Method for producing austenitic fe-cr-ni steel reinforced with oxide dispersion

Abstract

The invention relates to a process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion, said process comprising the following steps: a) supplying (100) Fe-Cr-Ni austenitic steel powder; b) carrying out (200) a grinding, called cryogenic grinding, of the Fe-Cr-Ni austenitic steel powder at a temperature between -50°C and -196°C; c) supplying (300) an oxide powder; d) carrying out (400) a co-grinding of the Fe-Cr-Ni austenitic steel powder with the oxide powder thus supplied to obtain Fe-Cr-Ni austenitic steel powder reinforced by an oxide dispersion; and e) producing (500) the Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion from the powder thus produced.

Description

Technical field of the invention

[0001] The invention relates to the field of metallurgy of austenitic Fe-Cr-Ni steels strengthened by an oxide dispersion. In particular, the invention relates to a process for producing such a steel. State of the art

[0002] Currently, the production of ODS (Oxide Dispersion Strengthened) austenitic steels using powder metallurgy involves a mechanical grinding step. This step is now essential for the production of certain classes of steel, such as ODS steels. ODS steels are steels strengthened by a dispersion of oxides, most often at the nanometer scale. They are of interest in industry for their advanced properties in terms of corrosion resistance and high-temperature mechanical strength, and also in the nuclear sector for their resistance to radiation damage.

[0003] Today, a process for producing ODS steels involves powder metallurgy and the co-milling (also called mechanosynthesis) of austenitic steel powder with oxide powder, for example, yttrium oxide (Y₂O₃). When mechanosynthesis is carried out correctly, the oxide is dissolved in the matrix. This step is well controlled for ferritic steels. However, the high ductility of austenitic steels makes this mechanosynthesis step difficult because the powders can stick and agglomerate together or coat the grinding media or grinding chambers without the mechanosynthesis step occurring.

[0004] Attempts have been made to solve this problem by varying the chemical composition of the powder and / or the manufacturing parameters.

[0005] There is currently no consensus on the manufacturing parameters that, to a first order, vary the kinetics or energy of the system. Parameters such as rotational speed, the ball-to-powder mass ratio, media filling rate, blade geometry, and many others are dependent on the technology and material being studied.

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

[0007] A solution has been proposed to reduce the predominant bonding: the use of surfactants or PCAs (Process Control Agents). However, these agents induce significant contamination in the finished material. For example, it has been shown that the use of stearic acid as a PCA leads to substantial carbon contamination, negatively impacting the behavior of the resulting steel. The addition of carbon causes the precipitation of coarse M23C6 carbides, known to impair corrosion resistance and weaken the material. Similarly, the precipitation of M7C3 carbides is detected at grain boundaries, potentially affecting mechanical strength. Resistance under irradiation is also reduced by oxide growth, which is accelerated by carbon contamination due to the diffusion of elements towards the carbides.

[0008] It has also been proposed to perform a two-stage co-milling process. This involves first co-milling ferritic Fe-Cr steel powder with oxide powder (lasting 20 to 40 hours), followed by a second co-milling stage (lasting 3 to 30 hours) with the addition of pure nickel powder. This yields an austenitic Fe-Cr-Ni alloy. However, the addition of a co-milling stage with pure nickel induces significant binding, thus impacting both the yield and the chemical homogeneity.

[0009] To prevent powder adhesion to the grinding media, it has also been proposed to use grinding media made of hard materials such as zirconia (ZrO₂) or tungsten carbide (WC). However, the fragments of the grinding media trapped in the powders, once consolidated, drastically degrade the mechanical properties, particularly the toughness and ductility, of the resulting austenitic ODS steels.

[0010] As a final example, cryo-milling of austenitic steels has also been proposed. Unfortunately, the resulting consolidated materials are porous and exhibit a bimodal microstructure. Furthermore, the oxides are particularly coarse (visible under a light microscope).

[0011] Due to these problems with the binding of the austenitic steel powder, 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 process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion which allows for improvement of the aforementioned yield.

[0013] To this end, the invention proposes a process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion, said process comprising the following steps: a) supply an austenitic steel powder Fe-Cr-Ni; b) carry out a grinding, known as cryogenic grinding, of the austenitic steel powder Fe-Cr-Ni at a temperature between -50°C and -196°C; c) supply an oxide powder; d) carry out a co-grinding of the austenitic steel powder Fe-Cr-Ni with the oxide powder thus supplied to obtain an austenitic steel powder Fe-Cr-Ni reinforced by an oxide dispersion; and e) produce the austenitic steel Fe-Cr-Ni reinforced by an oxide dispersion from the powder thus produced.

