Method for producing an austenitic steel Fe-Cr-Ni strengthened by an oxide dispersion

Abstract

The invention relates to a method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion, said method comprising the following steps:a) providing (100) an austenitic steel Fe—Cr—Ni powder;b) carrying out (200) a grinding operation, known as cryogenic grinding, on the austenitic steel Fe—Cr—Ni powder at a temperature of between −50° C. and −196° C. ;c) providing (300) a powder of oxides ;d) carrying out (400) a co-grinding of the austenitic steel Fe—Cr—Ni powder with the oxide powder thus supplied to obtain an austenitic steel Fe—Cr—Ni powder strengthened by an oxide dispersion; ande) producing (500) the austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion from the powder thus produced.

Description

TECHNICAL FIELD OF THE INVENTIONThe invention relates to the metallurgy of the austenitic steels Fe—Cr—Ni strengthened by an oxide dispersion. In particular, the invention relates to a method for producing such a steel.PRIOR ARTCurrently, the production of austenitic ODS (Oxides Dispersion Strengthened) steels by powder metallurgy involves a mechanical grinding stage. This stage is currently essential for the production of certain classes of steel, such as ODS steels. The ODS steels are steels strengthened by an oxide dispersion, usually nanometric. They are of interest in industry for their advanced properties in terms of corrosion resistance and mechanical strength at high temperatures, and also in the nuclear industry for their resistance to radiation damage.Today, a method for producing ODS steels involves powder metallurgy and the co-grinding (also known as mechanosynthesis) of an austenitic steel powder with an oxide powder, for example yttrium oxide Y2O3. When the mechanosynthesis is carried out correctly, the oxide is dissolved in the matrix. This step is well mastered for the ferritic steels. On the other hand, the high ductility of austenitic steels makes this mechanosynthesis step difficult because the powders can “stick” and agglomerate with each other or “butter” the grinding beads or grinding tanks without the mechanosynthesis step occurring.Attempts have been made to solve this problem by varying the chemical composition of the powder and / or the processing parameters.There is currently no consensus on the design parameters that primarily affect the kinetics or energy of the system. The parameters such as the rotation speed, bead / powder mass ratio, media fill rate, blade geometry and many others depend on the technology and material being studied.

[0006] However, for the dissolution of the oxides in the matrix and the mechanical alloy to be effective, the intensity of the mechanosynthesis must be high enough to deform the powder.

[0007] A solution has been proposed to reduce the preponderant sticking: the use of surfactant or PCA (Process Control Agent). However, these cause significant contamination in the processed material. For example, it has been shown that the use of steric acid as a PCA induces significant carbon pollution, with negative consequences for the behaviour of the steel obtained. The addition of carbon leads to the precipitation of coarse carbides M23C6, which are known to impair corrosion resistance and weaken the material. Similarly, the precipitation of M7C3 type carbide is detected at the grain boundaries, potentially impacting the mechanical strength. The resistance under irradiation is also reduced by the growth of oxides, which is accelerated by carbon pollution due to the diffusion of elements towards the carbides.

[0008] It was also proposed to carry out co-grinding in two steps. This involves an initial co-grinding of a ferritic Fe-Cr steel powder with an oxide powder (lasting from 20 to 40 hours), followed by a second co-grinding (lasting from 3 to 30 hours) with the addition of a pure nickel powder. The result is an austenitic alloy Fe—Cr—Ni. However, the addition of a co-grinding step with pure nickel induces a preponderant sticking, the yield as well as the chemical homogeneity are impacted.

[0009] To prevent the powders from sticking to the beads, it has also been proposed to use grinding beads made of hard materials such as zirconia ZrO2 or tungsten carbide WC. However, the fragments of the beads trapped in the powders once consolidated drastically deteriorate the mechanical properties, in particular the resilience and ductility of the austenitic ODS steels thus produced.

[0010] As a final example, it has also been proposed to carry out cryo-grinding on the austenitic steels. Unfortunately, the materials obtained once consolidated are porous and have a bimodal microstructure. Moreover, the oxides are particularly coarse (they are visible under an optical microscope).

[0011] Because of these problems with the austenitic steel powder sticking together, the yield is very low. This yield is relative to the quantity of powder actually used to produce the austenitic steel compared to the quantity of powder initially supplied.SUMMARY OF THE INVENTION

[0012] One aim of the invention is to provide a method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion, which allows to improve the above-mentioned yield.

[0013] To this end, the invention proposes a method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion, said method comprising the following steps:

[0014] a) providing an austenitic steel Fe—Cr—Ni powder;

[0015] b) carrying out grinding, known as cryogenic grinding, of the austenitic steel Fe—Cr—Ni powder at a temperature of between −50° C. and −196° C. ;

[0016] c) providing an oxide powder;

[0017] d) carrying out co-grinding the austenitic steel Fe—Cr—Ni powder with the oxide powder thus supplied to obtain an austenitic steel Fe—Cr—Ni powder strengthened by an oxide dispersion; and

[0018] e) producing the austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion from the powder thus produced.

