OPTIMIZED STEEL MATERIAL

Abstract

Steel material whose constituent grains comprise a matrix in which precipitates are incorporated, the precipitates comprising at least one metallic element selected from a metallic element M, a metallic element M′, a metallic element M″ or mixtures thereof; the microstructure of the steel being such that the grains are equiaxed and the average grain size being such that the average of their largest dimension “Dmax” and/or the average of their smallest dimension “Dmin” is comprised between 10 μm and 50 μm.

The steel material has optimized, stable and isotropic mechanical properties, in particular so that the steel material can best withstand mechanical and/or thermal stresses.

Claims

1. A steel material wherein grains of which it is composed comprise a matrix into which precipitates are incorporated, wherein: i) the material comprises, in weight percentage, the following elements: 16% to 20% chrome, 8% to 14% nickel, 0.001% to 0.030% carbon, 0.001% to 0.050% oxygen, 0% to 2% manganese, 0% to 3% molybdenum, 0% to 1% silicon, and iron; ii) the precipitates comprising at least one metallic element selected from a metallic element M, a metallic element M′, a metallic element M″ or mixtures thereof, the metallic elements being selected independently of each other among yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminium, hafnium, molybdenum or mixtures thereof, a microstructure of the steel being such that the grains are equiaxial and an average size of the grains being such that an average of their largest dimension (Dmax) and/or an average of their smallest dimension (Dmin) is in the range of 10 μm to 50 μm.

2. The material according to claim 1, wherein the grains are equiaxed in a plane parallel to a plane of the superimposed layers of the material resulting from the manufacture of the material by an additive manufacturing process.

3. The material according to claim 2, wherein the grains are additionally equiaxed in a plane perpendicular to the plane of the superimposed layers of the material resulting from the manufacture of the material by an additive manufacturing process.

4. The material according to claim 1, wherein the equiaxed grains are such that an average Dmax/Dmin ratio between the largest dimension (Dmax) and the smallest dimension (Dmin) of a grain is comprised between 1 and 2.

5. The material according to claim 1, wherein the metallic element M is titanium, iron, chromium or mixtures thereof.

6. The material according to claim 1, wherein the precipitates comprise at least one metal oxide, at least one metal carbide, at least one metal oxycarbide, at least one intermetallic compound, or mixtures thereof; each of this oxide, carbide, oxycarbide or intermetallic compound comprising at least one metallic element selected from the metallic element M, the metallic element M′, the metallic element M″ or mixtures thereof.

7. The material according to claim 6, wherein the metal oxide is selected from at least one simple oxide MO.sub.2-x with the index X comprised between 0 and 1, at least one mixed oxide MM′.sub.y′O.sub.5-x′ with the index X′ other than 5 comprised between 0 and 5 and the index y′ non-zero between 0 and 2, or at least one mixed oxide MM′.sub.y′M″.sub.y″O.sub.5-x″ with the index X″ other than 5 comprised between 0 and 5, the index y′ non-zero between 0 and 2 and the index y″ comprised between 0 and 2, or mixtures of these oxides.

8. The material according to claim 7, wherein the simple oxide MO.sub.2-x is selected from Y.sub.2O.sub.3, Fe.sub.2O.sub.3, FeO, Fe.sub.34, Cr.sub.2O.sub.3, TiO.sub.2, Al.sub.2O.sub.3, HfO.sub.2, SiO.sub.2, ZrO.sub.2, ThO.sub.2, MgO, MnO, MnO.sub.2 or mixtures thereof.

9. The material according to claim 8, wherein the simple oxide is TiO.sub.2.

10. The material according to claim 7, wherein the mixed oxide MM′.sub.y′O.sub.5-x′ is selected from FeTiO.sub.3, Y.sub.2Ti.sub.2O.sub.7, YTi.sub.2O.sub.5 or mixtures thereof.

11. The material according to claim 6, wherein the metal carbide is selected from at least one simple carbide MC.sub.8-x with the index X other than 8 comprised between 0 and 8, at least one mixed carbide MM′.sub.y′C.sub.8-x′ with the index X′ other than 8 comprised between 0 and 8 and the index y′ comprised between 0 and 5, or mixtures of these carbides.

12. The material according to claim 11, wherein the simple carbide MC.sub.8-x is selected from TiC, SiC, AlC.sub.3, CrC or mixtures thereof.