[0014] The process according to the invention may have at least one of the following characteristics, taken alone or in combination: The grinding step lasts between 10 minutes and 20 hours, particularly between 10 minutes and 15 hours, and more specifically between 10 minutes and 100 minutes. The process includes an additional step, between steps d) and e), consisting of sieving the austenitic steel powder reinforced by an oxide dispersion to a predetermined initial average particle size of less than 250 microns. Step e) comprises the following substeps: e1) encapsulating and vacuum-sealing the austenitic steel powder reinforced by an oxide dispersion, then: e2) performing hot isostatic compaction. Step e) is a sintering step, for example, flash sintering. Step e) is a hot spinning step. the oxide powder supplied in step c) is chosen from a powder of Yttrium oxides (Y 2 O 3 ), Zirconium oxides or Titanium oxides.the oxide powder supplied in step a) has an average particle size of less than 5 microns, preferably less than 1 micron, and even more preferably less than 100nm. Brief description of the figures

[0015] There figure 1 is a flowchart of a process for manufacturing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention; The figure 2 is a graph illustrating a comparative distribution of particle size distribution by laser granulometry; The figure 3 is a photograph illustrating the difference in particle size obtained by a process according to the prior art and that obtained by the process of the figure 1 ; There figure 4 is a photograph illustrating the comparison between a new grinding ball and grinding balls after implementation of the process of the figure 1 and of a prior art process. Detailed description

[0016] Throughout the following description, particle size distribution is based on dry laser particle size analysis. The principle of this measurement is as follows: a certain quantity of powder is placed on a vibrating platform. When the platform is activated, the powder falls into a vertical column (in this case, the column is filled with a gas, for example, air: dry method). A collimated laser beam passes through the vertical column, interacts with the powder, and becomes scattered as a result of this interaction. The angle at which the light is scattered then provides information about the powder size, using a physical model that relates the scattering angle to the powder size. This physical model is, for example, a Mie scattering model. Typical error margins on powder size obtained with dry laser particle size analysis are typically less than 5%.More specifically, the measurements provided in this description were obtained with a Horiba Jobin-Yvon LA-950 machine.

[0017] With reference to the figure 1 , we will describe a process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention.

[0018] The process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention includes a first step 100 of supplying an austenitic Fe-Cr-Ni steel powder.

[0019] Next, the process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion according to the invention comprises a second step 200 of grinding, referred to as cryogenic grinding, the Fe-Cr-Ni austenitic steel powder supplied at a temperature between -50°C and -196°C. This second step 200 lasts for a few tens of minutes, for example, between 10 and 100 minutes. However, it may be necessary for this step to last longer, for example, several hours. More generally, this step can be expected to last between 10 minutes and 20 hours, in particular between 10 minutes and 15 hours. Fe-Cr-Ni austenitic steel powder is obtained with a predetermined initial average particle size.

[0020] Then, the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention comprises a third step 300 of supplying an oxide powder, for example, a yttrium (Y₂O₃) oxide powder. Other oxide powders can be used, such as zirconium or titanium oxides. The supplied oxide powder has an average particle size of less than 5 microns, preferably less than 1 micron, and even more preferably less than 100 nm. The smaller the particle size of the oxide powder, the easier it is to incorporate the oxide powder into the steel powder.

[0021] Next, the process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion according to the invention comprises a fourth step 400 of co-milling the Fe-Cr-Ni austenitic steel powder obtained from cryogenic grinding with the oxide powder to obtain Fe-Cr-Ni austenitic steel powder reinforced by an oxide dispersion. This co-milling step employs a mechanosynthesis of the powders. This allows the oxides to dissolve within the Fe-Cr-Ni austenitic steel powder grains.

[0022] The result of this co-grinding step is illustrated, for example, in figure 3b ). This is a scanning electron microscopy (SEM) image of 316L steel powder reinforced with an oxide dispersion obtained by the process for producing Fe-Cr-Ni austenitic steel reinforced with an oxide dispersion according to the invention. It is compared to the same powder ( figure 3a)) obtained by a prior art process.

[0023] Finally, the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention includes a fifth step 500 of producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion from the powder thus ground and sieved. This fifth step 500 is the consolidation step. For example, it may include the following substeps: encapsulating and vacuum-sealing the ground austenitic Fe-Cr-Ni steel powder reinforced by an oxide dispersion, which therefore has a final medium particle size, and then performing hot isostatic compaction (HIC).