[0019] The method according to the invention may have at least one of the following characteristics, taken alone or in combination:

[0020] the grinding step lasts between 10 min and 20 h, in particular between 10 min and 15 h, and more particularly between 10 min and 100 min.

[0021] The method comprises a further step, between step d) and step e), consisting of sieving the austenitic steel powder strengthened by an oxide dispersion to an initial predetermined average particle size of less than 250 microns.

[0022] step e) comprises the following sub-steps:

[0023] e1) encapsulating and evacuating the austenitic steel powder strengthened by an oxide dispersion, then:

[0024] e2) carrying out a hot isostatic compaction.

[0025] step e) is a sintering step, for example flash sintering.

[0026] step e) is a melt spinning step.

[0027] the oxide powder supplied in step c) is chosen from a powder of Yttrium oxides (Y2O3), Zirconium oxides or Titanium oxides.

[0028] the oxide powder supplied in step a) has an average particle size of less than 5 microns, preferably less than a micron, even more preferably less than 100 nm.BRIEF DESCRIPTION OF THE FIGURES

[0029] FIG. 1 is a flow chart of a method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention;

[0030] FIG. 2 is a graph illustrating a comparative distribution of the particle size of the particles by laser particle size analysis;

[0031] FIG. 3 is a photograph illustrating the difference in particle size obtained by a method according to the prior art and that obtained by the method in FIG. 1;

[0032] FIG. 4 is a photograph showing the comparison between a new grinding bead and grinding beads after using the method shown in FIG. 1 and the prior art method.DETAILED DESCRIPTION

[0033] Throughout the following description, the particle size (also called particle size distribution) is provided on the basis of a dry laser particle size measurement. The principle of this measurement is as follows: a certain quantity of powder is placed on a vibrating plate then, when the plate is activated, the powders fall into a vertical column (in this case the column is filled with a gas, for example air: dry method) and a collimated laser beam passes through the vertical column, interacts with the powders and then becomes diffuse as a result of this interaction. The angle at which the light is scattered then provides information about the size of the powders, using a physical model linking the scattering angle to the size of the powders. This physical model is, for example, a Mie diffusion model. The typical margins of error on the powder size obtained using the dry laser particle size are typically less than 5%. More specifically, the measurements provided in this description were obtained using a Horiba Jobin-Yvon LA-950 machine.

[0034] With reference to FIG. 1, we describe a method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention.

[0035] The method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention comprises a first step 100 of supplying an austenitic steel Fe—Cr—Ni powder.

[0036] Next, the method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention comprises a second step 200 of carrying out a grinding operation, known as cryogenic grinding, on the austenitic steel Fe—Cr—Ni powder supplied at a temperature of between −50° C. and −196° C. This second step 200 lasts several tens of minutes, for example between 10 minutes and 100 minutes. However, this step may need to last longer, for example several hours. More generally, this step can be expected to last between 10 min and 20 hours, in particular between 10 min and 15 hours. An austenitic steel Fe—Cr—Ni powder is obtained with a predetermined initial average particle size.

[0037] Then, the method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention comprises a third step 300 of supplying an oxide powder, for example Yttrium oxide (Y2O3) powder. Other oxide powders can be used, such as zirconium oxides or titanium oxides. The oxide powder supplied has an average particle size of less than 5 microns, preferably less than a micron and even more preferably less than 100 nm. The smaller the particle size of the oxide powder, the easier it is to insert the oxide powder into the steel powder.

[0038] Next, the method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention comprises a fourth step 400 of co-grinding the austenitic steel Fe—Cr—Ni powder obtained from cryogenic grinding with the oxide powder to obtain an austenitic steel Fe—Cr—Ni powder strengthened by an oxide dispersion. This co-grinding step involves a mechanosynthesis of the powders. This allows the dissolution of the oxides inside the grains of austenitic steel Fe—Cr—Ni powder.

[0039] The result of this co-grinding step is illustrated, for example, in FIG. 3b). This is a Scanning Electron Microscopy (SEM) image of an oxide dispersion strengthened steel powder 316L obtained by the method of producing an oxide dispersion strengthened austenitic steel Fe—Cr—Ni according to the invention. It is compared with the same powder (FIG. 3a)) obtained by a prior art method.

[0040] Finally, the method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention comprises a fifth step 500 of producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion from the powder ground and sieved in this way. This fifth step 500 is the consolidation stage. For example, it may comprise the following sub-steps: encapsulating and evacuating the austenitic steel Fe—Cr—Ni powder strengthened by a ground oxide dispersion, which therefore has a final average particle size, and then carrying out hot isostatic compaction (CIC).

