CH715564B1 - Composition of austenitic stainless steel powder without nickel and part produced by sintering of this powder. - Google Patents

Abstract

The present invention relates to an austenitic stainless steel powder having a nickel content of less than or equal to 0.5% by weight and a specific carbon content which is greater than or equal to 0.05% and less than or equal to 0.11% by weight. It also relates to the method of manufacturing by powder metallurgy of this powder and to the part (3) resulting from the manufacturing method having the characteristic of having a deoxidized layer (1) on the surface of the part (3) extending over a thickness e greater than or equal to 200 μm.

Description

Field of the invention

The invention relates to a nickel-free austenitic stainless steel powder composition. It also relates to a part, in particular a cladding part in watchmaking, produced by sintering using this powder as well as to the sintering manufacturing process.

Invention background

[0002] Sintering of stainless steel powders is very widespread today. It can in particular be produced on blanks obtained by injection (Metal Injection Molding), extrusion, pressing or other additive manufacturing. In the most traditional way, the sintering of austenitic stainless steels consists in consolidating and densifying a powder of such steel in an oven at high temperature (1200-1400 ° C), under vacuum or under a gaseous protective atmosphere. The properties of the parts after sintering (density, mechanical and magnetic properties, corrosion resistance, etc.) for a given powder composition strongly depend on the sintering cycle used. The following parameters are particularly important: heating speed, sintering temperature and time, sintering atmosphere (gas, gas flow, pressure) and cooling speed.

For fields like watchmaking where aesthetics are particularly important, two characteristics of the microstructure after sintering are particularly important, namely the density and the presence of inclusions. The presence of non-metallic porosities or inclusions, in particular oxides, is indeed very harmful for the rendering after polishing. To obtain a polished part having a shine and a color similar to wrought stainless steels, it is therefore necessary to aim for a density close to 100% and a minimum of non-metallic inclusions.

With regard to density, solutions are known to eliminate any porosity, at least on the surface, in parts made of austenitic stainless steels without nickel formed by powder metallurgy. These solutions consist, among other things, of performing hot isostatic compression (HIP for Hot Isostatic Pressing) on sintered parts whose porosity is closed.

Regarding oxides, it is mainly the high manganese concentration in austenitic stainless steels without nickel which is problematic for shaping by powder metallurgy. Indeed, the great affinity of this element with oxygen, coupled with the high specific surface of the powders, requires a great mastery of the processes which are carried out at high temperature: it is first of all necessary to use a powder whose particle surface is the least oxidized possible; then the oxidation of the powder must also be minimized during the heating of the parts up to the sintering temperature by using a very reducing atmosphere. However, for austenitic stainless steels in the composition of which nitrogen is introduced in order to dispense with the use of nickel, the sintering atmosphere must necessarily contain nitrogen, with the consequence that it does not is not possible to work with an atmosphere containing only reducing agents; finally, it is necessary to reduce the most stable oxides at high temperature and to eliminate the reduction products before the porosity is closed. This is the most critical point because, during the sintering operations, the reduction of the manganese oxides is carried out at the same time as the closing of the pores, at a temperature generally above 1200 ° C. Consequently, parts are typically obtained with a well-deoxidized layer on the surface and numerous inclusions (oxides) at the core. Indeed, on the surface of the parts, the atmosphere of the oven is more easily renewed and the reduction products can be transported throughout the volume of the oven when, at the start of sintering, the porosity is still open. On the other hand, the atmosphere is not renewed and the conditions do not allow a total reduction of the oxides before the porosity closes. For nickel-free austenitic stainless steels, it is therefore very difficult to have a deoxidized layer whose thickness is greater than 200 μm. As the termination of the parts (machining, polishing) after sintering requires removal of material which may be greater than 200 μm, oxides may be present on the surface of the finished parts, which is harmful both from an aesthetic point of view and from the point of view of corrosion resistance for polished surfaces.

There is therefore a need to increase the thickness of this deoxidized layer for the parts formed by sintering austenitic stainless steel powders without nickel.

Summary of the invention

The present invention aims to provide a composition of austenitic stainless steel powder without nickel to obtain, after sintering, a particularly deep deoxidized layer, namely greater than or equal to 200 microns.

