary aspect);
  • FIG. 2 is a schematic view of an exemplary microstructure of high nitrogen steel powder, having nitride precipitates
  • FIG. 3 is an exemplary time-temperature cycle for fabricating HNS powder according to the some aspects of the current disclosure
  • FIG. 4 is an exemplary outline of the inventive steps for fabricating HNS powder according to the teachings of the current disclosure
  • FIG. 5 is a schematic composition map for adjusting the phase content in the HNS powder, according to the teachings of the current disclosure
  • FIG. 6A is a schematic perspective view of an exemplary batch processing system for HNS powder wherein the processing chamber includes a rotary tube according to the teachings of the current disclosure;
  • FIG. 6B is a schematic cross sectional view of an exemplary batch processing system for HNS powder wherein the precursor powder is being loaded into the rotary tube by a screw feeder according to the teachings of the current disclosure;
  • FIG. 7A is a schematic cross sectional view of an exemplary batch processing system for HNS powder wherein the precursor powder is being processed within the rotary tube according to the teachings of the current disclosure;
  • FIG. 7B is a schematic cross sectional view of an exemplary batch processing system for HNS powder wherein the powder is being removed from the tube after nitrogen dissolution and quenched into a collection chamber according to the teachings of the current disclosure;
  • FIG. 8A is a schematic perspective view of an exemplary continuous processing system for HNS powder wherein the system includes a heated rotary tube and powder precursor is loaded at one end and the processed powder is collected at the opposite end according to the teachings of the current disclosure;
  • FIG. 8B is a schematic cross sectional view of an exemplary continuous processing system for HNS powder wherein the system includes a heated rotary tube and the powder precursor is loaded at one end and the processed powder is collected at the opposite end according to the teachings of the current disclosure;
  • FIG. 8C is a schematic cross sectional view of an exemplary continuous processing system for HNS powder wherein the system includes a heated rotary' tube along with an auger and the powder precursor is loaded at one end and the processed powder is collected at the opposite end according to the teachings of the current disclosure;
  • FIG. 8D is a schematic cross sectional view of an exemplary continuous processing system for HNS powder wherein the system includes a heated rotary tube having a vibratory device and the powder precursor is loaded at one end and the processed powder is collected at tire opposite end according to the teachings of the current disclosure;
  • FIG. 9 is a schematic perspective view of an exemplary fluidized bed powder processing system wherein the processing includes gas phase agitation according to some aspects of the teachings of the current disclosure
  • FIG. 10 is a schematic cross sectional view of an exemplary fluidized bed powder processing system showing the precursor powder being loaded according to some aspects of the teachings of the current disclosure
  • FIG. 11 is a schematic cross sectional view of an exemplary fluidized bed powder processing system showing the precursor powder being processed according to some aspects of the current disclosure
  • FIG. 12 is a schematic cross sectional view of an exemplary fluidized bed powder processing system showing the processed powder being quenched according to the teachings of the current disclosure
  • FIG. 13 is the schematic process map for powder processing showing different effects of temperature and holding time on the powder microstructure according to some aspects of the current disclosure
  • FIG. 14 is the schematic cooling rates for powder quenching and their effects on the powder microstructure according to some aspects of the current disclosure
  • FIG. 15 presents the Scanning Electron Microscope micrograph for powders processed under different process conditions according to some aspects of the current disclosure
  • FIG. 16 presents the X-Ray diffraction patterns for as received solid precursor powder, and the powders processed under different conditions according to some aspects of the current disclosure.
  • processes forming high nitrogen steel and devices for performing the process are useful for creating dissolved nitrogen in a solid steel material, optionally a powder material.
  • the processes tailor heating and optionally holding periods at particular temperatures of the steel to form various phases, allow nitrogen to dissolve into the steel and prevent final formation of nitrides that hinder corrosion resistance and mechanical strength of the final high nitrogen steels.
  • HNS as used herein means steels having high nitrogen content specifically in dissolved solid solution form
  • the amount of nitrogen in the high nitrogen steel is optionally equal to or above the amount of nitrogen achievable in an equivalent steel alloy in a liquid state at atmospheric pressure of nitrogen.
  • Precursor as used herein means the starting steel powder used to make the HNS powder where the precursor powder has a lower nitrogen content that the resulting HNS powder.
  • Compound as used herein, means a material formed by reactions between elements having a stoichiometric ratio, illustratively, Cr 2 N and Fe 2 N, etc.