[0024] Alternatively, the fifth step 500 of the process for producing an austenitic Fe-Cr-Ni steel reinforced by 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" according to Anglo-Saxon terminology).

[0025] Step 500 can also be achieved by hot spinning or other compatible consolidation techniques.

[0026] It should be noted that between steps 400 and 500, an additional, optional step can be performed in which the Fe-Cr-Ni austenitic steel powder, reinforced with an oxide dispersion, is sieved to a predetermined final average particle size, for example, 250 microns or less. This can be useful if hot isostatic pressing (HIP) is to be performed in step 500. Other final average particle size values, such as 100 microns, can be chosen depending on the application.

[0027] It appears from the use of a process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion according to the invention as previously described makes it possible to obtain a greater quantity (yield) of Fe-Cr-Ni austenitic steel powder reinforced by oxide dispersion, moreover, without deterioration of the quality of the powder as can be the case for some prior art processes.

[0028] Incidentally, an improvement in performance can be noted by looking at the figure 4 In (a), the grinding ball is new. In (b), the grinding ball is simply capped and was used in the fourth grinding step 400 of the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention. In (c), the grinding ball has a surface on which the austenitic steel powders adhere during co-grinding by a prior art process.

[0029] Furthermore, as illustrated in the figure 2A volume distribution of the average diameter (particle size distribution) of Fe-Cr-Ni austenitic steel powders reinforced by an oxide dispersion is given for two batches of powders. The curve with circles represents the conventional production of 316L ODS steel using a prior art process. The curve with crosses represents the production of the same 316L ODS steel using the process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion according to the invention. Note that the co-milling in the fourth step 400 is carried out under the same conditions for both tests. A clear reduction in particle size is observed between the conventional co-milling and the double milling (cryogenic milling + co-milling) of the process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion according to the invention.To consolidate the material by Hot Isostatic Pressing (HIP), the powder is often sieved to less than 250 micrometers. The results of this sieving below 250 µm yield a fraction of 1.48% for conventional grinding and 70.26% for the double co-grinding process used to produce an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention. This demonstrates the potential of the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention to significantly increase process yield, since sieving to 250 microns allows for the retention of 70% of the ground powder, compared to barely 1.5% in the case of the conventional process.

[0030] Finally, by implementing the process according to the invention, in this case with a flash sintering consolidation step 500, SAXS (Small Angle X-ray Scattering) measurements were performed on the product obtained after consolidation. The SAXS technique allows the detection of nanometric objects in a matrix. For example, an average powder grain size (i.e., average precipitate diameter) of 4.55 nm was observed, for a volume fraction of 0.65%, corresponding to a number density of approximately 1.6 x 10⁻²² particles / m³.

[0031] The invention therefore makes it possible to obtain a state of nano-precipitation classically expected for an ODS steel, namely an average size of oxide powder grains less than 20nm, generally even between 4 and 10 nm, for a volume fraction of about 0.3% for a number density of the order of 10 +22< particles / m 3< to x10 +23< particles / m 3<.

Claims

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

2. A method according to claim 1, wherein the grinding step lasts between 10 minutes and 20 hours, in particular between 10 minutes and 15 hours, and more particularly between 10 minutes and 100 minutes.

3. A method according to any one of claims 1 or 2, comprising an additional step, between step d) and step e), consisting of sieving the austenitic steel powder reinforced by an oxide dispersion to a predetermined initial average particle size, measured by dry laser particle size analysis, of less than 250 microns.

4. A method according to any one of claims 1 to 3, wherein step e) comprises the following substeps: e1) encapsulating and vacuum-sealing the oxide dispersion-reinforced austenitic steel powder and then: e2) performing hot isostatic compaction.

5. A method according to any one of claims 1 to 3, wherein step e) is a sintering step, for example flash sintering.

6. A method according to any one of claims 1 to 3, wherein step e) is a hot spinning step.

7. A method according to any one of the preceding claims, wherein the oxide powder supplied in step c) is selected from a powder of Yttrium (Y2O3) oxides, Zirconium oxides or Titanium oxides.

8. A method according to any one of the preceding claims, wherein the oxide powder supplied in step a) has an average particle size, measured by dry laser particle size analysis, of less than 5 microns, preferably less than 1 micron, even more preferably less than 100nm.

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