Paramagnetic hard stainless steel and manufacturing method therefor

Abstract

One aspect of the invention relates to a paramagnetic stainless steel part (1) comprising a core (2) surrounded by a surface comprising at least a first zone (3a) and at least a second zone (3b), said core (2) and said second zone (3b) having a hardness HV1 between 500 and 900, and a microstructure formed of a sigma phase comprising a mass percentage between 40 and 80% and of an austenitic phase comprising a mass percentage between 20 and 60%, the part (1) being characterized in that the first zone (3a) is enriched in Ni compared to the core (2) and the second zone (3b), in that the first zone (3a) forms a layer consisting of 100% of an austenitic phase, said layer being called the austenitic layer (4) and in that the austenitic layer (4) has a hardness of less than 400 HV1. Another aspect of the invention relates to the manufacturing process of this part.

Description

Technical field of the invention

[0001] The invention relates to a part, in particular a watch component made of paramagnetic stainless steel having a core hardness of between 500 and 900 HV1 and a surface hardness of less than 400 HV1. It also relates to the manufacturing process of this stainless steel part. Technological background

[0002] Hard, non-ferromagnetic metal alloys find applications in numerous fields, primarily for components subjected to high mechanical and / or tribological stresses and requiring insensitivity to magnetic fields. This is particularly true for many watch components, such as wheels, pinions, axles, and springs within the movement. High hardness is also advantageous for external components, such as the case middle, bezel, case back, clasp, and crown. Indeed, a high hardness, exceeding 500 HV, generally provides better scratch and wear resistance, thus ensuring the durability of these components exposed to the elements. Alloys with such high hardness levels are generally ferromagnetic and therefore less suitable for watch components.

[0003] Recently, a paramagnetic stainless steel with a hardness between 500 and 900 HV10 was developed with a composition and microstructure disclosed in document EP 3 835 438. This steel comprises, by weight: 26 ≤ Cr ≤ 40%, 0 ≤ Mn ≤ 5%, 5 ≤ Ni ≤ 20%, 0 ≤ Mo ≤ 3%, 0 ≤ Al ≤ 5%, 0 ≤ Cu ≤ 2%, 0 ≤ Si ≤ 5%, 0 ≤ Ti ≤ 1%, 0 ≤ Nb ≤ 1%, 0 ≤ C ≤ 0.1%, 0 ≤ N ≤ 0.1%, 0 ≤ S ≤ 0.5%, 0 ≤ P ≤ 0.1%, The balance consists of iron and possible impurities, each with a content less than or equal to 0.5%. It has a microstructure consisting of a sigma phase comprising a mass percentage between 40 and 80% and an austenitic phase comprising a mass percentage between 20 and 60%.

[0004] It is produced using a specific process comprising the following steps: Provision or production of a blank having the aforementioned chemical composition and possessing a predominantly or completely ferritic structure, Heat treatment, known as hardening treatment, of the blank to obtain the part, the hardening treatment being carried out at a temperature between 650 and 900°C for a time between 30 minutes and 24 hours to transform the ferrite of said structure into an austenitic phase and an intermetallic sigma phase, the hardening treatment being followed by cooling to ambient temperature.

[0005] This particular microstructure, consisting of two non-ferromagnetic phases, makes it possible to obtain a very good compromise between hardness and toughness, good corrosion resistance and excellent polishability.

[0006] However, it is observed that the microstructure and composition of this steel could be optimized to improve the part's ability to withstand shocks in specific areas that are more stressed. Summary of the invention

[0007] The invention consists of optimizing the composition and microstructure of the prior art in specific areas of the surface of the part that are under greater stress.

[0008] To this end, the aforementioned steel manufacturing process is modified with a localized surface treatment step prior to the hardening heat treatment. This step involves selectively depositing a gamma-forming element, namely nickel, in specific locations and then performing a diffusion heat treatment, all before the hardening heat treatment. This diffusion heat treatment allows the nickel to diffuse to a given depth and transforms the ferrite into a 100% austenitic layer, which is characterized by its ductility, thereby improving impact resistance.