13. The material according to claim 11, wherein the mixed carbide MM′.sub.y′C.sub.8-x′ is selected from (FeCr).sub.7C.sub.3 or (FeCr).sub.23C.sub.6.

14. The material according to claim 6, wherein the metal oxycarbide comprises; a metal oxide selected from at least one simple oxide MO.sub.2-x with the index X comprised between 0 and 1, at least one mixed oxide MM′.sub.y′O.sub.5-x′ with the index X′ other than 5 comprised between 0 and 5 and the index y′ non-zero between 0 and 2, or at least one mixed oxide MM′M″.sub.y″O.sub.5-x″ with the index X″ other than 5 comprised between 0 and 5, the index y′ non-zero between 0 and 2 and the index y″ comprised between 0 and 2, or mixtures of these oxides, and a metal carbide selected from at least one simple carbide MC.sub.8-x with the index X other than 8 comprised between 0 and 8, at least one mixed carbide MM′.sub.y′C.sub.8-x′ with the index X′ other than 8 comprised between 0 and 8 and the index y′ comprised between 0 and 5, or mixtures of these carbides.

15. The material according to claim 6, wherein the intermetallic compound is selected from YFe.sub.3, Fe.sub.2Ti, FeCrWTi or mixtures thereof.

16. The material according to claim 1, wherein the material comprises 0.1% to 1.5% by weight of precipitates.

17. The material according to claim 6, wherein the precipitates of the metal carbide have an average size comprised between 10 nm and 50 nm.

18. The material according to claim 6, wherein the precipitates of the metal oxide and/or of the metal oxycarbide have an average size comprised between 10 nm and 100 nm.

19. The material according to claim 1, wherein the average density with which the precipitates are distributed in the matrix is comprised between 2 precipitates/μm.sup.3 and 100 precipitates/μm.sup.3.

20. The material according to claim 1, wherein the material further comprises, in weight percentage, at least one of the following: 0% to 0.11% nitrogen, 0% to 0.045% phosphorus, 0% to 0.05% sulfur, 0% to 0.0300% aluminum, 0% to 2% manganese, 0% to 3% molybdenum, and 0% to 0.003% vanadium.

21. The material according to claim 1, wherein the matrix comprises, in proportion by weight to the weight of the material, 0 ppm to 500 ppm of the metallic element M, the metallic element M′ and/or the metallic element M″.

22. The material according to claim 1, wherein the metallic element M, M′ or M″ contained in the matrix is selected from yttrium, titanium, tungsten, zirconium, thorium, aluminum, hafnium, silicon, manganese or molybdenum.

23. The material according to claim 1, wherein the material is of austenitic structure.

24. The material according to claim 23, wherein the matrix has the chemical composition of a 316 L or 304 L type steel.

Description

BRIEF DESCRIPTION OF TEE FIGURES

[0209] FIG. 1 represents a table indicating ranges of overall contents by weight for the chemical elements composing the steel powder of the invention as well as, by way of comparison, corresponding contents as defined in the standards ASTM A666 and RCC-MRx.

[0210] FIG. 2 represents a table detailing the content by weight and the atomic content of the chemical elements in the matrix and in the precipitates of a steel powder in accordance with the invention.

[0211] FIG. 3 represents a table indicating the content by weight of the chemical elements in the steel material of the invention, in the matrix, the oxide precipitates, the carbide precipitates and the oxycarbide precipitates.

[0212] and

[0213] FIGS. 4 and 5 represent images obtained by Electron Backscattered Diffraction of a steel material of the invention, respectively in a plane parallel to and in a plane perpendicular to the direction z of additive manufacture of the material.

[0214] FIG. 6 is a pole figure for the steel material of the invention obtained from data collected by Electron Backscattered Diffraction (EBSD).

[0215] FIG. 7 is an image obtained by Scanning Electron Microscopy (SEM) showing the cells present inside the steel material of the invention.

[0216] FIG. 8 is a micrograph obtained by Transmission Electron Microscopy (MET) of a thin slice of the steel material of the invention.

[0217] and

[0218] FIGS. 9 and 10 represent images obtained by Electron Backscattered Diffraction of a steel material which has undergone the treatment process of the invention, respectively in a plane parallel to and a plane perpendicular to the direction z of additive manufacture of the material.