By deoxidized layer is meant a layer having finely dispersed small oxides. Preferably, the diameter of the oxides is less than 2 μm and the surface fraction of these oxides is less than 0.1% in this layer. These finely dispersed oxides have no impact on the aesthetic level after polishing. Outside this deoxidized layer, the diameter of the oxides can typically reach 5 μm and the surface fraction of the oxides can range up to 1%.

To obtain this deoxidized layer of greater thickness, it is necessary to select an austenitic stainless steel powder without nickel having a carbon mass concentration greater than or equal to 0.05% and less than or equal to 0.11%. Carbon in fact allows a reduction in the most stable oxides, particularly the manganese oxides and the mixed oxides of manganese and silicon, at a temperature which can be less than or equal to 1200 ° C. Given that below 1200 ° C, the densification is low and the porosity remains open, the presence of carbon makes it possible to deoxidize more deeply, the elimination of reduction products and the renewal of the atmosphere inside of the room being favored. This reduction of oxides by carbon at high temperature is commonly called carbothermal reduction and responds, for example for a manganese oxide, to the following reaction: MnO + C → Mn + CO.

It has been shown during several sintering tests carried out on austenitic stainless steel powders without nickel that the carbon concentration must be chosen in the specific range 0.05-0.11% by weight. This optimal concentration makes it possible to obtain the thickest deoxidized layer possible, while avoiding problematic decarburization of the parts. In fact, when the mass concentration of carbon is less than 0.05% by weight, the carbothermal reduction is not complete and the deoxidized layer is reduced. Conversely, too high a carbon concentration is problematic because the decarburization resulting from the reaction with the hydrogen present in the sintering atmosphere cannot be controlled and large variations in carbon concentration between the parts are observed. With mass concentrations of carbon in the nickel-free austenitic stainless steel powder of between 0.05 and 0.11%, the possible variations in carbon concentration are sufficiently small not to influence the microstructure and the mechanical and physical properties of the parts after sintering.

Brief description of the figures

Other features and advantages of the present invention will appear on reading the detailed description below referring to the following figures. FIG. 1 schematically illustrates the transition between the deoxidized layer and the rest of the part according to the invention, and FIGS. 2A and 2B respectively represent a metallographic section of a part according to the prior art and of a part according to the invention.

Description of the invention

The invention relates to an austenitic stainless steel powder and, more specifically, an austenitic stainless steel powder comprising nitrogen in order to reduce or even get rid of nickel known for its allergenic nature. According to the invention, this austenitic stainless steel powder has a specific carbon content chosen to optimize the carbothermic reaction at the surface of the part during the sintering of the latter. The invention also relates to a process for the manufacture by metallurgy of powders of mechanical parts with a technical and / or aesthetic function and, more specifically, of a decorative article. More particularly, the mechanical part can be a timepiece covering part chosen from the non-exhaustive list comprising a middle part, a back, a bezel, a push-piece, a bracelet link, a bracelet, a pin buckle, a dial, a hand, crown and dial index. It can also be a piece of jewelry.

The nickel-free austenitic stainless steel powder according to the invention comprises by weight: 10 <Cr <25%, 5 <Mn <20%, 1 <Mo <5%, 0.05 ≤ C ≤ 0.11%, 0 ≤ If <2%, 0 ≤ Cu <4%, 0 ≤ N <1%, 0 ≤ O <0.3%, the balance being made up of iron and any impurities. The term “possible impurities” is understood to mean elements which are not intended to modify the properties of the alloy but whose presence is inevitable since they originate from the powder manufacturing process. Any impurities such as B, Mg, Al, P, S, Ca, Se, Ti, V, Co, Ni, Zn, Se, Zr, Nb, Sn, Sb, Te, Hf, Ta, W, Pb and Bi can in particular be each present in a mass concentration less than or equal to 0.5%. For the purposes of the present invention, the term “nickel-free austenitic stainless steel” is therefore understood to mean an alloy containing not more than 0.5% by mass percentage of nickel. Advantageously, the nickel-free austenitic stainless steel powder according to the invention also has a diameter D90 less than or equal to 150 μm.

Preferably, the nickel-free austenitic stainless steel powder according to the invention comprises by weight: 15 <Cr <20%, 8 <Mn <14%, 2 <Mo <4%, 0.05 <> ≤ C <> ≤ 0.11%, 0 ≤ If <1%, 0 ≤ Cu <0.5%, 0 ≤ <> N <1%, 0 ≤ O <0.2%, the balance being made up of iron and any impurities as mentioned above.