[0009] This results in a paramagnetic steel part with a core and a portion of the surface surrounding the core having a high hardness between 500 and 900 HV1, and with the other portion of the surface having a hardness below 400 HV1. The core and this portion of the surface comprise a microstructure formed of the sigma phase and the austenitic phase, while the other portion of the surface is formed of austenite without the sigma phase, which makes it possible to reduce the hardness while maintaining the paramagnetic character of the part.

[0010] More specifically, it is a paramagnetic stainless steel part comprising a core surrounded by a surface including at least a first zone and at least a second zone, the core and the second zone having a chemical composition comprising by weight: 26 ≤ Cr ≤ 40%, 0 ≤ Mn ≤ 5%, 5 ≤ Ni ≤ 20%, 0 ≤ Mo ≤ 3%, 0 ≤ Al ≤ 5%, 0 ≤ Cu ≤ 2%, 0 ≤ Si ≤ 5%, 0 ≤ Ti ≤ 1%, 0 ≤ Nb ≤ 1%, 0 ≤ C ≤ 0.1%, 0 ≤ N ≤ 0.1%, 0 ≤ S ≤ 0.5%, 0 ≤ P ≤ 0.1%, the balance being made up of iron and possible impurities each having a content less than or equal to 0.5%, said core and said second zone having a hardness HV1 between 500 and 900, and a microstructure formed of a sigma phase included in a mass percentage between 40 and 80% and of an austenitic phase included in a mass percentage between 20 and 60%, the part being characterized in that the first zone is enriched in Ni compared to the core and the second zone, in that the first zone forms a layer made up of 100% of an austenitic phase, said layer being called the austenitic layer and in that the austenitic layer has a hardness less than 400 HV1.

[0011] More specifically, it is the manufacturing process for a paramagnetic stainless steel part, which includes the following steps: a) Provision or production of a blank having substantially the shape of the part to be manufactured or being of a different shape, the blank having the aforementioned chemical composition and possessing a predominantly or completely ferritic structure, b) Deposition of a Ni layer over the entire surface or only over the first zone of the surface with a step b') of local dissolution of the Ni layer on the second zone if the deposition is carried out over the entire surface or with a local machining step on the second zone, c) Heat treatment, known as diffusion treatment, of the blank at a temperature between 1050°C and 1400°C to diffuse the Ni to a given depth of the blank below the first zone and to transform the ferrite within said given depth into a 100% austenitic phase forming the austenitic layer, d) Heat treatment, known as hardening treatment, of the blank to obtain the part,The hardening treatment is carried out at a temperature between 650 and 900°C for a time between 30 minutes and 24 hours to transform the ferrite within the core and the second zone into an austenitic phase and a sigma intermetallic phase; the hardening treatment is followed by cooling to ambient temperature. Brief description of the figures

[0012] There figure 1 represents a cross-sectional view observed under polarized light optical microscopy of a part according to the invention with a surface layer of ductile austenite. figure 2 This represents, for the same piece, a cross-sectional view observed under polarized light optical microscopy of a portion of the surface lacking the ductile austenite layer, with this portion of the surface and the core having the same microstructure formed of the sigma phase and the austenitic phase. figure 3represents a cross-sectional view observed by electron microscopy of a part according to the invention with a surface layer of ductile austenite that stops crack propagation under stress. figure 4 is a schematic cross-sectional view of the part according to the invention. The figure 5 This represents a schematic view of a crown cap, with the areas subjected to stress during an impact indicated by the arrows. figure 6 represents the work at break in Nmm for samples of different thicknesses with some samples comprising a ductile austenite layer on the surface according to the invention and some samples lacking the ductile austenite layer on the surface according to the prior art. Detailed description of the invention

[0013] The invention relates to parts made of paramagnetic stainless steel, predominantly with a hardness between 500 and 900 HV1, as well as the manufacturing process for parts made from these steels. HV1 hardness is defined as Vickers hardness measured according to ISO 6507-1:2018. For example, the part may be a watch component. This could be a case component chosen from a non-exhaustive list including a case, case back, bezel, crown, pusher, bracelet link, bracelet, pin buckle, clasp (e.g., a folding clasp), dial, hands, and dial markers. It could also be a movement component chosen from a non-exhaustive list including a gear, axle, pinion, spring, bridge, mainplate, screw, and balance wheel.

