Sintering process for austenitic stainless steel

Abstract

The present invention relates to a method for manufacturing a part in austenitic stainless steel comprising successively the following steps: 1) Provision of a powder and sintering of said powder to form a sintered alloy with an austenitic structure; said alloy having a nitrogen content greater than or equal to 0.1% by weight, 2) Treatment of the sintered alloy to transform the austenitic structure into a ferritic or two-phase ferrite + austenite structure on a layer (2) on the surface of the alloy, 3) Treatment of the sintered alloy to transform the ferritic or two-phase ferrite + austenite structure obtained in step 2) into an austenitic structure and to form after cooling the part (1) having on the layer (2) subjected to the transformations of steps 2) and 3) a density greater than that of the core (3) of the part (1).

Description

Object of the invention

[0001] The present invention relates to a sintering process for an austenitic stainless steel comprising a significant nitrogen concentration (≥ 0.1% by weight). The present invention also relates to the product obtained from the process, which is characterized by a very dense surface layer. Technological background

[0002] The sintering of austenitic stainless steel powders is now very widespread. It can be carried out on parts produced by injection molding (Metal Injection Molding), extrusion, pressing, or additive manufacturing. In the most traditional method, the sintering of austenitic stainless steels consists of consolidating and densifying the powder in a high-temperature furnace (1000–1400 °C), under vacuum or gas protection. The properties of the parts after sintering (density, mechanical and magnetic properties, corrosion resistance, etc.), for a given composition, depend strongly on the sintering cycle used. The following parameters are particularly important: heating rate, sintering temperature and time, sintering atmosphere (gas, gas flow, pressure), and cooling rate.

[0003] After sintering austenitic stainless steels, some residual porosity generally remains. When this porosity is limited (< 5%), it has little influence on the mechanical, thermal, or magnetic properties. However, this small amount of residual porosity is detrimental aesthetically, particularly when the parts are polished. They then exhibit a milky appearance, a lower luster, and a different color compared to completely solid parts. For applications where aesthetics are important, it is therefore necessary to find solutions to limit or even eliminate this residual porosity, at least on the surface of the parts.

[0004] To improve the density of sintered parts, several liquid-phase sintering methods can be used, for example: Sintering by adding one or more elements forming a liquid phase at the sintering temperature: the liquid phase thus provides a much faster diffusion path than in the solid and therefore results in better densification. In austenitic stainless steels, a small amount of boron can, for example, be added. However, after such sintering with a liquid phase, the microstructure and composition of the alloy are not homogeneous, which leads to problems with corrosion resistance as well as polishing. Supersolidus liquid phase sintering (SLPS): this involves carrying out the sintering at a temperature between the solidus and liquidus of the alloy, so as to have a fraction of the alloy in the liquid phase.However, for stainless steels, this technique is of little use because the temperature window between the solidus and liquidus is too narrow to control the process. Furthermore, this type of sintering generates significant grain growth, which is again detrimental to polishing.

[0005] In summary, the various liquid phase sintering methods are not suitable when polishability and corrosion resistance are paramount, as in the watchmaking and jewelry sectors for example.

[0006] As an alternative, for solid-phase sintered parts exhibiting closed porosity, i.e., a density greater than 90% after sintering, it is possible to perform a hot isostatic pressing (HIP) treatment to eliminate any residual porosity. This solution works well, but this technology is very expensive. Furthermore, austenitic stainless steel parts that have undergone hot isostatic pressing are oxidized after treatment, which generally necessitates machining or polishing all surfaces of the parts after treatment. Summary of the invention

[0007] The main object of the present invention is to propose a new solid-state sintering process that makes it possible to obtain very high surface densities of parts without having to resort to high-pressure processing (HIP). The process is specifically adapted for manufacturing high-nitrogen austenitic stainless steels, making it possible, where applicable, to reduce or even eliminate the use of nickel, which is known for its allergenic properties.

[0008] To this end, the present invention proposes a three-step process, with a first conventional step consisting of sintering the powder in the austenitic region. In a second step, the alloy from the first step is treated in the ferritic region or in the two-phase ferrite + austenite region to form a surface layer containing ferrite. The latter, thanks to its less compact crystallographic structure compared to that of austenite, allows for faster diffusion of the alloying elements and, consequently, better densification. In a third step, the alloy is treated in the austenitic region to remove the previously formed surface ferrite.