  • Solid solution means an alloy formed by dissolving one or more alloying elements) in a host solid without changing its phase.
  • y-Fe[N] wherein N is the alloying element dissolved in FCC-Fe, the austenite phase.
  • the addition of nitrogen improves the strength, ductility and impact toughness in austenitic steels, while the fracture strain and fracture toughness are not affected at elevated temperatures.
  • the strength of nitrogen alloyed austenitic steels arises from three components: strength of the matrix, grain boundary hardening, and solid solution hardening.
  • the matrix strength is not appreciably impacted by nitrogen
  • rattier matrix strength correlates to the friction stress of the face centered cubic (FCC) lattice that is mainly controlled by the solid solution hardening of the substitutional elements like chromium and manganese.
  • Grain boundary hardening which occurs due to dislocation blocking at the grain boundaries, increases proportionally to the alloyed nitrogen content.
  • SFE stacking fault energy
  • N additions on the SFE in Cr and Mn alloyed steels is reported to be non-monotonic, exhibiting a minimum SFE at -0.4 wt% N.
  • the decrease in SFE at low N content (e.g. less than 0.4 wt%) is believed due to the segregation of interstitial N atoms to stacking faults, however, at higher N contents (e.g.
  • the SFE increases as the bulk effect of interstitial solid solution becomes more pronounced.
  • the formation of nitrides such as Cr 2 N, at elevated N content, affects the distribution of alloy ing elements within the lattice and in turn diminishes the bulk effect of interstitial solid solution and the SFE.
  • the formation of nitrides occurs when the nitrogen content goes beyond certain threshold value (depends on the overall composition of the alloy) and should be discouraged to take advantage of the interstitial solid solution hardening phenomenon described above.
  • the high nitrogen steels of the present disclosure are optionally free or substantially free of any nitrides.
  • alloying elements e.g. Cr, Al, Mo, V, Ti, etc.
  • nitride formation occurs because these alloying elements are stronger nitride formers than iron.
  • nitrides of the type MxNy where M is Cr, Al, Mo, V, Ti, etc., x any y are chosen to arrive at proper stoichiometry
  • the high nitrogen steels produced by the processes as provided herein are optionally absent, optionally substantially absent a nitride of Cr, Al, Mo, V, Ti, or others.
  • High nitrogen containing austenitic steels also exhibit excellent resistance to atmospheric corrosion.
  • the corrosion resistance is also strongly influenced by the nitrogen content.
  • the formation of s phase (an intermetallic compound with Cr) at the grain boundaries as well as the formation of nitrides such as Cr 2 N at high nitrogen content are detrimental to the corrosion resistance of these steels.
  • Best corrosion resistance can be achieved if all nitrogen is in solid solution, i.e. no nitrides such as Cr 2 N are precipitated. It can be summarized that an optimal combination of toughness and corrosion resistance can be achieved by limiting the nitrogen content within a range, wherein a substantially or completely precipitation free homogeneous microstructure with N in solid solution form can be obtained. This range of dissolved N depends on other alloying elements present in the alloy as well as the process thermal history as discussed herein.
  • chromium addition significantly enhances nitrogen solubility in the melt, it is also a strong d-ferrite stabilizer.
  • d-ferrite solidification in iron alloys is associated with a wide solubility gap and a sudden drop 12 of nitrogen solubility in the material.
  • a melt containing dissolved nitrogen 13 will lose most of its nitrogen during d-ferrite solidification even though the subsequent lower temperature austenite phase can dissolve a much higher amount of nitrogen, 11.
  • enhancing the dissolved nitrogen content 14 in the liquid by alloying additions and performing the melting operation under high nitrogen pressure would not retain the dissolved nitrogen in the d phase due to the associated loss during d-ferrite solidification.
  • the d-ferrite solidification should be avoided.
  • the pores can be suppressed increasing the N content to some extent 15, and importantly after the d®g transformation, substantial amount of nitrogen 16 can be dissolved in the g phase; the extent of which depends on the holding temperature, pressure and time.
  • the austenite phase can have a significant amount of dissolved nitrogen 11’ and in order to achieve the saturation level 11’ the liquid may contain higher dissolved nitrogen 14’ to start with, which can be achieved only by high pressure melting and alloying adjustment. Further, under high nitrogen partial pressure the austenite can pick up more nitrogen 16’ and depending upon the temperature and time of holding the nitrogen content can reach the solubility limit 11’.