[0014] The parts 1 have a different chemical composition and microstructure between the core 2 of the part and a part 3a of the surface around the core 2 ( figure 4 This part 3a of the surface, also called the first zone, is enriched in Ni relative to the core 2 of the part and the remainder 3b of the surface, also called the second zone, to form, after heat treatment, an austenite layer 4. According to the invention, only certain areas of the surface are targeted. These are the areas most stressed during an impact. For example, the figure 5The diagram uses arrows to show the internal areas of a crown guard that are subject to stress and require a more ductile layer to improve impact resistance. The entire surface is not covered by the ductile layer because it would be detrimental to have a ductile layer on the outside of the crown guard; higher hardness is desired for the exterior of the part. Another example is the case. Certain parts of the case are used to close the case back and are characterized by their thinness. Increasing the ductility of these parts with an austenite layer would be highly beneficial.

[0015] The nickel-enriched austenite layer typically has a thickness of less than 500 µm, and more typically on the order of 10-20 µm. It should be noted that the choice of layer thickness will depend on the size of the part; the ductile layer must occupy a relatively small volume compared to the total volume to maintain the benefit of an overall hard part.

[0016] At its core and on the part of the surface not enriched with nickel, the part is made of stainless steel having the following composition by weight: 26 ≤ Cr ≤ 40%, 0 ≤ Mn ≤ 5%, 5 ≤ Ni ≤ 20%, 0 ≤ Mo ≤ 3%, 0 ≤ Al ≤ 5%, 0 ≤ Cu ≤ 2%, 0 ≤ Si ≤ 5%, 0 ≤ Ti ≤ 1%, 0 ≤ Nb ≤ 1%, 0 ≤ C ≤ 0.1%, 0 ≤ N ≤ 0.1%, 0 ≤ S ≤ 0.5%, 0 ≤ P ≤ 0.1%, the balance being made up of iron and possible impurities each having a content less than or equal to 0.5%.

[0017] Preferably, stainless steel has the following composition by weight: 28 ≤ Cr ≤ 38%, 0 ≤ Mn ≤ 3%, 5 ≤ Ni ≤ 15%, 0 ≤ Mo ≤ 3%, 0 ≤ Al ≤ 3%, 0 ≤ Cu ≤ 2%, 0 ≤ Si≤ 5%, 0 ≤ Ti ≤ 1%, 0 ≤ Nb ≤ 1%, 0 ≤ C ≤ 0.05%, 0 ≤ N ≤ 0.05%, 0 ≤ S ≤ 0.5%, 0 ≤ P ≤ 0.1%, with always a balance consisting of iron and any impurities, each having a content less than or equal to 0.5%.

[0018] More preferably, stainless steel has the following composition by weight: 30 ≤ Cr ≤ 36%, 0 ≤ Mn ≤ 3%, 5 ≤ Ni ≤ 10%, 0 ≤ Mo ≤ 1%, 0 ≤ Al ≤ 1%, 0 ≤ Cu ≤ 1%, 0 ≤ Si≤ 3%, 0 ≤ Ti ≤ 1%, 0 ≤ Nb ≤ 1%, 0 ≤ C ≤ 0.05%, 0 ≤ N ≤ 0.05%, 0 ≤ S ≤ 0.5%, 0 ≤ P ≤ 0.1%, with always a balance consisting of iron and any impurities, each having a content less than or equal to 0.5%.

[0019] This core with the part of the surface not enriched in nickel has a hardness HV1 between 500 and 900 and a microstructure formed of a sigma phase included in a mass percentage between 40 and 80% and an austenitic phase included in a mass percentage between 20 and 60%.

[0020] The other part of the surface has a composition similar to that of the core but with an enrichment in nickel. It has a hardness of less than 400 HV1, preferably between 150 and 350 HV1. It is composed of 100% austenitic phase.