[0009] Advantageously, the second step is carried out by manipulating the temperature and / or controlling the atmosphere to denitrify and / or decarburize the surface. Conversely, in the third step, the surface is carburized and / or nitrided to promote the formation of austenite.

[0010] Advantageously, the total holding time for the second and third stages is kept below 20 hours to gain productivity and avoid excessive growth of austenitic grain that is detrimental to polishing.

[0011] The process according to the invention thus makes it possible to obtain austenitic stainless steel parts that are completely dense on the surface, without the presence of residual ferrite, with a limited grain size, and with reasonable cycle times. After polishing, the luster and color of these parts are comparable to those of parts obtained by conventional metallurgy (casting + thermomechanical treatments) or having undergone a hot isostatic pressing (HIP) densification treatment.

[0012] Other advantages will become apparent from the features expressed in the claims, from the detailed description of the invention illustrated below with the aid of the accompanying drawings given by way of non-limiting examples. Brief description of the figures

[0013] <tb>Figures 1A and 1B <sep>illustrate, using a phase diagram for a stainless steel composition, the phase changes induced by the three stages of the sintering process according to the invention, with Fig. 1A showing the phase changes on the surface of the part and Fig. 1B showing the phase changes in the core of the part. <tb>Fig. 2 <sep>schematically represents a part of the part produced by the process according to the invention having on the surface a layer of density greater than that of the core. <tb>Figures 3 and 4 <sep>represent a cross-sectional micrograph of a sample prepared using the method of the invention. More specifically, Fig. 3 shows an enlargement at the junction between the dense surface layer and the porous core, and Fig. 4 shows a cross-sectional view of the entire sample. <tb>Fig. 5 <sep>represents a cross-sectional view similar to that of Fig. 3 for a sample produced according to the process of the invention with parameters adapted to increase the thickness of the dense layer. Detailed description of the invention

[0014] The present invention relates to a new sintering process for austenitic stainless steel. It also relates to the part produced by the process, which may, in particular, be a watch case component or a piece of jewelry.

[0015] The process according to the invention is applicable to all austenitic stainless alloys comprising a significant concentration of nitrogen or nitrogen and carbon, and for which ferrite can be formed during sintering. A significant concentration is defined as a nitrogen concentration greater than or equal to 0.1% by weight, optionally with a carbon concentration also greater than or equal to 0.1% by weight. This concentration of N or N + C may be present in the initial powder or result from the enrichment of the alloy in N and C during sintering under a controlled atmosphere. The initial powder is therefore not necessarily 100% austenitic but may contain ferrite.

[0016] Among the most common alloys, this includes grades 316LN (1.4406, 1.4429) and 317LN (1.4434), but also grades 1.4466, 1.4537, 1.4547, 1.4652, 1.4659, 1.4529 and 1.4565. In addition, all austenitic stainless steels in which the nickel is completely (Ni ≤ 0.5%) or partially compensated by nitrogen or by nitrogen and carbon can also be sintered according to the invention, such as, for example, grades 1.3816, 1.3815, 1.4456, 1.4452 or 1.3808 (UNS S29 225). In addition to these alloys, there are all stainless steels for which nitrogen or nitrogen and carbon would be added to the powder, before or during sintering.

[0017] The process according to the invention makes it possible to form a stainless steel part without residual ferrite having a surface layer with a density greater than that of the core of the part. This layer preferably has a thickness greater than or equal to 20 µm, more preferably greater than or equal to 30 µm, and even more preferably greater than or equal to 50 µm. Qualitatively, this difference in density between the surface layer and the core can be easily visualized by optical microscopy on a cross-section of the sample, where a clear transition between the very dense layer and the porous core is observed. Schematically, this transition on a cross-section of the part 1 is shown in Fig. 2, where the core 3, comprising pores 4, j, is seen, surmounted by a layer 2 devoid or virtually devoid of pores. Quantitatively, the pore volume of the core and the surface can be determined by optical microscopy image analysis of polished sections of the sample.Preferably, the surface has a relative density between 99% and 100% (≥ 99%). The porous core, on the other hand, has a relative density greater than 90% and less than 99% (< 99%). Furthermore, the absence of ferrite in the core and on the surface of the part can be confirmed by optical microscopy, X-ray diffraction, or magnetic property measurement techniques.