  • the elimination of d-ferrite solidification step can be achieved by carefully adjusting the composition of the alloy.
  • manganese addition plays an important role. While enhancing the nitrogen solubility in the melt, manganese also suppresses the formation of d-ferrite during solidification. As discussed above, the significant enhancement of strength in nitrogen alloyed austenitic steel comes from the formation of Cr- N SRO. Additionally, Cr enhances the resistance against atmospheric corrosion and hence is an important alloying addition. Further, the effect of manganese on enhancing nitrogen solubility is known to be two times less than the effect of chromium Hence, significantly higher amount of Mn compared to Cr may be present in order to provide equivalent nitrogen solubility, eliminate d-ferrite formation as well as achieve enhanced toughness and corrosion resistance. Another way to promote austenitic solidification and avoid degassing of nitrogen is to add carbon, however, carbon contents > 0.1 wt.% have negative influence on corrosion resistance and ductility of the material and hence should be avoided.
  • Nitrogen in Eqn (1) remains in solid solution depending on temperature of thermo-chemical treatment and nitrogen pressure. Nitrogen loss or nitrogen pickup may occur according to Sieverts’ law at a given set of temperature and nitrogen partial pressure parameters. Also, note that CrN in Eqn. (2) is a coherent precipitate and is beneficial in enhancing mechanical properties of the steel, whereas Cr 2 N is a precipitate that deteriorates the corrosion resistance. Further, due to slow diffusion rate of N in steel, nitrogen pick up is fast when the surface area is large which is achieved by exposing the powder to a nitrogen atmosphere. However, the presence of oxide layer on the steel surface inhibits the diffusion of nitrogen and should be removed for enhance the rate of diffusion. This can be achieved by treating the powder in a reducing gas atmosphere. Once the nitrogen is dissolved in the steel, it can be retained in the powder by quenching the powder to a temperature where the nitrogen diffusion is virtually absent.
  • the temperature and time cycles 30 and method 40 may include one or more of the following steps; providing a solid precursor, optionally in powder form, surface form, coating form or other, the solid precursor steel with a substantially low dissolved nitrogen content in step 41 and the disposing the solid precursor powder into a nitrogen gas or mixture of a reducing gas and nitrogen gas environment in steps 42-44, where the said precursor powder can undergo an exemplary temperature-time cycle 32 or 33 to obtain the nitrogen steel powder with a dissolved nitrogen content in step 45. Further descriptions on steps 42-44 and temperature-time cycles 32 and 33 are provided below.
  • the reducing gas can optionally be a mixture of nitrogen and hydrogen, argon and hydrogen, or anhydrous ammonia. Under the reducing gas, the oxides layers will be removed and facilitate nitrogen introduction.
  • a solid precursor material is in the form of a coating on another substrate or other steel type.
  • a coating optionally has a thickness, optionally from 10 nm to 100 micrometers.
  • a precursor steel is in the form of a powder.
  • the solid precursor powder material can optionally be obtained by atomizing a liquid steel alloy in atmospheric pressures.
  • the powder is continuously agitated to provide contact with the gas as well as prevent sintering.
  • Various methods for providing continuous agitation are described in this disclosure.
  • the precursor powder has a powder size.
  • the precursor powder size is optionally between 5 and 250 micrometers (mm), is optionally betw'een 5 pm and 150 pm, optionally between 10 pm and 75 mm
  • Powder size is defined as the size that is appropriately sieved through a desired sieve where powder below a certain size will pass through a first sieve and will have size that will be retained by a smaller second sieve. Choice of sieve size depends on the desired powder size.
  • the precursor steel is predominantly Fe (i.e. 50 wt% or greater Fe) and optionally includes one or more other elements that will promote FCC structure.
  • a precursor optionally includes Mn.
  • Mn when present, may be provided at a weight percent of 0 to 35.
  • the weight percent of Mn is less than 30.
  • the weight percent of Mn is 19-27.
  • the weight percent of Mn is 20-26.
  • the presence of N in such alloys serves to promote and stabilize a desired FCC structure even when the amount of Mn or other FCC promoting metal is less than 20 weight percent.
  • the dissolved N and Mn optionally work in concert to promote austenitic structure to the protective layer metal alloy.
  • the precursor powder includes Ni, which also promotes austenitic structure.