[0021] According to the invention, the process for manufacturing a stainless steel part comprises a step a) of providing or producing a blank having a composition within the aforementioned ranges. This blank has a predominantly ferritic structure or, preferably, 100% ferritic. The blank is obtained from a base material subjected to heat or thermomechanical treatment at a temperature within a temperature range between 950°C and 1450°C, followed by quenching. The base material may be in the form of a powder or a consolidated material. It may be produced by casting, pressing, molding, injection ( MIM: Metal Injection Moulding ), by additive manufacturing, and more broadly by powder metallurgy. It is conceivable to produce the base material and the heat treatment in a single step, for example, by an additive manufacturing technique with a laser ( SLM: Selective Laser Melting). These different techniques make it possible to create a rough draft with a base material having dimensions that are approximately equal to those of the part to be produced, in which case a subsequent shaping step is not required.

[0022] The composition of the base material is optimized to obtain a predominantly or completely ferritic structure when held at a temperature between 950°C and 1450°C for a period of 1 minute to 24 hours. The temperature is chosen to obtain an austenite mass fraction of 40% or less and a ferrite mass fraction of 60% or more. The presence of austenite ensures minimum hardness and maximum ductility, allowing for easy shaping, for example, by forging, cutting, or machining.

[0023] Heat or thermomechanical treatment in the 950°C–1450°C range can be used to perform homogenization, recrystallization, or stress-relieving treatments on cast base materials, or to sinter base materials in powder form. Treatment in the ferritic or ferritic-austenitic range can be carried out in a single cycle or involve several heat or thermomechanical treatment cycles. It can also be preceded or followed by other heat or thermomechanical treatments.

[0024] After being held in the ferritic or ferritic-austenitic state, the blank is subjected to rapid cooling, also known as quenching, to a temperature below 500°C to prevent the formation of new phases during cooling. Thus, the ferritic or ferritic-austenitic structure is preserved at room temperature. With the compositions according to the invention, the ferritic structure is sufficiently stable to be preserved at room temperature after rapid cooling but sufficiently metastable to be easily and rapidly transformed into a sigma phase and an austenitic phase during subsequent heat treatment at intermediate temperatures between 650°C and 900°C.

[0025] At the end of step a), the alloy exhibits low hardness and high ductility, allowing for easy shaping if necessary, for example by forging, cutting or machining.

[0026] After step a), the process includes an optional step of shaping the blank by machining, cutting, or any operation involving deformation, such as forging. This step can be carried out in several sequences. This step is not required if the blank at the end of step a) already has the final shape of the part to be manufactured. This step could also be carried out after the diffusion heat treatment step described below. As mentioned later, this step could also be used to selectively remove the Ni-enriched layer mechanically.

[0027] In addition to shaping, a plastic deformation operation can be used to increase the rate of ferrite transformation during the subsequent transformation step into austenite and the sigma phase. Furthermore, since work hardening is weak for ferritic structures and the alloy according to the invention is predominantly or entirely ferritic before hardening, this plastic deformation step does not cause problematic hardening for subsequent machining or cutting. This plastic deformation, in one or more sequences, can be carried out at a temperature below 650°C.

[0028] The process then comprises steps b) and c), which are more specifically the subject of the invention and are aimed at selectively depositing nickel on the surface of the blank and diffusing this nickel to a given depth within the part. It should be noted again that these steps could potentially be carried out before the shaping step, if there is one.