[0018] The sintering process according to the invention comprises the following steps carried out under a controlled atmosphere in a temperature range between 900 and 1400 °C selected according to the composition of the steel: <tb> 1) <sep>The first stage involves sintering the powder in a 100% austenitic environment under an atmosphere containing a nitrogen carrier gas (e.g., N2). The powder typically has a diameter D90 between 5 and 100 µm. During this stage, the nitrogen concentration in the alloy is fixed while simultaneously densifying the parts until open porosity is eliminated (density > 90%). The nitrogen concentration for a given alloy depends on the temperature and the partial pressure of the nitrogen carrier gas (≥ 0.1 bar). The carbon concentration in the alloy depends on the initial carbon concentration in the powder, any residues from organic binders used in powder preparation, and the reactions between the carbon and the atmosphere during sintering (carbon reduction by oxygen, decarburization, etc.).Thus, other gases can be used in addition to the nitrogen carrier gas for oxide reduction (e.g., H2), to adjust the carbon concentration (e.g., CO, CH4), or to adjust the total pressure (e.g., Ar). Generally, this single sintering step in the austenitic range is used in the prior art for sintering austenitic stainless steels. <tb> 2) <sep>A second step aims to densify the parts, at least on the surface, by forming ferrite from the surface. Indeed, since the diffusion of alloying elements in the body-centered cubic structure of ferrite is approximately two orders of magnitude greater than the diffusion of elements in the face-centered cubic structure of austenite, densification is much more significant in the presence of ferrite. Several solutions are possible for forming ferrite on the surface of the parts: <tb> <sep> A. <sep>The temperature should be set so that the alloy exhibits a two-phase ferrite-austenite structure or is entirely ferritic. At the surface, nitrogen and carbon, which stabilize the austenitic phase, can be released into the atmosphere by diffusion through the solid, and ferrite formation is facilitated, as the solubility of carbon and nitrogen is much lower in ferrite. In the core, where the nitrogen and carbon concentration has not been reduced by diffusion through the surface, the alloy composition remains unchanged because the porosity was closed in the first step. Preferably, the temperature should be set to obtain a two-phase ferrite-austenite or entirely ferritic structure at the surface and a completely austenitic structure at the core. However, depending on the alloy and the parameters used in these first two sintering steps, it is possible that some ferrite may also form at the core during this step. <tb> <sep> B. <sep>The partial pressure of the nitrogen-carrying gas is fixed, or even the process is carried out in a nitrogen-free atmosphere, in order to reduce the amount of nitrogen on the surface of the parts by denitriding and thus form an austenite + ferrite or a completely ferritic surface structure. In the core, where the nitrogen concentration has not been reduced by diffusion through the surface, the alloy composition remains unchanged and the structure remains entirely austenitic. <tb> <sep> C. <sep>The partial pressure of the carbon-carrying gas, such as CO or CH4, is fixed to reduce the amount of carbon on the surface of the parts by decarburization. Alternatively, a decarburizing atmosphere, such as H2, can be used if the alloy already contains carbon. Again, the atmosphere must be selected so that the alloy exhibits a two-phase austenite + ferrite structure or is completely ferritic at equilibrium. In the core, where the carbon concentration has not been reduced by diffusion through the surface, the alloy composition remains unchanged and the structure remains entirely austenitic. <tb> <sep> D. <sep>Use any combination of solutions A, B and C.

[0019] In summary, during this holding period, the goal is to form ferrite on the surface of the parts to obtain a very dense layer. Since this ferrite forms primarily through denitrification and / or decarburization, which are diffusion phenomena within the solid, the thickness of this densified ferrite-containing layer, for a given composition, depends on the temperature, the duration of the holding period, and the partial pressures of the nitrogen and / or carbon-carrying gases. At the core, where the nitrogen and carbon concentration has not been reduced by diffusion through the surface, the composition and therefore the structure remains unchanged because the porosity was closed during the first step. However, if the temperature differs between the first and second steps, it is possible that some ferrite may also form at the core, even though the composition remains unchanged. <tb> 3) <sep>A third step is required to remove the surface-formed ferrite. This removal is achieved primarily by renitriding and / or recarburizing the parts, carefully selecting the temperature, the partial pressure of the nitrogen-carrying gas (≥ 0.1 bar), and, if applicable, the partial pressure of the carbon-carrying gas. These parameters must be set so that the alloy exhibits a completely austenitic structure at equilibrium. Since nitriding and carburizing are diffusion processes, the thickness of the diffusion layer depends on time and temperature for a given alloy and atmosphere. Therefore, the parameters of this step are dependent on the previous step and must be set to transform all the ferrite into austenite. Indeed, the presence of ferrite in the parts is undesirable, as it reduces corrosion resistance and exhibits ferromagnetic behavior.The third step is conventionally followed by cooling with a cooling rate adapted according to the composition to maintain an austenitic structure at room temperature.