  • Ni when present, may be provided at a weight percent of 0 to 20. Since Ni reduces the N solubility in the protective layer, the Ni is optionally between 0 wt% to 5 wt%.
  • the precursor powder may optionally include C, that when present, may be provided at a weight percent of 0 to 0.2. While C improves N solubility, it also reduces the toughness of the resulting alloy.
  • the C is present in the precursor powder at 0 wt% to 0.1 wt%.
  • the strengthening mechanism in nitrogen alloy steel emerges from the formation of Cr-N SRO and hence Cr is optionally included in the provided N alloy.
  • Cr is a d-ferrite promoter as well as ferrite stabilizer.
  • the ferrite stabilizing effect of Cr may be countered by adjusting the amount of N and/or Mn, both of which serve as austenite stabilizers.
  • the precursor may include one or more other metals.
  • a precursor may include molybdenum.
  • Mo when present, may be provided at a weight percent of 0 to 5.
  • a precursor may include aluminum.
  • A1 may be provided at 0.01 wt% to 10 wt%.
  • A1 is optionally present at or less than 10 wt%, optionally at or less than 8 wt%, optionally at or less than 6 wt%.
  • T N 34d represents a temperature where nitrogen uptake in steel occurs through nitride formation.
  • the exact temperature and form of nitride depends on the steel composition.
  • T g 34c represents the temperature at which the steel transforms into austenite of FCC form. Again, the temperature depends on the steel composition.
  • T YN 34b represents the temperature at which all the nitride compounds dissolve and nitrogen in the steel exists in dissolved nitrogen form.
  • Tm 34a represents the melting point of the steel.
  • first nitrogen uptake is expected to occur through nitride formation during the ramp-up phase 32’, which will dissolve during holding the powder above 34b. Above 34b, all the nitrogen uptake is expected to occur elemental N form. The extent of N uptake will depend on the ramp-up time plus the holding time above 34b.
  • the quenching rate influences the final microstructure of the HNS. Under slow cooling the N may precipitate into carbide particles. The gas pressure, temperature and time are adjusted according to the desired dissolved nitrogen content in the final HNS of step 45 and the composition of the precursor.
  • T N nitride formation temperature
  • a temperature for a nitrogen uptake time is at or above the T N and below the T g of the alloy. Further, holding the powder at this temperature prevents sintering especially in the presence of powder agitation is used during the nitrogen uptake time as will be illustrated below. However, holding the powder at a temperature between the T g and T N may lead to the formation of undesirable nitrides.
  • the temperature-time cycle 33 comprises of a second heating step, where the temperature is raised above T YN and held there for a brief nitride conversion time to decompose the nitrides, yielding the desired dissolved nitrogen content, prior to quenching the powder.
  • the nitride conversion time above T YN is substantially shorter compared to nitrogen uptake time and thus prevents sintering.
  • T N , T g , T YN depend on the steel composition, and further the holding times also depend on the powder composition and size.
  • the temperature-time cycles 32 and 33 are exemplary illustrations and many variations can optionally be adopted to achieve sinter/nitride free powder with a desired dissolved N content.
  • a nitrogen uptake time is in excess of 1 second.
  • a nitrogen uptake time may be indefinite, but is more commonly 1 hour or less.
  • the nitrogen uptake time may be adjusted upward.
  • a nitrogen uptake time is from 1 second to 15 minutes, optionally 1 second to 100 seconds, optionally 1 second to 60 seconds, optionally 10 seconds to 100 seconds, optionally 30 seconds to 70 seconds, optionally 50 seconds to 60 seconds.
  • a hold time max' be sufficient to fully heat the precursor to the desired temperature or may hold the precursor at that temperature for the nitrogen uptake time.
  • a nitrogen uptake time may be at a constant temperature or may be at a varying temperature.
  • a varying temperature during a nitrogen uptake time may be at or between T N and T g .
  • the temperature may fluctuate or remain substantially constant, optionally varying by 5 °C or less.
  • the precursor with the nitrogen uptaken into the material may then be subjected to a further heating step whereby the precursor is heated to a temperature near, but not at or above the Tm.
  • the second heating step heats the precursor to a second temperature that is above a T YN of the precursor and below' a melting temperature for the precursor powder.
  • any nitrides formed in the uptake step or otherwise present in the steel material are converted into dissolved nitrogen. This further increases the weight percent of dissolved nitrogen and prevents unwanted characteristics that occur due to the presence of nitrides in the final high nitrogen steel.