[0029] In step b), nickel is deposited either over the entire surface of the blank or over a portion of it. Typically, the layer is deposited by electroplating or PVD. The deposited layer has a thickness of between 1 and 20 µm, preferably between 3 and 10 µm, and more preferably between 4 and 10 µm. When nickel is deposited only on a portion of the blank's surface, the surface is partially masked to target the nickel deposition. Masking can, for example, be achieved with a lacquer that will be subsequently dissolved. If nickel is deposited over the entire surface, there is a step b') to dissolve the deposit where it is not desired, or a machining step to selectively remove the nickel layer before or after the diffusion step, or even after the hardening heat treatment step.One way to achieve localized nickel plating is to mask the areas where the layer is desired, for example, with lacquer or simply by positioning an element such as a plug if it is a hollow area to be coated with an austenite layer, and then soak the part in an acid bath such as dilute HNOs for a few hours. The part is then rinsed. Following the localized nickel plating step, a heat treatment step (c) of the part is carried out at a temperature between 1050°C and 1400°C, preferably between 1200°C and 1300°C, for a time between 5 minutes and 5 hours, preferably between 5 minutes and 1 hour, to diffuse the nickel into the alloy and transform this area, previously composed mainly of ferrite, into austenite. The result is a piece with a layer of ductile austenite in certain areas of the surface.The layer is 100% austenitic, with a thickness depending mainly on two factors related to nickel diffusion in the alloy, namely the Ni deposition thickness and the high-temperature diffusion treatment time.

[0030] At this stage of the process, the core and the portion of the surface not enriched with nickel are still predominantly or entirely composed of ferrite. In step d), a hardening heat treatment of the blank is carried out between 650°C and 900°C, preferably between 700°C and 800°C, to obtain the final properties. The duration of the heat treatment between 650°C and 900°C is set to ensure complete transformation of the ferrite and thus the formation of a microstructure composed of a sigma phase and an austenitic phase in the core of the part and on the untreated portion of the surface, the nickel-enriched layer formed of austenite remaining stable, without transformation.

[0031] The rate of transformation of ferrite into austenite + sigma phase depends primarily on the alloy composition and its thermomechanical history, as previously mentioned. Generally, the treatment time ranges from 30 minutes to 24 hours. After the hardening treatment, the steel exhibits a sigma phase mass fraction between 40% and 80% and an austenite mass fraction between 20% and 60%, the percentages depending on the chemical composition and the heat treatments performed. The core and surface of the resulting non-Ni-enriched part have a high hardness of between 500 and 900 HV1 thanks to the hardening heat treatment.

[0032] As with all stainless steels, small amounts of non-metallic inclusions may also be present without affecting the mechanical and magnetic properties. Furthermore, inclusions that improve machinability, such as manganese sulfides, may also be present in small quantities in the alloy.

[0033] This heat treatment hardening step can be followed by a possible surface finishing step e) such as polishing.

[0034] Furthermore, if the blank has an austenite-ferrite structure in step a), the manufacturing process may include an additional step before the Ni diffusion heat treatment in step b), in the temperature range of 950°C–1450°C, to transform the austenite-ferrite structure into a 100% ferritic structure. Alternatively, this step may be combined with the diffusion step.

[0035] In summary, after high-temperature heat treatment (950°C-1450°C) followed by quenching, the steels exhibit the following properties: Hardness between 150 and 400 HV1. Good ductility with plastic deformation without cracking exceeding 50% under compression at room temperature. Ferromagnetic behavior, due to the presence of ferrite.

[0036] After localized nickel deposition with diffusion and hardening heat treatment, the steels according to the invention exhibit, in particular, the following properties: Hardness between 500 and 900 HV1 in the core and on the non-nickel-enriched surface. Hardness below 400 HV1 in certain surface areas. Non-ferromagnetic behavior. Excellent polishability due to its very fine microstructure. Good wear resistance. Improved impact resistance in high-stress areas. Good corrosion resistance.

[0037] Tests were carried out with a 5 µm thick galvanic nickel coating over the entire surface of the parts. The nickel layer was selectively dissolved after masking the areas where the layer was desired. Dissolution was achieved by immersion in a dilute HNOs bath, with 20 ml of HNOs in 100 ml of H₂O for 19 hours. The nickel diffusion heat treatment was carried out at 1250°C for several tens of minutes. The hardening heat treatment was then carried out at 750°C for 12 hours. As shown in the figure 1 The austenite layer 4 has a thickness of 10-20 µm. At the figure 2 , we observe the areas of the surface without an austenite layer. The austenite layer 4 helps to stop the cracks 5 that develop towards the surface of the sample during stress and thus to delay failure ( figure 3 ).

[0038] Flexural tests were also performed on samples with an austenite coating covering their entire surface, as opposed to samples without such a coating. The samples were fully coated with an austenite layer to simplify the tests. We observe at the figure 6 that the presence of an austenite layer results in a greater energy absorption during a bending test.