[0219] FIG. 11 is a pole figure for steel material which has undergone the treatment process of the invention, obtained from data collected by EBSD.

[0220] FIG. 12 represents a graph expressing the misorientation angle expressed in degrees as a function of the frequency, expressed as a %, for the steel material of the invention before (“unrefined”) and after having undergone the treatment process of the invention (“HIP”).

[0221] FIG. 13 represents a table indicating the overall content by weight of the chemical elements of a reference steel powder.

[0222] FIG. 14 represents a table indicating the content by weight of the chemical elements composing the matrix and the precipitates of a reference steel powder.

[0223] and

[0224] FIGS. 15 and 16 represent images obtained by Electron Backscattered Diffraction of a reference steel material respectively in a plane parallel to and in a plane perpendicular to the direction z of additive manufacture of the material.

[0225] FIG. 17 represents a table indicating the content by weight of the chemical elements composing a reference steel material.

[0226] FIG. 18 represents a pole figure for a reference steel material obtained from data collected by EBSD.

[0227] FIG. 19 represents un graph indicating, for a reference steel material and for a steel material of the invention (unrefined materials and materials after treatment(s)): on the one hand, up the left hand side ordinate is the average size of the grains, expressed in μm for the largest dimension and the smallest dimension of the grains, and on the other hand, up the right hand side ordinate is the ratio between these two dimensions.

[0228] FIG. 20 represents a table indicating the mechanical properties of the steel material of the invention and of a reference steel material (in a direction parallel to and perpendicular to the direction z of additive manufacture of the material), as well as the anisotropy of these properties.

DESCRIPTION OF PARTICULAR EMBODIMENTS

1. Steel Powder Used in the Manufacturing Process of the Invention

[0229] In general, the steel powder had a composition as shown in FIG. 1, namely a composition by weight encompassing that of a steel complying with the ASTM A666 and RCC-MRx standards (the RCC-MRx standard corresponds to the rules for the design and construction of mechanical equipment for high temperature, experimental and fusion nuclear facilities. It is a technical document for the production of components for Generation IV nuclear reactors).

[0230] 1.1 Characterization of Steel Powder.

[0231] 1.1.1 Chemical Composition.

[0232] A steel powder (316 L steel with reference FE-271-3/TruForm 316-3—batch no. 12-034043-10, sold by Praxair) was analysed by X ray microanalysis, more precisely by Energy Dispersive X-ray Spectroscopy (EDX) using a Scanning Electron Microscope (SEM)), as well as by Glow Discharge Mass Spectrometry (GDMS), by Inductively Coupled Plasma—Optical Emission Spectrometry (ICP-OES) and by Instrumental Gas Analysis (IGA).

[0233] The elemental composition of the matrix and of the precipitates of the steel powder obtained was determined by compiling these various measurements. The proportions obtained for each chemical element are expressed with a relative uncertainty of 3%: [0234] as a % by weight with respect to the total weight of the matrix. However, by convention, the unmeasured chemical elements are subtracted for the matrix. It was then assumed that the remaining percentage by weight was constituted by iron. [0235] as a % by weight and as an atomic % with respect to the total weight of precipitates contained in the steel powder.

[0236] These proportions were normalised by assigning a value of 100 to the total weight or the total number of atoms. They are reproduced in FIG. 2, which shows that the precipitates are rich in oxides of aluminium, titanium, silicon and manganese in the form of a simple oxide and/or mixed oxide. The precipitates may optionally contain carbides or oxycarbides of these chemical elements which, however, would not have been detected by SEM because of their small size.

[0237] 1.1.2 Morphology.

[0238] The steel powder had a 100% austenitic structure, as shown by an Electron Backscattered Diffraction (“EBSD”) analysis.

[0239] The particles of this powder comprise grains agglomerated into particles which are most usually substantially spherical. They have a diameter in the range 10 μm to 100 μm, and an average diameter of 34 μm. More particularly, the median diameters D.sub.10, D.sub.50 and D.sub.90 (for which, respectively, 10%, 50% and 90% of the population of the particles composing this powder had a size which was below the median diameter under consideration) measured by laser granulometry in accordance with the standard ISO 13320 (2009-12-01 edition) were as follows: D.sub.10=22 μm, D.sub.50=32 μm, and D.sub.90=48 μm.