  • the increased temperature above the T YN of the precursor is optionally held for a nitride conversion time.
  • a nitride conversion time is optionally any time to allow all, substantially all or any desired amount of nitrides within the precursor to be converted to dissolved nitrogen.
  • a nitride conversion time is optionally 1 hour or less.
  • a nitrogen conversion time is a short as possible so as to both convert the nitrides to dissolved nitrogen but also to prevent sintering (in some aspects).
  • a nitride conversion time is optionally less than 1 hour, optionally less than 20 minutes, optionally, less than 10 minutes, optionally less than 5, 4, 3, 2, 1, min. It has been observed that some sintering may occur when using particular precursor steel in the form of a powder at 10 minutes. As such, when a powder precursor is used, the nitride conversion time is optionally 10 minutes or less, according to some non-limiting aspects.
  • the powder is quenched to a temperature where the diffusion is virtually absent in step 44 follow'ing an exemplary temperature-time cycle 33.
  • the cooling rate is critical to avoid nitride formation or reformation.
  • the pow'der cooling rate is between 1 °C/s and 100 °C/s, optionally the powder cooling rate is between 5 °C/s and 50 °C/s, optionally the powder cooling rate is above 10 °C/s.
  • the atmospheric pressures used in the processes optionally are not required to exceed 1 atm as, in many aspects, 1 atm is sufficient to radically increase the amount of dissolved nitrogen in the high nitrogen steel relative to prior processes. However, in other aspects the atmospheric pressure is optionally above 1 atm, optional 2 atm or greater, optionally 3 atm or greater optionally 4 atm or greater In some aspects the atmospheric pressure is less than that typically required to dissolve nitrogen in liquid steel. As such, an atmospheric pressure is optionally less than 10 atm, optionally less than 9 atm, optionally less than 8 atm, optionally less than 7 atm, optionally less than 6 atm, optionally less than 5 atm.
  • the resulting high nitrogen steel is provided with an exceptionally low nitride content, optionally 0.01 wt% or lower.
  • the resulting HNS has a nitride content at or below 0.02 wt%, optionally 0.03 wt%, optionally 0.04 wt%, optionally 0.05 wt%, optionally 0.1 wt% or lower.
  • the resulting high nitrogen steel optionally has a dissolved nitrogen content of 0.05 wt% to 6.0 wt%, or higher or any value or range therebetween.
  • the dissolved nitrogen content of the HNS is at ore greater than 0.1 wt%, optionally 0.5 wt%, optionally 1 wt%, optionally 2 wt%, optionally 3 wt%, optionally 4 wt%, optionally 5 wt%, optionally 6 wt%.
  • the amount of dissolved nitrogen exceeds the solubility limit of nitrogen in the alloy (alloy of otherwise identical composition) in a liquid state at atmospheric pressure.
  • the resulting high nitrogen steel includes a ferrite (a) phase, austenite (g) phase, or a mixture of a + g phase.
  • the alloy is predominantly a single phase.
  • a single phase may be a ferrite phase or a gamma phase.
  • an alloy is predominantly or entirely a single phase, optionally a g phase.
  • the HNS produced by the processes as provided herein is optionally predominantly FCC structure, optionally 90% or greater FCC structure.
  • the HNS produced by the processes as provided herein are absent BCC structure throughout the HNS.
  • N_eq and Cr eq are tire two most impactful austenite stabilizers. Since addition of C bey ond 0.1 wt% is detrimental to the toughness, primarily the influence of N and Mn is considered here for exemplary illustration of alloy compositions.
  • N equivalent A x Cr equivalent - B.
  • A ⁇ 0.98
  • B ⁇ 11.5 for Ni free Fe-Mn-Cr-N alloy.
  • exemplary alloy compositions will lead to the following outcomes as presented in Table 1.
  • the impact of Mn content in stabilizing the austenite decreases as the content increases. For example, keeping the nitrogen concentration at 0.5 wt%, an increment of Mn content from 15 wt% to 30 wt%, decreases the N-eq from 5.27 to 3.65.
  • N concentration is the most influential factor in stabilizing the austenite. For example, by changing the N concentration from 0.5 wt% in alloy #4 to 0.7 wt% in alloy #5, results in an austenitic alloy even though significant amount of Cr (20 wt%) is present in the alloy. However, care must be taken not to increase the N content significantly beyond the stability zone especially when high amount of Cr is presort to prevent CraN precipitation as illustrated in FIG. 2.