Claims

1. Part (1) made of paramagnetic stainless steel comprising a core (2) surrounded by a surface comprising at least a first zone (3a) and at least a second zone (3b), - the core (2) and the second zone (3b) having a chemical composition comprising by weight: • 26 ≤ Cr ≤ 40%, • 0 ≤ Mn ≤ 5%, • 5 ≤ Ni ≤ 20%, • 0 ≤ Mo ≤ 3%, • 0 ≤ Al ≤ 5%, • 0 ≤ Cu ≤ 2%, • 0 ≤ Si ≤ 5%, • 0 ≤ Ti ≤ 1%, • 0 ≤ Nb ≤ 1%, • 0 ≤ C ≤ 0.1%, • 0 ≤ N ≤ 0.1%, • 0 ≤ S ≤ 0.5%, • 0 ≤ P ≤ 0.1%, the balance consisting of iron and possible impurities each having a content less than or equal to 0.5%, said core (2) and said second zone (3b) having a hardness HV1 of between 500 and 900, and a microstructure formed of a sigma phase in a mass percentage of between 40 and 80% and an austenitic phase in a mass percentage of between 20 and 60%, the part (1) being characterized in thatthe first zone (3a) is enriched in Ni compared to the core (2) and the second zone (3b), in that the first zone (3a) forms a layer formed 100% of an austenitic phase, said layer being called austenitic layer (4) and in that the austenitic layer (4) has a hardness less than 400 HV1.

2. Part (1) according to claim 1, characterized in that the austenitic layer (4) has a hardness between 150 and 350 HV1.

3. Part (1) according to one of the preceding claims, characterized in that said part (1) is a component of the exterior or of the watch movement.

4. Method for manufacturing the part (1) made of paramagnetic stainless steel according to one of the preceding claims, comprising the following steps: a) Provision or production of a blank having substantially the shape of the part (1) to be manufactured or being of a different shape, the blank having the chemical composition according to claim 1 and having a predominantly or completely ferritic structure, b) Deposition of a layer of Ni over the entire surface or only on the first zone (3a) of the surface with a step b') of local dissolution of the layer of Ni on the second zone (3b) if the deposition is carried out over the entire surface or with a local machining step on the second zone (3b), c) Heat treatment, called diffusion treatment,of the blank at a temperature between 1050°C and 1400°C to diffuse the Ni over a given depth of the blank below the first zone (3a) and to transform the ferrite within said given depth into a 100% austenitic phase forming the austenitic layer (4), d) Heat treatment, called hardening treatment, of the blank to obtain the part (1), the hardening treatment being carried out at a temperature between 650 and 900°C for a time between 30 minutes and 24 hours to transform the ferrite within the core (2) and the second zone (3b) into an austenitic phase and a sigma intermetallic phase, the hardening treatment being followed by cooling to room temperature., 5. Method according to the preceding claim, characterized in that the Ni layer has a thickness of between 1 and 20 µm, preferably between 3 and 10 µm, more preferably between 4 and 10 µm.

6. Method according to claim 4 or 5, characterized in that when Ni is deposited only on the first area (3a) of the surface, said surface is partially masked so as to target the Ni deposition.

7. Method according to one of claims 4 to 6, characterized in that step b') of local dissolution is carried out after having masked the first zone (3a).

8. Method according to one of claims 4 to 7, characterized in that step b') of local dissolution is carried out in an acid bath such as HNOs.

9. Method according to one of claims 4 to 8, characterized in that it comprises a step of shaping the blank if said blank from step a) has a different shape from the part (1) to be manufactured, the shaping step being carried out between steps a) and b) or between steps b) and c).

10. Method according to one of claims 4 to 9, characterized in thatthe structure of the blank in step a) comprises a mass fraction of austenite less than or equal to 40% and a mass fraction of ferrite greater than or equal to 60%.

11. Manufacturing method according to one of claims 4 to 10, characterized in that the structure of the blank in step a) comprises 100% ferrite.

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