[0240] The precipitates contained in the particles of the powder are most often spherical. Their maximum dimensions (which therefore most often correspond to the diameter of the spherical particle) were such that the size measured by Scanning Electron Microscope (SEM) imaging was generally in the range 24 nm to 120 nm. Their corresponding average size was 63 nm.

[0241] The density with which the precipitates were distributed in the matrix was measured by counting using SEM imaging: it was in the range 2 precipitates/μm.sup.3 to 100 precipitates/μm.sup.3. The corresponding average density was 6 precipitates/μm.sup.3.

[0242] 1.1.3 Properties.

[0243] The apparent density of the steel powder, measured by the standard ASTM B-212, was 4 g/cm.sup.3±0.01 g/cm.sup.3. Its true density, measured by helium pycnometry, was 7.99 g/cm.sup.3±0.03 g/cm.sup.3.

[0244] The Hall flow rate (capacity to make 50 g of powder flow through an orifice of fixed dimensions), measured in accordance with the standard ASTM B213, was more than 40 seconds.

2. Manufacturing Process in Accordance with the Invention

[0245] A part composed of a steel material in accordance with the invention was manufactured by additive manufacture with the Selective Laser Melting (SLM) process using a Trumpf TruPrint Series 1000 model printer.

[0246] In order to manufacture the part, on a substrate of stainless steel, the laser scanned a plurality of 4 mm sided squares in succession, distinguished by a scan direction perpendicular to that of the preceding layer. At the end of this first scan, a first layer n in the form of a checkerboard was obtained. After a second scan carried out on the layer n in which a fresh rotation by 90° of the laser scan direction had been carried out, a new layer n+1 was superimposed on the subjacent layer n.

[0247] The principal operating parameters of the SLM process were as follows: [0248] Yb fibre laser with wavelength of 1.064 nm; [0249] diameter of spot laser=55 μm; [0250] power of laser=150 W; [0251] scan speed of laser=675 mm/s; [0252] distance between two successive laser tracks (“Hatching distance”)=90 μm; [0253] thickness of bed of powder=20 μm; [0254] composition of gaseous medium of the build chamber=argon, with an oxygen content of less than 100 ppm during consolidation.

[0255] Five cylindrical specimens (length=40 mm and diameter=8 cm) built with axes X and Z and five parallelepipedal specimens (10 mm×10 mm×15 mm) were obtained. After manufacture, the parts were removed by cutting specimens from the base in order to separate them from the stainless steel substrate.

[0256] The cylindrical specimens were then machined for the tensile tests. The parallelepipedal specimens were used for all of the composition and microstructure analyses.

[0257] No supplemental treatments were applied to the unrefined material obtained.

[0258] The density of the steel material constituting the specimens was 7.93 g/cm.sup.3 (measured using the Method of Archimedes), i.e. a relative density of 99.25%, assuming that the theoretical density for a 316 L steel is 7.99 g/cm.sup.3.

[0259] This density could be increased by modifying at least one of the following parameters until a relative density of more than 99% was reached without, however, modifying the grain size of the steel material: [0260] power of laser=50 W to 400 W; [0261] scan speed of laser=50 mm/s to 3000 mm/s.

[0262] The density generally varies with the power of the laser or the scan speed of the laser in a parabolic manner. However, too low or too high a power or scan speed could possibly reduce the density.

[0263] The distance between two successive laser tracks (“Hatching distance”) was in the range 30 μm to 90 μm, for example.

3. Characterization of the Steel Material Obtained by the Manufacturing Process in Accordance with the Invention

[0264] 3.1.1 Chemical Composition.

[0265] The overall chemical composition of the steel material obtained by the manufacturing process described in the preceding example complied with the standards ASTM A666 and RCC-MRx indicated in the table of FIG. 1.

[0266] The elemental composition of this alloy was measured by EDX analysis. It was very similar to that of the steel powder used to manufacture the steel material. However, in the steel material, the chemical elements are distributed differently between the matrix and the oxide, carbide or oxycarbide precipitates (namely, for example, a mixture of oxide and carbide): their percentages by weight, measured locally on one or more of these precipitates, are indicated in the table of FIG. 3 with a relative uncertainty of 3%.