  • Mn addition can counter the influence of Cr and contribute towards the stability of austenite.
  • the N kept between 0.4 wt.% and 0.9 wt.%
  • Mn is kept between 19-27 wt%
  • the Cr is kept between 10-18 wt.%, the rest being iron.
  • a N alloy is or includes 13-14 wt% Cr, 20-26 wt% Mn, and 0.4-0.6 wt% N with the remainder being Fe.
  • an exemplary embodiment 60 for batch processing of powder shown in FIGS. 6A and 6B includes a ceramic processing tube 65 which is rotated using a motor-gear arrangement 67.
  • One end of tube 65 is connected to a powder feeding apparatus, which comprises of motor 61, precursor inlet 62 to load powder and an auger 63.
  • This feeding apparatus can be inserted or retracted using lead screw arrangement 69 to evenly distribute powder through the processing tube 65.
  • the other end of the tube 65 is operably coupled to a cooled collection chamber 66, which can be cooled by chilled gas or other fluid.
  • the powder treatment tube is heated to a desired temperature by a heater 64.
  • FIG. 7A and Fig. 7B Further details of processing and removal of the precursor is shown in Fig. 7A and Fig. 7B respectively.
  • fins 79 can be added to the processing tube 75.
  • the fins can be ceramic rods placed along wall of the processing tube 75 as shown in cross section 78.
  • a tilt angle is applied to the rotating processing tube 75 through a jack arrangement 72 which in turn empties the treated powder into the collection chamber 76.
  • FIG. 8A and 8B Another exemplar)' embodiment 80 for continuous processing of powder shown in FIGs. 8A and 8B, includes a ceramic processing tube 85 which is rotated using a motor-gear arrangement 87.
  • One end of tube 85 is operably connected to a powder feeding apparatus 81, via precursor inlet 82 to continuously feed powder.
  • the other end of the tube 85 is operably coupled to a cooled collection chamber 86, which can be cooled by chilled gas or other cooling fluids.
  • the powder treatment tube is heated to a desired temperature by a heater 84.
  • the tube assembly can be placed at different angles relative to gravity via an adjustment device 88. This angle as well as the rotation speed of the tube 85 determines the residence time of the precursor pow'der in the treatment tube. Accordingly, various temperature-time cycles presented in FIG. 3 can be adopted through this arrangement to continuously treat the powder to achieve a desired dissolved N content in the powder.
  • the auger 87’ extends from the powder reservoir 82’ till the deliver)' end of the processing tube 85’.
  • the processing tube remains stationary, while the auger continuously agitates the powder inside the processing tube.
  • the processing tube also rotates while treating the powder.
  • the pitch and rotational speed of the auger 87’ controls the feed rate of the precursor as well as the dwell time in the processing tube 85’.
  • the auger eventually pushes the precursor into the collection chamber 86’.
  • auger is made of high temperature compatible materials such as ceramic to be able to operate at high temperatures in the processing tube.
  • precursor feeding can be achieved using electromagnetic vibration 81”.
  • the feed rate of the precursor is controlled by regulating the vibration frequency.
  • the feed rate can be further increased by tilting the setup using a jack arrangement 83”.
  • the precursor is introduced into the processing tube 85” using commercially available powder feeder 82” such as thermal spray powder feeder. Due to continuous vibration of the processing tube 85”, the precursor powders are agitated and moved through the processing tube into the collection chamber 86”.
  • Embodiment 90 provides a methodology to alloy the said precursor with N via a fluidized bed reactor.
  • Embodiment 90 includes a heating element 92, a processing tube 93, a precursor intake 91, a vent for the exhaust gas 95, a downstream gate valve 98 and a cooled collection chamber 97, operably connected to the processing tube 93 via gate valve 98.
  • a known quantity of precursor 99 is introduced into the processing tube 93 via the intake port 91, while keeping the gate valve 98 closed.
  • preheated nitrogen gas is continuously injected through nozzle 94 which fluidizes the precursor as shown in FIG. 11.
  • the exhaust gas leaves the processing chamber through the vent 95, which can be recycled back to the chamber nozzle 94.
  • the combined heat input from the heating element 92 and the preheated fluidizing gas keeps the precursor powder temperature at a desired range.