More preferably, the nickel-free austenitic stainless steel powder according to the invention comprises by weight: 16.5 ≤ Cr ≤ 17.5%, 10.5 ≤ Mn ≤ 11.5%, 3 ≤ Mo ≤ 3.5%, 0.05 ≤ C ≤ 0.11%, 0 ≤ If ≤ 0.6%, 0 ≤ Cu ≤ 0.5%, 0 ≤ N <1%, 0 ≤ O <0.2%, the balance being made up of iron and any impurities as mentioned above.

The manufacturing process of the part consists in producing, by means of the aforementioned metal powder, a blank having substantially the shape of the part to be manufactured, then in sintering this blank. The blank can be produced by injection molding (MIM for Metal Injection Molding), pressing, extrusion or additive manufacturing. More specifically, in the case of injection molding, the blank can be produced by means of a mixture, also called feedstock, comprising the metal powder and an organic binder system (paraffin, polyethylene, etc.). Then the feedstock is injected and the binder is removed by dissolution in a solvent and / or by thermal degradation.

The blank is sintered in an atmosphere comprising a nitrogen-bearing gas (N2 for example) at a temperature between 1000 and 1500 ° C, preferably at a temperature between 1100 and 1400 ° C, even more preferably at a temperature between 1200 and 1300 ° C, for a time between 1 and 10 hours, preferably for a time between 1 and 5 hours. The characteristics of the sintering cycle, particularly the temperature and the partial nitrogen pressure, depend in particular on the composition of the alloy and are fixed so as to obtain a completely austenitic structure after sintering. It will be noted that the nitrogen content of the room can be adjusted by changing the partial nitrogen pressure of the atmosphere. In addition to nitrogen, other gases can be used in the sintering atmosphere, such as hydrogen and argon.

The sintering cycle can be carried out in a single step in the aforementioned temperature and time ranges. It is also possible to carry out the sintering cycle in two stages with a first stage in a temperature range between 1000 and 1200 ° C. for a time between 30 minutes and 5 hours, followed by a second stage in a range of temperatures between 1200 and 1500 ° C, preferably between 1200 and 1300 ° C, for a time between 1 and 10 hours. This first stage makes it possible to optimize the carbothermic reduction of the manganese oxides and / or the mixed oxides of manganese and silicon and thus to obtain a deeper deoxidized layer.

After sintering the nickel-free austenitic stainless steel powder according to the invention, additional heat treatments can be carried out on the sintered components such as, for example, a hot isostatic compression treatment to eliminate the residual porosity as much as possible. .

An additional heat treatment can also consist in eliminating the residual porosity at the surface of the sample. Thus, in accordance with application EP17202337.6 published under the number EP 3484009A1, the additional heat treatment can consist in treating the sintered part to transform the austenitic structure into a ferritic or two-phase ferrite + austenite structure on the surface of the part, and then to transform the ferritic or two-phase ferrite + austenite structure again into an austenitic structure so as to form a denser layer on the surface of the part. Furthermore, after the sintering step, the part can be subjected to a finishing treatment by stamping, also called forging.

The parts obtained with the nickel-free austenitic stainless steel powder according to the invention have, after sintering, a deoxidized surface layer whose thickness is at least 200 μm, this thickness preferably being greater than or equal to 250 µm and, more preferably, greater than or equal to 300 µm. In this deoxidized layer, the diameter of the oxides which are of substantially circular shape is less than 2 μm and the total surface or volume fraction of the oxides is less than 0.1%. FIG. 1 schematically illustrates the transition between the deoxidized layer 1 of thickness e and the rest 2 of the part 3 comprising oxides 4 of greater size and surface fraction. The line of demarcation between the so-called deoxidized layer and the rest of the sintered part and thus the thickness of the deoxidized layer can be easily determined by optical microscopy from one or more metallographic sections. Similarly, the surface or volume fraction of oxides can be determined by analysis of light microscopy images of polished sections of the part. The surface fraction corresponds to the ratio between the surface occupied by the oxides and the total surface of the deoxidized layer analyzed. The volume fraction can be deduced from the surface fraction by assuming that the oxides are circular. It will be noted that this transition between the deoxidized layer and the rest of the sintered part can only be observed if the parts have at least a near zero porosity on the surface, that is to say a relative density greater than or equal to 99%. . Indeed, in the presence of a large porosity, it will be difficult to differentiate the pore oxides by optical microscopy.