[0267] This table shows that the metallic oxides (in the form of a simple oxide and/or mixed oxide) are rich in chromium, iron and nickel in particular, but also in aluminium, titanium, silicon or manganese. The carbides (in the form of simple carbides and/or of mixed carbides) are rich in iron or chromium in particular, and silicon or manganese to a lesser extent. The oxycarbides are in fact rich in chromium, manganese, iron, and silicon, titanium and nickel to a lesser extent.

[0268] 3.1.2 Morphology.

[0269] An analysis by Electron Backscattered Diffraction (“EBSD”) showed that the steel material has a 100% austenitic structure.

[0270] The oxide, carbide and oxycarbide precipitates were incorporated into the matrix of the grains which constituted the steel material or into the spaces between these grains (grain boundaries). The average density with which these precipitates were distributed in the matrix was 6 precipitates/μm.sup.3.

[0271] The average size of the oxide precipitates was in the range 10 nm to 100 nm; that for carbide was in the range 10 nm to 50 nm; that for the oxycarbide precipitates was in the range 10 nm to 100 nm.

[0272] One of the particular features of the material of the invention is a microstructure such that the grains comprising this material are equiaxial in structure. In particular, when the material of the invention is obtained by additive manufacture, its grains may be equiaxial in a plane parallel to the direction of additive manufacture (which in general corresponds to a plane which is substantially perpendicular to the direction of the layers successively obtained during manufacture).

[0273] This particular microstructural feature of the material of the invention is illustrated in FIGS. 4 and 5, which show the equiaxial structure of the grains in a plane respectively parallel to and a plane perpendicular to the direction z of additive manufacture of the steel material. These grains had an average width (Dmin) of 16.2 μm±1.5 μm and an average length (Dmax) of 20.6 μm±1.5 μm, i.e. an average ratio Dmax/Dmin of 1.34±0.13.

[0274] Furthermore, the crystallites which form the grains of the steel material have a preferential orientation. As illustrated in FIG. 6, this texture of the material is reflected by the fact that the (110) directions are preferentially orientated parallel to the build direction Z, but also by a texture intensity equal to 1.9.

[0275] As illustrated in FIG. 7, the grains of steel material are themselves constituted by cells which are nanometric in scale (more particularly a size of less than the average diameter of 500 nm).

[0276] FIG. 8 also illustrates this cellular structure while also highlighting the precipitates incorporated into the matrix, which itself is paler in colour.

[0277] 3.1.3 Properties.

[0278] The mechanical properties of the steel material are as follows: [0279] Vickers HV1 microhardness (1 kg for 10 seconds): [0280] 202±3 HV1 in the plane parallel to the direction Z of manufacture and 202±3 HV1 in the plane perpendicular to the direction Z of manufacture; [0281] tensile tests at ambient temperature (25° C.) and irrespective of the direction of measurement with reference to the direction Z of manufacture: [0282] Rm (maximum tensile strength)=645±30 MPa; [0283] Rp0.2 (yield strength)=453±35 MPa; [0284] A (elongation at break)=54±9%.

[0285] Thus, advantageously, the steel material obtained directly from the manufacturing process of the invention (namely the unrefined material which has not undergone any supplemental treatment such as a heat treatment, for example) has optimised mechanical properties which are, furthermore, homogeneous in all directions (isotropic nature of these properties).

4. Treatment Process in Accordance with the Invention

[0286] The steel material manufactured in accordance with the preceding example underwent the treatment process of the invention comprising a step for hot isostatic pressing (HIP).

[0287] This HIP consisted of heating the material, under an atmosphere of argon at a pressure of 1800 bar, from ambient temperature (25° C.) to a constant temperature of 1100° C. which was maintained for 3 hours, and then returning it to ambient temperature. The temperature ramp-up or ramp-down rate was 800° C./hour.

[0288] As an alternative, the constant temperature could have been in the range 600° C. to 1400° C.

5. Characterization of Steel Material Obtained by the Treatment Process in Accordance with the Invention

[0289] 5.1.1 Morphology.

[0290] The treated material no longer had a cellular structure.

[0291] In contrast, phase mapping by Electron Backscattered Diffraction (“EBSD”) showed that the treated material still had a 100% austenitic structure, as well as equiaxial grains, as illustrated in FIGS. 9 and 10, which show the equiaxial structure of the grains in a plane respectively parallel to and perpendicular to the direction z of additive manufacture of the steel material undergoing the treatment process of the invention.