[0020] Ideally, these different steps are carried out during the same sintering cycle. However, they can also be carried out separately. The sintering principle presented can also consist of post-treatment on parts sintered differently (laser sintering, spark plasma sintering, etc.).

[0021] This sintering process has been successfully applied to several different powders. In particular, it is applied to nickel-free austenitic stainless steel parts shaped by metal injection molding.

[0022] By way of example, the three-step sintering process according to the invention is explained below using a phase diagram. For a given alloy, this diagram makes it possible to predict the existing phases as a function of temperature. For austenitic stainless steels, it is also useful to observe the influence of the nitrogen concentration, as it can be adjusted during sintering, particularly via temperature and nitrogen partial pressure. Such a phase diagram for Fe-17.5Cr-11Mn-3.5Mo-xN stainless steel is shown in Fig. 1A. The area corresponding to a 100% austenitic alloy is labeled FCC_A1. Ferrite corresponds to phase BCC_A2 and appears when the nitrogen concentration is too low at a given temperature or when the temperature is too high for a given nitrogen partial pressure.Finally, the influence of the nitrogen partial pressure on the nitrogen concentration in the alloy is illustrated by isobaric curves for nitrogen partial pressures of 100, 400, and 900 mbar. For this Fe-17.5Cr-11Mn-3.5Mo powder, the carbon concentration is low (< 0.1%), and sintering is carried out in an atmosphere containing a nitrogen partial pressure (N2). The process according to the invention makes it possible to obtain a very high surface density of the parts thanks to steps 1, 2, and 3, illustrated on the phase diagram using circles for the core and surface of the part, respectively, in Figs. 1B and 1A. In this example, the nitrogen partial pressure is maintained at 400 mbar in all three steps, and the phase change at the surface is achieved by changing the temperature. The three steps are as follows: <tb> 1) <sep>Nitriding of the powder and densification until open porosity is eliminated in the 100% austenitic range. Temperature and nitrogen partial pressure are used to adjust the nitrogen concentration in the alloy. The time required for proper nitriding of the powder depends on the powder size. However, for particles with a D90 less than 100 microns and at temperatures above 1000 °C, the nitriding rate is relatively fast, i.e., less than one hour. Densification until pore closure, on the other hand, is longer and generally requires several hours. Thus, the sample with a thickness of approximately 10 mm was maintained at a temperature of 1150 °C for 3 hours to reach equilibrium a nitrogen content in the alloy of 0.75% by weight (circle 1 in Figs. 1A and 1B) while densifying the sample until the open porosity was eliminated. <tb> 2) <sep>The temperature is increased to 1220 °C to obtain, at thermodynamic equilibrium, a two-phase ferrite + austenite structure, with a plateau of 1 h at this temperature. At the surface, nitrogen is released into the atmosphere and ferrite formation is rapid (circle 2 in Fig. 1A). For the core, where the nitrogen concentration has not decreased, the structure remains austenitic despite the temperature increase (circle 2 in Fig. 1B). <tb> 3) <sep>Removal of surface ferrite by renitriding at a lower temperature of 1100 °C with a holding time of 2 h (circle 3 in figs. 1A and 1B). The temperature and holding time are set to completely remove the surface ferrite.

[0023] With this process, parts with no residual ferrite and very dense surface are finally obtained, as shown in the micrographs in Figs. 3 and 4, with a dense layer having a thickness of 100–150 µm. By extending the treatment time in steps 2) and 3) to 4 and 8 h respectively, all other things being equal, the thickness of the layer is increased up to 400–450 µm (Fig. 5).