  • the temperature-time cycle to achieve a target dissolved nitrogen content in the steel powder is adopted according to the teachings of this disclosure as illustrated in embodiments 30 and 40.
  • the downstream valve 98 is opened to release the treated powder 99 into the collection chamber 97 through the cooled channel 96. It is understood that the fluidized bed treatment method as described herein can be achieved by alternative embodiments following similar mechanical and physical principles.
  • a precursor powder with composition of Fe, 12 wt% Cr and 20 wt% Mn was centrifugally atomized at Ervin Technologies, Tecumseh, Michigan, USA and classified to yield a size distribution of 10 mm to 60 mm
  • the powder was then processed according to the teachings of the present disclosure by utilizing an embodiment illustrated FIG. 8C.
  • Nitrogen uptake in the powder was determined using Leco TC436 combustion analyzer at NSL Analytical Services, Inc., Ohio, USA.
  • the phase of the powder was determined using a Rigaku Miniflex X-ray Diffractometer (Cu Ka radiation l ⁇ 1.5402A).
  • FIGS. 13-16 The precursor powder was subject to various temperature-time cycles under N2+5%H2 gas mixture environment. The observations are presented in FIGS. 13-16. As illustrated in FIG. 13, when the process temperature remained between Ti ( ⁇ 980°C) and T g , for example 930°C, nitride precipitated were always present in the final powder as shown FIG. 15D. In the early stages of the treatment process, especially when the treatment temperature was closer to T g , both a and g phases along with grain boundary precipitates were observed as shown in FIG. 15B. The corresponding x-ray diffraction pattern B shown in FIG. 16, indicates the presence of a and g phases.
  • the incoming precursor powder had single phase (a or ferrite), as demonstrated by the diffraction pattern A in FIG. 16.
  • the overall nitride precipitate content increased and the matrix alloy transformation into g phase. Longer holding time led to powder sintering.
  • Processing temperatures, between T g and T N formed nitride precipitates in the a matrix. No changes were observed below the threshold temperature Tth.
  • T2 >1080°C
  • 1130°C although the N uptake was accelerated, the powders sintered quickly.
  • the corresponding powder microstructure is shown in FIG. 15A. Temperature-time cycle within the shaded region as illustrated in FIG.
  • a treatment zone as illustrated in FIG. 13 can be constructed to achieve single phase homogeneous dissolved nitrogen alloy according to the teachings of this disclosure.
  • Table 2 processing at 980°C and 1080°C for a dwell time of 300s, followed by quenching to room temperature at 10°C/s, yielded nitrogen content of 0.3 wt% and 0.48 wt% respectively, without any precipitates and uniform austenite phase. Further, sintering of powder was also absent in these experiments.

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Abstract

L'invention concerne des procédés et des dispositifs pour la formation d'acier à haute teneur en azote. Les procédés comprennent le chauffage d'un précurseur d'acier à une température qui transforme l'acier en une austénite en configuration CFC, le chauffage se faisant dans une atmosphère contenant de l'azote. Après un temps d'absorption d'azote facultatif, le précurseur est encore chauffé à une température au-dessus de la TγN de l'acier mais au-dessous du point de fusion de l'acier, ce qui permet de conserver un solide et de créer une solution solide d'azote. La seconde température est éventuellement maintenue pendant un temps de conversion en nitrure, le temps de conversion en nitrure étant éventuellement trop court pour conduire au frittage de l'acier. Le procédé comprend en outre une trempe rapide de la poudre de précurseur pour maintenir la solution solide d'azote et empêcher la formation de nitrure, ce qui forme ainsi un acier à haute teneur en azote ayant peu ou pas de teneur en nitrure et comprenant de l'azote en solution solide.
PCT/US2020/019894 2019-02-26 2020-02-26 Poudre d'acier à haute teneur en azote et procédés de fabrication de celle-ci Ceased WO2020176616A1 (fr)

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EP20763825.5A EP3930942A4 (fr) 2019-02-26 2020-02-26 Poudre d'acier à haute teneur en azote et procédés de fabrication de celle-ci
CN202080031000.1A CN113840673A (zh) 2019-02-26 2020-02-26 高氮钢粉及其制造方法
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CN118543842B (zh) * 2024-07-29 2024-10-18 西安赛隆增材技术股份有限公司 一种制备高氮钢粉末的装置及方法

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US20220134424A1 (en) 2022-05-05

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