[0292] These grains had an average width of 15.9 μm±2.2 μm and an average length of 21.5 μm±1.2 μm, i.e. an average ratio Dmax/Dmin of 1.36±0.16.

[0293] As illustrated in FIG. 11, the treated material was still structured, since the (110) directions are preferentially orientated parallel to the build direction Z.

[0294] The misorientation angle between two grain boundaries was measured using the pole figure technique. In theory, after a treatment by hot isostatic pressing, the grains of a steel material are assumed to increase in size and/or in number. This change then modifies the distribution of the misorientation angles.

[0295] However, FIG. 12 shows that the misorientation angle between the grains did not vary significantly after the steel material had been treated by the treatment process of the invention. This provides evidence of an absence of modification to the size of the grains.

[0296] Apart from the cellular structure, these various data show that the morphology of the steel material is advantageously stable (in particular the variation in the size of the grains is minor), even after having undergone the treatment process of the invention which, however, combines the application of variations in temperature and in pressure.

[0297] This stability in the morphology of the steel material of the invention is advantageous, for example when its creep behaviour is considered when a pressure and/or temperature stress is applied to the material.

[0298] 5.1.2 Properties.

[0299] The mechanical properties of the steel material which has undergone the treatment process of the invention are as follows: [0300] Vickers HV1 microhardness (1 kg for 10 seconds): 175±1.5 HV1 in the plane parallel to the direction Z of manufacture and 172±2 HV1 in the plane perpendicular to the direction Z of manufacture: the microhardness with respect to the untreated material was thus reduced, but was advantageously isotropic; [0301] tensile tests at ambient temperature (25° C.) and irrespective of the direction of measurement with respect to the direction Z of manufacture: [0302] Rm (maximum tensile strength)=622±22 MPa; [0303] Rp0.2 (yield strength)=336±8 MPa; [0304] A (elongation at break)=76±4%.

[0305] These various mechanical properties are advantageously isotropic, namely irrespective of the direction in which they are measured. The isotropic tensile behaviour of the steel material of the invention is relatively stable over time.

[0306] The density of the treated steel material constituting specimens with a geometry similar to those of the unrefined steel material was 7.94 g/cm.sup.3±0.05 g/cm.sup.3 (measured using the Method of Archimedes), i.e. a relative density of 99.4%±0.06%, assuming that the theoretical density for a 316 L steel is 7.99 g/cm.sup.3.

[0307] The density of the steel material which had undergone the treatment process of the invention was thus almost identical to that of the corresponding unrefined material.

6. Comparative Example

[0308] By way of comparison, a reference steel powder (Stainless Steel 316L-A LMF powder—batch no. 201600024, sold by Trumpf) underwent an additive manufacturing process in accordance with the operating parameters of Example 2, in order to obtain a solid reference steel material.

[0309] The chemical composition of this powder was determined by energy dispersive X-ray spectroscopy (known as “EDX”) carried out by SEM, GDMS, ICP-OES and IGA; the compiled measurements are shown in FIG. 13 and FIG. 14. The relative uncertainty in the measurement was 3%.

[0310] The particles of this reference powder were essentially spherical. Their diameter was in the range 10 μm to 100 μm with an average diameter of 30 μm. Its median diameters D.sub.10, D.sub.50 and D.sub.90, measured by laser granulometry in accordance with the standard ISO 13320 (2009-12-01 edition), were as follows: D.sub.10=21 μm, D.sub.50=28 μm, and D.sub.90=39 μm.

[0311] The apparent density, measured by the standard ASTM B-212, was 4.39 g/cm.sup.3±0.01 g/cm.sup.3. The true density, measured by helium pycnometry, was 7.99 g/cm.sup.3±0.03 g/cm.sup.3.

[0312] The Hall flow rate (capacity to make 50 g of powder flow through an orifice of fixed dimensions), measured in accordance with the standard ASTM B213, was 16 seconds. A steel powder used in the manufacturing process of the invention has, for example, a Hall flow rate in the range 30 seconds to 500 seconds.

[0313] The reference solid material obtained had a chemical composition as indicated in FIG. 17 as a percentage by weight in the matrix and in two different metallic oxide precipitates.