[0024] It should be noted that by further extending the holding times, it would be possible in steps 2) and 3) respectively to form ferrite over the entire part and then to nitride or carbonitride the whole thing to ultimately form a 100% austenitic part with a relative density greater than or equal to 99% over its entire thickness. However, for parts such as watch case components, which typically have thicknesses greater than 1.5 millimeters, this would lead to very long cycle times of more than 20 hours, resulting in excessive grain growth that is very detrimental to polishing (orange peel effect).

[0025] Thus, preferably, the surface layer has a thickness of less than 0.75 mm and more preferably less than 0.5 mm and the austenitic grain has an average size of less than 300 µm, preferably less than 100 µm, and, more preferably less than 50 µm.< / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / sep> < / tb> < / sep> < / sep> < / tb> < / sep> < / sep> < / tb> < / sep> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb>

Claims

1. A process for manufacturing a part (1) of austenitic stainless steel having a density difference between its core (3) and its surface, the process comprising the following steps carried out under a controlled atmosphere: 1) Making available a sintered alloy with an austenitic structure or making available a powder and sintering the powder to form said sintered alloy, the sintered alloy having a nitrogen content greater than or equal to 0.1% by weight with optionally a carbon content greater than or equal to 0.1% by weight; 2) Treating the sintered alloy to transform said austenitic structure into a ferritic or two-phase ferrite + austenite structure on a layer (2) on the surface of the alloy;3) Treatment of the sintered alloy to transform the ferritic or two-phase ferrite + austenite structure obtained in step 2) into said austenitic structure and, after cooling, form the part (1) having on the layer (2) subjected to the transformations of steps 2) and 3) a density greater than that of the core (3) of the part (1).; 2. A process according to claim 1, characterized in that steps 2) and 3) are carried out by performing one or more of the following actions aimed, for step 2) at treating the sintered alloy of step 1) in the ferritic or two-phase ferrite + austenite range and for step 3) at treating the sintered alloy of step 2) in the austenitic range: adjusting the temperature, adjusting the partial pressure of a nitrogen carrier gas from the atmosphere, adjusting the partial pressure of a carbon carrier gas from the atmosphere.

3. A method according to any one of the preceding claims, characterized in that the cumulative holding time for steps 2) and 3) is less than 20 hours.

4. A process according to any one of claims 2 to 3, characterized in that the nitrogen content greater than or equal to 0.1% by weight is fixed during step 1) of sintering by controlling the temperature and partial pressure of the nitrogen carrier gas from the atmosphere.

5. A method according to any one of the preceding claims, characterized in that step 1) of sintering the powder and steps 2) and 3) are carried out continuously during the same cycle.

6. A method according to any one of claims 1 to 4, characterized in that steps 2) and 3) are carried out during separate cycles.

7. A process according to any one of the preceding claims, characterized in that the sintered alloy of step 1) has been previously shaped by injection, extrusion, pressing or additive manufacturing.

8. A process according to any one of the preceding claims, characterized in that the sintered alloy of step 1) was obtained by SPS flash sintering or by laser sintering.

9. Part (1) of austenitic stainless steel having a nitrogen content greater than or equal to 0.1% by weight, characterized in that the part (1) has on the surface a layer (2) having a density greater than that of the core (3) of the part (1).

10. Part (1) according to claim 9, characterized in that the layer (2) has a minimum thickness of 20 µm, preferably of 30 µm and, more preferably, of 50 µm.

11. Part (1) according to claim 9 or 10, characterized in that the layer (2) has a thickness of less than 0.75 mm and, preferably, 0.5 mm.

12. Part (1) any one of claims 9 to 11, characterized in that the layer (2) has a density between 99 and 100%.

13. Part (1) according to any one of claims 9 to 12, characterized in that the austenitic stainless steel has a combined carbon and nitrogen content greater than or equal to 0.2% by weight.

14. Part (1) according to any one of claims 9 to 13, characterized in that the austenitic stainless steel has a nickel content less than or equal to 0.5% by weight.

15. Part (1) according to any one of claims 9 to 14, characterized in that the austenitic stainless steel has an average grain size of less than 300 µm, preferably 100 µm, and more preferably 50 µm.

16. Part (1) according to any one of claims 9 to 15, characterized in that it is a watch or jewelry part.

17. Watch or jewelry comprising part (1) according to any one of claims 9 to 16.