To illustrate the effect of carbon on the deoxidation of nickel-free austenitic stainless steel powders, parts shaped by MIM (Metal Injection Molding) from two powders (D90 = 16 µm) having different concentrations carbon were sintered at the same time in the same oven. Metallographic sections were made to measure and compare the thickness of the deoxidized layer between the two parts. The sintering was carried out in a reducing atmosphere comprising 75% N2and 25% H2 under a pressure of 850 mbar with a first stage at 1200 ° C for 3 hours followed by a second stage at 1300 ° C for 2 hours. A comparative sample was carried out using a powder comprising 0.03% by weight of carbon. More specifically, it is a Fe-17Cr-11Mn-3Mo-0.5Si-0.1O-0.5N-0.03C powder (% by weight). A microscopic view of a cross section of the sample after polishing is shown in Figure 2A. A transition line is clearly observed between the deoxidized layer 1 and the rest 2 of the sample. The thickness of the deoxidized layer is approximately 196 µm. Measurements carried out on several sections have shown that the values are substantially similar from one section to another.

A sample according to the invention was carried out using a powder comprising 0.07% by weight of carbon. More specifically, it is a Fe-17Cr-11Mn-3Mo-0.5Si-0.1O-0.5N-0.07C powder (% by weight). A microscopic view of a cross section of the sample after polishing is shown in Figure 2B. A transition line is also clearly observed between the deoxidized layer 1 and the rest 2 of the sample. The thickness of the deoxidized layer is considerably greater, with a value of approximately 335 μm.

Tests have also been carried out with a powder comprising 0.18% by weight of carbon, ie a powder Fe-17Cr-11Mn-3Mo-0.5Si-0.1O-0.5N-0.18C (% by weight). The decarburization resulting from the reaction of carbon with hydrogen in the atmosphere gave rise to very large differences in carbon concentration within the same charge, i.e. starting from the same powder composition, with the consequence of differences in the microstructure of the parts.

It goes without saying that the present invention is not limited to the preceding description and that various modifications and simple variants can be envisaged without departing from the scope of the invention as defined by the appended claims. It will be understood from the above that the present invention relates to the sintering of parts using powders of austenitic stainless steels without nickel and that its aim is to provide such parts which have a surface layer as thick as possible devoid of maximum of its oxides. However, such steels include high manganese concentrations. However, manganese is a component having a strong affinity for oxygen and the oxides which it forms can only be eliminated at temperatures close to the sintering temperatures. However, at the sintering temperature, the porosity closes quickly, which makes it more difficult to remove the oxides in which the manganese is included. Faced with this problem, the Applicant has demonstrated that compositions of austenitic stainless steels without nickel containing a carbon concentration at least equal to 0.05% by weight and not exceeding 0.11% by weight made it possible to obtain sintered parts with a surface layer having a lower concentration of oxides and whose thickness is greater than what has been observed on sintered parts obtained by means of known austenitic stainless steels without nickel known. It has in fact been observed that for carbon concentrations of between 0.05% and 0.11% by weight, the presence of carbon makes it possible to deoxidize more deeply the sintered parts while avoiding problematic decarburization, this deoxidation being further reinforced by the fact that it can be carried out at temperatures below the sintering temperature for which the density is still low and the pores relatively open.

Claims (20)