[0314] Regarding its microstructure, the reference solid material had a cellular structure.

[0315] The grains constituting these cells comprised a matrix into which the precipitates had been incorporated. The corresponding structure is represented on the images of FIGS. 15 and 16, which show that the axial growth of the reference steel material produced a columnar and therefore anisotropic structure for the grains in a plane parallel to the direction z of additive manufacture of the material. These grains had an average width (Dmin) of 20.8 μm±2.7 μm and an average length (Dmax) of 68.6 μm±8.3 μm, i.e. an average ratio Dmax/Dmin of 3.2±0.1.

[0316] This microstructural characteristic is illustrated by FIG. 18, which shows that the reference steel material has a texture in the plane (110) parallel to the build direction Z resulting in a strong texture intensity equal to 5.4.

[0317] In order to compare the properties linked to the microstructure of the reference steel material with respect to the steel material of the invention, various treatments were applied: [0318] the reference steel material underwent a hot isostatic pressing treatment (denoted “HIP”) identical to that of Example 4; [0319] the reference steel material and the steel material of the invention underwent a heat treatment (denoted “TT”) consisting of a treatment in which each unrefined build material was maintained at a temperature of 700° C. for 1 hour (known as “stress relieving” treatment).

[0320] At the end of these treatments, the average grain size was measured by the intercept method for the reference steel material and the steel material of the invention as obtained at the end of: [0321] i) the additive manufacturing process (unrefined build), or [0322] ii) the additive manufacturing process followed by the stress relieving heat treatment (“TT”), or [0323] iii) the additive manufacturing process followed by the hot isostatic pressing treatment (“HIP”).

[0324] This average size was measured for the largest dimension “Dmax” (length) or the smallest dimension “Dmin” (width) of the grains constituting each material. The corresponding ratio Dmax/Dmin was calculated in order to evaluate the equiaxial nature of the grains of each material.

[0325] The results are presented in FIG. 19, which shows that the reference steel material (denoted “Trumpf”) underwent modifications in the geometry of its grains after the heat treatment, but especially after hot isostatic pressing. The distribution of the misorientation angles was thus modified for the reference steel material.

[0326] In contrast, the microstructure of the steel material of the invention (denoted “Material”) was extremely stable, because no substantial modifications to the geometry of the grains were observed (in particular, the grain size remained small, namely, for example, less than or equal to 50 μm for L and l), which provides the steel material of the invention with great robustness and mechanical and thermal stability.

[0327] The mechanical properties of the “Praxair” steel material of the invention (new measures taken) and of the “Trumpf” reference steel material were measured before and after having undergone the treatment of the invention comprising a step for hot isostatic pressing (HIP). The resulting table of FIG. 20 summarises the values obtained for the parameters Rm (maximum tensile strength, expressed in MPa), Rp0.2 (yield strength, expressed in MPa), A (elongation at break, as a %), both in a direction parallel (//Z)) to and perpendicular (⊥Z)) to the direction z of additive manufacture of the two materials. At the same time, the percentage anisotropy for each of these parameters is indicated: the smaller this percentage is, the more isotropic are the mechanical properties, namely that their value is homogeneous irrespective of the direction of measurement.

[0328] The analysis of FIG. 19 and FIG. 20 shows that: [0329] before the HIP treatment, the mechanical anisotropy (Rm, R p0.2 and A) was lower for the steel material of the invention (Praxair) than for the reference steel material. [0330] After the HIP treatment, the mechanical anisotropy was greatly reduced for the two materials (Praxair and Trumpf). However, only the steel material of the invention did not suffer an increase in the average size of these grains by the phenomenon of grain coalescence. This size remained stable and below 30 μm despite the HIP treatment.

[0331] In conclusion, the steel material of the invention obtained after the HIP treatment of the invention has equiaxial grains, isotropic mechanical properties, a fine microstructure (the average grain size is generally 50 μm or less, for example an average size “Dmax” and/or “Dmin” in the range 10 μm to 50 μm, or even in the range 10 μm to 30 μm) and precipitates which are generally nanometric in size (generally in the range 10 nm to 100 nm).

[0332] Clearly, the present invention is not in any way limited to the embodiments described and shown, and the person skilled in the art will be able to combine them and use their general knowledge to make many variations and modifications.