[0041] Alternatively, the fifth step 500 of the method for producing an austenitic steel Fe—Cr—Ni strengthened with an oxide dispersion according to the invention can be carried out by sintering, for example flash sintering (better known by the acronym SPS meaning “Spark Plasma Sintering”).

[0042] The step 500 can also be carried out by melt spinning or other compatible consolidation techniques.

[0043] It should be noted that between step 400 and step 500, an additional and optional step may be provided during which the austenitic steel Fe-Cr-Ni powder strengthened by an oxide dispersion is sieved to a predetermined final average particle size which is, for example, less than or equal to 250 microns. This can be useful if a hot isostatic compression (CIC) is desired for step 500. Other final average particle size values can be chosen depending on the application, such as 100 microns.

[0044] The use of a method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention as described above allows to obtain an austenitic steel Fe—Cr—Ni powder strengthened by an oxide dispersion in a greater quantity (yield), without deteriorating the quality of the powder as may be the case for certain methods in the prior art.

[0045] FIG. 4 shows an improvement in performance. In a), the grinding bead is new. In b), the grinding bead is simply capped and was used during the fourth grinding stage 400 of the method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention. In c), the grinding bead has a surface on which the austenitic steel powders adhere during co-grinding by a prior art method.

[0046] In addition, as shown in FIG. 2, a volume distribution of the average diameter (grain size) of austenitic steel Fe—Cr—Ni powders strengthened by an oxide dispersion is given for two batches of powders. The curve with circles represents the conventional production of an ODS steel 316L using a prior art method. The curve with crosses represents the production of the same ODS steel 316L using the method for producing an austenitic steel Fe—Cr—Ni strengthened with an oxide dispersion according to the invention. It should be noted that the co-grinding in the fourth step 400 is carried out under the same conditions for both trials. A clear reduction in the particle size was observed between conventional co-grinding and double grinding (cryogenic grinding+co-grinding) of the according to the invention. In order to consolidate the material using Hot Isostatic Compaction (HIC), the powder is often sieved to less than 250 micrometres. The results of this sieving of less than 250 μm give a fraction of 1.48% for conventional grinding and 70.26% for double co-grinding of the method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion according to the invention. This shows the potential of according to the invention to considerably increase the yield of the method, since sieving to 250 microns enables 70% of the ground powder to be retained compared with just 1.5% in the case of the conventional method.

[0047] Finally, by implementing the method according to the invention, in this case with a step 500 of consolidation by flash sintering, it was possible to carry out SAXS (Small Angle X-ray Scattering) measurements on the product obtained after consolidation. The SAXS technique can be used to detect nanometric objects in a matrix. For example, we observed an average powder grain size (i.e. average precipitate diameter) of 4.55 nm, for a volume fraction of 0.65%, i.e. a numerical density of approximately 1.6×10+22 particles / m3.

[0048] The invention therefore allows to obtain a state of nanoprecipitation classically expected for an ODS steel, i.e. an average size of the oxide powder grains of less than 20 nm, generally even between 4 and 10 nm, for a volume fraction of approximately 0.3% for a numerical density of the order of 10+22 particles / m3 à×10+23 particles / m3.

Claims

1. A method for producing an austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion, said method comprising the following steps:a) providing (100) an austenitic steel Fe—Cr—Ni powder;b) carrying out (200) a grinding operation, known as cryogenic grinding, on the austenitic steel Fe—Cr—Ni powder at a temperature of between −50° C. and −196° C.;c) providing (300) a powder of oxides;d) carrying out (400) a co-grinding of the austenitic steel Fe—Cr—Ni powder with the oxide powder thus supplied to obtain an austenitic steel Fe—Cr—Ni powder strengthened by an oxide dispersion; and,e) producing (500) the austenitic steel Fe—Cr—Ni strengthened by an oxide dispersion from the powder thus produced.

2. The method according to claim 1, wherein the grinding step lasts between 10 min and 20 h.

3. The method according to claim 1, comprising an additional step, between step d) and step e), consisting of sieving the austenitic steel powder strengthened by an oxide dispersion to an initial predetermined average particle size, measured by dry laser particle size measurement, of less than 250 microns.

4. The method according to claim 1, wherein step e) comprises the following sub-steps:e1) encapsulating and evacuating the austenitic steel powder strengthened by an oxide dispersion, then:e2) carrying out a hot isostatic compaction.

5. The method according to claim 1, wherein step e) is a sintering step, for example flash sintering.

6. The method according to claim 1, wherein step e) is a melt spinning step.

7. The method according to claim 1, wherein the oxide powder supplied in step c) is chosen from a powder of Yttrium oxides (Y2O3), Zirconium oxides or Titanium oxides.

8. The method according to claim 1, wherein the oxide powder supplied in step a) has an average particle size, measured by dry laser particle size measurement, of less than 5 microns.

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