1. Austenitic stainless steel powder comprising by weight: - 10 <Cr <25%, - 5 <Mn <20%, - 1 <Mo <5%, - 0.05 ≤ C ≤ 0.11%, - 0 ≤ If <2%, - 0 ≤ Cu <4%, - 0 ≤ N <1%, - 0 ≤ O <0.3%, - 0 ≤ Ni ≤ 0.5%, with a balance consisting of iron and possible impurities each having a content of between 0 and 0.5%. 2. Powder according to claim 1, comprising by weight: - 15 <Cr <20%, - 8 <Mn <14%, - 2 <Mo <4%, - 0.05 ≤ C ≤ 0.11%, - 0 ≤ If <1%, - 0 ≤ Cu <0.5%, - 0 ≤ N <1%, - 0 ≤ O <0.2%, - 0 ≤ Ni ≤ 0.5%, with the balance consisting of iron and any impurities each having a content of between 0 and 0.5%. 3. Powder according to claim 1 or 2, comprising by weight: - 16.5 ≤ Cr ≤ 17.5%, - 10.5 ≤ Mn ≤ 11.5%, - 3 ≤ Mo ≤ 3.5%, - 0.05 ≤ C ≤ 0.11%, - 0 ≤ If ≤ 0.6%, - 0 ≤ Cu ≤ 0.5%, - 0 ≤ N <1%, - 0 ≤ O <0.2%, - 0 ≤ Ni ≤ 0.5%, with the balance consisting of iron and any impurities each having a content of between 0 and 0.5%. 4. Powder according to any one of the preceding claims, characterized in that the particles which form the powder have a diameter D90 less than or equal to 150 µm, that is to say that 90% of the particles which form the powder have a diameter less than or equal to 150 µm 5. Austenitic stainless steel part obtained by sintering a powder according to one of claims 1 to 4 with a nickel content of less than or equal to 0.5% by weight, characterized in that it comprises carbon in a content greater than or equal to 0.05% and less than or equal to 0.11% by weight and in that it has on the surface a layer (1) called deoxidized, said deoxidized layer (1) comprising oxides (4) according to a surface fraction less than that of the rest (2) of the part (3), the diameter of these oxides (4) being smaller compared to the rest (2) of the part (3), said deoxidized layer (1) having a thickness greater than or equal to 200 µm. 6. Part according to claim 5, characterized in that the deoxidized layer (1) has a thickness greater than or equal to 250 µm. 7. Piece according to claim 5 or 6, characterized in that the deoxidized layer (1) has a thickness greater than or equal to 300 µm. 8. Part according to any one of claims 5 to 7, characterized in that the deoxidized layer (1) comprises oxides (4) having a diameter less than or equal to 2 µm with a surface fraction less than or equal to 0.1%. 9. Part according to any one of claims 5 to 8, characterized in that the deoxidized layer (1) has an almost zero porosity, that is to say a relative density greater than or equal to 99%. 10. Part according to any one of claims 5 to 9, characterized in that the oxides (4) are manganese oxides and / or mixed oxides of manganese and silicon. 11. Piece according to any one of claims 5 to 10, characterized in that the piece (3) is a cladding piece in watchmaking. 12. Piece according to any one of claims 5 to 10, characterized in that the piece (3) is a piece of jewelry or fine jewelry 13. Part according to claim 11, characterized in that the covering part is chosen from the list comprising a middle part, a back, a bezel, a push button, a bracelet link, a bracelet, a pin buckle, a dial, a hand, a crown and a dial index. 14. Watch comprising the part according to claim 13. 15. Method for manufacturing an austenitic stainless steel part comprising the following steps: - provision of a powder according to one of claims 1 to 4, - development of a blank having substantially the shape of the part (3) to be produced using, at least, said powder, - sintering the blank in an atmosphere comprising a nitrogen-bearing gas at a temperature between 1000 and 1500 ° C for a time between 1 and 10 hours to induce the carbothermic reaction between oxides and the carbon present in the blank and to densify the blank. 16. The manufacturing method according to claim 15, characterized in that the sintering takes place in two stages with a first stage between 1000 and 1200 ° C for a time between 30 minutes and 5 hours, followed by a second stage between 1200 and 1500 ° C for a time between 1 and 10 hours. 17. The method of claim 15 or 16, characterized in that the blank is produced by injection molding, extrusion, pressing or additive manufacturing. 18. Method according to one of claims 15 to 17, characterized in that it comprises, after the sintering step, a forging step. 19. Method according to one of claims 15 to 17, characterized in that it comprises, after the sintering step, a step of hot isostatic compression. 20. Method according to one of claims 15 to 19, characterized in that it comprises, after the sintering step, a step of transforming the surface of the austenitic stainless steel into a ferritic or two-phase ferrite + austenite structure, and then a step of transforming said ferritic or two-phase ferrite + austenite structure into said surface into an austenitic structure, so as to form on the surface of the part (3) a so-called densified layer having a higher density compared to the core of the part ( 3).